Do I need a new prop or a stabilizer fin?

[davethegeek]

I have a Merc 60 Bigfoot on a 20ft Fischer pontoon.
I get unbelievable gas mileage, but it seems a little slow (even for a pontoon).
With just me on board at WOT I am only able to get it to 14kt. I would like to pull the kids around in a tube this summer, but that seems a little slow.

I noticed when I gets over 4500rpm that I frequently get power loss (cavitation or ventilation, not sure which) The caviation plate looks good when on plane, so I don't think I need to adjust the height.

I read somewhere that I may need to re-prop. I am still using the 3 blade prop.
I could care less about hole shot, I just want more top end speed without changing out the motor. And I'm not talking much speed, I would be happy with anything over 20kt.

Should I go to a 4 blade prop? a stabilizer fin? What will get me more speed?

 

[Home Cookin']

Re: Do I need a new prop or a stabilizer fin?

although I don't know much about rigging a pontoon, I do know that they are a limited purpose boat, and speed is not a purpose. They're just not made for water sports; the compromise is a smaller motor, and fuel savings. One seldom considers "hole shots." I know, there are deck boats with better hydraulics and larger engines that are used for such, but that's not your rig.
I just don't see how a prop will get you 6 more knots, or add 50% to your speed. If you get a prop with a bigger pitch, your engine will strain to turn it and you loose rpm's, so you would end up in functionally the same place, with just a couple knots difference if any but no real advantage--certainly not for the money.
If you want speed, get a bigger motor; if you want water sports, get a different hull. Just changing the props is like putting racing slicks on a K-car.

 

[PROPer]

Re: Do I need a new prop or a stabilizer fin?

How many RPM are you achieving at WOT? Is a 2-stroke or a 4-stroke motor? What is the diameter and pitch of your current propeller? This will give us some insight into whether or not your set-up has more performance to offer.
As far as the lose of power you are experiencing 4500 RPM, I am assuming your motor is not loosing power but the prop is loosing traction? This most likely is ventalation and it occurs very often on pontoons because the wakes from the inside of each float are meeting right around the motor. This results in surface air disrupting the water near the prop. In these cases a 4-blade prop is helpful because you are putting more blade in the water, resulting in more efficient delivery of power.
With that said, it would be difficult to reccomend a size until you can answer the questions in the first paragraph.

 

[Cubbyblue22]

Re: Do I need a new prop or a stabilizer fin?

Most I would think want to run somewhere around 5000 to 5300 watt and you should be getting much more speed than 14. The prop sounds like it's not designed for a pontoon boat. I would start looking at your diameter and pitch. Most people run a 14 diameter and 11 inch prop, and yes like the previous post says, a 4 blade would be better if you can find one. However make sure your boat isn't too high on the transom or anything else because switching props can get $$$$$$$ expensive.

 

[steelespike]

Re: Do I need a new prop or a stabilizer fin?

The 4 blade will resist ventilation and a 14 X 11 should be close to what you need. The Solas Amita is an inexpensive
aluminum prop.
Because of your venting problems its difficult to use your rpm figures to select a prop. It is possible you might get to 20-21 mph. About 18 knots.

 

[Home Cookin']

Re: Do I need a new prop or a stabilizer fin?

I wonder, too, if you are getting power loss at high rpm (high load), if your hub is spun?

 

[davethegeek]

Re: Do I need a new prop or a stabilizer fin?

My gut tells me I am not getting power loss, but am getting ventilation.
If I just leave it on WOT I can run it up over 6000RPM, but not get any better speed than 4000RPM.
The numbers on my prop are
48 77338A33 11PL
It has through hole exhaust.
These number identify it as a 14x11 3 blade stainless steel prop with standard rotation.

I am not looking to turn this into speed boat, just fix this ventilation issue and maybe get a little more speed. I am not an expert on any of this, I am just questioning other setups that I have seen at the ramp. I see some with fins and some with fancy looking props. Will the stabilizer fin help at all with the ventilation issue?

 

[davethegeek]

Re: Do I need a new prop or a stabilizer fin?

Since I'm new at this. Any tips on how I can determine if the hub is spun? I see no obvious damage to the prop and I get good throttle response a slow speeds.

 

[Home Cookin']

Re: Do I need a new prop or a stabilizer fin?

so are you saying you get the same speed at 6K rpm as 4K?
I suspect you are probably right about air, but if you want to test for a spun prop, mark the hub and prop across the bushing; run it, and then see if the mark is still lined up, or if it slipped.

 

[davethegeek]

Re: Do I need a new prop or a stabilizer fin?

steelespike, I tried to look up the prop you recommended to price it, but it doesn;t look like solas amita makes any 4 blade 14" props.

 

[rallyart]

Re: Do I need a new prop or a stabilizer fin?

You have an 11" pitch prop on there. At that pitch range there is usually no advantage to go with a four blade as you can have a lot of surface facing back with just 3 blades. Your engine's WOT RPM is 5500-6000. It sounds like you've got the right prop on it but are getting air somehow
When you test to see if you are getting prop slip check the blades carefully for any bends That can easily cause cavitation.

 

[steelespike]

Re: Do I need a new prop or a stabilizer fin?

You can test for a spun hub but they don't slip like that.When they go you pretty much lose all push above no wake speed. might make it to 1/2 or 3/4 throttle but then it turns loose like you sheared a pin.

 

[davethegeek]

Re: Do I need a new prop or a stabilizer fin?

ok, so it sounds like the concensus is air. So does anyone know if a hydrofoil fin will fix this?

 

[dockwrecker]

Re: Do I need a new prop or a stabilizer fin?

Hydrofoils are useless. Especially for cavitation.

 

[ondarvr]

Re: Do I need a new prop or a stabilizer fin?

Hydrofoils are useless. Especially for cavitation.

He doesn't have cavitation, its ventilation and that's exactly what hydrofoils are designed to help eliminate. Will it help in this case......hard to tell.

 

[Mi duckdown]

Re: Do I need a new prop or a stabilizer fin?

Not on a pontoon boat. Waste of money.

 

[ondarvr]

Re: Do I need a new prop or a stabilizer fin?

Read the last sentence, I didn't say it would work, but don't count it out because of a misunderstanding of how the product works..

 

[eng24eng]

Re: Do I need a new prop or a stabilizer fin?

He doesn't have cavitation, its ventilation and that's exactly what hydrofoils are designed to help eliminate. Will it help in this case......hard to tell.

Have the same issue with a new to me 24' riveria cruiser with a johnson 90 ob 2 stoke. Pulls out great, throttles up but as soon as the wake thins out the water behind the boat it started venting. Put a hydrofoil on today and it did improve performance and eliminated some of the venting. However, it still vented at wot as soon as wake took the water away. going to try new 4 blade prop.

 

[jestor68]

Re: Do I need a new prop or a stabilizer fin?

I am on board with the addition of the Doel fin to help with the ventilation issue. There are many comments on other sites by folks that use them on pontoons with great success.

I also recommend increasing pitch to the next size(13). The 2.33:1 gear ratio of the Bigfoot should be able to handle it.

 

[jestor68]

Re: Do I need a new prop or a stabilizer fin?

Quote from a sample testimonial concerning Doel fin on pontoon boat: "I have 16 ft pontoon boat. It had a severe cavitation problem. This product completely fixed my problem".

I own a 24 ft Tracker pontoon that had the same problem. Being a long time user of the Doel fin on my other boats, I decided to try it on the pontoon. The ventilation went away almost completely, and helped the prop to stay hooked up at all speeds. With the Doel fin on there, I can actually pull a water skier(26 mph) with the 90 hp Merc.