Prop help please

[2004 Voyager]

My current set up is this- 18' foot pontoon with a 75 horse Mercury ELPTO. Prop is 13.75"x15 pitch. I've tried to use the prop calculators I found on the internet and get widely varying results. I know about the RPM vs MPH correlation, however I do not have a working speedo on my boat, so that method will not work. I'm primarily interested in what other pontoon owners have on their boats if they are of similar size and horsepower. Seems boat should go a lot faster with a motor of this size, it's the biggest legal size I can put on it!
Any and all thoughts welcome, Larry

 

[ahicks]

A tach and a portable GPS should be all you need?

 

[2004 Voyager]

Too late for any on water testing this year, I'm already winterized. Took the prop off yesterday, needs a little straightening and some nicks cleaned up. Was just curious if this prop was even in the ballpark for a set up like I have. Online prop calculators have me going anywhere from an 11" pitch to an 18" pitch! Ok, 15" is right in the middle. What do other small pontoon (18') owners have on their boats?

 

[The Rooster]

Larry, a 15" prop seems very big for your 75 hp. By your description, its not an Optimax, nor a 4-stroke, so the carb'd 2-stroke should run to a maximum of 5250 rpms. Need to be able to prop for those rpm's w/ your normal load on-board. Good luck !!!

 

[2004 Voyager]

Sorry, prop is 13.75" diameter, 15" pitch. It is a 75 hp 2-stroke, and yes, max rpm is 5250

 

[steelespike]

In the spring a Tach and gps will get us the info we need. A lightly loaded wot test for rpm and gps speed. with your present prop.
As suggested a 15 sounds like too much prop,but a good starting point.
This is more like a prop question so will likely do better in the prop forum.
just start a new thread over there in the springJ

 

[2004 Voyager]

I thought about posting this in the prop forum, but was hoping for feedback from pontoon owners, their boat size, motor size and prop size. My prop may be just about right for my combo, but the online prop calculators have me all over the place! Will some other pontoon owners chime in and tell me about their boats, something I can begin to develop a baseline from? Thanks, Larry

 

[steelespike]

History says usually not to many exact comparisons.
Also keep in mind that there is a pretty broad range of performance results especially when dealing with pontoons.
My info indicates your Merc has a 2.30 gear ratio.I believe the Yamaha 4 stroke 75 has a very similar ratio 2.31.
While you can't really compare the results you can get an idea of prop sizes and speeds at various rpm.
There are a few Merc 4 stroke tests that again should give some comparisons again similar ratios to your motor but 4 stroke.
Unfortunately the only 75 2 stroke I could find was on the Etec site but the ratio is 2.00.
Its fairly obvious that a 4 stroke at about 6000 will likely be faster than your 2 stroke at 5250 assuming the same prop and ratio.
Perhaps your 2 stroke may be able to turn more prop

 

[Silvertip]

Here is some data to chew on. The 40-50-60 HP Mercury Big Foot motors (intended for work boat and pontoon service) used conventional power heads attached to the 75 HP lower unit to give them deeper gearing and the ability to swing larger diameter props. The small gear case on the non-Big Foot motors was 10 inch. The 75 uses a 13-1/4 - 13-3/4 inch prop. Generally, these motors on a pontoon the size of yours would have a 40 - 50 HP Big Foot that was equipped with a 14 x 9. My neighbors 18 footer was so equipped as well. So -- if you extrapolate that data you can see that a 15P is probably a bit much for optimum performance. I'm guessing, but an 11 or perhaps a 13P is about as much as the 75 will twist and still reach 5500 rpm. Yes - 5250 is recommended but 5500 is ok. But do not throw money at a prop until you have made several WOT runs with what you consider an "average" load. Propping for a light load means you have an absolute dog if you load up more than average. Average meaning the number of people and gear you will carry MOST of the time. You need a tach and speedo or gps to make that determination. For all you know, the current prop may be ok -- but I'm betting it is not. Do not prop for top speed. You prop for 5250 - 5500 rpm. Speed will be whatever that prop delivers.

 

[Scott Danforth]

on a pontoon, number of logs, configuration, etc along with depth of leg in water has as much to do about performance as the prop does. have you optimally set the motor height? if not, get the motor height to the optimum when you do your WOT RPM/Speed trials in spring.

out of curiosity, what is your current speed? There was a post a few months back where a 'toon owner switched from a 50hp to a 90hp and was able to just beat the speed of the 50, however at a greater use of fuel. point is, until you get a pontoon up on plane, its a displacement hull and you hit a max displacement speed