no speed??

[toylet]

I have a 2010 18ft pontoon with a 40hp bigfoot Mercury. The prop the dealer installed was a 14x13 pitch. the first time out i could have swam faster than the boat would go. I had 5500 rpms at WOT. The dealer then installed an 11 pitch prop. I got more speed but it felt like it was still cavitating. When i made a turn the rpms would shoot up to 6000 rpm and when i straightened out it settled down to 4000 rpms. I was told it had to drop to a 9 pitch, which is where i thought it should be to begin with. I did alot of reading on here andi keep hearing of a pontoon special prop 14x9 Pitch with 4 ears. In looking in the prop book this prop is not found. Can anyone help me with a place here in Ontario or a part number as thisis supposed to be made by Mercury.

 

[steelespike]

Re: no speed??

18ft pontoon I wouldn't think a 13 would be out of line.What year is the merc? 4 stroke or 2 stroke?In fact 5,500 is in the ball park.Sure sounds like you have some serious slip/venting(cavitation).perhaps the motor needs to be deeper or something is disturbing the water in front of the prop.They do make a Solas Amita 4 blade in a 13 for a 2010 4 stroke big foot.Sort of tells me a 13 may be the one.A 4 blade surely will help solve any slip/venting issues.If the 13 seems too much you could have it pitched down to an 11. I would imagine you could get about 18-21 with that boat motor combination.

 

[toylet]

Re: no speed??

the motor is a 2010 four stroke 40HP merc bigfoot. i have a 9P on it now but havent had a chance to get it in the water yet. With the 11P i would only get 4000 WOT going in a straight line, but when i turned it slammed up to 6000WOT then back to 4000 thats why the dealer said to drop to a 9P. Am i going in the right direction here? Thanks for the 4 blade info that i will look up.

Any advice would be great.

 

[steelespike]

Re: no speed??

You have some sort of venting or slip issues to go down 2" and lose 1,500 rpm makes no sense. And now your 2,000 below what you need. Nothing adds up.I sure would like to see the actual speed with each prop.
If your not making rpm less pitch is one solution.a 9 does seem extreme to me ,on a big foot, and with a 4 stroke your looking for 6,000 I wonder if a 2" change will do it?
Even though a new install don't assume throttle is adjusted right or for that matter running on all cylinders.