Correct prop on 18 Parker

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Well-known member
Mar 16, 2006
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Denver and MV
On its maiden voyage last weekend, I had some nasty chop to deal with so I couldnt play with trimming the outboard or the tabs too much.

However, at WOT the tach showed 5100 RPM and I got to approx. 34MPH. Based on the service bulletin on the Yamaha site, the F115 should get to nearly 40MPH at 5900 RPM.

Any thoughts on trying a different prop? I couldnt find any obvious markings on the hub, just a marking that said 19-K (maybe the stock 13X19). I will play with it a little when I go back to the island next month.

If I need to produce higher RPMS to get to the proper operating range at WOT, do I need more or less pitch? The boat also seemed to take an overly long time to get on plane. Is this indicative of having the wrong prop, or just being new to 4 strokes?

I am also curious about replacing the prop. If trying a different pitch is called for, is it best to stay with a stock Yamaha product, or is there a quality aftermarket solution.


5,100 at WOT is too low and probably bogging the engine. 1" in pitch change increases/decreases the rpm approximately 300 rpm. Looks like you need to drop 2" off the pitch. Someone with a bit more experience should weigh in on this.

FWIW, my twin F-115s wind up to 5,700 rpm and 39-40 mph at WOT. Book says 5,500-6,000, so I believe I'm in the "sweet spot" right now.
I had the same question and wrote to the factory. Robin replied and said for the 18' Parker with 115 Yamaha, they recommend a 13x17 black stainless prop. I have the same boat as you and it made a difference changing from aluminum to stainless as I already had a 13x17 prop.
I had the same results as you with my 1801 and F115. 5100-52100 and about 34-35 mph. I installed a Permatrim and I'm getting 5800-5900 and 40-42 mph. Also have the 13x19 prop and was thinking about going to a 13x17 but it looks like I don't need to now.

Sounds like you found the solution, but what is a permatrim? Is it like a doel fin, or a planing wing that attaches to the cavitation plate? I am a little leery of drilling into the new motor.

Thanks for the suggestion. Look forward to hearing more about the permatrim.

Yes it is foil type deal you attach to the cavitation plate. I had mine for 3 weeks before I worked up the courage to mount it. My rig is new also (94 hrs.).
Go to the Hull Truth website and scroll down to the Permatrim section. Lots of info there. Good luck and let us know how it turns out for you.