Help me to pick a new prop for my pontoon

[Dmpaul89]

have an 88 Grumman pontoon 24' with a 70's Johnson 50hp. the motor runs at a lower rpm than I think it should, (no rpm gauge to know exactly) but Ive been around boats a lot and I know its not reaching peak rpm. Whats a good pitch to try and how can i tell if its in the proper rpm range? it has the large gear case I believe might be a 74' ill have to double check. boat does around 15 in smooth water/ light load. i bought the motor off of a little 15ft fiberglass fishing boat so i know its probably the wrong prop.

 

[Scott Danforth]

You will need to know the RPM your spinning at to pick the right prop....no way around it. You may need to pull the cowl and use a laser tach, however you need to know what RPM your at

 

[Dmpaul89]

just looked it has a 13- 3/4" x 15 pitch currently

 

[ahicks]

You're right. That's very likely way to much prop. 11" is probably closer, but the only way of even getting an educated guess is to know what the engine is turning with the pro that's on it. Or you could just wing it and go with the 11".

 

[Dmpaul89]

now the question is, 3 or 4 blade? I'm thinking since its a pretty large diameter already the 4 blade would be overkill. I think I will go ahead and order the 11 pitch 3 blade, id like to test the rpm somehow beforehand, but I need a prop for this weekend and I haven't a clue how to do a rpm test. I think I will just wing it. if I feel the rpms are excessive ill back off the throttle a little

 

[Sea Rider]

Not even having an educated musician ear will hardley know at which max wot rpm that OB is revving, before investing on a prop need to invest on a standard or an induction tach to tach current prop and then go from there maximizing props to get the max out of that 50 OB. Investing on a prop blindly will surely be dumping money into the deep blue.

Happy Boating

 

[Scott Danforth]

without knowing your current RPM, you may as well empty the contents of your wallet in the water now.... spend the $11 on a laser tach to save you hundreds of dollars guessing.

 

[Sea Rider]

If wanting to track the run hours along the rpm range that 50 HP is working, go for a Hardline or TT (Tiny Tach) and secure it on front tiller or front cowl if happens to be a center console boat. Are excellent tachs for programed preventive maintenances, maximizing props and monitoring rpm for least fuel comsumption at a given rpm while cruising.

Happy Boating

 

[ahicks]

Harbor freight, and I would imaging Amazon as well, have inexpensive hand held laser tachs that are aimed at a shiny piece of reflective tape that you attach to the engine's flywheel. Hopefully you can find a location that's easy to see/get to when the boat is underway - which may present a challenge on some boats if you can't find somebody daring enough to hang off the back while under full power. This would be necessary as I believe the tach's max range is about 2'.

That leaves buying, installing and configuring an outboard tach the most practical way I can think of to get an rpm reading that some folks would use only when selecting a prop, maybe once?

The last option would be to ask around for other's experience regarding what props have worked well for them with similar setups, though some might view that as wasting hundreds of dollars, or dumping money into the deep blue.

Sounds like some days, you just can't win.....

 

[Dmpaul89]

thanks for the tips, I didn't know about the laser tachs. I will definitely pick one up!

 

[Sea Rider]

If a laser tach must be hand held while reading OB's rpm is not practical. Are usually for shop uses, not for a guy pointing the laser tach while combo is at speed with cowl removed. Now imagine same scenario on choppy water cond.

A visible cheap induction tach apart from its stated benefits will show how much wot rpm gains or loses when playing with load/passengers. You could be lugging the OB if it was correctly propped for a light load and happens than many more friends wants to join an outing with you at full throttle..

Happy Boating