Confused about prop pitch? HELP!!!

[Redfisher53]

I have a 2001 - 2575 RL-2 25’toons powered with a mercury 90 horsepower 4 stroke. Came to me used w a Black Max 14 inch 13, pitch. 3 blade prop and has run great for the last four years. Had some prop damage & need a new Prop. The online Mercury prop finder, based upon the engine, size of the pontoon, load et cetera I should be using an 11 pitch. Most pontoon forums/online resources suggest higher pitches 13/15/19 etcetera? The engine plate indicates maximum RPM of 6000 with wide open throttle between 5,000-5,500. In most cases I never reach the 5000 to 5500 RPM's with the 13 pitch prop. With 2 of us on the boat wide open, my depth finder speedometer, we probably do 20 to 22 knots. Wondering if I should replace with another 13 pitch prop, drop down to 11 pitch as the mercury prop Finder suggests, or even go to a higher pitch?. #CONFUSED.

 

[Jack M]

If you were happy with the previous prop , I would replace with the same size .
If you feel like trying different sizes , you can contact, Ken at propgods

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[Redfisher53]

Thanks Jack. The internet is filled with Sooo much information & opinions, its hard to know the truth. I'll call him. I see you are in Michigan? We are planning an RV trip to your neck of the woods next month. be heading up the west shoreline then to Traverse City, Petrosky & on up to Macinaw City...and then to the UP.

 

[Jack M]

I lived in Petoskey for 15 years before we moved to Indian River . We are about 25 miles south of the bridge now.

 

[Michiman]

Ken has been a reliable source of information on props. That said, I definitely wouldn’t increase the pitch if you’re currently not operating in the suggested range now.

 

[DVW]

Let’s start with your confusion about prop pitch. The pitch is the theoretical distance the prop would advance with one revolution and no slip. Your max boat speed will be achieved with a pitch that allows your engine to run at the speed (rpm) that it makes max power. It might be tempting to go to a prop with more pitch so you can go farther and faster with every turn of the prop. Unfortunately that can lug the engine down to a speed where it makes less power. In the real world no prop advances forward the pitch distance with every turn. The fastest props have some slip. Less slip is not always better. The best prop choice involves calculation, experimentation and good advice. Finally most people are happiest when they prop for the most common load case. Hope this helps.

 

[PartyBarge]

+1 If the numbers you provided are close, you were getting very good performance (almost too good for the size and weight of your pontoon, maybe 25 MPH with a 25' RL package and 90 HP?). Expect performance to fall rapidly with added weight. For that combo, the best option is to run a pontoon rated prop. In the Merc/Quicksilver line those are the Spitfire/Nemesis designs. Considering our use of the 13p Nemesis, the likely best choice for you would be the 11p (maybe even a 10p). Also expect anything more than 13p in any design to be a bust with either excessively low RPM or excessively high slip, well beyond the manufacturer's intention. Check CALCULATED slip for yourself with the link below:

Prop Slip Calculator | Mercury Racing

Quickly determine slip for Mercury Racing propellers using our prop slip calculator.

www.mercuryracing.com

In general, slip only gets worse with added weight or drag and lower prop RPM.