A lot of the secret to the fun cruises I have experienced over five decades is to make the journey much of the reward, rather than just focusing on the places we cruise (those have been way cool, too) and enduring the passages.
And to keep the journey fun (assuming we decide we want a sailboat) we need to buy one that’s easily driven, but once we have the boat we also need to guard against inadvertently ruining her sailing capabilities.
After all, if we buy our first cruising sailboat and then promptly screw up her sailing abilities how will we ever know what we are missing?
And moving a boat from place to place, or even crossing oceans, maybe especially crossing oceans, efficiently, safely and skillfully under sail is one of life’s truly beautiful experiences. It’s a pity to inadvertently miss out on that.
And if, dare I say it, we have already screwed up our boat, or bought one that someone else screwed up for us, we should at least give consideration to fixing that (not that hard) and then trying her out, even if we then make the decision to revert…bet we don’t.
And if we cruise in a motorboat, or have a cruising sailboat but don’t actually sail much, these tips will still make getting there more comfortable and fun.
Let’s dig in.
Before I go any further, much of what I will write I always knew from experience to be true, but my case will be much more compelling in this article thanks to the real numbers brilliantly explained by Eric Klem and Matt Marsh in comments and articles. So this article stands on their shoulders. Thanks, guys.
#1 Understand How Little Power We Have
Eric Klem estimates that a 40′ cruising sailboat moving at six knots in 20 knots of apparent wind from forward of the beam is being propelled along by only about 1000 newtons or 225 lbf (pound-force).
To put that in perspective, that’s about 3000 newton meters/second1 or roughly the same propulsive force as an 8-hp2 engine.
I know, that low number stunned me, too—engineers, please check me on that and everything that follows.
That’s all we have to overcome drag from the water and air that the boat is passing through.
So anything we add to our boat that sticks up into the air flow, or down into the water flow, is going to eat away at that precious force.
Another way to think about this is that when we are sailing at a given speed, the drag from water and air are perfectly balanced by the propulsive force, so any drag we add will slow us down.
#2 Minimize Drag
Now let’s look at adding and removing drag-inducing stuff to and from our boats.
Air Drag
For example, let’s add a few things to our 40-foot boat and look at the drag with 20 knots of apparent wind from forward3:
| Added Gear | Area (m²) | Drag Coefficient | Newtons |
|---|---|---|---|
| Dodger | 1.5 | .6 | 55 |
| Cockpit enclosure added to dodger | 3 | .6 | 110 |
| Dinghy in davits | 1.5 | .6 | 55 |
| Arch with 300 w of rigid panels | 1 | .6 | 37 |
| Wind turbine* | n/a | n/a | 75 |
| Total | 332 |
*I guesstimated the wind turbine by looking at the predicted output in watts and doubling it to account for propeller inefficiency and drag from the unit itself. Propellers are high-lift and therefore high-drag devices.
Yikes! Just a few common additions have used up a third of our available propulsive force.
So does that mean we will be sailing at four knots instead of six all the time? No, even to windward it’s not that simple (more in a moment).
And as soon as the apparent wind is aft of the beam, all that stuff (sort of) becomes extra sail area rather than speed-robbing drag.
That said, we need to remember that even when the true wind is on the beam, the apparent wind, which is what counts here, is way forward of that—in real cruising the apparent wind is forward of the beam a lot.
There is no good drag, just drag we decide is worth it to get a given benefit.
Water Drag
Now let’s turn our attention to where drag hurts even more: in the water. Water is about 800 times denser than air and, worse still, any drag we add below the waterline reduces our speed upwind and down.
Let’s start with propellers. I used to be a PHRF handicapper, and we typically gave a boat 12 seconds per mile off her handicap for a three-blade exposed prop.
So instead of expecting her to sail at say six knots, it would be 5.88. That doesn’t sound like that much, but know that PHRF handicappers are notoriously unkind to those who choose not to fit low-drag props, so the actual speed decrease is more.
And I know from experience that just a good coating of slime (no shell) on the McCurdy and Rhodes 56 slows her by half a knot, and the J/109 is about the same.
#3 Don’t Rely On Hard Driving
There is good news to take into account when trying to quantify the negative effects of drag, both air and water:
Friction drag (air and water) is reduced as a percentage of overall drag as we get closer to hull speed (speed-to-length ratio of about 1.4)4.
So if we are driving the boat hard we (kinda) have propulsive force left over that can’t be used because of the huge increase in wave drag that occurs at hull speed.
So in that situation, adding air and water drag will slow the boat less than proportionally.
Or, to put it another way, at least on a reach and a run we can overcome drag by crowding on sail.
That said, driving a boat hard is tough enough on a full racing crew. For a shorthanded cruising crew, often not as young as we were, it’s foolhardy and leads to accidents.
Not having to drive hard and sail at high heel angles while enduring more motion is why we bought an easily driven boat in the first place, so let’s not screw that up.
#4 It’s Cumulative
None of these added drag items are that bad by themselves, but they add up. Let’s guesstimate that:
Broad reaching and running:
- 0.12 knot for a fixed prop, at least
- and .5 knot for a slimy bottom;
Equals .62 knot.
And going to windward I would conservatively estimate that all the air drag we added above would rob us of an extra knot.
So let’s call that 1.25 knots—yes, I’m bending over backward to be kind here.
But, hey, we got to add all that stuff on deck, saved our money by not buying a feathering prop, and didn’t have to dive and clean the bottom.
So maybe that’s a reasonable trade-off?
And, in fact, you often hear exactly that from a sailor who has calculated the drag induced by a piece of gear they want to add.
#5 Beware Simple Drag Calculations
But the problem is these simple drag calculations ignore three real-world factors:
- Deceleration in lulls
- Apparent wind
- Pitching
Add ’em together and the news gets worse, way worse.
#6 Think About Deceleration and Apparent Wind
Let’s start with the first two.
The wind is never steady, there are always lulls and puffs.
So what happens in a lull?
- The true wind drops.
