One of my first transits of the Cape Cod Canal in Massachusetts ended in an eye-opening experience when our small cruising boat was almost stopped by 2- to 4-foot wind-over-tide chop while exiting the west end into Buzzards Bay.
It wasn’t until years later that I realized why we had pitched so uncontrollably in such otherwise tame conditions:
Although the boat design wasn’t known for pitching, several adults in the cockpit made her very stern heavy, plus several modifications by the owner had greatly increased the pitch moment of inertia. Together, the improperly-distributed mass resulted in the boat pitching horribly.
Mass (weight¹), how much, and its location, affects the performance of all boats.
The goal of this article is to help you think about where and how you add or remove mass from your boat so you can improve performance without resorting to racer extremes.
¹Us lay people can for our purposes here, think of weight and mass as the same, although actually they are not. Eds.
We will examine mass, the moment due to the position of the mass, and finally the moment of inertia, which can all be additive to each other.
Mass (Weight) Alone
Let’s start by pretending that we add mass to the boat and can ignore where for now. This results in three important effects:
Great article, Eric.
This puts a lot of numbers to what I have mentioned in my posts so far, regarding reducing weight and taking care with its distribution. Our She 36 is of the same era and style as your CS36 and I’m sure yours is as sensitive as ours to weight and its distribution.
Two things. The first is that while a larger boat (with the same number of crew) will swallow the same amount of cruising gear with less performance penalty, that advantage can soon be lost with scaling everything up and adding complexity.
The second being the addition of kit like wind vane self steering gears and heavy duty anchors, left in place all the time. It helps to be able to remove a vane gear (they’re heavy!) when coastal cruising and removing an anchor when facing a long passage. An anchor (e.g. Spade) that can be disassembled and stowed low in the boat is a big plus here.
The current tendency to add endless amount of gear on the transom can’t help performance – thanks for pointing that out.
Maybe this have been discussed elsewhere, but I wonder about the safety of removing the anchor. Of course it will be of little use in the middle of the Atlantic, but you’ll reach shore again, and if you’re faced with having to reassemble it on the bow in heavy conditions….? Is this a normal practice? Do people put the lighter secondary anchor up in the meanwhile?
Hi Arne,
I think that depends a lot on the size of the gear. On a small boat I can perhaps see removing the anchor, but, for example, on our McCurdy and Rhodes 56 I went to a lot of trouble to have a safe set up to leave the anchor on the bow at all times. Bottom line, I don’t like making a landfall without an anchor ready to go on the bow. I learned this the hard way on my Fastnet 45 when we made landfall in a Force 7 and had to get the anchor back on the bow in very stressful conditions.
Botton line, if trying to move weight off the bow I would look at lighter chain or moving it aft.
This seems to align perfectly with my “fear” too. I think I’ll just not do that. All the potential disasters (carrying a heavy anchor on a moving foredeck, losing the anchor, losing the shackle/pin, damaging the hull with a heavy and pointy anchor….). Too much risk for somewhat limited reward as a non-racer.
Hi Arne,
I agree with John on not removing anchors in general. Maybe someone has worked out a system that really is foolproof but because the anchor alone isn’t that bad, I live with it. The chain makes up a big part of that rotational inertia unless the chain locker is far aft. As you point out, even stacking chain in the aft end of the locker can make a difference. On our boat, I modified it to have 2 lockers, the normal one and then another under the V-berth. Usually a good working amount of chain is in the normal locker and the rest is in the other locker ready to run out which it does with no extra help. On hauling up, we can fit everything into the normal locker just but if we use extra chain, we generally send someone below to pull the extra chain aft. When knowing that we have a nasty upwind time ahead of us, we have occasionally pulled all the chain aft of the 2. Moving the chain on our boat 4’ aft lowers the moment of inertia from it by 35% which is significant.
Eric
Hi Eric .
we have done a version of this. To be able to have 350 ft of chain on our 35 feet Linjett without gettin to much of a penalty from exessive piching ,We have a pipe going from the anchor locker to the bilge in top of the keel.
There we can store all the chain during crossings.
In the achor locker there is a 2 part nylon fitting that fits around the chain where it enters the pipe to make it watertight. The chain feeds out of the bilge compartment by itself but need to bee put in by one person inside while one feeds the chain from the anchor locker. That takes a littel effort but make it possible for us to store all the chain in top of the keel while doing crossings. and ready to deploy at landfall.
