Originally Posted by PCP
Back to boat stability:..
Obviously the Dynamic stability counts for much in the capacity a boat has to resist capsizing and contrary to a very popular opinion I don’t think it has nothing to do with the roll moment of inertia, or the boat mass. If it had, the Miniclass racers or the light OVNI (with little inertia) would be boats that would be easily capsized.
It has all to do with the way a boat dissipates the energy of a breaking wave:
if the boat transform all that energy in a rolling movement (tripping on the keel) the chances are that the boat will capsize. If the boat can dissipate the energy of the wave in a kinetic movement (sliding laterally) the chances are that the boat can resist capsizing.
Some clarification about this (taken from another thread):
There are other factors that contribute to dynamic stability and I don't think that one is the more important. Really important is everything that contributes to dissipate the wave energy in kinetic lateral movement and detrimental is everything that contributes to transform that energy in a rotational movement.
I would say that the main factors that contribute to allow the dissipation of energy through a lateral movement are a small area of submersed surface, a small freeboard and a big righting moment (not necessarily at big angles of heel because when boats slide sideways in a wave the heel is not much) and a low inertia.
Regarding inertia and roll moment of inertia, let’s consider two boats with the same positive area under the RM curve, and therefore needing the same energy to be capsized. One is a long keeler, small draft, narrow heavy weight boat the other one is a beamy light boat with a big draft and all the ballast on a bulb at the end of a fin keel.
As I had said before the dynamic behavior of these boats when hit by a breaking wave will be very different in what regards the capacity to dissipate the wave energy moving sideways, but let’s consider that the full keeler would not trip in its keel, that the extra surface would not have a damping effect on the rotational movement and that the low mass and small under water surface would not permit the lighter boat to move much more easily sideways.
Let’s consider that the same amount of energy of the wave that hits the boats results in a rotational movement for both boats.Both boats require the same energy to be capsized so in what regards the results (capsizing or not) the effect would be the same but the kind of movement due to inertia would be very different.
The heavier boat would start to roll much more slowly but because it has much more inertia once started the rolling movement it would be much harder to stop it. On the lighter boat the roll movement will be faster but will be also stopped faster because the inertia is much smaller. Both boats will roll to the same point but the duration of the roll movement (to capsize at 90º and back to its feet) will be much longer on the heavier boat.
We could say that the slower movement is a more comfortable one but the fact is that in what regards seaworthiness the heavier boat will be much more time deeply heeled, exposing its side much longer to another breaking wave that will catch him with little stability left and therefore will have potentiated effects, resulting probably in a capsize.
That’s why I think roll moment of inertia is not a determinant factor in Dynamic stability and that its effect in Dynamic stability effectiveness is many times hugely overrated.