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Good morning Whaler Nation! I have a 1993 Montauk 17 with a new Yamaha F90 onboard. I had a painted aluminum 13x19 prop installed on it while waiting on my SS prop to be delivered. I got good performance at 5800 RPMs WOT. WOT range for Yamaha F90 is 5000-6000. Top speed was about 38 mph on the GPS.

At the 20 hour service I had the dealer install my new SS prop which is a Yamaha Performance Series GYT 3R 13.25x18. But now my max RPM is 5000 at WOT with a max speed of about 34 mph on the GPS.

The drop in top speed with a decrease in pitch makes sense but the 1” drop in pitch should not have dropped the WOT RPM by 800. Even with 1/4” larger diameter. If anything, the RPM should’ve increased with the drop in pitch right? I just don’t think I’m getting the max performance out of my new engine at 5000 RPMs WOT.

Or could there be a different issue created during the 20hr service? Fuel flow? I can’t think of anything. Other then the RPM drop the motor seems to be running flawlessly.

Thanks for any insight you can provide before I go back to the dealer!

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Typically a Montauk 17 with a 90hp engine running solo and light would reach 43-44 mph.

Describe the mounting height of the engine - How many empty holes above the top bolts?

Is there bottom paint?
T- Top?
Bimini deployed?
Do you runi with a light load? if not describe what is in the boat.

Think. What else changed between the old and new prop?

Are you trimmimng the engine out as far as you can before the speed starts to fall?

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Keep in mind...

Prop performance is not linked to size. The blade geometry of the Performance Series is quite different that the aluminum prop.

It is for this reason we offer specific make, model and size recommendations.

You appear to have the recommended prop.

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Thanks Phil! The motor is mounted second hole from the top (1 hole above the mounting bold). I understand the motor should be mounted one more hole up (2 holes above the bolt). The dealer was uncomfortable installing it in that position, I can’t remember why, but we compromised on the current setup. I realize I’m leaving some MPH on the table.

No T-top or Bimini top. An old bottom paint job. I keep the boat trailered so I haven’t repainted it. Just me on the boat (180#). Two batteries installed in the center console. About 15 gal of gas. Maybe 50# of gear (cooler, anchor, life jackets, equip bag). I was running into a stiff wind on somewhat chop with the new SS prop. I’ll have to run it on smooth water…but that shouldn’t drop the RPMs by 800 right? Only thing changed between the old prop and the new prop was getting the 20hr service complete.

Thanks Phil T!

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A) I don't appreciate dealers that don't do what you tell them. You spent over $9,000 and they will not mount it the way it should be.

This tells me either they don't know whalers OR they don't care about customer service. In the old days, the customer is ALWAYS right.

B) You just spent over $9,000 and the boat is not performing as it should.

You have the right prop, the engine is a bit low but regardless, you should be over 40mph.

Double check the make/model and size of the prop. Is it stamped on the outside? Cant remember if the Performance Series is or not. If not, remove and find the part #.

If prop is still correct ---

Go out again, with NO gear, only fuel and you on calm water, no wind.

Wind her up and trim out till the rpms and speed start to fall. Then bump the trim in 2 quick times.

Report back rpms and speed via GPS.

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So I did what you said. Today was a much better day on the water. Calm winds, smooth water. Got the boat as light as possible and trimmed it out the best I could, even crouched down behind the center console. Got a solid 5500 RPMs and just shy of 43mph on the GPS. Way better then what I saw a few days ago.

I’m happy with the result but the 300 RPM drop from the 19 pitch aluminum prop to the 18 pitch SS prop still doesn’t make sense. (5800 to 5500) Shouldn’t the RPMs increase going from a 19 to a 18 pitch prop? The SS prop is .25 inches bigger and heavier. The SS prop also seems to have more cupping. The answer probably lies in there.

Thanks again!! If you have any other advice I’d love to hear it. I’m a HUGE fan of whalercentral.com. Thanks for everything you do!

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I think if you raise the engine another hole, you will gain another 200 rpm's at WOT just from less drag in the water.

Keep in mind we do the WOT testing because if the engine reaches it's sweet spot (5700-5800 rpm's for the F90) running solo and light, it will run efficiently at all various speeds with passengers and gear.

Now if you run most of the time with two guys, 50 lbs of ice and gear, I would drop down 2" of pitch in the same model of prop.

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Forgot - The RPM change has to do with blade geometry. They are completely different designs. The SS is less flexible and more efficient.

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As mentioned, your problem relating to RPM vs prop pitch is the SS propeller.

Several years ago, Yamaha bought out their propeller supplier/manufacturer, a company called Precision Propeller, I believe. This company also marketed their own props under various names, including "Stiletto" and "Turbo". As was determined many years ago,and detailed on this site and others, and when used on Etec 90 powered Montauks, the pitch of their SS props was determined to be understated. For instance their 15" pitch mid sized prop (4-1/4" dia gear case) performed like other brands 17" props.

I am suspecting that is the case with the Yamaha prop you were furnished - it is performing like a 20" pitch prop. You should have your Dealer exchange it for a 17" pitch (which will run similar to the 19" aluminum), and you should be just right on top end RPMs

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Thanks! That’s really interesting. That would make sense.