I finished reading through Marchaj’s “Seaworthiness: The Forgotten Factor” a few weeks back, and although it doesn’t explicitly cover multihulls, it’s markedly changed my thinking on the subject of hull cross-sections. (If you have the time, read it and see for yourself!) In a way, this post is directed especially at proponents of barge and other flat-bottom hull shapes (that means you johannes! 😊 ).

Basically, pretty much anything with a flat, or even circular, shallow-draft bottom is terrible from the standpoint of seaworthiness (and seakindliness) because the sea is effectively far rougher in all respects. Other things being equal, the more draft your hull has, the gentler the accelerations in a seaway, the lower the effective wave height (can be as much as a 20% difference), the lower the kinetic energies which have to be dissipated in each wave-cycle (proportional to the square of the effective wave height), the lower the effective wave slopes, and the more seaworthy the boat will be in general. Draft being equal, a deep-v provides even gentler accelerations, and better course stability (and thus a lower danger of broaching in a seaway), than a flat or round-bottomed boat.

In one set of data from monohull models in a tank test with following seas, a shallow draft (3.2% of LWL) round-bottom hull encountered vertical accelerations of more than 0.8g in the forward sections of the hull. The same test with a deep-v hull was much less dramatic, especially the hull with increased draft (6.4% of LWL) which resulted in vertical accelerations of at most 0.1g. If you don’t have the book, you can go to the book’s page on amazon.com and “Look Inside”; scroll down till you get to page 190 of the book, and you’ll see the graph which I’m referencing.

The encountered vertical accelerations not only influence how quickly and to what extent the crew gets sea-sick, and whether it’s possible to go on the fore-deck at all or not, but it also markedly influences the stability of the boat. If you think about it for a moment, then you’ll realize that in the extreme case, where a boat and the surrounding water are in free-fall together, the boat literally has zero stability. While that can only occur near the crests of waves just in the process of breaking, large vertical accelerations of the boat at the wave-crest, even from non-breaking waves, can massively reduce the overall stability of a boat. That combined with the fact you are also not in the wind-shadow of the wave anymore can make you very vulnerable to capsize at the wave-crest.

While I can’t cite all the relevant evidence from ‘Seaworthiness’ (I didn’t take notes! :( ), reading it altogether brought me to the conclusion that if you want seaworthiness and seakindliness, then beyond the really obvious point of wanting more LOA, you also want more displacement (a heavy boat is more seaworthy than a light one, provided the center of gravity is low), a deep-v cross-section, and perhaps a little surprisingly, as much hull draft and lateral plane area as you are willing to put up with.

Some of those points stand diametrically opposed to the goal of high performance, but for a coastal / offshore cruiser it in my opinion it makes no sense at all to drop a deep-v geometry in favor of a barge, sharpie, or trapezoid cross-section hull, for the benefits of marginally reduced hull draft. On the light cruiser I’m currently designing for the Wadden Sea, it was 32cm for the trapezoid I had initially, and 45cm for the convex deep-v version I am designing now. The total sailing draft, which at the very least for upwind courses means boards down, is pretty much the same anyway though. The 50cm deep-v might be just marginally sail-able upwind even without the boards, so you could argue that although it has more draft where the hull is concerned, it’s actually the better shallow-water boat of the two either way. The deep-v takes better to beaching as well, and allows fitting of a replaceable wood beaching shoe.

Flat-bottom boats might be easier to build, and both them and round-bottom boats might have less wetted-surface area than a deep-v, but after reading ‘Seaworthiness’, I wouldn’t use either one for anything other than sailing in protected waters.—And the Polynesians I think, did the same. 😉

Marco

(that means you johannes! 😊 ).