Ola
Hi Ola,
That sounds like a good system to me. I especially like that you haven’t sacrificed your ability to anchor in a hurry as some systems do.
Eric
Hi Colin,
Yes, unfortunately while bigger boats should often have less of these issues, it seems that the natural tendency is to scale complexity with boat size and cancel it out. I hadn’t thought about vane gears but you are right that they are in a bad place and weigh quite a lot.
Eric
Maybe not very relevant for most, but a major part of why I bought a yacht in the first place, is to go scuba diving in harder to reach places. Doing technical diving (rebreather and lots of extra bottles), the total gear easily weighs 100 kg per person. It is hard to find good places to store the majority of this gear, apart from on the transom/sugar-scoop. I can absolutely tell the difference the ~150 kilos back there makes on my ~40 foot OVNI. Reading this makes me realize I should make a bigger effort in finding more suitable locations for the heaviest items during passages at least. It may also be yet another reason to try to fit the dinghy under the boom instead of in front of the mast, and maybe even trying to lay the chain towards the aft part of the anchor locker. I’m unsure how much the chain will impact, but I’m sure I can move the center of mass at least half a meter aft by taking a bit of care when I lift the anchor if comparing a worst case with a best case chain storage.
I don’t think the motion is very bad as it is, but that may be due to the fact that the mast on my boat is fairly low. I suspect this lets me get away with more than a racer/cruiser would, if it was loaded up the same way.
Interesting article, and very good to get some numbers on it, even though they are not directly transferable.
Arne
Hi Arne,
Interesting about the diving gear.
It is worth noting that weight is only part of the equation in terms of motion comfort, the volume distribution and how the boat is sailed are equally important. From pictures only, it seems like an Ovni 395 or similar has a good shape up forward to reduce pitching and then back aft they are a bit wide which can force the bow down but definitely not too extreme like some other boats.
Eric
In 2009 we overwintered our Ovni 435 in France. On the pontoon right behind us was another 435, just returned from the Caribbean.
Our Ovni floated to her waterline exactly – I had worked hard to keep the weight out of the bows and stern and ensure she was trimmed as best we could. The French Ovni was down at the stern and six inches up at the bow. Which was hardly surprising as he had a big alloy bottomed RIB with a 15hp outboard in davits off the arch, wind generator, dive bottles in the aft lockers etc.
One day, the charming French owner of the other Ovni came down and we got chatting. He complained of handling issues upwind, and I (politely) suggested that all the kit on the stern might be having an effect, as ours didn’t have such problems. ‘But they’re not the same models’ he protested. It took quite some persuading to convince him they were the same boat……
Hi Colin,
That sounds kind of like our boat when we bought her. The previous owners had a big RIB with a 15hp outboard in davits plus a pretty heavy oxygen tent and a bunch of other stuff. The pre-sale pictures showed a lot of bottom paint in the bow and a scuzzy boot stripe back aft. We dealt with most of it before the first year but could still feel it was off and finally got it where we wanted it by relocating the house batteries about 15’ further forward and right under the mast. The boat sails so much better now.
Eric
Hi recently saw a podcast about a boat with a feature i wondered why do it.
You’ve explained perfectly.
The chain locker was rather far inboard, fairly close to the mast. A hawser pipe led forward to the bow.
Now this makes sense. Moved half or so of anchor-rode-chain mass aft.
I wonder if it is practical to move windlass back or perhaps even inside chain locker? Weight aft and shorter heavy cable runs.
Hi Robert
yes – every Boreal has the windlass just ahead of the mast and the chain locker below it in the bilges.
Hi Eric and all,
A variation on having the chain below the forepeak berth:
It is not possible for every boat, but Alchemy, a Valiant, has nice sized limber holes and a deep bilge. I was able to lay PVC pipe/conduit, just large enough to accommodate 5/16 g4, along the crotch of the hull from the anchor well to a mid-ships bilge. The conduit is to provide a protected channel for the chain to travel while still allowing “stuff” to be stored around and on top of the conduit.
Coastal cruising, 100 feet or so of chain stays in the locker: usually enough for most anchoring, at the bow while 200 feet stay in the mid-ships bilge. The extra chain is able to be pulled out easily if needed from the bow along the conduit.
Passage making, I can pull all but about 40 feet back to the bilge and the 77-pound Spade stays on the bow well secured, but easily available at passage end as is the chain.
