Prop for 20' pontoon

[muskiemike12]

I am having an issue with high slip when I run over 4000 rpm's. Ventilation seems to be the issue.

Prop is a Solus Rubex 4 13.25 dia X 13" pitch, Aluminum

Light Load 2300# 5000 rpm's 19 mph
Avg load 2800# 4900 rpm's 18 mph
Hvy load 3400# 4750 rpm's 18 mph
Towing a tube 4600 rpm's 18 mph
1979 Mercury 900, 90 hp
2011 Lowe Suncruiser SF214, 1701# dry

At 3700 rpm's I'm running 16mph with a heavy load. Adjusting the trim seems to have no effect on speed.

Do I keep it under 4000 rpms for cruising? Since it seems that ventilation or slip is the problem, will a 3 blade prop help?

 

[steelespike]

Re: Prop for 20' pontoon

As is the case with most boats as prop speed gets lower slip goes up. But your boat speed for prop setting is likely more favorable.
Your slip(not ventilation) at 5000 is about 30%A planing hull would usually be 6-12% Your slip at 3800/ 16 mph heavy load is about 41% But your throttle setting is more favorable. As you add weight slip goes up. 4900 at 19 is 47%
I would think that 3800 with a light load your pretty close to a planing hulls minimum
planing speed but at more rpm.Some tests indicate that at 6 to 8 mph some pontoons can get better mileage than some planing hulls.
A 4 blade should be ideal for a pontoon. Not only for slip but speed and control especially around the dock

 

[muskiemike12]

Re: Prop for 20' pontoon

So from 4000 rpm's on up I'm just increasing my slip. Is there anything that can correct this? Just seems odd that through the upper rpm range I gain no speed, just prop slip.
Thanks!

 

[steelespike]

Re: Prop for 20' pontoon

Perhaps a stainless prop might help a little but here is the problem; as you add weight more and more boat is dragging in the water.
It will only go just so fast so you create a lot of slip.You would be lucky to gain a mile per hour.
If you put a skin on the bottom it will help.
also a third log will really help. There are aftermarket kits to do this.They say the pontoon will go almost like a runabout.

 

[muskiemike12]

Re: Prop for 20' pontoon

It was suggested to me in the pontoon forum that adding a hydrofoil may help the prop slip issue by keeping the very turbulent water between my tubes from effecting my props bite. Ventilation is the issue here as air is being drawn into the water from the turbulence caused by the tubes on the pontoon. The ventilation is what causes the prop to slip. I have read many reviews and it seems that the Sting Ray XRIII hydrofoil really limits ventilation. If this doe not help it looks like I'll have to switch props. 14 x 11 - 3 blade is what I'll have to try.

BTW I don't have the issue when pulling a tube. I run at 18mph @4600 rpm's. The motor feels good and there is not the high slip when I go over 4000 rpm's. It was mentioned that because of the towing of the tube that is pulling my motor a little lower in the water getting my prop out of the turbulence near the surface.

 

[muskiemike12]

Re: Prop for 20' pontoon

I found a brand new 14 x 13 - 3 blade pontoon specific prop for $82 out the door at Cabela's the other day. Will test different setups and post results in a couple of weeks.