- The boat slows, and the more drag we have added the faster she will slow.
- The apparent wind drops by the sum of the true wind speed and boat speed.5.
- Wind pressure on the sails, and therefore propulsive force, drops off by the square of the apparent wind speed change—that’s a lot.
- The sails have the same area, but are producing less forward force since they are feeling much less pressure.
- This is made even worse on a sensibly sailed cruising boat because we have reefed early so there is little “spare” propulsive force to be had.
- The boat slows some more.
- Repeat steps 2 to 7 until the wind picks up again.
The wind increases back to mean and then overshoots into a gust (typical):
- The added drag prevents the boat from accelerating as quickly.
- So the added force on the sails heels the boat, slowing her acceleration even more (excessive heel is slow).
- The boat tries to round up more because of the added heeling.
- This requires more rudder angle to counteract.
- That adds yet more drag—any more than 4° of rudder angle is super-slow.
Adding more drag is a double-edged sword, because it makes the boat slow a lot more in lulls and take longer to accelerate in puffs, which means that the loss in average speed is way more than a simple drag calculation would indicate.
#7 Watch The Weight
All of the above assumes smooth water, but what happens when we start sailing offshore in waves? To understand that we need to think about weight, and where we put it.
Luckily for us, Eric Klem has already done a masterful job explaining that in an article—one of the most important to a good cruising experience that we have ever published. If you haven’t read it, do.
So I’m going to build on that by sharing a vicious circle of sailing performance robbing decisions that I see far too often out there:
#8 Avoid Stern-Down Trim
All weight added to a cruising boat has a performance impact, but if the boat is well designed and the amount of weight added is kept within reasonable bounds in relation to the displacement of the boat, all is well and we can still make fast and comfortable passages.
This is why filling the tanks does not matter much, and for example, the weight saving of lithium batteries helps less than one might imagine, as long as both were sensibly sited near the middle of the boat by the designer.
But what often happens out there in the real world of cruising is that we fill the lazaret with heavy gear, and then add an arch, davits and a big RIB with a huge outboard.
But wait, Eric has shown us that weight aft is not really that bad for pitching moment because the axis of pitch is usually quite far aft on most boats.
And that’s true, but here’s the problem:
It’s the distance from the centre of gravity, which is about the middle of the boat, that affects trim6.
So now we are down by the stern, and immersing the transom more than the designer intended, which is super-slow and also sucks for deceleration in a lull.
So, surely, we can fix that trim problem just by adding the same amount of weight forward, so not a problem?
Right, but there’s an even worse problem lurking:
#9 Watch Out For The Sucky Physics
This is where the good news that the axis of pitch is quite far aft becomes bad news:
- To get the boat back in trim we must add the same weight the same distance forward of the centre of gravity as the weight we added aft of it7.
- But because the axis of pitch is way aft of the centre of gravity, the weight we added forward to trim the boat to her lines is way further from the centre of pitch than the weight aft that started this.
- And pitching moment scales by the square of the distance from the axis of pitch.
The result is that if we load up the stern, bringing the boat back into trim by adding weight forward has a huge negative effect on pitching moment.
And this is added to the gear that a cruising boat must have up forward, like a big anchor and a good chain rode.
Let’s summarize:
- Stern-down trim is bad.
- Added pitching moment makes the boat both slow and uncomfortable.
- Because we need to keep the boat in trim, adding excessive weight far from the centre of gravity is always bad, whether it’s forward or aft, although generally weight aft is not quite as bad.
#10 Think About Passages
All this is bad enough when coastal cruising, where the speed reduction of adding a bunch of drag might be .6 knots downwind and 1.5 knots upwind (adjusted for deceleration), when compared to the speed of the same boat with a feathering prop, clean bottom, and a reasonable amount of stuff on deck.
But this assumes smooth water, which is why a lot of cruisers never realize they have ruined their boats until they leave sheltered waters and head offshore, where there is always swell and where we can’t go anchor when the going gets tough.
How much more when we add swell and waves? Probably impossible to calculate with any precision, and certainly way past my pay grade to even attempt, but I can tell you from experience that it’s at least half as much again, but let’s be kind and call it one knot downwind and two knots upwind.
Now let’s imagine a passage to Bermuda from the US East Coast, where the wind will almost certainly be forward of the beam for over half the passage.
Let’s say that the high-drag boat will average 1.25 knots slower than her low-drag sister ship—again I’m being kind here.
For our average 40 footer that’s at least a day longer before we hoist a beer at the White Horse Pub in St. George’s. And that’s also another day for bad weather to catch us at sea.
#11 To Windward Really Sucks
But wait, it gets worse.
Suppose we get unlucky and most of the trip is close reaching with a day and half hard on the wind—pretty common on that passage.
When the wind comes on the bow, even our fully optimized 40 footer will only make 3 knots (at best) toward Bermuda.
But on our high-drag boat with a bunch of weight in the ends, VMG (velocity made good) will be cut in half, (again at best) because not only is the boat going slower, it’s not pointing as well—as the boat slows and pitches, the keel and rudder become less efficient. VMG is a fragile thing.
It’s a long way to Bermuda at 1.5 knots VMG!
That means a windward passage can easily take twice as long just because we added a bunch of stuff on deck, were too cheap to buy a low-drag prop, and didn’t clean the bottom. And the passage will be way less comfortable.
That’s bad enough, but many boats are so badly screwed up that they simply can’t make any headway to their destination once the wind and waves are up and forward.
This is probably why more than half the boats we see out there these days going to windward or even close reaching, even on a coastal daysail, are motor-sailing, and even then they are less comfortable and going slower than a boat that has not been ruined.
And that’s not fun.
#12 Prioritize
So does this mean that we can’t have a dinghy in davits, or an arch, or a cockpit enclosure, or put the gear we need in the lazaret?
Of course not. Each of us bought our boats with our own money and can do with them whatever the heck we want.
But the key takeaway is that if we just add stuff without really understanding the negative effects, or worse still, kid ourselves that said effects don’t exist, we will ruin our boats and sailing will no longer be fun, or, worse still, we may never find out how much fun sailing is.
