900 inline 6 mercury prop question

[ChrisD2725]

Ive got a merc 900 (1965 vintage) on a 16ft 1986 Monark Mcfast bass boat. It is currently swinging a 13.5X15 aluminum 3 blade prop. She jumps out of the hole like a scalded cat and she maxes out at 33mph and 6000rpm (500 over redline i believe?) She porpoises in choppy water at anything less than wot unless you trim in so far that bow steering becomes a huge problem.

My question is i think i need to increase my prop pitch but how much? And maybe a prop with stern lifting qualities?

Chris

 

[jimmbo]

Your engine rpm range was 4800 - 5200 so you are about 800 over. Yes you do need more pitch. I will assume your current prop is a Mercury prop. If not, ignore the following paragraph. and get a 19 inch pitch

If it is a mercury prop
Any idea how old your current prop is? If it a pre 1979 prop it complicates the selection a bit. In 1979 Mercury made major changes to their propellers, new blade shape, progressive pitch, cupping, and very importantly a new thrust washer. So you if you have pre 79 prop, you would need a new thrust washer as well. The pre 79 props weren't cupped so it makes it a bit difficult to say just get a prop with 4 inches more pitch, the rule of thumb being, every inch pitch change adds/subtracts 200rpm. Cupping can add 1 to 2 inches pitch to a prop. If your current prop is pre79, then a new 17 should be fine. If your prop is post 78, then a 19 inch is what you need.

 

[ChrisD2725]

From looking at the thrust washer, how can i tell if its the new style or old style? I believe its a solas prop, cant find any markings on it other than the diameter and pitch along with a pn for a chicago wheel hub insert. Looking at the blades, they do look cupped.

I dont know what ratio the lwr unit is, just that it was removed from a 125hp tower of power by the previous owner and installed on this motor.

Anyone here have any expirience with piranah composite props?

 

[jimmbo]

If it is a Solas, ignore paragraph two, that was for Mercury brand props. As for the gear ratio, I can't find any info, so you are going to have to rotate the engine by hand for say, 4 or 5 revolutions, counting how many times the prop rotates. 1.87:1 was a ratio used back then, 2:1 was also used. Simple math will provide the ratio. Piranah Props? They are less rigid and therefore flex more that AL or SS.