I'm a numbers guy, so I can appreciate the work! That said I'm in the "some things are better left unknown" camp.

If you must do this, you've might consider factoring in residual value at the end of your loan. In that case the payment only matters for calculating your interest, and your cost/hr is driven off depreciation. You've got it structured like a lease right now, which is fair if you always plan to have a boat payment I guess.