So, sure, add a dinghy on davits if that’s what works for you, but buy a light one, and a lighter and smaller outboard for it—do we really need a huge RIB driven by a 15-hp motor?
And I would be the last person to suggest going to sea without a dodger, and I have even owned a cockpit enclosure—not sure I would have done that if I had done the above calculations—but really think about whether you need both, and even if the answer is yes, make them as small and streamlined as you can.
Then buy a feathering prop, repaint the bottom regularly, forgo the wind generator, and cut the solar array back from the 1 kW we were lusting after, to half that or less.
Having some stuff will not ruin our cruising sailboats, but having it all surely will.
Further Reading
- In this article I focused on drag and weight, but sails and rigging matter a lot too.
Comments
If you have questions, ask away. And if you are an engineer or have relevant technical training, please speak up if you think I got any of the numbers materially wrong.
That said, please don’t nitpick every detail, since, given that I rounded the negative effects way down, focusing on small differences is meaningless to the overall point.
Foot Notes
- 6 knots is 3.09 meters/second. ↩︎
- Very approximate, but close enough to illustrate the point. Assumes 50% gearbox and prop combined efficiency. ↩︎
- Again, the numbers are very approximate, particularly since I guessed the drag coefficient big time. ↩︎
- I don’t like the term hull speed much and prefer to think in speed-to-length ratios, but hull speed is the generally used term and I’m keeping things simple here. ↩︎
- Approximately, actually depends on true wind angle. ↩︎
- This is a simplification that’s probably just OK on most boat’s, but on boats that have much wider sterns than bows things get more complicated. See Matt’s excellent comment for a good explanation of why. ↩︎
- Once more I have simplified. In fact, we could add half the weight twice as far forward or double the weight half as far to balance the weight aft and bring the boat back into trim. Read Eric’s post for the details. ↩︎
The propulsion hp estimate for a 40ft boat sailing upwind at 6 knots sounds quite low to me. In the ship world we call such an estimate Effective Horse Power or EHP. This is only the drag on the hull (basically wavemaking + fiction) and is used as the basis for calculating powering requirements for engine sizing and ultimately propeller optimization, usually done through scale model testing. It doesn’t account for other forms of drag. For instance, a boat sailing upwind will have induced drag as it takes on leeway. A boat heeled up will have more induced drag. Add the air drag of the sails and rig and that required driving force climbs. EHP is quite a bit lower than Brake HP of the engine and Delivered HP which is what actually is required to drive a ship accounting for other losses, so it’s not like you can stick an 8hp engine over the side and expect 6 knots of speed. I know that wasn’t the intended inference but it gets complicated. Anyway, I haven’t done this stuff in 20 years so some of my details might be a little off but the general concept is right.
Hi Jesse,
I’m sure you are right, but as you say, the general concept is right, and that’s what matters given that all I’m trying to do with that number is show how little propulsive force we have here in a way that will be more intuitive than newtons or lbf.
I also took into account propeller and gear box losses, see footnote.
Also, in my experience with a clean 40 footer, 8HP is about right assuming no wind and a flat sea. For example my J/109 does about 6 knots at less than a half gallon an hour, which using the old rule of thumb of 18 hp = 1 us gal an hour works out close, as does the power curve for the engine.
And amazing though it is, the M&R 56, which displaces at least twice as much as a good 40 footer does six knots while only using about 15 hp at the flywheel.
So this also passed the real world experience test too.
Hi John and all,
I had a 38-footer with nice lines and an engine with gremlins.
One season we were without an engine on multiple occasions. My inflatable as a yawl boat with an 8hp engine could get her up to 5-6kn boat speed without opening up the throttle in flat water, no wind, with ease.
Good article. Thanks, Dick Stevenson, s/v Alchemy
Hi Dick,
That’s a good real world data point, particularly since the props on outboards are far from optimized for that kind of use. In fact, the more I think about it, the more I think I may have calculated/estimated high on the HP comparison.
Hi John,
Along similar lines, I have relished, and been very pleasantly surprised, when coastal cruising, to be out early in the quite light morning wind, the seas are flat and before power boats get out and, to be going 5-6 knots close hauled in 6-8kn of wind or so and a comfortable 8 degrees of heel. Magical.
And, if really lucky, the smell of bacon cooking is wafting up from below.
My best, Dick Stevenson, s/v Alchemy
I’d say John’s figures are ballpark correct; I’ve seen comparable figures come out of the resistance calculation codes many times now, and fuel flow data from a wide range of boats supports this. It’s surprising just how slippery a sailboat hull is at speeds where it isn’t making much of a wake. They have to be that way; there simply isn’t very much power in the wind close to the surface at the wind speeds most of us like to sail in.
Hi Matt,
Good to hear. I always hold my breath after publishing one of these technical articles in case you or Eric find a glaring error! On the other hand, I would not want to take this kind of thing on without your support and auditing.
Hi All,
I don’t have access to the calculation that came up with the 1000N where I am right now but my memory is that I at least included all hull drag (including drag coming from generating lift to counteract leeway). This was for calm water as stated, waves would make this quite a bit different and it becomes a dynamic problem not a static one. What I can’t remember is how aerodynamic drag was handled and it may have been ignored at these comparatively low windspeeds for everything but the sails. Drag on surfaces like a dodger can get tricky because much of a dodger does not see the full AWS, especially as you heel. For that reason, when calculating the drag on a solar array, I specified that it was tall so that it was up in the full airstream. I don’t know of any rules of thumb for these items that are close to the deck, maybe someone like Matt does.
From the standpoint of the article, I think the important thing is as John mentions that the amount of power is remarkably low so we need to make sure not to waste it. I know that I have noticed it in the real world and see it regularly on boats that give up sailing to weather in anything but ideal conditions. Which is really too bad as for a few hour coastal sail, my favorite thing to do is to see what I can get the boat to do upwind.
