It depends what you're going to do with it really.
The only advise I can offer is that my F4 was a little tail heavy, although I was right on the limit with the engine I had on it (55HP). Tony Lee-Elliott, the designer, added about 18 inches to the front in the form of a conventional bow to up the HP on the F4GT.
If you are using the 15HP mentioned you'll no doubt be OK, especially if you use the standard space in the bow for the fuel tank and other heavy gear.
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