My best, Dick Stevenson, s/v Alchemy
Hi Eric and all,
Reading all of this reminds me of running into a friend many decades ago in Martha’s Vineyard who had just brought his mast-less boat (longer story, but John, a Fastnet 45) down from Boston. He thought it would be no big deal to motor down, but found the motion of the boat without a mast and rigging at best very uncomfortable and, when in open water, quite scary. He had bruises in quite a few places where, as he put it, the boat snuck up and hit him unexpecatably and with force.
I have never forgotten that story.
My best, Dick Stevenson, s/v Alchemy
Hi Dick,
Your chain system sounds good and not all that dissimilar to what we do although it seems like you are moving it even further aft than us.
I suspect that your friend who delivered their boat without a mast was struggling with roll as the mast is a much bigger factor there. Taking the mast away when it represents over half of the roll inertia will really change the motion. I have never seen a boat with too little inertia in pitch but when we motored our family boat without the mast, that definitely had too little in roll. Roll not only has much less inertia but the waves interact with the boat differently.
Eric
Hi Robert,
The Adventure 40 is designed that way too: https://www.morganscloud.com/category/boat-design-selection/adventure-40/
Hi Robert,
Yes, several well thought out boats have the chain locker further aft and all of the ones that I have seen move the windlass aft as well. At some point the anchor will cease to be self launching and the mud on deck can become a pain but both of these just require a little thought and can be overcome in design.
Unfortunately, I have also seen owners completely ruin this feature by putting really heavy stuff in the bow where the chain used to go which shouldn’t really have much at all, maybe a few fenders or something. I saw one boat with 2 large outboards in the forward locker making moving the chain aft completely pointless.
Eric
Thanks, Eric. I have a question about the starting point for calculating additional impact of stuff we add. Are we to assume that any well designed boat is designed to be perfectly balanced when it it is built and that any additions must be equally balanced? If not, how do we know if a boat (without stuff added) is currently balanced? A related question, in my deams, I will one day commission a Bestevaer 53 which doesn’t have a place for lots of solar panels. If I ask Mr Dykstra (or any designer) to add a big rig at the back to take solar panels and other gear, do I assume he will design the boat differently to adjust for the additional weight and therefore keep the boat in balance out of the gate?
Hi Michael,
Ignoring really high end race boats that get a lot of simulation and analysis in design and then get any issues corrected through testing, cruising boats don’t come out perfectly balanced but come out close. There are many examples of boats with bad weather helm, literally tons of trim ballast, etc. That said, a good design including appropriate analysis with careful manufacturing that follows the design will come out in a place that you shouldn’t mess with too much. If the boat’s boot stripe hasn’t been moved, comparing the distance to the water at bow and stern should give you a good idea of if the boat is on her lines. A smart designer will take into account expected added weight but you will notice that this is not a normally published number so there is no real emphasis on this. If you are thinking of having a boat built with some custom design involved, then this is the sort of thing that you should ask the designer about as part of the process. If they can’t answer your question, then that would be very worrying to me, this is not the sort of thing you can eyeball in design.
Eric
Hi Michael, I happen to own a Bestevaer 53, building no 2. There is actually a lot of space for solar panels when you put them on the Bimini frame that covers the entire cockpit. This is what I plan to do. It should be fairly easy to put some 900 Watts. The original,owner had a stainless steel frame build for solar panels, a radar scanner and a heavy windgenerator. This was a really sturdy frame adding more than 250 kgs at the far back end. It was the first thing to leave the boat when I bought her, for all the reasons mentioned in the above article. On my first Dijkstra 56 we had removable stainless Davids carrying a heavy dinghy and outboard, weight more than 200 kgs, even on a 56 foot 20 tons yacht we could clearly notice the difference when these were removed for a long crossing. Dijkstra does carefully design their yachts, all the heavy items are placed as central and low as possible, fuel and water are carried in integrated bottom tanks around the keel and mast. The anchor is in a chain box some 3 meters behind the bow. The accommodation ends at the back of the pilot house. The ends of these yachts are “empty” if we don’t put loads of toys and “non- essential” gear in these empty spaces. You should realise that this design was intended to be a “retirement cruiser” so two plus two occasional guests. Sort of the interior volume of a 40 ft fitted into a 53ft hull, doing so it is possible to have a well balanced boat that is isn’t prone to pitching when you respect the initial intentions of the designer.Gerard Dijkstra is an expert in this discipline. If you really want an ugly big rig at the end of a Bestevaer, you might want to consider a different design or go for a heavy steel boat where the impact of stuff in the “wrong” place has less impact on the movement.
