Yamaha F70LA prop advice?

[phmatt]

I just purchased a Bennington 20SLI Pontoon with the F70LA 4-stroke.
WOT RPM range 5300 to 6300, gear ratio 2.33 / 1.
It came with a Solas 4 blade 13x15 prop that would only get to 5000 RPM and 18+ MPH.
The Published Yamaha Performance Guide uses a Yamaha 13 5/8 x 13 prop which I
bought and have now ran. Boat runs 6600 RPM gets 21+ MPH.

Yamaha's performance guide for my exact boat configuration has WOT RPM of 5700
and a speed of 22.6 MPH.

I have verified the tachometer and am using a Garmin GPS for speed.

I have now ordered the Yamaha 13 1/2 x 15 prop hoping to reduce RPM by 400
and speed by several MPH's.

Questions?
1. Why the difference in my boat and the performance guide assuming that they
are the same as I say?

2. Will the 15 pitch prop do what I hope?

3. Does the yamaha like high RPM's ( I was told Mercury 4 strokes do)?

Any advice will be welcomed.

I do want to add I am very pleased with the boat and motor overall, I just would like to
see it do it what it said it would do. I was talked out of a 90 HP because of the
Yamaha published performance guide.

Thanks,

Matthew

 

[steelespike]

Re: Yamaha F70LA prop advice?

Not sure why it doesn't follow the performance guide.Was the guide based on your exact brand and model? with the same weight aboard.When you change prop brands your results can be unpredictable.If you went from a 4 blade to a 3 blade would change results.Even allowing going from a 4 to a 3 and dropping 2 inches a 1600 rpm change indicate some sort of problem like high prop slip numbers.The Solas props generally have had good predictable results and 4 blades are effective pontoon props.Is this a new boat/motor?Generally 4 strokes are run very near or at their rated rpm.

 

[phmatt]

Re: Yamaha F70LA prop advice?

The boat is 1 month old.2 hours on the Tach.
It should be the same weight as the performance guide and I
even had 2 people on the boat. The boat feels and sounds
fine. I think it feels better with the 3 blade prop.

I am most curious about the difference in my numbers and the performance guide.

 

[steelespike]

Re: Yamaha F70LA prop advice?

The 1600 rpm change makes me think that for instance the 4 blade prop likely could run higher and with more trim than a 3 blade.Usually trim be used only slightly on a pontoon regardless of the prop.
I really think you were experiencing slip.Its likely the turbulance of the boat caused the 3 blade to slip.not like venting prop still pushes but doesn't have a good grip.A pontoon, like any boat that is hard to push, will have high slip numbers but not to the tune of 1600 rpm. change.
Basically the motor likely was setup wrong for the 3bld 13" likely too high.
I would go back to a solas 4 blade in a 12" pitch and see what you get.The 4 blade will also handle better at the dock if there is a choice of diameter go with the larger number.
This may only take you to 5600 but at least in the ball park.You could get your solas 15 repitched to a 13 may be more cost effective than a new prop.
Don't forget when you change brands and styles you introduce unknowns and you may need to change the motor setup to get the prop to work.

 

[Silvertip]

Re: Yamaha F70LA prop advice?

You just learned why prop selection is such a crap shoot. All the charts in the world cannot predict with 100% accuracy what prop works best on a given boat. Even in one tests the same exact boat as was used in the chart or test, YOUR boat may be set up differently and have subtle differences that performance varies. Prop selection is not just about diameter and pitch. Blade shape, number of blades, prop material, rake, finish, blade thickness etc., are all "design" factors. Two props from two different manufacturers will not generally perform identically. Some designs favor hole shot over top end while some favor top end over hole shot. Others favor neither end and concentrate on midrange performance. A 13P four blade sounds more reasonable than a 15P three blade. Lowering rpm doen't necessarily translate to a loss of speed. Nor does an increase in RPM translate automatically to an increase in speed. Engine speed is determined by pitch. Pitch determines (theoretically) how far the boat is pushed forward with each prop revolution. A bigger bite (more pitch) requires fewer rpm to maintain the same speed as a prop with less pitch turning faster. Only if the engine cannot twist that extra pitch do you lose speed. And that lugs the engine which is why you want to select a prop that puts WOT rpm at or near the top of the RPM band.

