I know the question sounds silly, but how do you determine how to position the engine. I've got a deep V hull 1020 mm long and I'm putting in an older brand new Zenoah 23cc engine. Will be using a surface drive. I've been told 1/4 of the boat length from the rear and also 1/3 from the rear. Thats a reasonable difference.
I've got the rails cut, ready to be glassed in, so need to make a decision soon. Photo of hull to date is attached.

You want to mount all the rest of your hardware first, then place the engine so that the center of gravity is set at about 30% from the transom. There is no set mounting position to do this.

That is correct. You will be looking for a CG of 30-33%. With a stock 23, 30 will most likely be right on. For the record, in most cases the engine ends up being very close to the CG of the hull, since the engine is the heaviest component in the hull.

marc wrote:
Yes in some cases that is true... BUT, people setting up boats especially new boaters need to truely find the CG when setting engine placement..instead of simply going by transom to CL of engine placement. There is also more to balance point in keeping a boat happy on the water. It dont matter if you get the balance point dead on...if you mount the engine so its an inch above the floor with the pipe sticking 70 degrees up...its still gonna be a tipsy one.

I just wish boat manufactures would stop giving engine placement measurements and give a CG % the boat likes.

Not telling him to mount it at a certain point, just giving him a general idea of where engine will most likely end up, so he can plan his construction accordingly. :drinks:

Thanks all. That helps a lot. I assume that I should allow for a full fuel tank when establishing this. It won't be the flashest rig around but once set up, wait for number 2 to be built. Have to get the missus sign off on 260PUM or a RCMK.

I like to balance without fuel. Most of the time you can put the fuel (if you use a bag) right on the CG so it doesnt effect anything. If the fuel is in front of the CG.. it will just balance a little heavier when full... but if the balance point is good without, the boat should not become flightly once it has started burning off most of the fuel.

A good way to ballance things is to do it with half a tank of fuel that way nothing changes too drastically wether your tank be full or empty .
Mart