Eric
This figure from Yachting Monthly shows a calculated 75 kg hull drag for an Oceanis 325 at 6 kt. Add in a little for the propeller drag then scale up to 40 feet, then 1000 N looks resonable.
https://www.yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807
Hi William,
That looks like a really interesting article. I will put some time aside to read through it, but first glance at the article says that we PHRF handicappers, as I suspected, do not give near enough credit for fixed blade props.
From Sails Magazine’s article on the Dehler 30 OD:
“The so-called Stealth Drive is usually seen on custom high-tech race boats, but Dehler have managed to do it on a 30-footer, the drag saving equivalent to an estimated 7–8 seconds a mile. The shaft and prop are on a retractable strut, easily operated by a lever in the cockpit. When lowered, the three-blade prop pushes the boat along well; over six knots with ease. When retracted, the strut is hermetically sealed behind a cover.”
https://www.sailsmagazine.com.au/multiple-personalities/
Hi PD,
That’s interesting, but probably not a level of complication I would want on an offshore boat. To me the best compromise between motoring efficiency and drag is a feathering prop.
I actually think the best compromise is a folding prop like Flexofold. More efficient when motoring due to curved rather than flat blades, less drag, and nearly impossible to snag lines when sailing with it folded. And half the cost of a max prop.
Yes reverse is worse than with a feathering prop but I find a few 100 more RPMs easily does the trick when docking, and frankly the time we all spend in reverse is less than 1/1000th the time we spend sailing or motoring.
The only remaining advantage with feathering is the ability to tweak the pitch.
But regardless, I agree that fixed props should be replaced on any boat that seriously wants to sail.
Hi Michael, have you tried both? I’m trying to decide between folding and feathering and I agree the reasons you stated above, also folding ones doesn’t need any maintenance unlike the others.
Yes I had a variprop feathering on my prior boat but that required a feathering prop since it had a closed aperture. I liked it, but always preferred the robust and simple nature of a modern geared folding prop.
Flexofold is maintenance free except for anode, and is much less expensive. I truly don’t notice any reversing issues other than the usual problems with crosswinds which have nothing to do with the prop. I’m sure a feathering prop would give me more thrust at lower RPMs but I have plenty of power in reserve so the folding prop stops me just fine when I need it.
My current boat (Tartan 40) came with a fixed 3 blade and I didn’t notice any difference in reverse when I switched to a Flexofold.
I had an old Maxiprop on my boat when I bought it but it basically fell apart after the first lift and anode replacement. I opted for a cheaper Flexofold and I have noticed a little bit less power in reverse but nothing the Volvo 2040 can’t handle with a few more revs.
Hi Yigit,
Having owned both folding and feathering props over some 40 year time span, I would always default to feathering for a cruising to get the better stopping and reverse power. A good feathering prop is generally even better in reverse than a fixed prop because the blades rotate through 180 degrees so the leading edge is used in both forward and reverse.
Hi Everybody,
The folks at Yachting Monthly did quite some comprehensive testing in 2015: https://www.yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807 – Based on these tests I opted for a Flexofold (leaning to the overpropped size according to some anonymous advisor) for my semi-custom Berckemeyer 40, and I am very satisfied.
Hi Hedrik and all
I did exactly the same and switched a maxprop for a Flexofold. It gave higher speeds in forward and seems to drag less and minimally. Only “drawback” is less propwalk in reverse.
Best wishes
Charles
Charles L Starke MD
s/v Dawnpiper
Hi Henrik,
Yes, I read that report. Very well done. In fact you have reminded me that I need to do a tip recommending it.
Dear Henrick
Thanks for the reminder of the excellent article I had forgotten but which influenced my choice of a new prop at the time.
I grew up crewing in the early Seventies on ocean racing boats which mostly had a five or ten gallon fuel tank. In the late Seventies I begun cruising in my own boats which had similar sized tanks and a couple of cans lashed on deck. The engine was mainly for going in and out of harbour. My principal bête noire was then and to a large extent still is manoeuvring in tight spaces.
Going backwards is when bad things happen. The prop that performs best in reverse is the one for me. And damn prop wash. The more extreme the more it is predictable.
PS God bless the previous owner of my boat who installed a bow thruster – something I was too much of a purist to ever have done… I like to think I use it sparingly.
PPS The chap who did the purchase survey on my boat enthusiastically recommended the Flexofold. I chose to spend half the cost of a new Flexofold on the service of the old Maxprop that has served the boat the last thirty odd years… Time will tell.
PPPS Ultimately its an aesthetic decision. Maxprops, Variprops, Featherstreams et al just look right.
Hi Mark,
I understand the appeal of a bow thruster, but it probably should be noted, for those who might be considering one, that a bow thruster is a significant drag on hull slipperiness: especially embedded in an essay on maintaining one’s easily driven hull. Hull shape and the quality of the fairing when constructed makes a difference, but my sense is that it is in the ballpark of fixed blade prop. Dave Gerr has written about this, but I have been unable to locate the article.
It is my experience that most of us, with practice, can get along fine in marinas and I would point everyone to the series John/AAC did on just this subject a while back as a place to get some thoughts to take into practice.
My best, Dick Stevenson, s/v Alchemy
Oops. Touché, Dick.
Hi Mark,
I hope not “touche” as in any kind of contest: I had no intention along those lines. And I understand the appeal and wisdom of a bow thruster for many cruisers, but, like many (all?) things on a boat, there is no free lunch and too many of these “labor savings” devices have down-sides that are little discussed.
And, John’s advice on marina maneuvering is well worth time spent on the articles, even with a bow thruster.
My best, Dick Stevenson, s/v Alchemy
Great points on the bow thruster, Dick. I am thinking of having one on my next bigger boat and hadn’t considered the drag. Also agree on John’s maneuvering articles. Before studying them, I was a fairly nervous marina driver (much happier at sea). But putting in to effect the advice John gives made last season a real pleasure (although not removing all nervousness). I even had the confidence to park my 41 foot boat within a couple of feet of a multi-million dollar yacht in front and an angry plastic-fantastic owner behind (I have convinced myself he was just jealous at my maneuvering skills) all on an angled pontoon in a swell with a wind blowing off. The Flexofold did a great job at bringing the stern in just right at the final moment with the prop walk.
