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post #524 of Old 10-04-2012
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Re: Full or fin keel?

Originally Posted by BryceGTX View Post
I have also pointed out that the use of a stability diagram on a boat in a wave results in totally the wrong conclusion.

Keep in mind that the values in the stability diagram are the related to the bouyancy type force and the weight acting through the center of gravity. The force acting through the center of gravity always acts vertically. But the bouyancy force acts perpindicular to the water.

As long as the water is horizonal, the effective bouyancy force acts vertically and the weight vector acts vertically. In this case, the righting moment diagram is correct. However, when you place the boat on a wave, the effective bouyancy force acts at an angle equal to the wave angle. But the weight still acts vertically. This dramatically reduces the righting moment. There are additional secondary effects that further reduce the righting moment.

Furthermore, when we consider the dynamic effects, the axis of rotation is no longer at the metacenter. This has profound effects on how the boat heels. Some people want to analyze the dynamics as if the dynamic axis is at the metacenter. This is a severe error, particularly as the boat goes onto a wave. In the most extreme case, the dynamic center falls below the center of gravity.

So basically, we need to throw the stability diagram and metacenter out the window when we have dynamics or when we place the boat on waves. On the other hand that is why they call it the STATIC stability diagram.
Well, you are wrong and it is not needed any complicated explanation to show that. Your explanations should adapt to reality and not the other way around.

Just have a look at two boats, with similar types of hulls, about the same size, one with 4.5T and a substantial part of its weight down on a bulb at the end of a 3m draft, the other one with 9T with much more proportion of weight in its hull and 1.55m of draft. Both are well designed boats the total RM would not be very different (due to the much bigger mass of the heavier boat) and the lighter boat will be massively stiffer.

We are talking here about a 40 class racer and about an Oceanis 41. Go to an experienced sailor that know both boats and tell him that the Oceanis is more seaworthy because it is a tender boat with a bigger mass and the guy will start to laugh. When he understands that you are not kidding he can go away shaking his head or if he his a patient guy will try to explain to you that the 40 class racer was designed to race solo on the Ocean on the worst conditions, that several, while circumnavigating higher than the furious 40ís were caught in big storms and that all survived with flying colors. He may also say to you that the Oceanis is not designed to be sailed on high latitudes and that taking one there would not be a very smart thing to do.

He could also point out that even if the 40class boat is a racing boat, because the boat is fast and has a huge safety margin, older boats that are not competitive anymore are being bought by cruisers to circumnavigate with their families and that in this moment there are several doing that without any problem.


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