Help with LCG LCF and LCB!!

[dougfrolich]

I found some Simpson Tables from way back. Just fill in the offset in the B column and the station spacing (CI) and see the results.

[fede]

Thank you all guys,
one of the exercise I have to do is something like this:
a yacht has a "light" (empty yacht) displacement of 3000 kg (3 tons), if trim = 0 what's the LCG?
From what I get I should just calculate the LCB for the Water LIne that gives me a Displacement of 3 tons?
correct?

[DaveB]

Yes... but it's not that simple... how do you know where to draw the waterline? If the instructor's a nice guy perhaps he'll tell you... otherwise it can be quite a chore to calculate several, interpolate and then verify...

Good luck with it....

[Stephen Ditmore]

I thought the question was how to calculate the center of gravity of a hull so that it can be entered in a weight study to calculate the vessel's lightship center of gravity. Let's say there are two scantlings, one for bottom and one for topsides, plus some additional structural elements such as additional framing in the bow, a stem & structural keel, and transverse floors. I would start by establishing the total distribution of hull surface area, and then see how this divides into areas by scantling. This will eventually be part of the overall weight study, and only a part - but I thought it was the part that fede was interested in.

Lets say we strike a diagonal (such that it lies in a single plane), then project it to establish its true length and where each station intersects it. We could then draw a straight line representing the diagonal's true length and transfer the locations of the stations along that straightened length. I haven't actually performed this exercise, but it seems to me we could take a half girth using a tic strip at each station, keel to sheer, marking on each strip where the station intersects the diagonal. Then we could plot the distance from diagonal to sheer and diagonal to keel at each station along the line that represents the diagonal's true length. Then the actual lengths of the sheer and rabbet line between stations can be taken from the plan and profile views, and triangles can be elliminated from the flattened shape to approximate the flattened surface. Then that flattened shape can be divided into scantling areas, and the area and center of each can be found.

There's a CAD program that lends itself to doing things like this. See http://www.touchcad.com
There may be others that will tell you the center of a surface area without unwrapping it.

Cheers
Stephen

[dougfrolich]

In regard to the tables I posted before, Line B38, the value description, should read the 1/2 area of the immersed station, not half girth. Cut and Paste, and no editing sorry about that.

[Chris Krumm]

Determining CG coordinates for something as complex as a boat hull using manual methods is way too tedious.

Most surface modelers geared toward boat design will allow you to assign a mass/unit area and give you the CG coordinates for any surface, and will even allow you to create groups of surfaces and extract the CG for the group. I know New Wave Systems' Pro series or AeroHydro's MultiSurf can do this.

If you want to get more precise, export your surfaces in IGES format to a CAD solid modeling program such as Solidworks or Cadkey (maybe even TurboCad Pro?), shell them into solids and add solids for other major scantlings and equipment groups. The programs will let you assign density values to all solids and generate CG's for individual solids or combined groups of solids.