After extensive testing shallow draught flat bottom barge and ASP hull shapes and comparing them to my deep V proa I agree with every point you mentione above. Deep draught, higher weight (within reason) deep V shape with overhanging sides and ends have a very soft and kind motion. It is far faster then my barges and sharpie proas.
I have about 250 GB of videos of them moving through waves and I have spent many hours looking at them in slowmotion over and over again, comparing them, sometime even frame by frame.
It is a very educational process to do this and slowly work through different variations and making iterative changes and watching the results.

My results are in total agreement with that book (I own a copy and have read most of it).

I will start reading it again. It is a really great book.
Thanks for a great post about seaworthiness!!!

Cheers,
Johannes

Sorry I have to be a bit of a Devils Advocate here. I can’t access the graph on page 190 so am speaking from a shaky platform BUT.

I think there is some danger in applying any general rules of thumb developed primarily for monohulls to proas that operate in a totally different spectrum of loadings (a position arrived at the hard way in some respects 😉).

Two things to consider in that haven’t been mentioned yet are waterplane loading (which has an enormous impact on vertical acceleration) and moments of inertia (put all that extra weight very far from the CG and you’ll quickly see whats what).

Extra weight I’m really not sure about. Not ever ever added any in hard circumstances but have tossed stuff overboard and smoothed the motion of my boat in stormy conditions.

Regarding hull cross section, a deep V may well have an edge, but at similar waterplane loadings I suspect there’s not much difference one shape to another, I could easily be wrong on this one.

In spite of these comments I think this is a very worthwhile topic and hope we can arrive at some fundamental truths.

cheers,
Skip

I agree with you in that one has to tread very carefully when trying to draw conclusions about the characteristics of proas from monohull data, some things are applicable, and others aren’t applicable at all.

For monohulls you naturally get large stability bonuses from increasing the displacement of the vessel (provided the CG stays the same or becomes lower), but there are two further arguments given in the book why increased displacement improves seakindliness and seaworthiness:

(1) A heavier boat has more intertia, hence accelerations will tend to be smoothed out more (obviously there’s a limit to this, you don’t want to become a submarine after all)

(2) For two boats similar above the waterplane, the amount of kinetic energy imparted to the boat when hit by the jet of a plunging breaker, beam on near the sheer for instance, is dependent on the mass of the boat. The amount of momentum imparted on impact, to two boats which are similar above the waterline, will be more or less the same (momentum = mass * velocity), but the amount of kinetic energy transferred will be different, because E_k = 1/2 * m * v^2. Thus if a boat H weighs 1000kg and a boat L weighs 500kg, then the lighter boat L will have twice as much kinetic energy imparted to it as H. Marchaj points out that being hit by a plunging breaker is obviously far from an ideal elastic collision, and not all the kinetic energy from the impact is converted into rotational kinetic energy, but the point remains the same, that you have a lot more kinetic energy transferred to the lighter boat, which is potentially available to capsize it.

Heeling a boat is sort of like compressing a spring, in the heeled position there is potential energy stored in the boat, which is converted back into rational kinetic energy the moment you let it go. Even if the righting moments of the two boats L and H were the same for all angles of heel, the lighter boat L, which has received twice as much kinetic energy from the impact, will heel a lot further before all the kinetic energy has been stored in the ‘spring’, and the boat (hopefully) starts coming back again.

This explanation is a little crude, but I wanted to keep it short. The essential point which was made, was when hit by breakers, the amount of kinetic energy imparted to the boat is higher for a lighter boat (other things being equal), and the maximum heel angle is also higher. With some tanks tests done, this literally made the difference between IOR yachts which rolled through 360 degrees multiple times in such an impact, and nothing more than a knockdown for a heavier boat. The reason Marchaj look at this case in particular, is because lying abeam dismasted and getting hit by the jet of plunging breaker beam on is just about the worst thing you can realistically design for.

