Ventilation on pontoon boat - New prop? Doel fins?

[Tompen]

Hello. I'm relatively new to the boating world, but I've been learning a bit from these forums. I have a 1989 Stratos 24' pontoon boat. I bought it a few yrs ago with a 90 hp Johnson. I had some moderate ventilation problems at 'high' speeds - mostly occurring when turning or too much weight in the front. I recently blew up my Johnson and just replaced it with a 115 hp Evinrude (1996).

The new motor has much more power, which I'm very happy with, but the ventilation is actually worse than my previous motor. This is probably explained by the fact that my new motor goes faster and it ventilates at the same speed as it did before, only now I have more room above my ventilation speed - around 21-22 or so. My mechanic said both motors had the same length shaft (20" I believe). Also, both motors were/are set as low as possible.

So a couple questions...

I got the new motor from a 19 ft glasspar fishing boat and I have not yet changed the prop (I believe its a 14x15 or possibly a 14x17). The prop is in great shape, but it is likely original and is not cupped or anything special. At WOT my tach reads 4700 and thats with some ventilation occuring - so its probably more like 4300. So I need a new prop. I'm thinking of getting a hydrus 14x15 prop thats supposedly cupped and designed for pontoons. Can I expect a SIGNIFICANT improvement in ventilation simply from going to this prop?

Question 2- I've read all the opinions on the Doel Fins, and they seem promising. But... it seems like they work best for hole-shot, and my problem is the opposite; my ventilation only occurs when I'm going full speed. Does anyone think these might help my ventilation problem?

Sorry to be so wordy, and thanks in advance for any advice.

 

[jestor68]

Re: Ventilation on pontoon boat - New prop? Doel fins?

The Hydrus comes in 9,11,and 13 pitch sizes. The regular aluminum props are cupped too(at least the new ones are).

I have seen many testimonials of folks putting the Doel Fin on their pontoon boats to help eliminate excess ventilation. They all claimed that it helped quite a bit.

Since your WOT rpm range is 4500-5500 rpm, ,verify your present prop pitch, so we can recommend a better size; as 4700 rpm is not quite enough for your motor to develop full power (at 5000 rpm).

I am leaning toward a 4 blade prop for better holding.

 

[Tompen]

Re: Ventilation on pontoon boat - New prop? Doel fins?

I am leaning toward a 4 blade prop for better holding.

Thanks for the response.

I realize 4 blades means better grab/holding, but what exactly does this mean? Does it just mean less slip? Or do do 4 blade props (generally) cut down on ventilation as well? I've wondered this when reading about 3 vs 4 blades, but haven't come across an answer.

If a certain amount of air is hitting your prop, its not going to matter if its hitting 3 blades or 4 blades, right? But perhaps the 4 blade prop will improve turbulence (or whatever) that is causing the ventilation?

 

[joncbrand]

Re: Ventilation on pontoon boat - New prop? Doel fins?

I would strongly recommend a 4 blade. I have a playcraft 24ft with a 2.6L 235hp johnson. I tried many different 3 blade props and ended up with a 4 blade solas 14.5 x 17 solas. I have no issues with cavitation no matter how weight is dispursed on the boat.
As for the dolphin/whaletail thats all snake oil to me. And I wouldnt waste my money.

You should also verify motor height. The cavitation plate "flat surface just above the prop" should be level with or below the lowest point on your center tube or transom for a twin tube boat. I ended up installing a jack plate on mine to get it dialed in.

 

[Tompen]

Re: Ventilation on pontoon boat - New prop? Doel fins?

The Hydrus comes in 9,11,and 13 pitch sizes. The regular aluminum props are cupped too(at least the new ones are).

I am leaning toward a 4 blade prop for better holding.

I actually meant to say I was considering a Hydrus 14x13 (not 14x15). Sorry about that.

I ran my fingers over my current prop and did not feel any cupping (I WOULD feel it and know if it had an edge to it, right??). So... even an aggressively cupped 3 blade would likely be a (big?) improvement on ventilation over my current non-cupped prop, correct?

However, it sounds like I should probably get a 4 blade prop. I don't have a lot of money to spend and I'm physically unable to work on the boat myself, so whatever direction I go in to address the ventilation, I'd like to be safe, yet aggressive with my choice. (I don't want to go through 3-4 props to find the 'right' one, which seems to be the norm around here )

Do you know of any 4 blade props you'd recommend for an evinrude/pontoon?

