how to get more rpms besides prop

[s2ntburd]

i just bought a 18ft pontoon boat and it has a rebuilt 50hp mercury on it. The wot rpms should be around 5500 from what i have read. i was only getting about 4500 rpms with a stainless 13p prop so i bought a blackmax 11 pitch and it did not really go up at all. could it be the prop? From what i have read 11 pitch on the motor should be enough to get it up to 5500 rpms. If i adjust the high speed carb would that get the rpms up at all? if not is there anything to try? Also i am getting a lot of cavitation the motor. is it mounted to high or low? would a hydra foil help? thank you.

 

[ondarvr]

It doesn't have a "high speed carb", and no adjustments to the carb, other than making sure it's clean and adjusted correctly will make it go faster.

​The prop isn't cavitating, it's ventilating, they're very different things, but commonly confused with each other.

​It's not uncommon for pontoon boats to have some odd issues with ventilation at certain speeds, lowering the motor can help, and sometimes a foil may help too.

​Normally if it ventilates the RPMs climb rapidly.

 

[Chinewalker]

11 pitch on a big toon is way too much pitch. Ask your dealer, but I'm pretty sure you should be running a 9P. 18-ft toon is a lot of load for a 50. In automotive terms, Think of it like you're downshifting to tow a trailer uphill and keep your engine from pinging.

 

[Scott Danforth]

11p is way to much prop for a 50hp on a toon

 

[Chigwalla]

I won't restate what others have said, but I don't think I've ever worked on a toon that did NOT vent the prop sometimes.
That said, we don't see many of them in the salt chuck, but we do get the odd laker.
Assuming the motor isn't too high, a dolphin might ease the problem by enlarging the anti-vent plate.

"From what i have read 11 pitch on the motor should be enough to get it up to 5500 rpms."
Generally, ignore it - anything you 'read' is a guideline to get you started when you don't have the experience to judge it for yourself.
The only way to prop the boat is to prop the boat.

If your motor is EFI, it'll prevent you from over speeding so just keep dropping the pitch until you're in range.

After a while of running your new-to-you boat your ear will be plenty accurate, but until then you might want to beg, borrow or rent an inductive tach to use for propping. The boat's tach is only a rough guideline - they can be out either way.

 

[s2ntburd]

well i just got a 10 pitch 4 blade and a fin so once i install those ill see how things go. So pretty much the only thing to do is keep lowering the pitch of the prop then?

 

[Faztbullet]

What year 50hp???

 

[s2ntburd]

2005 50hp oil injection

 

[Silvertip]

Is it a "Big Foot" or regular lower unit. Standard lower units have a 1.83:1 gear ratio. Big Foot has a 2.34:1. If it is a Big Foot, you need a 14x9 pontoon specific prop (great big rounded blades). If it is a standard lower it would run a 10x13 for a lightweight planing hull but you need way less than that for a pontoon. You probably didn't help yourself much with the 10P 4-blade. One more blade saps power.

 

[Faztbullet]

Dang it Silvertip you seen where I was going !!!

 

[jerryjerry05]

Pics
IF?? the motors mounted right at the right height.(cavitation)
​If the props the right one and the hub not too small(cavitation)
​This motor new to the pontoon? did it ever run right?(I see you just bought it)
​Get all the info. Weight, size, gearing(lower unit)and then contact Michigan Propeller.
​They are the experts.