Got my toon back with new motor, only getting 15MPH! will changing prop get more spee

I had downsized from a ?87 I6 115 Merc. To a ?99 60 bigfoot.

The Bigfoot has a WOT range of 5000-5500.

The marina put a used 14/13 Aluminum prop on it. My old merc had a 14/15? SS prop.

This is on a ?87 parti-barge, 24-foot pontoon, just me and 10gal of fuel. And only having the gauges on the dash to go by.

At WOT, the RPM was 5800 when trimmed out. The max speed appeared to say 15MPH. I don?t think the motor liked that, it was screaming. It appeared to be happy around 4000 RPM, which moved it at 10MPH.

I?m going to this weekend put my original prop on to see what happens. That would be a 2? increase, and if I recall that would result in a 2*200RPM decrease, or 400, resulting in 5400 RPM, within the range.

What should I get for speed with this motor? Should I get a 14/16 prop, that would put the WOT RPM down to 5200RPM. Would that get my speed up?

Are there special props for pontoons?

Re: Got my toon back with new motor, only getting 15MPH! will changing prop get more

24' toon with a 60hp would max out around 14 - 18mph deoending on load IMO


You need to change your prop out and your math is right. I would not expect more speed out of it though.

Re: Got my toon back with new motor, only getting 15MPH! will changing prop get more

Something is dreadfully wrong with your numbers. Either the tach is not correct, speed is way off, the prop is not a 13P or the hub is slipping. A BigFoot lower unit has 2.33:1 gears. At 5800 RPM and a 13P prop, the speed on a PLANING hull would be 26 MPH at 15% slip. On a pontoon speed will be considerably less than that but it should be more than 15 MPH. Speed needs to be checked with a GPS and I would have a look at the tach calibration. As far as screaming goes, the 60 BigFoot (assuming its a 2-stroke) is a three cylinder engine whereas the 115 was a four cylinder so it would have different sound. SLowing the engine rpm down does not increase speed. It merely means the engine RPM is less at the same speed you were going when it was running 5800 RPM. If you step up in pitch and the engine is STILL able to turn 5800 RPM, then yes, speed will increase. The idea is to prop the engine so it runs at or near is recommended WOT rpm. Speed will be whatever that combination provides.

Re: Got my toon back with new motor, only getting 15MPH! will changing prop get more

yes, I would not be surprised if the speedo is off ( That's why I said look like). The mechanic said the tachometer is correct. I'm not too concerned about the speed ( seems fast enough for me - I'll use my other boat for tubing). Guess my concern is should I change the prop to bring the RPMs down? Sounds like I should, but don't expect more speed.

Other then the RPM issue, I'm a happy camper. The motor is a lot quieter, you can carry on a conversation at the back of the pontoon without yelling. And it started with little effort.

Once thing is, you can't see the oil level through the window on the front of the motor, I fill the tank, and almost overflowed it ( The Marina had already filled it). I noticed that there is a warning buzzer under the helm, I hope to get the low oil warning signal working, as running out of oil is the last thing I want to do.

Untill I get the low oil warning system working, I will keep topping up the oil tank.
Should I expect to add oil at the same rate as if mixing in a tank? (IE. When I go through 6 gal of gas, put in the same amount of oil as if I mixed it in the tank?)

Unfortunately, I will be pulling the boat out next weekend and putting it away for the winter.

Re: Got my toon back with new motor, only getting 15MPH! will changing prop get more

"I had downsized from a ?87 I6 115 Merc. To a ?99 60 bigfoot.
The Bigfoot has a WOT range of 5000-5500."

I have a 2005 Merc 60 Bigfoot on my 21' PartyBarge.
I don't know the details on the prop but WOT is 5000 with just 2 of us onboard. When I load it up with 6 or more people, the WOT seems to drop to 4800. It never "screams" but it does eventually get up and go pretty good. No Speedo so I;ll have to take the GPS out nextime for MPH

Re: Got my toon back with new motor, only getting 15MPH! will changing prop get more

I found out they are charging me 160 for a used prop!!
I told them to give my my old prop back, and they argued that I will reck the motor. I can buy a new prop for less!

SunTrackerPB21: sounds like you should reduce your pitch to up the RPMs. I read somewere that overloading the motor will shorten it life expectancy ( Or is that only for 2-strokes!?) It will also get up and go faster.

Re: Got my toon back with new motor, only getting 15MPH! will changing prop get more

SunTrackerPB21: I just put my old SS prop on, and now the WOT RPM with just me is 4800, so I'll have to see if this is really a big deal or not.
I asked the question in the prop forum.