More general points on seaworthiness

Larger boats, with a greater LOA and displacement are significantly more seaworthy than small ones. I think that’s not news to anyone. As Skip mentioned, where that weight is located also plays a great role in general. Obviously you want the CG to be low, and you want to keep weight out of the bows and make sure you have a low pitch moment of interia to avoid a rocking motion building up (which can get a lot worse through constructive interference if the wave period is about right), and to ensure that the pitching moments are damped quickly by the bows (this recommendation was made in Norwood’s “High Speed Sailing” as well). If most of the mass is near the center of the boat, the boat will tend to conform to the sea-way and you reduce the chance of burying the bows.

What do do with the heeling moment of inertia is pretty obvious, you want that to be high so you don’t capsize due to wind action. You need a very large stability reserve in rough conditions, because high vertical accelerations encountered in steep seas can greatly reduce your effective stability (can be by 80% and more at the crests of steep waves). That said, from everything we’ve read about Russell’s boats; a bit of water ballast, and a storm-jib with a low center of effort seems to make that a non-issue. There’s no need to go way over the top there, but in rough conditions you definitely shouldn’t be anywhere near the static stability limit.

That leaves the yawing axis, where the biggest worry is probably avoiding broaching. Having a large lateral area gives the boat better course stability, but as has already been stated in another post here on the forum (and was stated in Marchaj as well), having a deep stern and forefoot (little to no rocker), can lead to a danger of broaching then running from a following sea, because with the boat on the forward face of a wave, the flow velocities of the water (flowing toward the wave-crest behind you) can be much higher at the bow than at the stern (an example given in Marchaj was (net) 4 knots near the crest and (net) 11 of flow speed at the trough), which can produce a powerful broaching tendency in a boat with a deeply immersed bow and stern. If the boat is at even a slight angle to the wave-train in that situation, which given the non-perfect nature of real wavetrains, will always be the case, then the bow in the high speed flow has a larger lateral resistance force (higher speed = higher drag force) than the stern, which will create a moment which attempts to turn the boat beam on to the sea.

What of non-rotational inertia though?—One thing I have been thinking about since I started reading ‘Seaworthiness’, is if having a water-ballast tank in the middle of the vaka underneath the floor, would be beneficial for the boat’s seakeeping ability. When it gets rough, you’d fill the tank, or fill it partially, to increase inertia, to reduce the accelerations encountered in the boat (more tendency to plow through the wave rather than strictly conform to the surface), reduce the imparted kinetic energies from wave impacts (because the boat is heavier), and to get the boat riding lower (with all the seakeeping benefits the increased draft brings). The moments of inertia of the boat would be relatively unaffected by that. On the negative side, you’d also reduce freeboard and pod-clearance though. What do you guys think on that point, as a trick to make a somewhat small and light boat a bit more seaworthy?

There’s one other point from ‘Seaworthiness’ which I want to bring up as well, is that having a skeg infront of your rudder significantly increases it’s range of operating angles of attack, making it much less likely to stall. That was news to me. The skeg obviously adds lateral-plane area which doesn’t really count towards your effective rudder area though, at least not fully, but to me it seems like Newick-style rudderboards placed closer to amidships than normal rudders, like the ones on Cheers or on Russell’s boats, are a pretty good tradeoff because you get improved stall characteristics, the total lateral area helps create lift to windward to counteract leeway, and for a pacific proa the extra lifting area in the stern half of the boat also helps counteract the weather helm resulting from the drag of the ama.

All in all, while I admit to definitely being vulnerable to confirmation bias here, reading ‘Seaworthiness’ further convinced me that overall the configuration of Russell’s proas is very seaworthy, and explained why. I think I’ve mentioned the most relevant points in these posts, but there’s definitely more good stuff in the book, so I highly recommend reading it, even if it does take a while. The book is geared solely towards monohulls, so I may have made errors in my judgement of what is applicable to proas and what isn’t, I’m definitely no expert by any means, so this is by no means as definitive as something quoted straight from the book would be (and even there a healthy amount of skepticism should be exercised 😉).