Thank you, Douwe. This is very useful and I certainly don’t want that heavy steel thing on the back of any boat. I just sent you a LinkedIn invite (well I hope it was you) to catch up offline. Would like to pick your brains on the Bestevaer.
Great article!
Thanks for the bits in blue. Not much of the rest made much sense.
Great article thanks!
So think twice before installing a 2.2lb masthead camera?
Is there anything we can do to reduce roll at anchor? And if so, is it to the detriment of sailing?
I wondered how the pitch moments might compare between a ketch-cutter & a sloop with the same sail areas. Looks pretty close.
Hi P D,
A 2.2 lb masthead camera on its own may not be the end of the world but if it is one of many gear additions like that, they will really add up quick.
You are likely to prefer the motion at anchor if you add mass to the top of your mast but you definitely wouldn’t want it underway. However, designing a mass that can be hoisted aloft at anchor and doesn’t slam around is not trivial. In practice, I think that flopper stoppers which act as dampers work much better than adding roll inertia.
Comparing a ketch and a cutter is going to be design specific. For example, the ketch you have pictured also has a bowsprit which is a big contributor to moment of inertia in pitch as is the weight of the gaff. Split rig boats can be a lot of fun if you are into that sort of thing and they are often fast reaching but I find a sloop or cutter pretty hard to beat for all around practicality in normal cruising boat sizes.
Eric
Thanks Eric,
I agree, single mast cutter is the rational choice.
It took me a while to understand that mass near the fore-aft centre (like a mast) increases the moment as it gets higher. Eventually a metronome came to mind and it made sense. Move the weight up the pendulum, and the metronome slows down. So, with a musician’s eye the sloop’s extra mast height looks just as detrimental as the ketch’s bowsprit.
The A40 gets it right. Minimal pitching moment for a given sail area. Nice.
Cheers,
Paul
Hi Paul,
thank you!
Roll at anchor can ruin your evening for sure. We have successfully used a hinged stainless steel roll-stopper from Magma. We hang it from the whisker pole at right angle to the hull so it’s about 8-10′ outboard , and 10-20′ deep. It immediately suppresses roll. I don’t think there’s enough force on it to stress the rigging, at least on our stoutly rigged cutter. Also, a stern anchor can be used to pull the boat into a more wave perpendicular position, but this is usually much more involved, especially when you are ready to depart the anchorage.
Being a bit of a nerd, I could not resist doing some sums on this, but I will spare you the formulae. I have the hydrostatics for my Ovni 345 and the weight:WSA ratio works out a bit less – about a 5% increase for a 10% increase in weight, so at moderate speeds about a 2.5% increase in resistance, which results in a speed loss of about 0.05kts at 6kts. Not so much.
The effect of weight distribution was more interesting. There are a number of technical papers around that enable you to estimate the added resistance in waves. Note that this takes no account of the loss of driving force in waves due to the pitch motion, so is an underestimate of what we actually experience, since high added resistance in waves is directly correlated with large pitch amplitudes and thus loss of sail drive.
These papers use a number called the gyradius as the basis for the results, which is directly reated to the pitch axis moment of inertia. In one paper, increasing the gyradius from 0.2 to 0.24 increased the added resistance in waves by 50%, in another going from 0.23 to 0.31 increased it by 70%. OK, so what do these numbers mean?
First off, the added resistance in waves (not especially big ones) was around half or more of the calm water resistance – a lot!
In the case of my Ovni, 200kg of anchor and chain at the bow increases the gyradius by around 15%, from 0.23 to 0.26, so increases the added resistance by about 25%.
These calculations put numbers to what we know from experience – wherever possible keep weight out of the ends in order to reduce pitch amplitude and added resistance.
This is a very, very broad brush extract from the research results, which go into a lot of detail about how the motions (and associated addded resistance) vary with heading relative to the waves, wave height and wave period as well as hull form characteristics such as displacement:length ratio (high is bad!). The results say nothing about motion comfort.
One last observation, the data also says that slender hulls expereince a lot less added resistanc in waves.
Hi Colin,
Those numbers are interesting and show just how important weight placement can be as I suspect 200kg is a pretty small fraction of the overall displacement. It also shows just how much difference there is between flat water and waves, you can sail pretty much anything in flat water.
