Re: Mid 70's Mercury Thunderbolt 500 RPM Question
The only way to get rpm up in the recommended powerband, assuming the motor is 'hitting' on all Four and running the way it should, would be to go to a prop with less pitch.
You've got a 50hp on a heavy tri-hull. It'll plane out much better and likely go faster with a prop that allows the motor to get to the top end of its recommended rpm range, at Wide Open Throttle (WOT) with only the driver in the boat.
There should be some numbers stamped in the main body of your prop, if it's a factory item. A long series of numbers with a 2-digit number at the very end, this'll be something in the range of "10", "11", "12" thru maybe "15". A 50 hp, unless it's on a very light high-performance hull, won't usually carry much more than a 15-pitch prop very well.
On your boat, a 10" or 11" pitch at most is more in tune with the hull weight and design.
If you can't find any numbers on the body of the prop, try removing the prop and see if there are any numbers on either the front or rear thrust hubs.
Failing that, take the prop to your local Propeller repair shop and they have measuring tools to determine prop pitch.
One other thing, most hulls run best with the tilt pin in the 2nd or 3rd hole. Trimming the motor out will lift the bow, allowing the hull to 'air out' and gain more speed. Of course, your "T-Hull" probably won't air out much with a 50hp hanging on the back, but any little bit will help!
HTH & let us know what you find.........ed