Hi Michael,
I suspect John would not want to remove all nervousness: a certain amount of trepidation sharpens the senses. But I also suspect all those who manage boats in tight quarters would benefit just as you did.
My best, Dick
Haha no danger of that, Dick. I was even nervous leaving the dock with that big expensive boat in front of me.
Hi Michael,
Understood: I have a certain amount for anxious trepidation whenever I am around anything hard. Exacerbated a good deal by those things being expensive and breakable and made worse by the likelihood of having an audience. (and now CCTV cameras and people with smart phones and a direct line to “Funniest Home Videos”).
It is enough to make one turn around and go to sea…
My best, Dick
Hi All,
While I find the Yachting Monthly test to be the most quantitative and easy to digest, I find it extremely hard to interpret. Some of the things that I find hard to understand are:
Given these issues, I came away thinking that some generalizations can be made but nothing more. A much more meaningful test would be with a variable flowrate lab rig that gave plots for power, efficiency at different slip rates and speeds. I remember there being a test tied to MIT that tried to do this a while ago but I also remember the results taking much more effort to interpret. Even with a lab rig, it can be quite frustrating as the matrix of data you want to take is amazingly large, for example, I would love to add shaft angle and some rough hull shapes to make the data much more realistic.
Eric
Hi Eric,
As so often happens with your comments, a whole bunch of good points I never thought of, but should have. Your first about pitch really hit me between the eyes! I know from my own experiments with a maxprop that getting the pitch right can make a huge difference to performance. And even there it depends on what you are trying to accomplish. For example we found that we could get quite a lot more speed at a given RPM in smooth water with a corser pitch, but that it cost us in stopping power and punching up wind, so the sweet spot was a bit finer. And that with a prop that has quite large pitch steps. Point being that a vendor that just lucked out on the pitch chosen could be a big winner without it actually meaning anything.
Big take away for me: select a prop with user changeable pitch. Way more important to efficiency than brand, at least in m most cases. I think I will do a tip on just that, unless you see a hole in my logic?
Hi John,
I agree completely that pitch is very critical to get right and that a vendor could have really lucked out in this test and another might have really lost out. In fact, the different tests run would suggest that different pitches would be optimal for the different tests. I actually think that the general recommendations for pitch are too simplistic as they assume you are optimizing around 1 thing and people may wish to optimize around something else but that is a different subject.
To me the requirement is to have a plan for getting the pitch right. One very reasonable solution is to have a prop that allows user adjustment. Planning on a repitching is perfectly possible and I have had it done several times but it costs money and takes diligence to actually do it. Another factor is how good the prop supplier is at getting the pitch right the first time, Flexofold seems to be quite good at this for example but I can think of another prop manufacturer which is really quite poor and all of them that I know of have needed to be repitched.
Eric
Hi Eric,
Thanks for the fill on that. As you say, I may be out on the drag for things like the dodger, but given how much I rounded drag and speed reduction down, together with yours and Matt’s thoughts, I think if anything I have underestimated the effects. My thinking was better that, than be too alarmist to the point that people would write off the whole article.
And I agree, getting the best out of a good boat up wind is hugely rewarding.
John,
Fantastic article illuminating a surprisingly complicated issue! Despite a fair amount of experience, my intuition regarding drag on sailing performance accounted for only a small fraction of the factors that you’ve explained here. Much thanks for describing the origins of drag, its initial effects and knock-on effects in such an accessible way.
I now understand much better the motives for keeping the Adventure 40 as sleek on deck and as lightly loaded as possible. The tradeoff of features such as stern arch with solar panels is a far worse “deal” than I imagined.
Hi Scott,
Thanks for the kind words. And yes, part of the inspiration for writing the article was to have an explanation we can link to when someone wants to know why the A40 does not come with an arch and davits.
What your figures show, which is demonstrated in the real world, is how easy it is to stall, or inefficiently power the boat through practises e.g. pinching close hauled, poor sail shape, sheet angles. The small amount of power available can easy be consumed through parasitic losses or inefficient practises. For example, I changed my blown Dacron sails for vectran, a world of difference in speed. In light airs a ghoster on my forestay will power the boat along, the new vectran furling head sail is much slower as it is a compromise compared to the original sail plan design. We see the same between fixed blade, folding and variable pitch props. Small changes in area that have dramatic impacts on speed, which of course is derived from power. I don’t know if your figures are correct, but they feel right knowing how small changes can impact performance significantly.
At such low power availability, marginal gains practises are worth their weight in gold i.e. 1% improvements add up significantly: hull friction, weight in the ends, props, sail shape, steering efficiency, crew comfort , halyard stretch, auto helm set up and underway adjustments et cetera. More or less what AAC is about.
Hi Alastair,
Good point on all the other things that matter. Reminded me that I should add a further reading link from the article above to our sails and rigging online book, thanks.
Very useful and interesting. I put my solar on an arch because I did not want to give up deck space. Maybe a bad compromise. Perhaps I should be able to figure the answer to the following question, but instead I will pose it here. I like rowing and sailing a hard dinghy. I have a nesting dinghy, 5′ long when nested. I carry it forward of the mast. That adds windage up there, which means I use a riding sail at anchor earlier than others might. Anyway, what does that windage do to performance going up wind? I could revert to a rolled up inflatable, but they are less satisfying to row and they don’t sail. Thanks,
Hi Terence,
I hear you on the joys of hard dinghies. We considered that for years on the M&R 56, although in the end stayed with inflatable primarily because we wanted to be able to strip the decks when going to sea. so there is that aspect to the decision too.
No question that a hard dinghy forward will slow you down, both from windage and added pitching moment since having it forward is further from the pitching axis. My guess is the added pitch would be a lot more significant than the windage.
