There is quite a bit of word play going on but none of it comes from me. The movement of G in response to movement of weights is not a belief, it is an unassailable fact of physics. Apparently, many members do not understand that extending a hull aft adds weight to the hull aft of its original CG. That moves the CG aft. Buoyancy has nothing to do with the location of G unless, as I repeat, flooding is involved. Here is a start, based on vessels most closely approximating yachts. http://www.imo.org/blast/blastDataHelper.asp?data_id=27389&filename=A267(VIII).pdf Just because you choose to measure the location of the CG from the new transom location does not mean the CG moves forward, it only means the number of measurement units from the new transom to the new (but still further aft) CG has increased. CG is what and where it is, changing the point of reference does not change the physical location of the CG on that vessel, it only changes the number you choose to use to describe it. There is a reason why alterations are based on a measurement from a point or perpendicular that has not changed. LCG is measured by builders, class societies, maritime authorities, and navies by either the forward perpendicular, the midpoint between perpendiculars, or the aft perpendicular. When a competent designer moves a perpendicular he will use the other perpendicular or the original midpoint because the distance from the new one is meaningless when compared to the previous measurement from that point. For purposes of stability and trim you must calculate the CG of the extension itself and use that as the basis for the location of the new CG, exactly as you would for any other trim or stability calculation. I understand that small boat operators or designers don't have a requirement to produce or reference a stability book but that doesn't change the fact that the physics are the same for a 300m box boat as they are for a 15m runabout. Your sketches illustrate that fact very clearly.