For me personally, the benefits of increased hull draft, and just how much of a difference hull cross-sections can make, definitely surprised me. I also found the contrast between design trends in racing yachts, especially monohulls (even the developments since the end of the IOR era), on one hand, and that what constitutes a seaworthy boat on the other, quite surprising as well. One could go as far as to say that beyond a certain performance requirement, the two are diametrically opposed for monohulls. I think for multihulls the gap between what features constitute high performance, and what features make the boats seaworthy, is not as large, but even there you need only start the discussion of what aspect ratio a foil should have, and the two paths start diverging.

This is the best I have read on this forum to date!!
Thank you Manik for a very good technical desciption of the behaviour of deep v hulls and seaworhiness.

Cheers,
Johannes

Manik - 12 October 2013 06:10 AM

What of non-rotational inertia though?—One thing I have been thinking about since I started reading ‘Seaworthiness’, is if having a water-ballast tank in the middle of the vaka underneath the floor, would be beneficial for the boat’s seakeeping ability. When it gets rough, you’d fill the tank, or fill it partially, to increase inertia, to reduce the accelerations encountered in the boat (more tendency to plow through the wave rather than strictly conform to the surface), reduce the imparted kinetic energies from wave impacts (because the boat is heavier), and to get the boat riding lower (with all the seakeeping benefits the increased draft brings). The moments of inertia of the boat would be relatively unaffected by that. On the negative side, you’d also reduce freeboard and pod-clearance though. What do you guys think on that point, as a trick to make a somewhat small and light boat a bit more seaworthy?

 

I think that idea has a lot of merit, the only way I know to really check it out would be with some real world experience. For anyone contemplating such an action I’d recommend serious thought about securing the ballast. Any reduction in freeboard/pod-clearance likely to be nominal unless you’re thinking about a lot of ballast 😉. Along those same lines some ballast in the current stern could help broaching tendencies in rough going. At one time I had considered trying this on an automatic basis where a venturi would fill up a small ballast bag in one direction and empty it in the other. A desire to keep things simple set the idea off to one side. There’s some mature tech using water ballast by the wake surfing powerboat crowd, it’s not my bag but I’ve helped my son with rigging his boat.

Good discussion, thanks
Skip

It’s a long time since I read ‘Seaworthiness’, but one of the things I remember is that low freeboard is better than high freeboard because it allows broken waves to wash over the deck rather than impacting on high hull topsides, which causes an overturning moment. My dim memory is that ‘Seaworthiness’ was written partly in response to the infamous 1979 Fastnet race in which 18 lives were lost on high freeboard, light, shallow IOR boats.

However, other studies have shown that for multihulls it can be advantageous to have a light shallow hull with low lateral resistance. The reason being that this allows the hull slide sideways down the face of the wave, which dissipates the energy and prevents the boat from being overturned. Model tests showed that there is an interesting phenomena whereby when a catamaran is lying ahull side on to the waves, the wave crest will lift the windward hull clear of the water (heeling the boat over) and pass under it, then the wave crest will then strike the leeward hull, lifting it and rapidly bringing the boat back to horizontal. I’m not too sure how good this experience would be if you were on board, but it did show that a light catamaran could be highly resistant to capsize in waves. I don’t have a reference for that study, but it may be on the net somewhere.

Mal.

Here’s a link to some notes by John Shuttleworth:

http://www.john-shuttleworth.com/Articles/NESTalk.html

this allows the hull slide sideways down the face of the wave, which dissipates the energy and prevents the boat from being overturned. Model tests showed that there is an interesting phenomena whereby when a catamaran is lying ahull side on to the waves, the wave crest will lift the windward hull clear of the water (heeling the boat over) and pass under it, then the wave crest will then strike the leeward hull, lifting it and rapidly bringing the boat back to horizontal

As I see it there are two different approaches to this. A light weight, shallow draught, high freeboard hull with less lateral area will be much better at lying ahull side on to the waves or with a sea-anchor from the stem, but a heavy(ier) deep V hull with low freeboards will be much easier to controll and keep on sailing through really rough weather. You can optimize the boat for either way. If you plan on sailing in storm-infested waters I belive it is better to design the boat primarly to maintain controll at all times. If you want speed in lighter winds you have to accept giving up controll and resort to a series-drough/sea-anchor sooner when the wind increses.

