engine wont reach operating range

[ggpostal]

Hi, I have a 24' pontoon boat with an 1989 Johnson 88spl running a 14x11 pitch prop at WOT the engine will not go past 4800 rpms with two people on board, I understand the operating range is between 4500&5500 rpms wondering why with an 11" pitch prop it won't get there. Any help will be appreciated.

 

[Barramundi NQ]

The WOT range is just that, if you're in it your engine is in the correct operating range. You buy a smaller pitch prop as a thrust type prop as a spare or when you have more people and or weight on the boat. Assuming that your motor is running well? Compression and fuel system all good? The dry weight of your boat? All these things of course factor into the way in which a prop can work, or not work.
If you went to a 14x10" prop you'll probably find it will rev closer to 5300 or thereabouts. Hard to tell without the weights a engine condition.

Hope this helps

Kerry

 

[ahicks]

You're trying to put the engine at/near the top of the operating range for the most punch available from it? Agree it's about your prop (assuming engine is running properly). An inch of pitch (one way or the other) is generally = 500 rpm. If you don't already have an extra prop, a new one might be a good plan - unless you can find a used one? If you have an extra prop, consider sending it out to a prop repair shop to be re-pitched (if it's close)? Some of the better shops will work with you in your attempt to get max performance available. Will let you try props out if you're buying a new one?

Last thought, many pontoon owners like the extra thrust available from a lot of blade area. That will be maxed with a 4 blade prop and/or going with the largest diameter available to get your engine at max rpm. Thrust is about blade diameter, speed is about pitch.

 

[Jeep Man]

Two people should not have a noticeable effect on your WOT rpm. I would try a 9p just to see the result. Personally, my banged up 14x11 kept me under my ideal 5800 mark. I tried a 9p and was over 6000. Tried a new 4 blade 11p and was at the 5200 mark. Finally settled with a 4 blade 10p which put me at 5700. If you go to a 4 blade, your pitch should be 1p lower than a comparable 3 blade.

 

[Barramundi NQ]

Is the prop in good condition? Nicks or chunks? Is the prop stainless or aluminum?

 

[Silvertip]

Before spending money on props, ensure the tach is accurate -- verify it with a shop tach. Then ensure the engine is making fully power. Are the carb butterflies opening fully and has a link & sync been done. Are the pontoon tubes crusted with algae. Fix all of those, then go for a prop change. There are a multitude of pontoon props on the market. They can be identified by their big "elephant type" blades. Push is not always about diameter. The diameter you need is determined by the gearcase size (not gearing). Props are designed to fit a specific HP range of engines. Get the diameter specified for your engine, then work on the pitch which is a matter of gearing.

 

[ggpostal]

Thanks for all the replies, boat toons are 24" diameter and clean, engine runs strong and smooth, butterflies are open full and advanced. Talked to another person pulling his out and he was running a 15" pitch and a 90 with no problem that's why I was concerned about rpms, will look for maybe a 9" pitch and go from there.

 

[ggpostal]

Prop is aluminum with no nicks or dings

 

[Barramundi NQ]

Another little piece of information about props, is going to a stainless steel prop with the identical diameter and pitch can lift the rpm because of the blade design. Stainless steel props are thinner and stronger (less flex and slippage) therefore are able to cut the water better and push the boat a little better. If your local marine dealer can give you a loan prop (some do this as a try before you buy kinda deal) then give that a go. Have an accurate tachometer is of course critical, but it sounds okay.
Keep on boating

 

[ggpostal]

Thank you all for the information.

 

[steelespike]

First of all pontoon prop sizes and speed are all over the place A 15" prop would the the exception and without details the 15 results can't be evaluated.
Do we know if it is ss or al prop 3 or 4 blades weight of the boat etc etc.
A 11" is pretty typical for your size. You are within range though a little low. The gps speed would help evaluate your setup.
Is the timing set right and advancing as it should.
Checked some pontoon tests with 99 hp evinrude same gear ratio.
The lightest boar was a 20' 1325 lbs with a 13" prop made 27 @ 5400 and 18% slip .
a 24 at 2500lbs. 13" prop 26.3 @ 5200 17%
23' 1980 11" 23.3@5400 17% slip
24' 1980 11" 23.5 @5600 16%
Very different size and weight can yield similar numbers.
Slip seems kind of all over not necessarily related to size and weight.
It is fairly obvious that it seems your rpm is about 600 to 800 low
Assuming the tach is accurate.

 

[gpfishingdude]

When I started adjusting my carburetor to the smoothest idle adjustment I couldn't get peak rpm at wot so I adjusted them again to the specs in the manual and picked up 300-500 more rpm. Same prop and all. Would have liked to tuned them to a vacuum gauge but I haven't figured out how yet.

 

[BatDaddy1887]

In my opinion, a 13.25 x 17 prop is the perfect prop for that engine.

 

[WaterDR]

My experience on pontoons is that switching to a SS prop always LOWERS the rpms NOT raises them. In fact, on my current toon, I had to go from a 15P to a 13P when going from an alyum to a stainless Why? Well two reasons. First, an alum prop will flex. Second, a stainless prop is more efficient (which is related to the fact that it will flex and lose shape).