21 ft pontoon prop

[Joetomass]

I just bought a 21 ft Lowe pontoon with a 90 hp Johnson. I wanted to see if it had the right prop on it. It has a 13.75 x 13. Also I'm running about 23 mph (gps) with 2 people on it and with 8 about 20mph if I Change the pitch could I get a few more mph out of it without blowing the motor? Current rpm wot is 4600 witch sound low for this motor
Thanks

 

[BLC]

Re: 21 ft pontoon prop

Good question, I am wondering the same thing. I also bought a 21 foot suntracker party barge with a 2009 e-tec 90 and I am getting 23mph on gps with a 13.25x15 prop and was wanting to get some more speed out of it. Didn't mean to hyjack just letting you know what a similar boat is doing with a different prop

 

[Jeep Man]

Re: 21 ft pontoon prop

What you have to pay attention to is your RPMs. Are you running at your recommended RPMs for your motor at WOT with 2 souls on board? If so, you have the correct prop. If you will be having more passengers on board on a regular basis and cannot achieve proper RPMs at WOT due to the increased load, then consider dropping your pitch by 2 points. We need to know the numbers before recommending further.

 

[Bamaman1]

Re: 21 ft pontoon prop

Joe: I've noticed that many people work on changing props, when they may have an engine mounted too low on the transom that's plowing through the water.

Unless the engine is at the right height, re-propping your boat may be a waste of time. Get your boat up to speed, and get someone to drive while you look at the anticavitation plate. If it's under a bunch of splashing water, you're too low. You should be able to see the plate plainly running cleanly. On the opposite side, if you were getting ventilation in a tight turn, you'd need to drop the motor. It's a fine balance.

I would think you're a little low in mph, and your rpm's need to be more like 5500-5800. You're way off.

 

[BLC]

Re: 21 ft pontoon prop

This all is starting to make some sence. I just checked the manual for my motor and it is supposed to be reving at 4500-5500 rpm at WOT and it is only reving at about 3800rpm(if I remember corectly). I have also been reading on another post about the motor being too low. Mine seems to fairly low also and throwing lots of water around the back and I was wondering how to determine what the proper height should be. According to what has been said should I check the motor height first in the manner in the above post and adjust if nessisary before addressing the rpm issue.

 

[Joetomass]

Re: 21 ft pontoon prop

More often then not I have 6-8 people on board and running at 4600. The book says 4800-5200, my motor is at the lowest setting but I don't think I can lift it because of cavitation, if I trim up at all and try and turn it cavities very bad.
Also forgot to mention I'm going to be adding a 3rd pontoon in the next few weeks so not shore if I should wait till I do that before trying to mess with the prop.

 

[Old Screwball]

Re: 21 ft pontoon prop

This has been my experience. I just bought a 2012 18' Party Barge. Came with a 75hp Merc and a 13 pitch prop. I had a 15 pitch stainless (same diameter) from a previous boat, so I had it re-hubbed to fit the Merc. Didn't work worth a damn. At about 4000rpm it would cavitate, slip, or ventilate or what ever you call it. Bought a 17 pitch for the boat. Runs 22mph BUT will only rev to 4200rpm. Looks like a 15 pitch aluminum 3 blade will do the trick for my boat. The 13 pitch could over speed, the 17 is too much. The 13 that came with the motor is at the Prop Dr. now getting re-pitched. Good Luck, props are expensive to experiment with.