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CG is normally referred to as % of boat total length (transom to tip). Say you want a 32% CG and your hull is 45 inches long. You would take 45 and multiply it by the desired CG. So 45" x 0.32 = 14.4". You would then measure 14.4" from the transom forward and mark the hull at this point. You then layout everything in the hull to get the boat to balance at this point.
On the other hand if you're trying to figure out what the CG of a boat is currently at, you find the point where it balances and mark the hull at that point. Measure from the transom to that point and divide it by the total length of the boat. So for the same example, let's say the boat balances at 14.4" from transom, and the total length of the boat is 45". You would do the following calculation to find the actual CG... 14.4" / 45" = 0.32 (32%). Hope this helps.
See this article for some more info on the subject:
www.modelgasboats.com/Magazine...Proper_Balance/