Need more rpm but don't want to lose speed

[TrickDickle]

Hello gentlemen -

I've been coming to these forums for years for research and finally decided to post a question.

I have a 22' Manitou Pontoon with a 1999 Suzuki DF70TLX (70 hp 4 stroke) running a 14 X 13 aluminum prop.

My recommended rpm range is 5200-5800 with a WOT of 5800/5900.

With current configuration i'm only hitting 5000 rpm at WOT so I'm fairly certain I need to pitch down 1 or 2 inches but I don't want to lose top speed (don't care about holeshot).

My question is can I go down .5 or .75 in diameter, keep the 13 pitch and still pick up the needed rpms? Or with such a big boat do I need to stick with the 14 diameter and just drop the pitch sacrificing top end? Finally, will a 4 blade configuration make a difference?

Thank you in advance for any comments or suggestions.

 

[Bondo]

Re: Need more rpm but don't want to lose speed

Hello gentlemen -

I've been coming to these forums for years for research and finally decided to post a question.

I have a 22' Manitou Pontoon with a 1999 Suzuki DF70TLX (70 hp 4 stroke) running a 14 X 13 aluminum prop.

My recommended rpm range is 5200-5800 with a WOT of 5800/5900.

With current configuration i'm only hitting 5000 rpm at WOT so I'm fairly certain I need to pitch down 1 or 2 inches but I don't want to lose top speed (don't care about holeshot).

My question is can I go down .5 or .75 in diameter, keep the 13 pitch and still pick up the needed rpms? Or with such a big boat do I need to stick with the 14 diameter and just drop the pitch sacrificing top end? Finally, will a 4 blade configuration make a difference?

Thank you in advance for any comments or suggestions.

Ayuh,... Welcome Aboard,.... Prop Diameter is more a function of blade design, than rpm control,....

A 4 blade may help, others who know better will be along,...

Gettin' yer rpms Up, could increase yer speed, even with a smaller prop,...
Seen it happen, Alota times,...

 

[sweet addiction]

Re: Need more rpm but don't want to lose speed

I think what you will find is by dropping some pitch you will actually gain some top end as well as hole shot. It sounds like you are working the engine too hard due to too much pitch.

 

[steelespike]

Re: Need more rpm but don't want to lose speed

A hard to push boat like your pontoon needs a large diameter prop.Perhaps a pontoon dedicated prop.
What is your present wot rpm gps speed?Do you know your gear ratio?
There is a possibility as you increase rpm slip may go down a little making up for part of the pitch reduction.
The increase in rpm will help to maintain speed as you add weight.
Your motor will definitely be happier at about 300 to 400 rpm higher.
I think an 11" pitch will work.If there is a choice a larger diameter is better.

 

[TrickDickle]

Re: Need more rpm but don't want to lose speed

A hard to push boat like your pontoon needs a large diameter prop.Perhaps a pontoon dedicated prop.
What is your present wot rpm gps speed?Do you know your gear ratio?
There is a possibility as you increase rpm slip may go down a little making up for part of the pitch reduction.
The increase in rpm will help to maintain speed as you add weight.
Your motor will definitely be happier at about 300 to 400 rpm higher.
I think an 11" pitch will work.If there is a choice a larger diameter is better.

Unfortunately I don't know gear ratio or speed but agree the motor will be much happier running higher rpm.

I appreciate the responses so far...sounds like 14X11 may be the way to go at this point.

 

[steelespike]

Re: Need more rpm but don't want to lose speed

Gear ratio is 2.59. If we knew your gps speed we could tell better if your setup works. Prop slip is all over the place with
pontoons. But 20% is a high normal with a pontoon. If we assume 20% slip at 5000 with a 13" we get about 19 mph.
If we go to an 11" at 5400 and 15% slip( as rpm goes up slip may go down) we get about 18.5.
Of course that's a lot of guessing. Actual results could be surprisingly bad or surprisingly good.

 

[TrickDickle]

Re: Need more rpm but don't want to lose speed

Gear ratio is 2.59. If we knew your gps speed we could tell better if your setup works. Prop slip is all over the place with
pontoons. But 20% is a high normal with a pontoon. If we assume 20% slip at 5000 with a 13" we get about 19 mph.
If we go to an 11" at 5400 and 15% slip( as rpm goes up slip may go down) we get about 18.5.
Of course that's a lot of guessing. Actual results could be surprisingly bad or surprisingly good.

Steelespike -

I'm a numbers guy so i really appreciate the above analysis. I'm also ok with the assumptions because I'm just trying to get directionally correct.