Thanks.

 

[Tompen]

Re: Ventilation on pontoon boat - New prop? Doel fins?

I would strongly recommend a 4 blade. I have a playcraft 24ft with a 2.6L 235hp johnson. I tried many different 3 blade props and ended up with a 4 blade solas 14.5 x 17 solas. I have no issues with cavitation no matter how weight is dispursed on the boat.

Thanks for the response.

Did your switch to a 4 blade correct/improve ventilation problems specifically? That is my number one consideration at this time, since I don't want to have lower the motor (requiring work/money).

 

[BobGinCO]

Re: Ventilation on pontoon boat - New prop? Doel fins?

The guy that owns the prop shop near me, told me that 4-bladed props don't work as well on pontoons, as on other boats, and recommends that I stick with a 3-bladed prop. He should know - he obviously would make more money selling me a 4-blade prop, so I'm inclined to believe that he knows what he's talking about.

 

[steelespike]

Re: Ventilation on pontoon boat - New prop? Doel fins?

The Solas amita and Rubex props have a good reputation. .
If you check present day Evinrude tests. Your lucky in that they are essentially the same motor but direct injected.
These tests are running 11 and 13" pitch.
I think an 11" 4 blade will do well.

 

[Tompen]

Re: Ventilation on pontoon boat - New prop? Doel fins?

The guy that owns the prop shop near me, told me that 4-bladed props don't work as well on pontoons, as on other boats, and recommends that I stick with a 3-bladed prop. He should know - he obviously would make more money selling me a 4-blade prop, so I'm inclined to believe that he knows what he's talking about.

Thanks for the response.

LOL! Thats the problem with these forums - it seems there is a lot of conflicting info/opinion.

But thanks for the advice. It does seem like you heard from a reliable source.

Seems I'm back to square one.

Does anyone else have advice on a 3 blade vs. a 4 blade for a 24 ft pontoon boat (evinrude 115) with mild-moderate ventilation problems?

Relevant info
1. My current prop is a stock 3 blade non-cupped prop, so any pontoon-specific prop will likely be an improvement
2. I do not want to lower my motor

 

[Tompen]

Re: Ventilation on pontoon boat - New prop? Doel fins?

The Solas amita and Rubex props have a good reputation. .
If you check present day Evinrude tests. Your lucky in that they are essentially the same motor but direct injected.
These tests are running 11 and 13" pitch.
I think an 11" 4 blade will do well.

Thanks. What tests are you referring to? Do you see no problem with a 4 blade on a pontoon boat?

I just looked - I don't think solas amita or rubex makes a 4 blade 11" pitch with diameter of 13" or greater. That seems strange?? Does this suggest that a 4 blade might not be applicable for my motor? I'm confused...

 

[jestor68]

Re: Ventilation on pontoon boat - New prop? Doel fins?

There are pontoon specific props that have four blades; like the Mercury Spitfire "Pontoon" 4 blade prop.

Perhaps that prop shop guy thinks that he knows more about pontoon prop design than Mercury.

 

[MH Hawker]

Re: Ventilation on pontoon boat - New prop? Doel fins?

I may be missing something but ventilation is a sign that the motor is mounted to high and is sucking air, So i would drop the motor first and deal with the ventilation problem first before messing with props.

 

[Tompen]

Re: Ventilation on pontoon boat - New prop? Doel fins?

I may be missing something but ventilation is a sign that the motor is mounted to high and is sucking air, So i would drop the motor first and deal with the ventilation problem first before messing with props.

From what I understand, pontoons have a tendency to ventilate and dropping the motor is not always the only or best answer. Both of the motors I've had on this boat were mounted all the way down and both ventilated in the same way - while turning and near top-end. I've read quite a bit of testimonials today and I'm pretty hopeful that a 4 blade prop will help me out. But now I'm having a hard time finding one the right size. Mercury spitfire seems to be the only one with the 4 blade with a diameter/pitch near what I want - 14x11. But mercury won't fit my motor.