Thanks.
Eric
Thought-provoking article – many thanks. I appreciate the clear explanation of the mathematics which deepened my understanding of Colin’s articles on the choices he made in fitting out his new boat and those about the A40.
Everything is a compromise but it’s really useful to understand the science behind the options of we’re to properly determine the costs & benefits of any changes on our own boats. I needed to add extra solar and a radar, plus my wife felt insecure at the helm so we thought an arch was best for our quite light 40’ Hanse that we cruise together for 6 months/year from France-Norway and everything in-between. It’s achieved the effects we wanted but the added 40+kg right at the back made me wonder if I should have balanced that with a similar amount forward or if that would just compound the penalty. From this article I understand now that, provided overall trim is still ok, seeking to balance that weight would simply encourage the boat to hobby-horse in waves. Have I missed something, please?!?
Hi Iain,
You haven’t missed anything. Provided that fore and aft trim is still good, then you have added some pitch inertia which means you will pitch more than before but otherwise are fine. If you were to add 40kg to the bow to try to offset, that would be even worse as it would add a bunch more pitch inertia.
If you find that your trim is out, then rather than adding weight, the best thing to do is to look for weight you can remove from the stern or if you can’t do that, look for weight that is in the stern that you can move forward closer to midships but not to the bow where you will cause pitching issues. When considering trim (moments), it doesn’t matter where a weight is that is being moved, only how heavy that weight is and how far it is moving. Moving 1kg from the stern to 3m ahead of the stern is the same as moving a 1kg weight from midships to 3m ahead of miships.
Another thought is that some of the stern arches are built out of heavy wall stainless which is really heavy, if you need to have an arch, an aluminum one may well be better for performance.
Eric
Hi Eric,
An interesting article that reminded me of some things I knew, made me understand some things clearer and thought me things that wasn’t clear to me. Thanks!
One small detail in your comment here, which you’re definitely aware of, but others might misunderstand from the wording: When moving mass lengthwise, we can’t refer to midship or the centre of flotation, but rather the rotational centre of the pitching motion, which perhaps isn’t super easy to define, but usually it’s well aft of midship. Somewhere around one third of LOA (total boat length) from the stern could be an approximation?
That means moving a heavy item forwards to the rotational centre will minimise its detrimental effect on pitching, but it will still contribute to a heavy stern, just a bit less than if it was further aft. Also it means that adding weight forwards of the flotation centre to compensate for a heavy stern, means that compensation weight will be about twice as bad for pitching as the same weight on the stern side of the balance. It’ll move about twice as far.
I’ve always been a weight saving fanatic, but this reminds me why.
Hi Stein,
You make a very good point about the location of the pitch axis being further aft than most people would expect. The good news is that this makes mass in the stern a bit more forgiving than mass in the bow when comparing pitch performance but both are obviously bad.
As a multihull sailor, I am sure you are aware how these subjects are a bit more complicated for you but that just means you need to be even more careful about mass.
Eric
Hi Eric- really grateful for your comments; they made me have a good, hard look at all the weight aft and gave me the motivation to do some serious shifting around which has made a surprising difference. Its been useful to read your technical appraisals in conjunction with Colin’s on the practical reasoning he’s employing in fitting out his new boat. Our cruising style is similar to his and we’re now questioning if we really do need that super-heavy liferaft and not the single-chamber alternative when in permanent range of some of the best SAR networks in the world.
Hi Iain,
I am glad to hear that you have successfully made a difference. Yes, these things are all trade-offs. I am currently pondering whether I should use our current 150lb hard dinghy as a mold to make a much lighter but more fragile version.
Eric
Thanks for the excellent and clear article, Eric. I would point out that holding tanks often seem placed in the bows under the forward berth, and the weight of the grey or black water could be significant. I now need to sell my spare Rocna 33 which is sitting in the anchor locker, and move that stainless steel flopper stopper device (which we use frequently) to a more central location. And maybe I don’t really need 1000′ of 3/4″ rope rode up there…
Hi Brian,
Yes, unfortunately holding tanks do often seem to end up quite far forward. Not only does the mass hurt pitch inertia but it also means that the boat’s trim changes a lot depending on tank level.
I bet 1000’ of ¾” weighs a lot too. Some of the boats I used to work on had halyards that were 4-500’ of 7/8” or 1” and handling that stuff was not trivial.
