Re: mph to be expected from 16.5 foot fiberglass runabout
I don't know anything about old mercs, but are you sure you have a 1:1 gear ratio ?!? (Mine is 1.9:1) If so, you ought to have a real rocket on your hands....<br /><br />Here's the math:<br /><br />prop pitch of 17" = 17" travel through water per rotation of prop (this is notional, I'll allow for slip later)<br /><br />1 mile = 5300 feet = 63600 inches<br /><br />63600/17 = 1 mile per 3741 prop rotations <br /><br />no compensation for gear ratio of 1:1 so <br />4500 rpm/3741 rpm = 1.203 miles per minute and so for 60 minutes = 72.18 mph.<br /><br />Now deduct slippage, which is the amount by which the drag caused by a boat hull prevents the prop from achieving maximum efficiency. For most planer hull boats under 20' it should be between 10 and 20%. Assume 20% and you should be moving at 57 mph at 4500 rpm. Assume 10% slip and it should 64.9 mph.<br /><br />Either way if your shallow v early 60's boat does this speed, hang on tight and avoid wakes if you want to stay out of orbit! If it doesn't go so fast you are afraid for your life then I suspect your quoted gear ratio is wrong (and I also wonder if 4500 rpm is not too low for the motor's optimum power range but that's for a merc guy to say).
