Tried new props on Bigfoot, Pitch change did not work very confused???

chrisg

Chief Petty Officer
Joined
Jun 29, 2004
Messages
476
If you have read my other treads, you know I had downsized to a ?99 60HP 2-stroke Bigfoot from a I-6 115 and have been trying to get the correct prop for it. The Marina originally put a 14/13 on it, which produced a WOT RPM at 5800. (and you could tell ? the motor was louder)

I then put my old SS prop, which I was told was a 15P. That gave me 4800 RPM, but the speed appeared faster, and the motor was quieter. (No I do not have a GPS unit and the speedo on the boat reads between 15 and 18MPH). I figured that the SS prop was not a 15p, as it dropped the RPM by 1000 (and that?s with just one person)

Well I went and got a 13.75 /15P Al. prop from the marina, and put it on, The prop was a new one from Mercury). But when I went out, it only indicated 4800 RPM WOT and acted the same as my SS prop.

So I went from a 13P to a 15P, which you guys say should have dropped the RPM by 2*200 or around 400rpm, which would have placed the WOT RPM in the range of the motor. (Which I was told is 5000-5500).

I was told you should not allow the motor to lug, so I went back to the 14/13P and WOT RPM again indicated 5800, so I just won?t run WOT. (It does appear to run slower, but that may be a perception, as I have no way of accurately measuring speed ? I put a hand held GPS on my Christmas wish list.)

I guess this 200RPM per 1? pitch change does not apply for pontoons, as nothing else changing on the pontoon and both props were new.

In either case, I took the wife out for a drive, and she liked it its quieter and it goes fast enough for her.

Now to go fix my broken trailer so I can get the pontoon out of the water!
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: Tried new props on Bigfoot, Pitch change did not work very confused???

You are ignoring the fact that you are switching between stainless and aluminum props, and also between manufacturers. You need to understand that a 15P prop from one manufacturer may not, and generally does not perform the same as a 15P prop from another manufacturer. Pitch is not the only factor in designing a prop. Rake, cupping, blade shape and a host of other things affect the design. Stainless props are generally more efficient than an equivalent aluminum and just as manufacturers have different designs, the situation is the same for stainless. Prop selection is a crap shoot as you are finding out. If the recommended wide open throttle rpm for your engine is 5000 - 5500 you want to be at 5500 with an average load. It would appear to me that 5800 with just you on board is ideal because adding people will pull the rpm down. When alone you can throttle down to 5500. Propped incorrectly you cannot throttle up to 5500 when more heavily loaded so you end up lugging. Put the 13P back on and call it good. Speed will be whatever you get at 5500 rpm.
 

chrisg

Chief Petty Officer
Joined
Jun 29, 2004
Messages
476
Re: Tried new props on Bigfoot, Pitch change did not work very confused???

I realized the differnce in changing types, the two I tried were both Aluminum, I Just said it 'acted the same as the SS' regarding the RPM setting. I did not know there is a differece between manufactures. Now I do. And I did put the original 13/14 back on.
I just want to let others know that things don't always work out, and there appears to be other variables involved. and yes, the props were not from the same manufacturer.

So thanks for your feedback silvertip.

I do have one last question. With this 13/14 Aluminum prop, I notice a vibration in the motor at low speeds, which I did not notice when I put the 13/15 ( either the SS or the aluminum). they appeared to run smother.
 

Randybeall

Petty Officer 1st Class
Joined
May 2, 2008
Messages
319
Re: Tried new props on Bigfoot, Pitch change did not work very confused???

If you notice a vibration don't run it. You will just tear up the drive unit and spend a lot of money for putting it back together. Take the prop to a prop shop and tell them your problem. They can make sure all blades are balanced, pitch matched and cup and rake matched. All are part of the problem.
 
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