The Offshore Voyaging Reference Site

12 Tips To Avoid Ruining Our Easily Driven Sailboat

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 GearArea (m²)Drag CoefficientNewtons
Dodger1.5.655
Cockpit enclosure added to dodger3.6110
Dinghy in davits1.5.655
Arch with 300 w of rigid panels1.637
Wind turbine*n/an/a75
Total332

*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?

  1. The true wind drops.
  2. The boat slows, and the more drag we have added the faster she will slow.
  3. The apparent wind drops by the sum of the true wind speed and boat speed.5.
  4. Wind pressure on the sails, and therefore propulsive force, drops off by the square of the apparent wind speed change—that’s a lot.
  5. The sails have the same area, but are producing less forward force since they are feeling much less pressure.
  6. 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.
  7. The boat slows some more.
  8. Repeat steps 2 to 7 until the wind picks up again.

The wind increases back to mean and then overshoots into a gust (typical):

  1. The added drag prevents the boat from accelerating as quickly.
  2. So the added force on the sails heels the boat, slowing her acceleration even more (excessive heel is slow).
  3. The boat tries to round up more because of the added heeling.
  4. This requires more rudder angle to counteract.
  5. 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

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

  1. 6 knots is 3.09 meters/second. ↩︎
  2. Very approximate, but close enough to illustrate the point. Assumes 50% gearbox and prop combined efficiency. ↩︎
  3. Again, the numbers are very approximate, particularly since I guessed the drag coefficient big time. ↩︎
  4. 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. ↩︎
  5. Approximately, actually depends on true wind angle. ↩︎
  6. 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. ↩︎
  7. 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. ↩︎

More Articles From Online Book: How To Buy a Cruising Boat:

  1. The Right Way to Buy a Boat…And The Wrong Way
  2. Is It a Need or a Want?
  3. Buying a Boat—A Different Way To Think About Price
  4. Buying a Cruising Boat—Five Tips for The Half-Assed Option
  5. Are Refits Worth It?
  6. Buying a Boat—Never Say Never
  7. Selecting The Right Hull Form
  8. Five Ways That Bad Boats Happen
  9. How Weight Affects Boat Performance and Motion Comfort
  10. Easily Driven Boats Are Better
  11. 12 Tips To Avoid Ruining Our Easily Driven Sailboat
  12. Learn From The Designers
  13. You May Need a Bigger Boat Than You Think
  14. Sail Area: Overlap, Multihulls, And Racing Rules
  15. 8 Tips For a Great Cruising Boat Interior Arrangement
  16. Of Cockpits, Wheelhouses And Engine Rooms
  17. Offshore Sailboat Keel Types
  18. Cockpits—Part 1, Safe and Seamanlike
  19. Cockpits—Part 2, Visibility and Ergonomics
  20. Offshore Sailboat Winches, Selection and Positioning
  21. Choosing a Cruising Boat—Shelter
  22. Choosing A Cruising Boat—Shade and Ventilation
  23. Pitfalls to Avoid When Buying a New Voyaging Boat
  24. Cyclical Loading: Why Offshore Sailing Is So Hard On A Boat
  25. Cycle Loading—8 Tips for Boat and Gear Purchases
  26. Characteristics of Boat Building Materials
  27. Impact Resistance—How Hull Materials Respond to Impacts
  28. Impact Resistance—Two Collision Scenarios
  29. Hull Materials, Which Is Best?
  30. The Five Things We Need to Check When Buying a Boat
  31. Six Warnings About Buying Fibreglass Boats
  32. Buying a Fibreglass Boat—Hiring a Surveyor and Managing the Survey
  33. What We Need to Know About Moisture Meters and Wet Fibreglass Laminate
  34. US$30,000 Starter Cruiser—Part 1, How We Shopped For Our First Cruising Sailboat
  35. US$30,000 Starter Cruiser—Part 2, The Boat We Bought
  36. US$30,000 Starter Cruiser—How It’s Working Out
  37. Q&A, What’s the Maximum Sailboat Size For a Couple?
  38. At What Age should You Stop Sailing And Buy a Motorboat?
  39. A Motorsailer For Offshore Voyaging?
  40. The Two Biggest Lies Yacht Brokers Tell
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