[Public-list] Longitudinal Center of Gravity

[FINNUS505 at aol.com]

In a message dated 9/26/2005 8:03:23 PM Eastern Standard Time,  
jg1111 at msn.com writes:

Mike,
I looked at the lines plan on Roger's site and I think the 16'  1" 
measurement is the Lateral Center of Buoyancy. I'm looking for the  Longitudinal Center 
of Gravity. There are two Centers of Gravity measurements  the Verticle Center 
of Gravity and the Longitudinal Center of Gravity. The  centers of gravity 
are what I am interested in to make the calculations to  design the hoist  to 
lift the boat out of the water. Jack probably  determined what these centers of 
gravity measurements were. Do you think  you  can get these?
Thanks,
John
----- Original Message  ----- 



Very interesting problem. Would an assumption that the center of gravity of  
the ballast would be at the same place as the center of gravity of the  entire 
boat be incorrect? I guess the center of bouyancy would more pertinent in  
placing the ballast keel, and that might not correspond to the overall center of 
 gravity.
 
We had an old (1913) racing sloop when I was in HS. It was a P Class, which  
was designed to the same rule as the J's, so it had the same profile. Just  
alot smaller. :) We got a copy of the original blue prints, because it was so  
interesting. The plans showed lifting rings on the keel in the bilge. One was  
above the ballast keel, and it appeared that it was just forward of what must  
have been the center of the lead keel. The other ring was just forward of the 
 intersection of the wood keel and the wooden rudder post. There were two  
slings that rose to one point, where a single lifting hook would lift the entire 
 boat. The aft sling went through the main companion, and the forward sling 
went  through a hatch that was about 1/3 aft along the cabin  trunk. 
 
If the concern is a stable lift of the boat, and not exactly even weight on  
both slings, I would think that one sling beneath the ballast,  forward of  
the center of the ballast, and one just forward of the rudder, should safely  
lift the boat. 
 
Interesting problem. Let's see what people who know what they are talking  
about come up with. :)
 
Lee

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