Solas rubex has a 4 blade 13.25 x 13 and I'm wondering if that would be comparable to the 14x11 that I'm looking for. I'm just worried about going with the smaller diameter considering I'm trying to improve ventilation.

 

[muskiemike12]

Re: Ventilation on pontoon boat - New prop? Doel fins?

I had the exact same problem with my pontoon. The 4blade gives you stern lift, which compounds the problem. I went with a 14x13, 3 blade black max with extended cup and I don't have ventilation issues any more.

 

[jestor68]

Re: Ventilation on pontoon boat - New prop? Doel fins?

Mercury spitfire seems to be the only one with the 4 blade with a diameter/pitch near what I want - 14x11. But mercury won't fit my motor.

Solas rubex has a 4 blade 13.25 x 13 and I'm wondering if that would be comparable to the 14x11 that I'm looking for. I'm just worried about going with the smaller diameter considering I'm trying to improve ventilation.

Mercury makes a Flo Torque hub(#835266Q1) to mount any Merc prop (that uses a Flo Torque hub) on a Johnson/Evinrude motor with the V-4 and V-6 gear cases(V-6 would use a different hub#).

A 13 pitch 4 or 3 blade would work IF your present prop is 15". If it's a 17", you'd need a 15".

All Solas props are cupped.

The Solas New Saturn line of SS 3 blade props are billed (by Solas) as being good for high thrust applications, including pontoons.

 

[steelespike]

Re: Ventilation on pontoon boat - New prop? Doel fins?

Thanks. What tests are you referring to? Do you see no problem with a 4 blade on a pontoon boat?
The Evinrude Etec site the 1st 4 115 pontoon tests are 24 footers.I don't think they are using 4 blade props But I think gives
an indication prop size and the fact that pontoon prop selection varies for no obvious reason.If you include 22 footers prop size varies from 15" to 11 inch.
Have you tried moving some weight toward the stern or for that matter perhaps its too low in the stern causing turbulence.
Sometimes a Doelfin will help certainly an inexpensive cure.
Usually as you move to a 4 blade diameter is reduced Perhaps the Spitf

I just looked - I don't think solas amita or rubex makes a 4 blade 11" pitch with diameter of 13" or greater. That seems strange?? Does this suggest that a 4 blade might not be applicable for my motor? I'm confused...

Usually as you move to a 4 blade the diameter is reduced. Perhaps the Spitfire is larger.
The Etec site has 4 24 ft 115 tests using 11" and 13" props, no 4 blades but lots if good results
If you also look at the smaller toon tests you may note a broader prop size result and speeds all over the place.

 

[Bamaman1]

Re: Ventilation on pontoon boat - New prop? Doel fins?

We seldom see any 4 blade props on pontoons. I had the same problem on my old Starcraft with a 115 hp Yamaha 2 stroke. Everyone gets on the front of the boat, and it lifts the rear end.
Your options:
1. Get the riders to move to the rear.
2. Put a jack plate on your boat in a way where you can lower the motor. Manual plates are not that expensive.
3. Find a good prop shop that will allow you to demonstrate 3 bladed props and return the props that ventilate or don't suit your needs. Their advice comes free when purchasing a prop.

 

[Tompen]

Re: Ventilation on pontoon boat - New prop? Doel fins?

Thanks to everyone for all the advice.

I had the exact same problem with my pontoon. The 4blade gives you stern lift, which compounds the problem.

Are you speculating, or did your boat actually ventilate worse with a 4 blade?

I know a 4 blade is good for acceleration and early stern lift, but does a 4 blade also cause stern lift (compared to a 3 blade) at higher speeds once you've 'planed'? This is the only time my boat ventilates. If so, your point would seem quite valid.

 

[muskiemike12]

Re: Ventilation on pontoon boat - New prop? Doel fins?

Real life situation. When I switched to the 3 from the 4, I dropped 300 rpms and gained 4 mph.

 

[Tompen]

Re: Ventilation on pontoon boat - New prop? Doel fins?

Real life situation. When I switched to the 3 from the 4, I dropped 300 rpms and gained 4 mph.

Thanks, thats helpful information.

I've played with moving my passengers from the front of the boat to the back and that does seem to help the ventilation quite a bit, so additional stern lift (from a 4 blade prop) is not something I want. I'm now leaning toward a 3 blade for this reason.