I am curious about the Japanese designs as well, looking at one the engine was in the middle of the boat! They are large and fairly lightly constructed which means they cant be that stiff. Considering many of them run without rear sponsons it makes me wonder how they can work. I noted Hammer's most recent design, the 12 boat, had the engine pretty much over the trailing edge of the front sponsons, and also running rear sponsons, so this is quite a departure from the typical Japanese designs.
The CG of some quite conventional design is further back than 10% tho. My 45 SG was 20-24% depending on whether you measure afterplane to the rear sponsons or the prop. My old Zenoah powered Cobra was about 18% from memory, and that ran quite well in the rough.
It is my belief that CG is not as important a variable on a rigger as it is on a mono or tunnel. On a rigger the amount of lift generated by the hull can be controlled independantly of the CG placement, but in a mono as you move the CG back the aero lift from the hull has less weigh to balance it and the whole hull can run a higher angle of attack until it blows off the water! By playing with the front and rear sponson angles and heights, strut height and angles, and also prop lifting characteristics you can change how a rigger runs in terms of how flighty and balanced it is. So if a boat is too flighty at the front, try adding boom shims to the rear boom or removing them from the front boom.
Personally I dont think the sponson top angle is that important compared to the riding surface angle. SG's are the fastest in the world and run flat tops....
Ian.
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