Originally Posted by Southern Star
Tom, sorry for the delay in responding.
No need to apologize -- I promised to add to my post but haven't gotten around to it so I'm already way behind in the karmic accounting on this one. And here I am rushing again...
Nevertheless, even in multihulls, increased displacement increases resistance to capsize; a lower CE for the sailplan and a lower SA/D ratio for the sailplan does the same. Furthermore, assymetric boards, when deployed, not only reduce leeway in comparison to LAR's keels, but also increase the risk of 'tripping' when suddenly overpowered.
Yes... But, design is holistic and I think sometimes it's possible to miss the big picture when concentrating on specifics. Quickly here are some design balancing acts that deserve some consideration. It's true that increasing displacement increases the resistance to capsize but it is also true that additional beam multiplies the effects of displacement and inertia also adding to resistance to capsize. But drag through the water is a function of displacement. Roughly speaking resistance to capsize is a function of displacement times effective beam and even more roughly speaking drag is proportional to speed to some exponent times displacement. So, while it is true that displacement increases resistance to capsize it is also true that a boat that is lighter but wider can have an equal or greater resistance to capsize and less drag. All else being equal the wider boat will be faster with the same amount of sail set and a similar exposure to wind capsize risk and, as a result of the added inertia, less exposure to breaking wave capsize. In theory at least and all else being equal
the wider, lighter boat with the same righting moment costs more than the narrow heavy one by most metrics but it is both faster and more seaworthy
for the similar levels of wind capsize risk. Also, when adding displacement what happens to the center of gravity will have effects on the boats resistance to wave capsize. Higher CG's increase the risk of capsize in waves. And the distribution of the weight also effects wave capsize resistance. So, particularly when looking at catamarans, it is not enough to look at D or even SA/D when considering wave and wind risk.
You're right that lowering the CE of the sail plan reduces the heeling moment. But I think you may be assuming that boats with lower SA/D's have lower centroids as a matter of course. However, it often happens that as designs get heavier they get taller -- bridge deck accommodations grow up and out and fore and aft. The rig then gets pushed up to accommodate the accommodations. A yacht with minimal bridge deck clutter can have a lower boom and lower and lower cut jibs and a lower air draft
for the same sail area. Also taller yachts have greater air drag makes them slower upwind and, very slightly, increases their vulnerability to wind capsize. So, a designer
could draw a yacht with less accommodation and more sail area that would have the same CE and less drag and a similar wind induced capsize risk than a yacht with a taller accommodation plan. So, lowering the CE is a good thing in terms of capsize resistance but one should not assume that cruising designs have significantly lower CE's as a matter of course.
I'm not sold on asymmetric boards for long range cruising. I understand the performance argument but even on racing boats that allow them and race
much of their time dead upwind their performance is not totally game
changing (eg. A class cats). I would expect in most cruising where even the "upwind" passages tend to be fetches that the additional realized performance would be very small. They cost more to build and need more attention when sailing and reduce your options... Hmmm... If I were to go that route
I would consider a design where the tip was flexible enough to wash out as the loads increased which would help with some of those issues but they still seem darned expensive in money
and opportunity to me.
Yes, by reefing one can lower both the CE and SA/D ratio. However, it stirkes me that for many cruising couples (who are/can be inattentive), a heavier cat with a lower SA/D ratio and LAR's keels would be more forgiving and hence, in a sense, more 'seaworthy' for their purposes. Yes, one can get into a reductio ad absurdum analysis and I am not proposing that the mulithull equivalent of a Westsail 32 would be the best option (indeed, in terms of performance and a comfortable motion, weight is the enemy of any mulithull).
FWIW, low aspect ratio keels may increase "tripping" more than a typical deployed dagger board... I'm thinking I'm already being over wonkish but low aspect keels are surprisingly effective in increasing roll resistance... OTOH, experience in beach cats suggests that boards may not be notably faster on the points of sail that many of us go out of our way to sail on... So many choices
Anyway, yes a design with a lower CE and lower SA/RM (D does not imply RM in multies) with plain sail set will be more forgiving than a an equivalent design where either of those factors is raised. But, there is not way to make a conventional multi immune from wind capsize so the operators will still need to be aware of their risks. And, it doesn't always follow that the faster, lighter boat has higher CE's or SA/RM's...
The stability of multis is all "form" stability so rules of thumb that work for keel
yachts are not always valid. I'm not sure what a Westsail equivalent cat would look like but it might end up super wide rather than super heavy...
Nevertheless, I still believe that due in part to the light displacement/relatively high SA/D ratio and performance underbody, the Atlantic would be more susceptible to capsize than many of her more pedestrian cousins unless particular attention is paid to correct sail and board trim.
Perhaps, but as rigged the designer
gives her a wind capsize number around 60 knots... That's a lot of resistance to wind capsize. No? Is it really obvious that a more "conventional" cruising cat of similar size would have done better? At some point all multis become "unforgiving". I feel the same way about reefing my cat as I do about turning the gas off at the bottle each time we use the stove. You can get away with lots of mistakes
, maybe a lifetime of them and the odds of any single
lapse causing disaster are very small. But the potential disaster is so great that it should never be ignored.
Anyway, bottom line: Displacement alone is a poor indicator of multihull
seaworthiness. I've yet to see a number that encompass all the important factors in multihull
wind or wave risk. You have to consider whole designs.