F90 Prop Question

[kgetch2000]

I have an Alumacraft MV1860 with a Yamaha F90. I recently changed out the factory aluminum 3 blade prop (13 in X 19 pitch) which would run WOT at 6100 RPM which is slightly above the recommended range. I replaced it with a Solas stainless steel 3 blade prop (13 in X 19 pitch) which now runs WOT at 5400 RPM. I did pick up about 2 MPH on the top end speed.

My question is related to the WOT RPM of the new prop. It's right in the middle of the recommended range, but I was hoping it would be closer to the upper end. Do I gain anything by repropping to get another 400 or so RPM or is this good enough?

[CatCam]

I'm running pretty much the same setup - Lowe Roughneck 1860 with a F90, same size prop Stainless prop. At WOT under perfect conditions I will tach at 5600 RPM, If the boat is weighed down, i.e. 4 people and ice chest loaded with trout it will drop to 5300ish...

IMHO, I think you are fine and you will not gain too much by switching.

Just curious, what is your top speed and what year is your motor? Mine is a 2004, last year of the carbureted F90's. Knock on wood - 10 years going and NO fuel issues!

[boscoe99]

A motor (particularly a four stroke) should be propped so that the WOT RPM, with the boat loaded, is at or near its red line RPM. Particularly if the model does not have a knock sensor. The F90 does not.

If you want more top end speed you need more HP. If you want the motor to perform its best all around, and to live a long and fruitful life, then prop it as Yamaha suggests. At or near red line when at WOT with the boat loaded.

If the boat is lightly loaded or there is another time when it might go over its red line then just reduce the throttle to prevent this from happening.

[almetelo]

On my Key Largo 16 (CC) with my 2004 F100 (carbed) with 13x19k Yamaha prop I got 6000/6100rpm's at WOT with 1 or 2 persons so reproped to 12 5/8-21K and now get about 5700rpm's WOT with 1 or 2 persons: my usully load is me, my wife, the kid and 1 or 2 portable tanks of gas with 25L/6.6G and few more light stuff.
Time of WOT is short as gas is very expensive here
At wot I can get 73Km/h or 45mi/h (76km/h on a good day) fuel consumption about 33L/h or 8.7G/h
I can cruise with my usually load at about 3200rpm's 30km/h or 18.5mi/h with fuel consumption of 10L/h or 2.6G/h

When I replaced the 19K prop to the 21k prop got more top end speed and low rpmīs when cruising but as Boscoe posted the most important is rpm's at WOT with usually load

I'm just a little over proped nothing to worry a lot I guess but I'm looking for a 2nd hand 20K pitch prop to see what will happen.

Good luck