[Public-list] Longitudinal Center of Gravity

Tue Sep 27 21:38:06 PDT 2005

John,
Oh! I see.  Well I lift my boat about the same as Don Campbell.  Aft strap about 4" in front of the rudder shoe and fore strap just at the forward keel cutaway.  I tie the straps together fore and aft both sides.  Took pictures so that I have a reference at the deck level when the boat is in the water as to where to position the straps.  

The spreaders should be wide enough to allow the strap to be tangent to the hull at its widest point.  Normally this is about a foot below the toe rail.  If the strap travels upward from the keel and just touches the hull at the tangent point and then continues outward away from the hull it will support the hull enough to prevent the hull from flopping to one side or the other.  This configuration raises the boat safely and places the least athwartships compression on the hull.

I have witnessed wooden boats crushed inward from careless lifting without adequate or no spreaders.  The Alberg 30 would probably withstand this inward pressure but why risk it?

Michael

JOHN GRAVES <jg1111 at msn.com> wrote:
Michael,
No, I'm not planning on leaving the boat hanging on the straps. I will need some type of trolley system to carry the boat to the bluff. The depth of the water at the lift will be the minimum I need at high tide so I can get the boat as close to the bluff as possible so the trolley will only have to travel the very minimum needed to put the boat on land. I think I will be able to get it so close that the bow will almost touch the bluff. I already have a great cradle sitting on the bluff that I found on the docks at a port terminal near to where I live and I tracked down the owner and bought it from him for $100. It is made of very strong steel with hinged pads on threaded adjustable stands. It was professionally made and is the strongest I've seen. It was used to ship boats on ships. It is the perfect size for an Alberg 30. I eventually want to put wheels on it or put it on some kind of trailer so once the boat is on land I can roll it into a shed to work on it. I also have 5
 jackstands.
John
----- Original Message ----- 
From: Michael Connolly 
To: Alberg 30 Public List -- open to all 
Sent: Tuesday, September 27, 2005 11:04 AM
Subject: Re: [Public-list] Longitudinal Center of Gravity

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 > 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
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