2008 Yamaha 90hp 2 stroke max rpm

[Lcavallaro]

I have an 08 90 hp Yamaha on an 18ft ranger bass boat that only revs out to about 4200 rpm. It has a 15p stainless 4 blade prop. Also the motor hits 4200 at about 3/4 throttle and the rest of the travel of the throttle doesn’t give any more rpms. Any suggestions would be appreciated. Thanks

 

[Sea Rider]

If not mistaken that OB revs 5000-5500, if so OB is 1300 wot revs off from max wot rpm range revving towards the lugging side as loaded which is no bueno LOL!! Need at least a 13 pitch 4 blade prop to pull wot revs higher and check with a wot water test what rpm dials at full hammer down. From there can fine tune a prop if needed..

Yam has excellent OEM 3 blades SS props, why is it that near everybody goes straight to 4 blade props when have not tested 3 blade SS ones with less pitch...

Happy Boating

 

[Lcavallaro]

The 4 blade stainless came on the boat when I bought it a week ago. I was also given the 3 blade aluminum 15p as well. Do you think that would make a difference?

 

[ClassicAQ]

The 4 blade stainless came on the boat when I bought it a week ago. I was also given the 3 blade aluminum 15p as well. Do you think that would make a difference?

No. At least not the difference you're needing. The number of blades and material of prop does have a factor in the rpm equation, but just barely. Engine height and prop pitch have a far greater influence on rpms.

Spec rpms @ WOT for your engine is 4500-5500. Rule of thumb for gauging change of rpm with change of pitch is 1" = 150-200 rpms. So per this rule if you were to drop down to a 13" your rpms would increase by 300-400. But this only gets you to the bottom of spec....and you wanna be at the top spec.

Of course, all of the above is assuming your engine's inability to reach spec rpms is NOT due to poor engine performance. Because if it is, you've gotta first address that issue before addressing prop pitch.

 

[Sea Rider]

Yep, OB needs to be in top running state before playing with props. Install the 3 blade one and check max rpm achieved at wot loaded same as when tested with the 4 blade prop. If the 3 blade prop revs higher will be much better, but probably won't reach max wot revs you need to aim for.

The described prop rule of thumb does not work that way, works well on very heavy large applications. To illustrate your particular case will need to drop 6 pitches in order for OB to rev towards max wot rpm range. that's out of this world. Going 2 max 3 pitches down will do the trick. Only checkable on water tests as theory most times doesn't match practice.

Good to read that we're on same page as ClassicAQ regarding correct OB/transom height is a must dial before playing with props. Most boaters sits their engines on transom and voilá...

Happy Boating