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Re: A tech few questions




Sean,

Some answers to your questions:

1) I have been reading a book titled "Heavy Weather Sailing" by Peter Bruce.  
In it he devotes a chapter to the stability of yachts and presents an 
imperical formula.  It is  called the Angle of Vanishing Stability (AVS) and 
is approximately 

                        AVS ~110 + 400/(screening value -10)

The screening value (SV) is given as

                    SV = B^2/(RxTxV^(1/3))

where B is the maximum beam in meters, R is the ballast ratio: keel 
weight/total weight, T is the hull draft @ B/8 from centerline in meters, and 
V is the displacement volume in cubic meters = Displacement (kg)/Density of 
water (1000 kg/cubic meter).  For my 1977 SJ23 I calculated the following: B 
= 2.44 meter, R = 0.343, T = 0.279 meter, V = 0.123 cubic meters, giving SV = 
57.97 (no units) and AVS ~ 118 degrees.  

A number of boat types with their righting moment and AVS are plotted in his 
discussion.  For a long keel the AVS is about 160 degrees with maximum 
righting moment about 50% of that or 80 degrees.  For a fin keel boat of low 
freeboard the AVS is about 135 degrees with the maximum righting moment about 
48% of that or 65 degrees.  The higher the Vertical Center of Gravity (VCG) 
is above the intersection of the keel with the hull, the smaller the AVS.  
The SJ is not a fin keel boat and is not a long keel but some where in 
between with a low free board.  From the low number for the AVS I would judge 
that the VCG is above the water line.  At any rate 50% of 118 degrees is 59 
degrees for the maximum righting moment.  Any angle up to 118 degrees and the 
SJ will right itself.  If the SJ is broadsided by a wave and rolls past 118 
degrees, then it is going to roll over.  

2) Knowing my mast height above the forecastle and scaling from the drawing 
in my owners manual I would judge that the height from water line to mast top 
is near 34 feet.

Bill Sharp

San Juan 23 Internet Fleet:  http://www.geocities.com/Yosemite/Rapids/5493/
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