Pontoon prop size Captivation/Ventilation?

[Reg312]

1990 20' Lowe pontoon with same year 40 hp Johnson. Motor seems to run fine but at times at 4500 - 5000 rpm if feels like the toon is spinning its tires. I take a look at the motor when this happens and looks like not a lot of water around prop. I tilt up and down and I may get a little more bite tilted down. I tried to pull a tuber and just seems to spin all the time and can't get traction. We've gps and at around 5000 rpm we are going 13 mph. That just seems a little slow. Now a few weeks ago at very slow speed I did hit something that put a small rip in 2 of the blades so MAYBE I've spun a hub. So I plan order a new hub and at "Propellers 101" they suggest a 3 blade 12-1/4 x 15 for top speed and a 12-1/2 x 13 for heavy loads and tubing, which we really don't do that much. Does anyone have a good recommendation on prop size? 3 blade? 4 blade? And what speed should I be getting out of that 40 Johnson on the 20' toon? Usually only 2 to 5 people on boat. Toon is new to me this year so we've only had it out a few times before I hit whatever it was. Damn Lake of the Ozarks just can't keep a consistent water level. But our maiden voyage I don't remember feeling the "Spinning tires feeling" so maybe its just a spun hub. But we still only got 13 mph with 4 people.

 

[reddogg]

Re: Pontoon prop size Captivation/Ventilation?

Sounds like prolly a spun hub. As far as 3 blade Vs 4 blade, the 4 blades are better at holeshot and "power", the 3 blades are better for top speed though they do great for "power" if pitched for it.Can't really tell you what speed you'll get out of any given prop, there are just to many variables, but pitch is really measured in inches so a 15 pitch prop will move your boat approx 15" in one revolution. That's not an exact figure due to slip but close. Also, going up or down 1" of pitch is worth approx 200rpm's.

red

 

[winterfunguy]

Re: Pontoon prop size Captivation/Ventilation?

If the prop has rips or cracks in the blades it is killing you! My 28 foot manitou gets to 24 with a 90 hp and a 14x13 3 blade prop...Im having cavatition issues as well.

 

[Silvertip]

Re: Pontoon prop size Captivation/Ventilation?

Neither of you are having "cavitation" issues. You are experiencing "ventilation" issues. A 15P prop on that size engine on a pontoon is too much pitch. 12-1/4 x 15P is the typical prop used with a planing hull boat. Put a 4-blade prop on for better bite.

 

[Reg312]

Re: Pontoon prop size Captivation/Ventilation?

From what I've read it is ventilation. So ---- 15" to big? I don't have a lot of knowledge on prop sizing so I have to go with recommendations. You recommend a 4 blade and I cannot and will not argue that fact. What size? what pitch? They recommend a 3blade 12-1/2 x 13 for water sports. Would that be a good overall prop or what would you recommend in 3 blade? 4 blade?

 

[Silvertip]

Re: Pontoon prop size Captivation/Ventilation?

Most engines of that size on a pontoon will run a three or four blade prop in the 11P range. Since you have no reliable performance data I will not make a recommendation other than 13P absolute maximum and probably 11P. Forget worrying about diameter. You buy a prop for your engine and then select the pitch. Diameter takes care of itself. In other words you can't put a 16 inch diameter prop on a Johnson lower unit designed for 11-3/4 - 13 inch props. It is called prop design. As pitch goes up, diameter comes down a bit and vice versa. I will tell you this however. Many of my past boats have been powered by 40 HP Johnson and Evinrudes. I've experimented with 13, 15, 17 and 19P props (aluminum and stainless) and regardless of the boat, always came back to the 15P. Best hole shot comfortable cruise and good top end. Those boats were PLANING hulls -- not pontoons. You need to step down in pitch to push those barges.

 

[Reg312]

Re: Pontoon prop size Captivation/Ventilation?

Still not to sure which way to go. But after doing some more reading on toons and props they say you need to prop the toon to hit proper RPM's at WOT. I'm not one to WOT anything, I always run it (boats in particular) 500 rpm's lower than rated. If I've read correctly the 1990 Johnson 40 WOT is 4500 - 5500 RPM. The few times I have had the toon at WOT with 2 people onboard it was running at just under 5500 rpm. So --- this tells me if you buy a prop for a toon to reach proper motor RPM's, then I need to replace the one thats on there with one thats the same Pitch and Diameter? Silvertip says not to worry about dia. so if available I'm guessing increase the pitch a degree?Or am I going the wrong way? I just think 13 mph on a 40 is just a little slow? I want at least 15 ??? lol.

 

[homeinspector]

Re: Pontoon prop size Captivation/Ventilation?

Anyone that owns a pontoon should install a Blackfinskeg, u will notice tighter turning ,less cavitation and slippage.