93 21' Pontoon over proped

[btroy]

Hi guys finely switch boats and picked up a 1993 21' Pontoon with a 1993 60 Johnson, it has been converted to pre-mix.

Have a questions thought you could help me with.

At WOT the motor is only running at 4000 RPMs(around 15mph) I was reading and it appears this motor should be running 5000 Min and 6000 Max at WOT. Any reason you can think it is only running at 4k? Currently it has a 13 -3/4 X 15 prop but the boat runs really good pushes thru the water nice.

Any idea as to what size prop I should be using, I am thniking it is over propped.

Starts right up and runs smooth.

 

[Maclin]

Re: 93 21' Pontoon over proped

If all figures are accurate, than a 12 pitch would get you to 5000rpms at the same 15mph, and it may rev higher that that and get you a couple more mph.

Right now your numbers indicate the engine is lugging, and eventually that will cause internal engine problems. Getting a lower pitch prop on there will let the engine work easier.

 

[btroy]

Re: 93 21' Pontoon over proped

that's what I was thinking so if min is 5k and max 6k if I put on an 11 pitch that should put me around 5300ish. That should be fine correct?

 

[Maclin]

Re: 93 21' Pontoon over proped

Here is a prop calculator that may help you work out some numbers. You really need the drive ratio, but with yours I used 25% slip, 4000rpm, 15mph and 15 pitch to "back in" to the drive ratio of 2.8 That may not be exactly accurate but can be used as a constant when comparing what rpm gives you the same mph, that kind of thing. I then changed the rpm to 5000 and blanked out the pitch then re-calced and got a 12 pitch.

I would use 25% as slip and the drive ratio of 2.8 for all calcs. However, if you know the exact drive ratio then you can calc the slip first, then use those as constants for re-calcs.


http://www.marksmarineinc.com/prop_calc.aspx

I reddid my calcs on an 11 pitch and came up with 5450 rpm. Again, speeds of 15mph and 25% as constant. This is all approximate and real world prop action may be different, like the slip can change on any given prop so it is still kind of a dartboard toss.

 

[btroy]

Re: 93 21' Pontoon over proped

ok great, thanks for all the help. I will check it and and go from there.

 

[Rancherlee]

Re: 93 21' Pontoon over proped

Id say 14x11 as a good starting point also. That 15 pitch prop is good for a boat capable of 30-35mph with that engine. Also, if its a factory OMC prop from that era then its probably has smaller blades than required for a good pontoon prop. The 14x11 and 14x13 factory props had big "elephant ear" blades which work great on a toon. You have the same 2.42 gear ratio as I do if that's a 3 cylinder 60hp. I think with No water in the tubes and a light load you should be expecting 18-20mph out of it. A 14x11 pitch prop is good for 24mph @ 5500rpm with ZERO slip, with typical non-planing pontoon slip of ~20-30% it should put you right at 17-20mph with the correct RPM!

 

[btroy]

Re: 93 21' Pontoon over proped

thanks guys that was the info I was looking for.

Really appreciate it!

Happy boating