Need Propeller Advice

[Scott C]

Hey Guys,

I ended up having to put a new motor on my boat recently.....kind of a long story, but the hard lesson I learned is to get a boat checked out by a mechanic before buying it!!  UGH!!

Anyway....the boat is a 1990 Fisher SV-18 GT (18' aluminum bass boat style, not a deep vee or a jon).  It used to have a 90 HP Merc on it, but now has a 1994 75 HP on it.  

The prop on it is a 17p, and at WOT I'm running about 4950 and doing 33.3 mph max (GPS), when its just me in the boat.  So from what I have seen online, the suitable rpm range for this motor is 4750-5250, so I am already in that range.  Also what I have found from calculations is that I am getting 3%-4% slippage.  That sounds pretty dern good to me!

So here's my question....will my motor be happier, more fuel efficient, or better off in general running at 5150 w\ a 16p prop??  I know my top speed won't change much if at all.  Or, is running at 4950 w\ the 17p going to be just fine?  The gear ratio is 2.30:1, btw...

I would appreciate any input that you guys have on the matter!!

Thanks!

[Wayne P.]

Simple answer for fuel ecomony: run your motor at or under 4000 rpms no matter what prop you run. The top 1000-1500 rpms is where you have the least ecomony.

[Scott C]

ah yeah, thats a good point.  I guess I'm more wondering about the health of the engine...is it better to run closer to the upper end of the RPM range, or is lower ok....I'm really leaning towards keeping it at 17..mainly because its cheaper to just leave it.   ;D  

[Wayne P.]

Use Seafoam all the time or a shock treatment of some de-carbonizer occasionally and your motor should be trouble free no matter what rpm range you use.

Lots of saltwater fishermen troll with that size motor or larger with no difficulties.