2000 Johnson 50hp on 24' pontoon?

[sutor623]

Hey fellas,

I just got a nice 2005 Bentley pontoon. Its a whopping 24' long. It did not come with a motor, but I had a nice 50hp laying around for a rainy day.

Anyways, this is going to be a temporary motor and I will likely use it for this season until I muster up the cash to get a bigger motor. For now, she should do.

Anyways, I do not currently have a working tach, but the prop pitch on it now is 17. I was wondering what a good starting point would be on pitch/diameter for the motor once I put her on the pontoon. Id imagine the 17 is gonna lug her down pretty good...........

 

[ahicks]

My WAG - 9" pitch if loaded down with a lot of furniture, or maybe an 11" pitch for a lighter boat with just a console and lawn chairs. 11" might be the safer move when considering no tach and over rev potential.

Note this is totally unscientific and based on no more than what you told us.

 

[NYBo]

You are correct: 17 is way over-propped. Definitely install a tachometer. I agree with ahicks that 9" or 11" is in the ballpark. Don't expect more than about 15 MPH with the 50. What is the boat rated for?

 

[sutor623]

Thank you guys. I appreciate you taking the info at hand and coming up with a good guess, vs. bashing me for not having tach readings.

Boat is rated for a monster at 135hp. I rarely see toons rated for motors that big around here. I believe you are correct about the speed of 15mph. Im cool with that for the time being.

Boat is loaded down good, so I guess I need to decide between a 9 and 11..........

 

[sutor623]

Oh one more thing guys. I have heard to increase the diameter of the prop when putting an engine on a pontoon. Should I increase the diameter of just keep a 12 1/2" diameter, which is what I have on there now? Thanks!

 

[ahicks]

You don't have a lot of choice when it comes to diameter. You make the choice of pitch and take what you get for diameter as a rule. There's some difference among manf's, so you do have some say.

Larger diameter props have more bite. This can be a big help when maneuvering at low speeds, but that advantage will be at the expense of extra drag at higher speeds.

A 1000hp tug boat will have a very large diameter prop without a lot of pitch. A 1000hp blown fuel hydro will have a prop you can pick up easily with one hand! So yes, pontoon specific props will have larger diameter props with more blade area. Will you notice the difference between a pontoon specific prop and a more conventional prop on your boat? My guess would be no.....

By the way, congrats on the new boat! Hope it works out well for you. -Al

 

[sutor623]

Thanks for the kind words and explanation AHicks!

I talked to the past owner and he had a Mercury 60hp EFI bigfoot on there with an 11 pitch prop. Motor spins up to 55-5800, depending on load. He said 5500 rpms with 7 people on the boat and he is moving at 17mph. Gear ratio is 2.33:1, which puts him at 31% slip.

My 50hp 2 stroke has a gear ratio of 2.42:1 and is supposed to spin up to 5000-5500. I figured even if the lil guy only spins up to 5000 rpms with an 11p on there, it will put me at 15mph. Best I can hope for with a 9p is 13mph.

Thank you guys for all the great info. It sounds spot on. I went ahead and got a solas prop 13x11p. Ill let you guys know how she does. If she cant spin the 11 up to speed Ill be on the lookout for a bigger motor sooner than later.

 

[Sea Rider]

Assume you have ordered a tach to play with the new prop, right ? if not, will never know if engine revs right inside its factory wot rpm range rpm as loaded and that's a very bad starting point. If so can go from that 13 x11 prop to a less pitch one if in need to pull wot rpm up. As under powered toons only achieves fast displacement speeds prop it right to achieve full 5500 wot rpm range.

Happy Boating

 

[sutor623]

Thanks for all the advice guys.

Took her out this weekend. I have a full set of gauges in the dash, but they arent set up for the engine that I have on the boat. I would like to try and get a conversion harness to get my Johnson engine to hook up to the Faria gauges.

Anyways, the boat did great. That 13x11p prop pushes the boat along very well. At an idle there is plenty of torque and control.

I had my wife and daughter on board, and the boat was cooking at open throttle at 19.6MPH. That is certainly faster than I was expecting. I have a good ear for these motors as I have been running them for a very long time. I feel like she is spinning up to 5000-5500.

Boat runs great and rides high in the water. Really, at any throttle setting she feels good in the water. Thanks for all the help. Certainly would love any feedback/criticism.

 

[Sea Rider]

Nice to read that with a 11 pitch the combo performed much better. Just for the record, when at 19.5 MPH was the toon on plane or at just fast displacement speed riding parallel to water level ? Install a tach to know exact wot rpm range the OB is revving at as currently loaded, from there can fine tune the OB to better its water performance if in need to better something.

Happy Boating

 

[sutor623]

Nice to read that with a 11 pitch the combo performed much better. Just for the record, when at 19.5 MPH was the toon on plane or at just fast displacement speed riding parallel to water level ? Install a tach to know exact wot rpm range the OB is revving at as currently loaded, from there can fine tune the OB to better its water performance if in need to better something.

Happy Boating

Thanks! The boat never got on plane. I just couldn't believe my eyes. I checked my speed via 2 different GPS just to be sure. She felt great and it certainly felt good to have that big ol girl moving 20mph. To be fair, there was no wind and the water had a slight chop. I will have 3 more people for a total of 5 1/2 on the boat next weekend, and a total of 10 on board the following weekend!! I imagine that is going to change things some :faint2: I am one of those overly obsessive/strange guys that will get a 9 pitch as well for when we have more folks aboard. :lol:

 

[Sea Rider]

Excessive load will kill prematurely any OB water performance powering a toon that has gone through a load and prop maximization to rev to its max wot rpm range as factory stated. A toon achieves way more hull drag, loading it severely will make OB to rev towards the lugging side of the wot rpm range which is no bueno for any engine. Too much stress on piston rods and pistons as well. It's same as going a steep slope starting from a dead stop on second gear...

It it were my toon wouldn't dare load it excessively with 10 boaters if the engine was propped right to rev to its full wot rpm range while being propped to handle light loads. If plan buying a 9 prop buy an induction Hardline hour/tach model HR-8062-2 for 3 and up cylinder OB's. Without a tach won't never know how well a 9 or 11 pitch prop performs while playing with variable loads nor how much wot rpm the OB loses while ading extra boaters.

It's not a good to start boating activities playing with variable loads, prop pitches and rpm without a tach. Check Iboats, Amazon for Hardline hour/tachs, best investment ever, take my word for granted..

Happy Boating

 

[sutor623]

Thanks bud!