So, as always, it comes down to a trade off that only you can balance off.
That said, given you have an arch and that dinghy, it would be a good idea to be really selective about anything else you add and to make sure you have good sails, the rig is tuned, the bottom clean, a feathering prop, and that all weight you can is moved out of the ends of the boat.
Hi Terence,
I too, have a nesting dinghy and love to row. It lives between the mast and the dodger where it fits pretty well and is more protected from boarding seas and leaves the foredeck clear for working the asym, whisker pole etc. I am sure there is still a hit from windage, but being right in front of the dodger and lower profile, I suspect it is minimal overall. Visibility from the cockpit is some compromised which gets us on our feet with greater regularity.
I was willing to “notch” my dinghy to get around my mast collar, but did not need to.
Those considering a nesting dinghy might work out storage ahead of time: many nesting dinghies are bespoke and the design can be massaged to make storage easier.
My best, Dick Stevenson, s/v Alchemy
While the average size of a cruising boat used to be well below 40’, it should be stated that size does matter. Larger boats are more tolerant of a dinghy in davits, a few extra cases of rum, and extra solar panels and crew, etc. Morgan’s Cloud is no exception. It appears to me that boat size is increasing too. The catamaran wave is perhaps part of that? It is perfectly good seamanship to solve some of the wants and needs we sailers have by simply choosing -and paying for – a boat large enough to sustain them without losing trim. No shaming needed.
Hi Robert,
Sure, the bigger the boat the less this stuff matters, but that does assume that the we don’t scale the stuff with the boat. From what I see out there many bigger boat owners buy bigger ribs with bigger outboards and fit bigger arches with more solar etc, so the benefit of having a larger boat goes away.
Other cruisers were always amazed that Phyllis and I did not have a big RIB in davits on the M&R 56 but passing on that sort of stuff was a lot of why she sails so well.
Anyway, I agree that there are benefits to bigger boats and more load carrying ability is one of those: https://www.morganscloud.com/2019/08/15/you-may-need-a-bigger-boat-than-you-think/
Hi John,
I think it is great that you are highlighting performance instead of focusing on adding gear through spending money which seems to be the standard way to look at boat “upgrades”. There are some places where spending money to get better gear can really improve performance such as sails, props, etc. but this isn’t the normal cycle of add more electronics, need more solar, need a generator, etc.
What got me thinking about all this years ago was watching the difference between similar or identical designs that were used for cruising and racing and seeing how much faster the racing boats were. Another interesting observation was how rarely we would be passed to weather when sailing old fishing schooners which are not exactly the pinnacle of weatherliness. Every now and then someone would blow by us to windward but most of the time we could pass any cruising boat we came across.
You mention wallowing which is always interesting to me as you can be going along just fine and then an almost imperceptible decrease in windspeed can leave you with a horrible motion and very little progress. Often just shaking a reef makes it feel like the wind increased 5 knots when in truth it was just that you reached a critical power threshold again.
And it is possible to horribly screw up a boat. I always think of the Windjammers doing the nitrate trade in the late 1800’s and early 1900’s. Their route westward around the horn basically consisted of tacking back and forth attempting not to lose ground then hoping for a favorable shift so that they could make their westing as many of those ships could barely tack through 150°. In a few examples, after tacking back and forth for more than a month and not succeeding in rounding, they turned east and went the long way round the bottom of the globe.
This whole exercise has me back to thinking about specific things on our boat that we could improve on. The boat has much better performance than when we got her but there are still some big concessions that hurt performance.
Eric
All good points, Eric.
I’ll emphasize again the importance of being able to make good angles to windward. This is one of the first things you lose when you start adding too much air drag. Here on the Great Lakes, two-thirds of every trip is spent close-hauled. Not reaching, or close reaching — if you want to make real progress, you are hard on the wind to within 5° of luffing for a fair chunk of each day. If you mess up your boat’s ability to go upwind here, you may as well just skip the sailing entirely and enjoy burning that $2.60/L diesel.
Hi Matt,
I look at a lot of boats and say to myself, “why do you even bother with the added cost and complication of a rig? If you want all that stuff on the boat, just buy a motorboat and be done with it”. And most sailboats that are so encumbered motor most of the time, so they would actually save money, and probably fuel and carbon too, by converting to a relatively efficient motorboat.
Hi Eric,
Good point on the difference between the same two designs, one optimized (racing or not) and one cluttered and poorly optimized is eye popping. When I was a PHRF handicapper I would get sailors bitching at me about their handicap but then when I went and looked at the boat one glance showed that their poor results were nothing to do with the handicap.
And yes, being under-canvased is horrible and (counterintuitively) uncomfortable too, particularly at sea.
Hi John,
Agree completely about being under-canvassed: true anytime, but particularly in a waning wind with leftover seas. Much better, more comfortable and predictable (and predictable translates into safety), to have the boat driving through the water/seas than having the seas dominate.
Among the cruising lore I occasionally challenge is the oft mentioned/recommended habit of reefing down at night: pretty much choosing to be under-canvassed. In early days, we occasionally did so, but quickly found the boat less conducive for sleeping.
In random discussions over the years, I have had the impression that many who adhere to the reef-at-night as-a-habit lore (there are situations where anticipatory reefing at night makes sense) did not have reefing systems that were easy to use and/or did not trust the less experienced crew to make the call when on watch.
My best, Dick Stevenson, s/v Alchemy
I wanted to estimate how much time a world cruiser would spend upwind. I used Predictwind’s route planning software and a good all-round sailboat that wouldn’t cause the software to distort the optimal route to match a distorted performance profile. I chose a J109 that seems to travel at about the same speed regardless of TWA* certainly not a screaming sled reaching nor a wallowing pig close hauled, just a nice balance everywhere. I told Predictwind I’d not use the motor.