Based on available props for my motor 14" diameter is the way to go I just need to decide on pitch. Other than potentially losing another .5 mph or so any other issues going down to a 9" pitch? I would really like to get in the 5800 rpm range.

Thanks again for the thoughts and suggestions.

 

[steelespike]

Re: Need more rpm but don't want to lose speed

My slip guess for the 11" prop was 15%I don't think we can assume another 5% improvement. So at 15% with a 9" prop
You will lose some meaningful speed. 5800 9" 15% =16.2 at 12% 16.8.
As you go to lower pitches the boat is not being forced beyond its more reasonable speed,this will help improve slip but will lose speed.

 

[smokeonthewater]

Re: Need more rpm but don't want to lose speed

don't forget that he'll be gaining rpm

 

[smokeonthewater]

Re: Need more rpm but don't want to lose speed

I think I'd look for a 10" 4 blade prop

I BET you'll gain a little speed and hole shot will be night and day better

 

[TrickDickle]

Re: Need more rpm but don't want to lose speed

I think I'd look for a 10" 4 blade prop

I BET you'll gain a little speed and hole shot will be night and day better

I actually spoke with a prop guy at the Atlanta Boat Show yesterda and he also suggested either a 14X11 3 Blade or a 13X10 4 Blade. He's actually going to let me try both and return the one that doesn't work out.

Thanks again for all the responses. I'll circle back after next weekend with my results.

 

[wrench 3]

Re: Need more rpm but don't want to lose speed

I know my prop supplier is pushing the four blade props for pontoons and dropping a pitch from the same diameter three blade. I've installed a couple of them and the people have been real happy.

 

[dazk14]

Re: Need more rpm but don't want to lose speed

I know my prop supplier is pushing the four blade props for pontoons and dropping a pitch from the same diameter three blade. I've installed a couple of them and the people have been real happy.

The middle of the rpm range is normally where peak hp occurs, giving you effectively a higher HP motor.

That results in more top speed all other things being equal.

Nice to be able to try props!

 

[wrench 3]

Re: Need more rpm but don't want to lose speed

Nice to be able to try props!

It takes a lot of the pressure off. My supplier does the same thing. They rebuild their props "in house" so I think it makes them more flexible.

 

[walleyehed]

Re: Need more rpm but don't want to lose speed

The middle of the rpm range is normally where peak hp occurs, giving you effectively a higher HP motor.

That results in more top speed all other things being equal.

Nice to be able to try props!

Not normally the case with this engine, especially at 5000rpm...The reason I would not recommend more than 1 step down in pitch because the torque curve will really kick in about 5300-5400. I believe with an 11p 3bld, you will see "close" to 5800. The HP is "taken" in the middle of this range.

 

[jestor68]

Re: Need more rpm but don't want to lose speed

The on line specs listed for the 1998-2008 DF 70 is rpm range-5200-5800; gear ratio is 2.42. It's hp peak should occur at about 5500 rpm.

The Solas 14 X 11 should be your best bet.

 

[walleyehed]

Re: Need more rpm but don't want to lose speed

You're going to find the best overall performance at 5800+a few. This engine was engineered around a better quality fuel in 1999 than what is available now. In fact, I'm sure they want this engine to run 5800 with your normal load. it's only been 5-6 years ago, Suzuki went from a recommended 5800 to 6000 due to carbon build-up and higher CHT and EGT at lower rpm. If you get a chance, read through some of my prop testing results several years ago that are in a sticky thread at the top of the prop forum...I think I posted some noted changes in temp rise when you pull too much pitch.

 

[jpwilly]

Re: Need more rpm but don't want to lose speed

How about that update. Your findings might help someone else.

 

[TrickDickle]

Re: Need more rpm but don't want to lose speed

Well, I just returned from second trip to the lake without being able to try a new set up. My prop guy has ordered the wrong equipment again. He insisted on Quicksilver product with the Flo Torgue hub system that would allow me to use a wider range of props. Well the hub fit fine but both props were too big for the gear unit so the props wouldn't fit fully on the shaft.

I'm frustrated because my boat is at my lake house which is 2 hours from my home so i have to find a free weekend to try again. I'm still planning to run the 14x11 and the four blade and will definitely come back to the forum with my results.

 

[steelespike]

Re: Need more rpm but don't want to lose speed

Is the hub too big for the gear case or do the blades strike the housing?If they hit the trim tab/anode it can be removed or replaced
with a clearance anode.