Re: What prop for Evinrude 40 HP Big Twin?
I had a pair of 1958 ERude 35hp Bigtwins, with 14" pitch props on a 15 foot glassmaster trihull, for a couple of years. Your boat is a foot longer, but is of similar design. You also have motors with five more hp each, so the match up should be similar.
The correct rpm range for your motor is probably 4000 - 5000 rpm at WOT. My guess is that the 14" wheels will get you within that range. I also recommend that you look into buying a set of Tiny Tach brand tachometers, since your motors don't have rectifiers to send signal to regular tachs. With these, you can tell what rpm you are running at for assistance in making decisions like this one, and you can "marry" the throttles to gain even rpm, while underway. These tachs are only about $37.00 each, but you will have to have the cables lengthened ($10 each) if you want to mount the tachs on the dash of your boat.
If you find that you are over-revving the engines, a prop shop should be able to pitch your wheels up as much as an inch. I mention this because there isn't a 15" wheel for your motor - as far as I know, the maximum was 14".