Starting from Opua (NZ) I went to Papeete & Hiva Oa in French Polynesia, Ecuador, Panama, Puerto Rico, Trinidad & Tobago, Haiti, Bermuda, Azores, Southampton, Norway, Alaska, Japan, Darwin, Port Moresby, New Caledonia, & back to Opua. The total trip was about 200 days. Depending on the routes chosen minimum upwind was 20% (40 days) and maximum upwind was 45% (89 days.) Obviously, the real world would be a bit different. Nonetheless, I think the numbers useful. If we’re going to spend at least 20% to windward then all the attention to reducing drag seems worth it.
On the other hand, I decided to see where I could get to without ever going up wind: Starting again at Opua I ended up at the Great Barrier Reef via Tierra Del Fuego, Falklands, South Georgia, Cape Town, St Helena, Barbados, Martinique, Dominica, Montserat, St Marten, British Virgin Is, Dominican Republic, Bahamas, Jamaica, Nicaragua, Panama, Galapagos, Hawaii, Philippines, Singapore, and Darwin: 180 days with just 8 hrs upwind between Singapore & Darwin. So it seems if we don’t have to go anywhere specific, we can sail forever with eased sheets.
Why do all this analysis? Well partly it’s fun dreaming about the future voyages, but also it seems important to find the right balance between weather protection and safety (and I think sailing performance is a safety issue.) I’m not interested in carrying a big dingy on davits, anything on deck, a wind generator, or permanent solar panels mounted high. But a pilot house would do a lot against melanomas, it’d keep the rain off, and minimise fatigue. It would also increase drag and raise the COG. On balance I think I’d have put the enclosure on the McR 56 too.
Cheers,
Paul
*J109 polar summary:
4.5 – 6 knots at 45deg,
5 – 8.5 at 135, and
3 – 7.5 at 180
Hi PD,
That’s interesting, but keep in mind that air drag hurts any time the apparent wind is forward of the beam, not just beating, and water drag and pitching hurt performance regardless of apparent wind direction.
The other issue is that sure we can avoid going upwind a lot of the time with careful planning, but none the less we will have to go to windward sooner or later and when that happens if we have ruined the boats performance it will be truly horrible and may even be dangerous.
I would love to be on a voyage where you can choose to not go upwind. We don’t have that luxury. A boat that isn’t easily driven when hard on the wind, here in the Great Lakes, is just a motorboat with a really expensive stick.
And let’s not be too quick to dismiss pilot houses. A really elegant, well-thought-out one with rigid flat glass windows and with laminated solar panels bonded to its top surface might have a drag coefficient on the order of Cd = 0.35. Compare to a drag coefficient of 0.55 to 0.75 for a soft dodger, bimini, and tubular steel arch with rigid solar panels mounted horizontal on the arch. The well-engineered pilot house / deck house also has a smaller projected area than the clutter of steel, canvas, and off-the-shelf rectangular panels. And it can be built of very light foam- or honeycomb-cored composites to minimize the impact on weight distribution. And, if you give it a watertight hatch that is actually kept closed, it’ll contribute favourably to the righting moment should there ever be a full capsize.
Considering how many boats leave their canvas and clutter up 24/7/365, I think there are non-negligible gains in performance to be found by integrating all that stuff in a more elegant, streamlined fashion, right from the factory, with a well-engineered hard dodger or pilot house.
Hi Matt,
Agree completely with regards to pilot houses. They seem to get easier to design as a boat gets bigger: I am not sure what the lower limit is.
And, I think one element plays against their being designed into modestly sized boats and that is that most modestly sized boats are not designed for cruising. They are designed for day-sailing, occasional weekending in fine weather and not to be sailed on the shoulders of the season.
Boats with even a modest nod in design to cruising, unlike the A40, are a rare entity.
My best, Dick Stevenson, s/v Alchemy
Hi Dick,
So true. I think the lower limit for a good pilot house that is not too boxy and has good sight lines is around 48 to 50 feet. Under that I think a hard dodger is a better bet.
“I am not sure what the lower limit is.”
16′ apparently;-)
https://www.offcenterharbor.com/videos/a-16-foot-pilothouse-motorsailer-youve-got-to-be-kidding/
Hi Matt,
I totally agree on hard dodgers and pilot houses, as long as they are done well they can actually reduce the clutter and windage. The A40 is a case in point.
On points #8 & #9 about trim and loading:
When we get as far into the physics as John is leading us here, we need to consider both the centre of water plane area (CWP) and the centre of gravity (CG) The latter will be vertically in line with the centre of buoyancy (CB). Sailboats with a blunt end and a pointy end generally have the CWP somewhat aft of the CG & CB. (On boats with two pointy ends, they are much closer together.)
Forces and moments relating to changes in static pitch and trim act about the CWP, not the CG. Adding one ton of cargo a metre in front of the CG might trim the boat bow-down quite significantly. Add the same one ton a metre aft of the CG instead, and you might see zero change in trim at all; in this case, the weight will be very close to the CWP.
Things get fun when you start looking at dynamic motions. The total moment of inertia of the boat in each axis — i.e. the thing we are trying to minimize by keeping weight out of the ends and the rig — is measured with respect to the CG. But the forces exerted on the boat by the water depend on the second moment of area of the waterplane with respect to the CWP.
One consequence of this is that, on a real boat, there is no such thing as a pure pitch motion or a pure roll motion. Motions are always coupled. Even in the totally idealized, infinitesimal-limit case, you cannot have pitch about the CWP without heave.
Another consequence is that the effect of the placement of weight on trim is not the same as its effect on dynamic pitch and roll. Consider a 10 m boat with its CG exactly at midship (5 m) and whose pointy-bow, blunt-stern waterline yields a CWP farther aft, at the 6 m station mark. It’s sitting nicely on its lines as designed.
Overload it with one ton of extra stuff in the fore cabin, at the 3 m station mark. That’s 1000 kg * (5 m – 3 m)^2 = 4000 kg*m^2 of moment of inertia, and 1000 kg * 9.81 m/s^2 (6 m – 3 m) = 29.4 kN*m of trimming moment (bow-down).