Eric
Hi, Eric. After reading your article again and also the relevant chapters in Desirable and Undesirable Characteristics of Offshore Yachts there are still some questions in my head. They came up this week after having to motor 250 miles from near Oslo to the South of Sweden against the winds generated by ex-hurricane Nigel. During that voyage, with the many and varied wave conditions that are to be found in the Skagerrak and the Kattegat (no point in even speaking of wave duration in those seas and those conditions because you usually have 3 waves all from different directions under the boat at once), I had lots of time to contemplate the pitching behaviour of my Sweden Yachts 41. So here are the questions:
I studied the pitch of the boat for hours and could not come to a conclusion as to whether the pitch was good or bad or in between.
Thanks
Michael
Hi Michael,
Pitch, roll and yaw are all about axes so they could intersect at a single point although I suspect that this is rarely the case.
Identification of the location of these axis is a bit tricky. While it is possible to calculate it, it is quite a complex calculation so it is generally easier to determine by observation. Unfortunately observation is a bit tricky because you are on the moving object. To make it more complicated, the axis moves around a little in relation to the boat as it is dependent on the wave interactions so we generally pick an average location. I have only ever tried to determine the pitch axis and having done it on a few boats, I find that if you sit athwartships watching the horizon and slowly slide yourself fore and aft, you can eventually get a pretty good idea of where it is, remember that it is likely well below you too.
Knowing the exact location of any axis isn’t super important unless you are trying to figure out what bunk will be the most comfortable, what is important is knowing where you need to watch mass. On most boats, the pitch axis will be much further aft than most people expect, like 2/3-3/4 of the way aft and it is almost always aft of the center of buoyancy. Knowing this, we can say that for a given mass, the worst place to put it is the top of the mast, the next worst is the bow and then the stern is still bad but not nearly as big a deal if trim is okay. On a typical 40’er, I would be trying to keep all unnecessary mass out of the forward 10’ of the boat at a minimum which includes tanks, batteries, heavy stowage, etc.
There is not a definitive way to say that pitch is out of whack as it is a function of many factors, mass distribution only being one of them but also the one that we can control before setting out. If the ends of your boat have virtually nothing in them that wasn’t from the factory, you know you are doing pretty well for the boat you have. From there, I would watch how your boat behaves relative to others, if it seems to do worse, then that is motivation to try to make improvements knowing that moving stuff around is only one of a few variables.
I have done a few long motors into nasty choppy seas and it can be quite unpleasant even with a good boat but it is downright awful or even undoable in the wrong boat. Getting the mass distribution right is a great step before leaving and then once you are out there, you have to play with your angle to the waves, your speed, sail combination, etc. to get it right. As an example, last Sunday I was out sailing on a 122’ boat that a good friend captains in about 5’ seas that were badly misaligned with the wind and when I made a course change of maybe 30°, suddenly we stuffed the bow under which was quite surprising to me for such a large vessel. My friend found it quite funny as I suspect she knew the boat well enough to know what would happen in seas with that shape and as soon as I changed course again, it was smooth again and the bow wasn’t threatening to dive into the comparatively very small waves.
Eric
Hi Eric,
Thanks for the fill on that.
I would add one thing for others, which you already allude to:
Sure weight in the stern is not terrible as long as the boat floats to her mark (trim) but what I see so often out there is boats trimmed down by the stern which is SLOW as any race boat sailer will tell us—dragging the stern adds a lot of wetted surface and wave drag too.
So, what many will say is “no problem, I will just move weight forward to compensate”. But the problem here is that said weight must be moved the same distance ahead of the centre of flotation, not the pitch axis, and the result is a it’s a long way forward of the pitch axis and that effect is squared, so horrible.
I have always kinda known this from observation, but it was a huge light bulb moment for me when reading your article above.
Hi John,
Great addition, I agree completely. Thanks.
Eric
Thank you both and very helpful. And you answered what was going to be my next question, Eric which was, if you are designing a boat, isn’t knowing the moments of roll and pitch inertia a good thing for deciding where to put the sea bunk/s.
Hi Michael,
Sure, the closer you can get the sea buncks to the pitch axis the better. More thoughts here: https://www.morganscloud.com/2011/03/20/the-perfect-seaberth-2/
Thanks for the great article Eric. It’s provided quite a bit of clarity for me on the “why” behind practical guidelines. I have a question on the effect of height in the equation. You mention it briefly in the description of roll but not in pitch. Why is that?
