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post #3887 of Old 02-23-2013 Thread Starter
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Multihull Stability

While I wait for a comment on those movies and boat, a paper and some comments on Multihull stability that I have posted already on another thread.

Not many studies around on cats. I had read this one many years ago and I had read it again. In my opinion the study, that is very interesting even if not always enlighten, suffers from a flaw: They determined that in the vast majority of capsizes wind was the main factor in what regards to capsizing and then when studding rolling and capsizing trough testing they considered only waves that have a pretty low incidence in capsizing catamarans, except when associated with wind.

I confess that I don't also understand this conclusion:

"84% of the catamaran casualties were the result of wind induced capsize or pitchpoling, whereas only 47% of the trimaran casualties were directly attributable to the wind. This does not indicate that trimarans are less vulnerable to capsize by the wind, because there are twice as many trimarans as catamarans in this sample"

They are talking about percentages so if the numbers are meaningful the fact that they have more samples from one than another is not relevant in what concerns percentages.

In fact it makes sense to say that Trimarans are less prone to capsize than catamarans by the wind. A simple compassion between two typical stability curves will show that while a cat after reaching its max righting moment (between 10 and 15 of heel) will lose rapidly stability, a Trimaran has a much more wider range of stability and even after reaching max RM, the loss of stability is much more gradual.

Here we have to stability curves of two cruising multihulls, or better several Cat GZ curves and a RM curve from a fast cruising trimaran (the shape of a GZ and RM curve is the same since they are proportional).

We can see that the cat MAX GZ (and RM) will be obtained with 15 of heel and then you have a strait line, from 45 till 80.

Regarding the Cat we will see the the Max RM (and max GZ) will be obtained with 22 and then it is not a strait line but a slightly convex line that goes till 90.

That's why when a cruising cat lifts one hull from the water it is a dangerous situation (a side wave can complete the loss of stability) while on a Trimaran sailing with one ama out of the water is normal and even with the central hull partially up (light on the water) the boat is still on control. this gives much more time for a sailor to react in a trimaran than in a cat. That is also why big ocean racing multihulls are today almost all trimarans.

Of course, they found out in that study (with testing) what it was obvious:

"Of the various catamaran configurations tested, the higher VCG (vertical center of gravity) conditions and the narrow beam configuration proved most vulnerable...
Although the narrow beam model was 20% narrower than the reference design, it is by no means unrepresentative. The length to hull separation ratio of the narrow model was 3.1 ..Previous tests with monohull models, indicated that, in general, they could be capsized by a breaking wave of a height equal to or greater than the beam of the yacht. "

and here he can see a basic difference between monohulls and multihulls, I mean on the last paragraph. Since the size of the wave, among other factors, relates with the beam of the boat in what regards the size needed to roll it, multihulls are much more resistant to be capsized by a breaking wave.

That is why when we study the factors that lead to the roll of monohulls we have by far breaking waves while on multihulls that is very rare (but not unheard) being by far the main factor, wind or wind associated with waves, a factor that has practically no relevance in rolling monohulls.

Here we can see that through these stability curves (all cats):

We can see that the values of Max GZ go from 1.8 to 3.0 (that has to do with beam). The cat they use as basic model (From where were derived the other models, with less and more beam, higher or lower CG) was a scale model of a 13.6m cat. A monohull with the same size has a Max Gz between 0.8 and 1.2. Since for the same size cats and monohulls have approximately the same weight that can gives us an idea of the different static stability and about the potential to carry sail.

Off course, the cats have to maintain a much bigger safety margin of security in what regards the amount of sail it is safe to carry. On a monohull a broach and a knock down is not a problem, on a cat it means a capsize.

Also, as was expected, they found out that Keels have a negative effect in what regards capsizing multihulls, but not with the relevance that Tropicat seem to attribute to them as a causing factor:

"The addition of the keels appeared to result in a slight increase in the vulnerability to capsize. For the narrow model it increased the capsize incidence from 14% to 60%, and for the standard model with the second VCG increase it increased from 17% to 25%. ...These results support the theory that it is the resistance to sideways motion that provides the couple to convert the breaking wave energy into rotation."

As we can see only on the narrow than average Cat the effect of the keels was very significant. On the standard model the increase was only 8% and in a sportive cat, that is wider than the average it is expected the increase would be even smaller. besides this is an old study from the nineties. Today almost no cats use keels, but daggerboarders that have a smaller lateral resistance so it is expected the negative effect in what regards stability to be smaller.

Regarding trimarans, not surprisingly and contrary to the wind factor, they have found that they are slightly more prone to capsize with a breaking wave, found out that smaller floaters in proportion with the main hull will make them easier to capsize by a breaking wave. Here, contrary to catamarans, the weight is a negative factor (and that's why I like trimarans).

"Of the trimaran configurations, those with the smaller floats were most vulnerable, with a 28% capsize rate for the standard displacement and a 38% capsize rate for the higher displacement....The tests confirmed the common opinion that small floats tend to become fully immersed if the yacht is struck by a breaking wave. Their high resistance to sideways motion then encourages rotation."


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Last edited by PCP; 02-23-2013 at 03:45 PM.
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