Hydro Junkie
Posts: 1965
Joined: 11/2/2004
From: Marysville,
WA, USA
Status: offline
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The engine is normally set at or as close as you can guestimate to the CG location. Where a hydro differs from all other boats is you have to balance it like and aircraft, With some of the smaller nitro classes. you can get away with using the engine as a balancing weight, but with the design being build, this is very unlikely. With the hydroplane being balanced slightly behind the sponson transom, a majority of the weight needs to be there or slightly forward to take as much weight as possible off the prop, which supports the rear of the boat.
Something else that must be taken into consideration is the length of the flexcable. Since all flexcables shrink under load, the shorter the cable is the better. Also, there is a greater chance for breakage of both the tube and the cable using a flex that is that long, which is something a vee and cat don't have to consider.
This then brings me back to the sponson tips. The first run of the boat will tell you if the boat's balance is off. If it's close, a little lead mixed with epoxy can be put in the sponson tips to fine tune the boat's ride. By using solid woodblocks in the tips, more weight will be needed to offset any tail heavyness due to it's being that much closer to the balance point. For example, you have a 6 meter long board balanced in the middle. Now you put a 5kg weight on one end. To keep the board level, you need to put 5kg of weight on the other end. Now, lets move the counter weight in 50cm, to keep the board balanced will take 6kg. This is the same way as it works on the boat, though not as extreme. Hope this answers your question
< Message edited by Hydro Junkie -- 12/22/2007 12:46:55 PM >
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Printable Version
All Forums >> RC Boats >> Speed - RC Gas Boats >> RE: Scratch building a 1/8 Hydro 
RE: Scratch building a 1/8 Hydro - 
. But I believe it is easier to shape and build solid wooden tips in the first place and modify them to your needs later. Besides the tips are very small anyway. Thanks for the tip. BTW, I changed the sponson transom to what you suggested and I noticed a big difference in stiffness, right away.
unglued. The next day when the epoxy had cured the boat was still flat as a board. then I continued and glued the front section on and again clamped it to the frame and the nose piece that shapes the ram wing. Believe me, everything is still according to planes. She is still sitting in her cradle without rocking. With every piece of skin I put on her she will be clamped down on the jig. Next set of pictures will be coming soon. 
I always love a good buildingthread. 
Beautiful work.