Specific to our boat, we have an Ericson 38-200 which has a shallow chain locker “pan” sealed above an empty locker below it in the bow. I’ve been thinking of removing it and moving the chain lower in the bow for a few reasons but one is that I’ve thought it would help to have the weight lower in the bow. Is this correct or is it irrelevant? The way the spaces are configured the change wouldn’t move the weight aft at all, just lower.
Hi Jim,
When calculating moments of inertia, we use radius to the axis in question. Unless we are mounting stuff up the mast, the dominant factor in the radius calculation at the ends of the boat where mass is a big deal is the horizontal component but there is absolutely a vertical component and we should not omit it. For something like a stern arch, it can actually make a significant difference. Up the mast, the dominant term in calculating the radius is the vertical component.
We have a similar anchor locker setup and use the area in the upper locker above the pan for the windlass and our snubber while the lower area under the pan is where our chain goes. From moment of inertia standpoint, moving the weight lower will help a little as the axis is even lower but it won’t make a huge difference. But we shouldn’t just think about moment of inertia, the vertical center of gravity is very important to sail carrying ability so everything we can do to lower it is a good idea. One thing to watch in this conversion is how everything drains, in many boats the upper area drains directly overboard while the lower drains into the bilge. Unless you have a really well sealed deck hatch which would be rare, getting rid of the pan could allow you to hold a lot of water up there and have it drain down below.
Eric
Thanks Eric, measuring the radius from the axis clarifies it for me and the vertical COG concern makes more sense as to a “why” for potentially lowering the chain. Drainage to the outside and closing off the current access from the v-berth to the space are my big concerns. I’m leaning toward leaving things as is while I’m still comfortable hauling the chain with our manual windless. We don’t have a ton of weight – 20kg anchor, 100’ of 5/16 G40, and 200’ of 5/8” nylon rode and it’s pretty easy to keep the chain at the aft end of the locker.
Hi we have a Malo 42 1997. We are thinking of fitting an Arch for solar and a D400 and possibly the radar. However we are also wanting davits. How would all this work ?
Hi Jon,
That is a lot of stuff to put on the stern and if it were me, I would try to find another way to do it as it will definitely have impacts on how the boat performs. The weight will impact pitching and trim which usually results in even more pitching. Also, the windage of that much stuff back there is quite significant and could really hurt performance.
I would recommend starting by looking at whether you can ditch/move some or all of the renewable generation. This always starts with conservation which can mean anything from lowering loads through improved icebox insulation, an improved autopilot, etc. to behavior modification such as reading a book as opposed to watching movies on a big screen. Then, make sure your battery bank is big enough as seen here https://www.morganscloud.com/2023/01/24/balancing-battery-bank-and-solar-array-size/. Then try using some of the tricks John has mentioned in the latest 2 articles such as maximizing engine charging especially looking at his real world examples found here https://www.morganscloud.com/2023/09/26/replacing-diesel-generated-electricity-with-renewables-part-2-case-studies/. Then, I would see whether the solar could be flexible panels mounted on existing surfaces like the dodger.
Radar usually isn’t too bad provided it is not mounted above a solar panel although some of the older units can be quite large and heavy, I find our 19” solid state unit from Furuno to have perfectly good performance for the sailing we do. The dinghy is a tricky one and davits are definitely handy. Some boats allow you to hide the dinghy much better from a windage standpoint than others and getting this right definitely helps. For offshore work, I wouldn’t have a dinghy in davits anyways.
I hope this is helpful.
Eric
Hi Jon,
I 100% concur with Eric’s advice and I used to be an arch fan—I had one in the original A40 specification—but after learning more from the engineers (Eric, Matt and Maxime) here at AAC I would look for pretty much any way to avoid adding that much weight and windage to the stern of a nice sailboat like the Malo 42.
Keep in mind that when you add all that weight aft, you will need to move a lot or weight forward to get her back in trim, and that weight, because the pitch axis is aft of the axis of flotation, will have an even worse effect on pitching. And that effect is squared by the distance from the pitch axis. The physics conspire against us on this one!
And finally, many Malos have in mast furling which is already a huge hit to pitching because of the increased weight up high. If that’s the case on your boat I can see adding an arch and davits may create a situation where your boat simply won’t sail properly with the wind forward of the beam and any sea at all and will be uncomfortable and inefficient when motoring.