Move the same one ton of stuff to the cockpit, at the 7 m station mark. It contributes the same 1000 kg * (5 m – 7 m)^2 = 4000 kg*m^2 of moment of inertia. But it produces only 1000 kg * 9.81 m/s^2 * (6 m – 7 m) = -9.8 kN*m of trimming moment (the – implying stern-down).
Both cases are equally bad for moment of inertia, and therefore for dynamic pitch behaviour once you get out in a seaway. But only one of them has a noticeable effect on trim at the dock.
The moral of the story is:
Keep all the weight you can control as close to the CG as possible, to keep the moments of inertia to a minimum.Keep weights that change over the course of a voyage (eg. fuel, food) longitudinally close to the CWP whenever possible, to minimize the change in trim and in dynamic behaviour as they are consumed.Excess weight forward is always bad, and is often obvious from the trim.Excess weight aft, particularly on a broad-sterned boat, can have a brutal impact on moment of inertia (and therefore on comfort and on performance to windward) without having a visibly obvious effect on static trim.
Hi Matt,
For my purposes, although I dug into it a bit, I decided to ignore the difference between centre of gravity and centre of water plane, primarily to keep things simple and also because I would not have been able to discuss the differences as well as you just did, so thanks for the fill.
Based on that I will change a couple of words to make clear that weight aft is not as bad as forward on most boats, but still not good.
Hi Matt, that’s a very clear explanation. Thanks for both qualitative and quantitative descriptions!
With “no pure motion”, does that mean that roll will generally be accompanied by some degree of pitch (in the forward direction for a boat with pointy bow and blunt stern)? And that overloading forward will cause the bow to dig in even in beam seas?
An aside to you and all the other very knowledgeable and experienced commenters– the quality of AAC articles + discussion continues to astound. Many times it’s like dropping into the office hours of a bunch of friendly professors! 😄
Thank you.
And yes. In mathematical terms, one of the notable characteristics of boats that John would describe as “easily driven”, in the sense we’re using the term here, is that they should have a relatively weak coupling between roll, pitch, yaw, and heave motions. The boat pitching back and forth should not cause it to bob up and down; the boat rolling should not cause a reaction that induces a yaw that the autopilot or helmsman would have to fight.
We’ll get into that a little more in an article that’s coming soon. The essence of this argument is that symmetry (not necessarily in the mirror image sense of a double-ender, but in the sense that the hull lines and areas are balanced around the critical points) tends to yield low couplings between the different motions, thus a predictable and easily managed boat. Whereas, if the different motions are strongly coupled (as is likely if symmetry is broken either through design, or through add-on modifications, or through bad loading), there are some tricks that can be exploited to improve performance and speed, but the potential for undesirable and unintuitive dynamic behaviours goes up.
Hi Matt,
Not sure if you will see this but I will give it a shot.
Do you know where the actual pitch axis tends to be on these wide stern boats say motoring into 4′ chop? If we were out in space with a one time impulse, it would be about the CG but in actual waves, it is going to be very dependent on wave shape, hull shape and mass distribution. And to make it worse, it isn’t actually 1 axis, it has to move around although I have never been able to discern it moving around when observing so it probably isn’t huge. I can imagine how you would write the dynamic code to simulate it but it would take a lot of tuning with real data to get right. I have found that most boats with more balanced ends have the pivot point well aft of the CG in the mentioned conditions but I don’t have enough experience on wide stern boats in the example conditions to know how they behave and that much volume aft is definitely going to impact it. Thankfully, with the parallel axis theorem, it isn’t difficult to translate the axis location.
Eric
Great stuff, John. My contribution to the number crunching going on here is to point out that you have 2 #3s in the article (i.e. your number #3 Minimize Drag is a typo and should be #2). That is about the limit of my contribution to this kind of conversation (but I can kind of follow).
Hi Michael,
Oops, thanks. Fixed now.
Having a feathering propeller is a wonderful thing. Keep in mind, though you have to know how to make it feather. We have an Autoprop which we dearly love. Learned a long time ago that by putting the transmission in forward, which, in our case locks the prop shaft, we will gain 0.75 kn boat speed. So if we’re sailing at 5 kn with the propeller freewheeling, lock the shaft we will now be doing 5.75 kn increases speed happens very quickly that’s a 15% increase in boat speed. For me that’s huge .
I wish I would have read this article and the other article on easily driven hulls before buying my first sailboat! Converting from power boating to sailing, I convinced myself I needed the heaviest, traditional, full keel boat I could find. I tried everything to make her sail better: changed the fixed blade prop to.a Campbell sailor (did not work because the blade area is too small to drive a heavy boat) could not fit hi tech props because of the small aperture. Reduced weight, new sails, reduced engine weight, Li battery bank – nothing helped with poor sailing abilities and the terrible hobby horse. Downeast 32
I fixed my problem – bought a Wauquiez Pretorien, installed a Flexofold prop on the saildrive, put my tools and heavy items in the storage locker in the middle by the mast and….boat sails like a dream!
Hi Dan,
That’s a sobering story indeed! Glad you have a boat you can enjoy now, but sorry it was such a long path. That said, seems like many of us make a mistake on the way to the right boat: https://www.morganscloud.com/2012/03/17/are-refits-worth-it/
Your thoughts and comparison of air drag vs water drag make me once again reconsider the 3 blade prop and the opening water strainer that projects from the hull… something I’ve wanted to fix for a long time.
Then there is all the turbulence from the standing rigging. There is a significant difference in the Reynolds number for a round stay compared to a more streamlined foil of similar size. So why aren’t we clipping on neatly designed feathering lightweight foils to our stays?
Hi Rick,
Foil shaped rod was tried back in the day (1970s if memory serves) it proved to have higher drag than round, not lower. Again if memory serves, the problem is that the foil rod needed to be oriented fore and aft, but the apparent wind is at an angle, so the foiled rod actually has a higher drag that round. The other problem was that foil rod was incredibly expensive and also the sharp edges were dangerous to both sails and crew. And finally, foil rod could not be rolled for transport, so had to be delivered on flat bed trucks.