[Public-list] Longitudinal Center of Gravity

[Michael Connolly]

John,
Are you planing to leave the boat hanging on the straps? while on the lift?  What is your depth of water at the intended lift?   Better to make a cradle and have the boat rest on her keel.  Use side boards to fit the hull to keep her from falling over in the lift.  You can then use a bow stop to align her fore and aft prior to lifting or if you want a drive through lift use a reference point on the dock or lift surrounding the boat.  Vertical posts with PVC pipe on them can act as athwartships alignments.
 
Michael #133 Lorrie Rose

JOHN GRAVES <jg1111 at msn.com> wrote:
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 
To: 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 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
_______________________________________________
These businesses support your Association:
http://www.alberg30.org/store/A30supporters.html
Please support them.
_______________________________________________
Public-list mailing list
Public-list at alberg30.org
http://alberg30.org/mailman/listinfo/public-list
_______________________________________________
These businesses support your Association:
http://www.alberg30.org/store/A30supporters.html
Please support them.
_______________________________________________
Public-list mailing list
Public-list at alberg30.org
http://alberg30.org/mailman/listinfo/public-list

 1127833445.0