Very interesting to learn some of the facts/math behind the concepts. I am especially interested in the weight of the anchor chain in the bow. I may look to follow the ideas presented by two about setting up a conduit to have extra storage of chain under the v berth which seems quite helpful. Question for you- if one can’t set up the conduit from under v berth to chain locker, what is your view on having the first 75′ in the bow and then having the next 75-100′ under the v berth and ready to be attached in the same manner one attaches the anchor chain to the actual anchor ie screw in shackle wired down? Thank you, Ken
Hi Ken,
It does not happen often, but, in a middle of the night “fire drill” of some sort, I would want to have all my rode available without the time necessary to link chain together, especially with fumble-fingers when hurrying. Where weight was a concern, I believe I would consider (in most cruising grounds) to have the 75 feet of chain you mentioned to be then attached to a length of nylon rode.
Moving beyond what you have written, perhaps you have one of those enclosed anchor wells/chain lockers with a little drain overboard. I have always been suspicious of these in blue water sailing. Most seemed to allow random water into the well with little restraint and it would not take too much mud or seaweed to clog that little drain hole and allow the well to fill.
I know of one windlass failure (installed inside the well where it was “protected”) that was attributed to this happening.
I was fortunate as there is an ample limber hole at the bottom of my chain locker (no outside drain) which allowed me to pull the chain aft and into PCV conduit and bring the extra chain amidships, but still easily available from the bow. (BTW, if a limber hole into the bilge of the boat, it is a good idea for the chain to be retrieved clean making a deck wash for spraying the chain important).
I suspect a limber hole like that could be added to most of the bulkheads separating an enclosed anchor well from the boat interior: perhaps a little added reinforcing around the hole might be considered.
My best, Dick Stevenson, s/v Alchemy
Thank you Dick for the insights, good points for sure and will see if I can arrange as you suggest. Tks Ken
Hi Ken,
This is one of those compromises that you have to figure out. In my opinion, I would always want to have an anchor and rode that is reliable in tough situations ready to go so if something happens like a loss of steering, I can quickly let it out. 75’ of rode would only be sufficient for specific shallow places, in many others the minimum is more like 150 or 200’ ready to go. Depending on where you are and the type of cruising you do, it may be reasonable to use a hybrid rode but that is a big compromise in itself.
I am not aware of any chain connectors that fulfill the need of quick to put together, go over a windlass gypsy nicely, full strength and corrosion resistant although there are connectors which meet some of those needs. The other thing that comes to mind is if you can’t make a pipe work, how will the chain get to the bow as it is usually quite messy?
I like moving the chain aft but when that is not possible, I tend to like a combination of a good and quite heavy anchor, high strength chain sized for strength not weight and really thinking about how much length of chain you need. A heavy anchor at 4:1 scope in deep water (say 50’) is likely to have much higher holding power than a light anchor at 7:1 and the difference in length can easily be 150’ of chain which is quite heavy and easily offsets the extra anchor weight. We actually size for 5:1 scope in 40’ of water with a 5’ freeboard allowance but there are other places where we might need more. John has more info on this sort of sizing in other chapters.
Eric
Eric,
Thank you , that makes sense and as I will be in the S Pacific another article on this site strongly suggested all chain given coral so I think I need to figure out the pipe configuration with heavy anchor. I had planned on 3/8 G40 so would be interested if you have further thoughts on strength vs weight that you mention. Tk you, Ken
Hi Ken,
I would suggest choosing chain by strength and not for weight.
The argument for weight is that it produces a useful load-absorbing catenary. However, I believe that, in gale conditions, when you want it most, that catenary is no longer available as the chain becomes fully stretched out (no longer able to absorb load) in gusts and sea state.
I would look to load absorption from a snubber, choose chain by strength, and put ground tackle weight into the anchor where, pound for pound, weight is most effective at keeping the boat attached to the seabed.
For example, I have been using g40 5/16” inch chain for decades with (last 8 years or so) a 77-pound Spade on a 40-foot heavy displacement sailboat.
Lighter chain also allows for greater length as the weight difference between 3/8ths and 5/16ths is substantial and greater length allows for more options when anchoring deep as some cruising grounds necessitate.
My best, Dick Stevenson, s/v Alchemy
Hi Ken,
We have a chapter on chain grades and the strength to weight trade off: https://www.morganscloud.com/2013/09/25/which-anchor-chain-should-we-buy/