Bertram31.com General Bulletin Board

Re: Project B20 *LINK*

Posted By: Chiles
Date: Wednesday, 23 March 2005, at 11:23 a.m.

In Response To: Project B20 (Kurt)

Taken from a Boat US article. (I wish I was this smart)

Center of Buoyancy (CB): Often called Lateral Center of Buoyancy (LCB), this is the center of the underwater volume of the vessel and can be expressed as a distance abaft the forward end of the LWL, abaft midships, or as a percentage of the LWL from the bow end. If the boat is to float on her LWL, the Center of Gravity (CG) must be in line vertically with the CB, both fore and aft and athwartship. If the two centers are not in line, the boat will change trim and so change her underwater shape, until the new CB lines up with the CG.

If your boat is floating perfectly in trim and you add 100 pounds of davits and dinghy aft, for example, you will move the center of gravity of the boat aft. The vessel will sink by the stern and the bow will come up until the underwater shape changes enough to move the CB over the new CG.

The same applies athwartship. With luck, the CB and the CG are both on the centerline of your boat, so she floats level without any heel angle. When you move to the starboard rail, you move the CG off centerline to starboard, so the boat will heel until the change in underwater shape moves the CB vertically above the new CG.

Now then...

I wish I had a nice formula to give, but the best I can come up with is the sheet we use to determine the effects on center of gravity on flying small planes. If you are a pilot, this is old news and center of CG is not CB. This excel spreadsheet shows how you can load a plane and the effects it has to it's flying ability within a "safety margin"

http://www.flyingclub.org/aircraft/017wb.xls

The basic principal here is, one pound moved an inch back only changes CG a little. That same pound could effectively feel like 40 pounds to the CG if it were hanging off the end of a pole behind the boat. Think the fulcrum effect.

Now that I have confused even myself, can anyone bail us out with a bow to stern formula that effects center of boyancy and center of gravity?

Chiles

Boat Formulas from BoatUS

Messages In This Thread

Project B20
Kurt -- Wednesday, 23 March 2005, at 8:44 a.m.

Re: Project B20 *LINK*
Chiles -- Wednesday, 23 March 2005, at 11:23 a.m.

Re: Project B20
Kurt -- Wednesday, 23 March 2005, at 12:44 p.m.