Pontoon prop

[mrloring]

Last spring I bought a Grumman Funship 24(24 foot pontoon) with a 1999 Nissan 50HP(NS50D). The current prop is an 11.0x12.4 Nissan# C13.5. I need to replace it because I am only able to get to 4400rpm's with a light load and about 4000rpm's with my normal load. The NS50D's WOT RPM range is 5000-5700rpm's. The blades on the current prop are banged up some as well. I am looking to stay with aluminum because of the potential to hit submerged objects where I boat. Any advice on what dia., pitch, and brand would be best.

Thanks in advance
Aaron

 

[Silvertip]

Re: Pontoon prop

Before buying a "different" prop, make sure the engine is developing full power. If its not running as it should, a different prop may be a disappointment. If you are confident the engine is running ok, it would appear something else is wrong as you would need to change pitch by roughly eight inches to get RPM in the proper range. That just does not seem logical. I think the toon has something dragging (extremely dirty logs) or something hanging loose under it. The other possibility is that the tach is not reading correctly.

 

[mrloring]

Re: Pontoon prop

Last summer the outboard seemed to run well. It idles fine and isn't running rough. The tach is a factory Nissan tach and seems to be measuring properly while at idle. The logs are not extremely dirty, just the normal crude from sitting on the lake all summer, they will be cleaned before hitting the lake in May. I am confident that nothing is hanging loose under the toon as it is currently sitting on a trailer and we swam under and around it a lot last summer.

What would I check to insure the engine is making full power?

 

[mrloring]

Re: Pontoon prop

I do agree that it doesn't seem logical to need to reduce the prop by that amount. I have been researching on the net with prop selectors and such and it seems like I should be in the ballpark with that prop. How much would the dinged prop blades affect the rpm?

But my other boat defied logic with a prop change.

14' Runabout 28hp Johnson
From 9x12 to a 9x9 went from struggling to get 3000rpm's with one person to pulling 5000rpm's with two people towing a tube.

Anyways, I will try any suggestions you give before buying a new prop.

 

[kp913]

Re: Pontoon prop

Since your current propeller needs repair anyway, why don't you have it changed to a 10 pitch when you have the repairs done. Theoretically you should gain about 200 RPM's per inch of pitch you change, so this should give you 400 more R's to work with, some boats gain more, some less, but usually with outboards it's more like 200 or 300 more. This will not only help the performance of the boat, but will give you a good guide as to what prop you would need, knowing the true RPM gain of the repaired prop with lower pitch.

Most pontoon boats are way underpowered, and need the lowest pitch available, and some cup to help keep them from cavitating. Try the pitch change and see if this helps your situation.

 

[Silvertip]

Re: Pontoon prop

He needs to pick up 1300 - 1700 rpm to get the engine in its WOT band. Something is wrong and its more than just a dinged prop. A 50 HP engine on a 24 foot toon is certainly not underpowered. Make sure the throttle cable is adjusted properly and is fully opening the throttle plate in the each carb. At wot the throttle plate in each carb should be perfectly horizontal in the carb throat.

 

[rickdb1boat]

Re: Pontoon prop

What speed are you running now? If you are running 15 MPH or better, then I'm betting the Tach. is off. They can show pretty close at idle, but be way off at full throttle if not for the correct number of poles...

 

[mrloring]

Re: Pontoon prop

I have no idea how fast it is going. I will check the throttle cable as soon as we pull it out of storage in a couple weeks. The tach is factory nissan equipment and was professionally installed by the dealer.

 

[kp913]

Re: Pontoon prop

Since when is a 50 hp motor pushing a 24 foot boat considered not underpowered?

It's common for them to have this sort of set up, but it is indeed underpowered. Usually the motor height is wrong, leaving to much of the lower unit in the water, it wont plane out, so all of whats left under water is creating drag. You need a propeller that has the most blade area you can find, and the lowest pitch available. You may never get the correct RPM, but this set up will at least let the boat perform.

 

[kp913]

Re: Pontoon prop

Enter in your data at this site, and see what comes up.

The have a nifty propeller calculator: http://www.turningpointpropellers.com/

 

[walleyehed]

Re: Pontoon prop

I would be cautious as to how far you drop in pitch...the powerband of that engine will throw the 200RPM per inch out the window due to the fact you are well below the range now.
Considering we run a 15P on a 24ft 'toon with a 115, I don't think a 10P would be far off...I've seen 600-700RPM gain with 1 step in pitch when below the powerband to start with..

 

[mrloring]

Re: Pontoon prop

Dia x Pitch Item # Price Each Price Prop & Hub
* HUB KIT REQUIRED 13 *Hub 13 required for all props

12-1/2 x 8 H1-1208

12 x 10-1/2 H1-1210

[colour=red]10-7/8 x 11 speed H1-1011
With my standard load it picks this prop.[/colour]

10-3/4 x 12 H1-1012

10-1/2 x 13 H1-1013

10-3/8 x 14 H1-1014

10-1/8 x 15 H1-1015

 

[rickdb1boat]

Re: Pontoon prop

If it were me and the gearcase will handle the larger prop, I would go with the 12 x 10-1/2 H1-1210. A larger diameter prop on a toon is always a good thing....

 

[steelespike]

Re: Pontoon prop

Last summer the outboard seemed to run well. It idles fine and isn't running rough. The tach is a factory Nissan tach and seems to be measuring properly while at idle. The logs are not extremely dirty, just the normal crude from sitting on the lake all summer, they will be cleaned before hitting the lake in May. I am confident that nothing is hanging loose under the toon as it is currently sitting on a trailer and we swam under and around it a lot last summer.

What would I check to insure the engine is making full power?

Check that the throttle plate is opening fully and that the cable is adjusted properly.
then do a compression check.Have you tried to adjust the trim.