I will install my Zenoah M5 clutch type in that Scarab 55 '' very soon
but what is the best lenght between the spark plug and back of boat ?
Andre

In most monos, getting the boat to balance at about 30-33% of total length will get you in the ballpark. The stronger the motor, typically the more forward you will go. You measure from the transom. So in other words, on a 55" hull, 30% would be 16.5". So measure 16.5" from the transom forward and make a mark on both sides of the boat. Then lay the stuff in the boat and mount so that the boat balances on this point.

Here is a good article explaining how to adjust CG. www.modelgasboats.com/Magazine...Proper_Balance/