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post #205 of Old 12-30-2012
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Re: Production boats- justified bias?

Originally Posted by JonEisberg View Post

As to Lou's question, Nigel Calder has a pretty thorough discussion of these issues in the first chapter of his CRUISING HANDBOOK... It includes a table of key parameters of a small assortment of about 2 dozen contemporary US & Euro boats between 38-42 feet... For example, the LPS of a Catalina 400 MK II is only 111 degrees, whereas for a Pacific Seacraft 40, it is an impressive 143... Numbers like that could someday translate to a VERY meaningful difference, say, somewhere between Newport, and Bermuda...
From the same book:

There is much more from a stability curve than an AVS. A boat can have a high AVS and not be a good sailing boat. Not saying that the Pacif Seacraft is not a good sailboat but it is better than the Vailant 40? Well certainly the one of the Vailant is better than the one of the Sabre 402 but without looking to the one of the Pacific Seacraft I would not know.

Certainly a sailing boat should have a good AVS, at least over 110 but ideally around or over 120, but that is not what defines the power and stiffness of the boat needed to sail in bad weather, specially upwind. what defines that is the first half of the curve, specially till 45.

Also the value at 90 is very important and in some soft curves with a low max GZ and a high AVS the value at 90 can be a lot smaller than in boats with a hard curve and a high GZ and for instance an AVS of 115.

The max GZ and the downflooding point are also very important in what regards safety as also the area under the positive part of the curve. Other important factor is the proportion between the area under the positive and negative part of the stability curve.

Here you have the stability curve of the new Elan 400 that I believe will be a very good overall sailingboat and particularly good in upwind sailing:

This is a RM curve while the others are GZ curves, they are proportional and have the same AVS but to obtain a GZ from a RM curve we will have to divide the values by the boat displacement.

If you do that we can see that the Elan GZ values are much better for a similar AVS value, considering the Vailant 40. We can see that the Max GZ of the Vailant is about 0.7m and the one of the Elan is about 1.0m.


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