pontoon with low rpms

[fireman57]

It makes me wonder when I had a buddy, who works on boats with me too, change it to a 9pitch if somehow he didn't seat it on the thrust washer properly and that cause the vibration that loosened engine bolts. Like I said I liked the performance of the 9 but still didn't turn the rpms the engine should. That could be due to weight and too deep in the water.

 

[steelespike]

It makes me wonder when I had a buddy, who works on boats with me too, change it to a 9pitch if somehow he didn't seat it on the thrust washer properly and that cause the vibration that loosened engine bolts. Like I said I liked the performance of the 9 but still didn't turn the rpms the engine should. That could be due to weight and too deep in the water.

The prop calculator says the 9" is turning about 5500 rpm. It is a fact of physics that it can't make 17 mph at 2800 rpm.
9" at 2800 rpm = 8.6 mph.

 

[jbcurt00]

And too deep doesnt effect RPMs

 

[Sea Rider]

Is there any clue to whether the OB is underpowered for that size toon ? if so, by how much. That would be the first thing to check, transom/engine height/water flow passing by in second.

Happy Boating

 

[steelespike]

The calculator says the 9" is making 17 at about 5500 rpm with 4 people and gear.It is a 79 so the tubes are probably about 19"
His tach says 2800 a 9" at 2800 would be about 8 mph. He has to replace or repair the tach.
It's quite possible that the 9" is too light but without an accurate tach there is no way to know.

 

[Sea Rider]

Is 75 HP the max HP that toon is rated for ? probably underpowered ? if so, needs a prop ,maximization with a healthy accurate tach.

Happy Boating

 

[fireman57]

I know it is underpowered but do not know what it is rated for. I was just trying to get the rpms up on the engine.

 

[Sea Rider]

You cam maximize prop to some extend, but if underpowered expect slower top speeds and less overall water performamnce compared to a much nicer performance if boating near the top of the HP range.

Happy Boating

 

[steelespike]

You cam maximize prop to some extend, but if underpowered expect slower top speeds and less overall water performamnce compared to a much nicer performance if boating near the top of the HP range.

Happy Boating

He has to get a working tach.Its possible the 9" is too small;from his description it may be over revving. "vibrating to the point of loosening nuts and bolts".
If the tubes are 19" and you add 4 people 2 batteries,fuel and gear it might even be dragging the underside in the water.

 

[Sea Rider]

He should get a tach in excellent working order and test that toon lightly loaded, on the other hand if he goes for a prop max to be near or full wot range for that OB, boating with 4, assume a family leisure combo with throw his equaton deep into the blue. That's why said that a underpowered OB can only be prop maximized to some extent, It's not same propping right an OB for 1 than for 4, propped right for one will over lug the OB with 4, propping right for 4 will over rev OB badly with one.

Happy Boating

 

[steelespike]

He should get a tach in excellent working order and test that toon lightly loaded, on the other hand if he goes for a prop max to be near or full wot range for that OB, boating with 4, assume a family leisure combo with throw his equaton deep into the blue. That's why said that a underpowered OB can only be prop maximized to some extent, It's not same propping right an OB for 1 than for 4, propped right for one will over lug the OB with 4, propping right for 4 will over rev OB badly with one.

Happy Boating

If he runs mostly with 4 aboard he should prop for that and with a working tach be aware of rpm with a light load.

 

[Sea Rider]

Yep, propping right for 4 will shoot rpm to mars when solo boating, keep both eyes on tach and throttle...sometimes it's s nightmere propping right a under powered OB on a toon which has more water drag to overcome fast for 3 passengers or more use.

Happy Boating

 

[steelespike]

Come to think of it most pontoons aren't intended to plane so power selection is more to get good "pontoon" speed without having to over tax the motor.
A 40 or 50 would produce pretty good "pontoon"speed but at a higher throttle setting than say a 75.The 75 wouldn't produce much additional speed but
would produce the smaller motors speed at a more relaxed throttle.
This pontoon is likely at a disadvantage with its more primitive under sized tubes.Less likely to get up any and being lower and closer to the water,
especially as you add weight.

 

[fireman57]

The tach is correct and the engine is lugging. It will be May before I can get it back on the water. With 4 people on it any 1 ft wave washes over the front unless everyone is toward the middle/back of the boat. Very inconvenient. If the engine wasn't lugging I know that my speed would increase and be able to bring the front end up more by proper weight distribution. Trimming the engine all the way out is the only way that I can bring the front up even a little and it seems to optimize the rpms that I have.

 

[steelespike]

What is the diameter of your tubes?
You state the 9" made 17 mph at 2800 rpm.
​ That is not possible. A 9" at 2800 would make about 8+ mph.
A 9" at 17 mph would have to be about 5000 to 5,500.
I think your boat as setup has a top speed around 17 or 18 with 4 aboard.
It just plain isn't likely yo go faster so the 13 is too much prop
Calculator says if the boat could make 25 with the 13 rpm would be around 5400.
Take all the weight off the boat in the spring And see what the 13 will do speed and rpm.
You, a small battery and the motor.
The 17 and 13 were so far off to what the boat can do the rule of thumb doesn't apply accurately.

 

[steelespike]

What is the diameter of the props? Motors were available with small or large gear case.