Most (all) boats that are forced to regurlarly operate in heavy seas are heavy, deep draught steel or aluminum. I don’t see any ultralight supply-ships going out to the oil-rigs in the north sea or any ultralight whaling boats in the south seas.

Cheers,
Johannes

I don’t think that proas are necessarily meant to be “seaworthy” in the classic sense , They are safe because they are fast and of shallow draft. Storms move at about 12 to 15 knots. I have been cruising on a lead mine for the past 40 years and can count on one hand the times I have been caught out in high winds and heavy seas. My 40’ boat draws 4’ board up and only approaches 10 knots on a broad reach.  I have crossed the Gulf Stream from Florida to the Bahamas at least 30 times and never seen a wave over 8 ft.  I wait for weather. If I had a proa I never would have been caught out. I could have easily run for shelter. A boat that can reach speeds of 20 knots and can cross reefs and sand bars can almost always find shelter before a storm hits. 99.9% of cruising is spent in coastal waters or at anchor.  Speed and shallow draft are your best friends.  Long lean light and flat. If you want to sail around the world alone, and less than 200 people in history have, then perhaps a proa is not the right boat for the voyage.

Johannes - 13 October 2013 11:05 PM

Most (all) boats that are forced to regurlarly operate in heavy seas are heavy, deep draught steel or aluminum. I don’t see any ultralight supply-ships going out to the oil-rigs in the north sea or any ultralight whaling boats in the south seas.

 

Ultra light ships carrying cargo is an oxymoron.

These types of boats are usually well over 100 ft long and have large physical volume (to carry load) and therefore must have deep draught and heavy scantlings. To do them in anything other than steel or aluminium is uneconomic. It has nothing to do with safety or seaworthiness.

Are you planning to go whaling in your proa Johannes???

Ultra light ships carrying cargo is an oxymoron.

Lighter boat = more cargo for the same total displacement. I can not see why they would not design very light cargo ships? Half the weight is half the amount of material and half the price, and lots more cargo. Still, every kind of ship designed to operate in the Arctic and/or Antarctic is strong and heavy. I have never seen or heard of anyone sailing to Jan Mayen or the South Georgian islands in a Melges 24 or any other light sailboat. I belive it is because people are not very interested in dying. I have tested to load my deep V proa down with 4 kg extra rocks and gravel, and of course it is not as fast as without it, but the difference is much smaller then people seems to believe it should be.

Are you planning to go whaling in your proa Johannes???

No, I am planing on running supplies to the oil-rigs outside Norway in late November and early december… 😉
Whaling just seems sooo 1900…

Cheers,
Johannes

Johannes - 15 October 2013 05:03 AM

No, I am planing on running supplies to the oil-rigs outside Norway in late November and early december… 😉
Whaling just seems sooo 1900…

Did someone forget to CC Japan on this?  Their “research” claims about commercial whaling are just plain embarrassing.  I kind of like seeing living things in the sea, especially endangered species.

A Jordan Series Drogue might be a good alternative to modifying the boat.  It slows the boat to reduce the chance of pitchpoling down a wave.  It tends to keep the stern perpendicular to the waves and decreases the chance of rolling or broaching.  A negative of the approach is that the boat can occasionally be pooped by a breaking wave from behind.  This is probably less of a problem on a proa than traditional monohulls.

I have never seen or heard of anyone sailing to Jan Mayen or the South Georgian islands in a Melges 24 or any other light sailboat.

Check out Roger Taylor and MingMing:
http://www.thesimplesailor.com/voyages.html
I enjoyed his second book too. He’s not crazy, just very pragmatic and disciplined.