LCB vs LCF

[John Klimek]

What is the difference between longitudinal center of buoyancy LCB, and longitudinal center of floatation LCF?

[Stephen Ditmore]

LCF is the center of the waterplane, LCB is the center of the immersed volume (LCB is therefore the weighted sum of the centers of all the waterplanes from the waterline down). Let's say you have a vessel with a deep bulbous bow and a broad shallow stern. The LCB is then likely to be forward of the LCF. If you are looking at the vessel's overall center of gravity and its relationship to bouyancy, and therefore trim, you would use the LCB. But if you're looking just at the effect of the addition of new weight somewhere, you would use the LCF.

[HOWdie]

Steven: Isn't the LCB as you described better understood as describing the VCB (vertical center of balance) or am I way off base?

[Stephen Ditmore]

They're the same thing, except the L indicates that you are refering to its longitudinal position, wheras V indicates you are refering to its vertical position (so if you were to assign values to them they would be different numbers, i.e. they would be distances to the same point, but from different reference planes). TCB is transverse center of bouyancy, LCG is longitudinal center of gravity, etc.

I might quibble and insist that the B is for bouyancy in the references I'm familliar with, but that's just semantics.

-S

[HOWdie]

Thanks for the note: Stephen

[Stephen Ditmore]

In the sailboat forum Paul ("Polarity") recommended http://www.tedbrewer.com/yachtdesign.html, and it really is quite nice. I encourage you to give it a look (though it says "L" in LCB stands for "Lateral"; I think it stands for "Longitudinal").