[Public-list] Longitudinal Center of Gravity

[JOHN GRAVES]

I pulled out an old book I have from when I went to sea years ago titled "Marine Cargo Operations" by Capt. Charles Saurbier. I looked up trim calculations and came across this: "When a ship is on an even keel and at rest the points G (center of gravity) and B (center of buoyancy are vertically one over the other. If they are not, the ship will revolve slightly causing B to move until it comes under G. Hence, if we take the ship on an even keel and place a weight aft of point B or G, point G will be moved aft."

So, if the Lateral Center of Bouyancy is the same thing as the center of buoyancy referred to in the book, then the center of gravity should be on a vertical line with it being that the boat is on an even keel. So, the Longitudinal Center of Gravity does equal the Lateral Center of Bouyancy located 16' 1" aft of the tip of the stem (bow). If the boat was down by the stern the center of gravity would be further aft.

Based on this information I would think the best place for the straps to go to lift the boat would be just forward of the rudder on the keel for the aft strap and the same distance forward of the Lateral Center of Bouyancy as the aft strap is aft of it for the forward strap.

>From these two points on the hull I can measure the width of the beam and get the measurement needed to make the spreader bar. Does anyone on the list have any idea what the measurements for the spreader bar should be? Should it be the same as the width of the boat at the points that the straps go? Or, slightly more? Slightly less?

After I get the spreader bar measurements I can determine the lifting points for the lift, the angles, and calculate the stress on the pilings so I can design the framework for the hoist. I am trying to figure out how to use the least amount of pilings to get the boat to the bluff. Here's the tricky part. I want to be able to do it with the mast on. I also don't want a jungle of pilings blocking the view. Any ideas?

Thanks,

John #15

  ----- Original Message ----- 
  From: FINNUS505 at aol.com<mailto:FINNUS505 at aol.com> 
  To: public-list at alberg30.org<mailto:public-list at alberg30.org> 
  Sent: Monday, September 26, 2005 9:48 PM
  Subject: Re: [Public-list] Longitudinal Center of Gravity



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