 

[phmatt]

Re: Yamaha F70LA prop advice?

I see above I type .2 hours and I meant 12 hours on the Tach.

I like what is being said. I have ordered the Yamaha 13 x 15 3 blade prop and the
Solas 13 1/2 x 13 4 blade prop. They should be in next week and I will run my
trials and post the results. Hopefully those 2 props will both fall in the 5300 to 6300
RPM range and one of them will be better than the other. It appears most think the
4 blade will be the one. I have a good friend who says "WE'LL SEE".

I will have a brand new Yamaha 13 5/8 x 13 3 blade and a Solas 13 x 15 4 blade for
sale if anyone is interested.

Thanks for all the advice.

Matthew

 

[phmatt]

Re: Yamaha F70LA prop advice?

I got the Yamaha 13 x 15 prop on today.Expecting it to lower my RPM's by 400 and put
me in range. It lowered them to 5200 and reduced top end to 18.4 MPH.

It is quite confusing to me. The 15 pitch prop must be too much for the motor.
The 4 blade ran 4800 max and the 3 blade ran 5200 max.

Last hope is the 4 blade 13 pitch which will arrive Monday.

What could I have wrong that makes the RPM drop from a 13 to a 15 be 1600 RPm not 400 as
most suggest?

 

[steelespike]

Re: Yamaha F70LA prop advice?

As S said prop selection can be a crap shoot and when you throw in a pontoon you add more variables.As I discussed earlier prop selection can require a change in motor setup especially height. Allmost without exception a 4 blade will run higher and with less venting and slip issues whether on a runabout or toon. I feel your 1600 rpm difference was a result of prop slip either from a poor prop match or perhaps the need to change the motor height/trim.Again pontoons do experience higher slip numbers even with the ideal setup.
Is the 4bld 13 a solas?If the 13 doesn't work I would go back to the 15 solas and have a good prop,shop remove some pitch or cup if there is any.I've seen some amazing results here removing a little cup from a prop.

 

[phmatt]

Re: Yamaha F70LA prop advice?

A recap.

2011 Yamaha F70LA 4 stroke on a 2011 Bennington 20SLI Pontoon. (25 " toons).
I don't know for sure but the Tach seems consistent and appears to be working.
I have Garmin (cheapest) GPS that is giving consisted speed readings. Motor
is mounted on hole position 1. I have checked and rechecked everything.
The boat and engine seem to be running fine with any of the below props. If you
weren't looking you would not know.

Engine RPM range 5300 to 6300. Gear ratio 2.33. I am trying to get a prop
to run WOT at 6000 RPM. Is that right?

The Yamaha Performance Bulletin on this exact rigged boat and motor uses
the Yamaha 13 5/8 x 13 prop. It claims WOT 5700 RPM & 22.6 MPH.

Prop 1: Solas 13 x15 4 blade. WOT 4800 RPM & 18 MPH
This prop follows the performance Bulletin #'s up to 4500 RPM.
At 2500 RPM 7.3 MPH and so in.

Prop 2: Yamaha 13 5/8 x 13 3 blade. WOT 6800 RPM & 23 MPH
This is the prop in Yamaha's Performance bulletin.
It is never close to their chart.

I like the speed. Is the high RPM that bad for the motor?
Yesterday with my kids and grand kids (8 people) WOT was 6300 18 MPH.

Prop 3: Yamaha 13 1/2 x 15 3 blade. WOT 5200 & 18.4 MPH

Prop 4: Ordered not in yet Solas 13.25 x 13 4 blade. Surely this is the on?

As we say in Alabama, "I have'nt had so much fun since the hogs ate my little brother".

Any more advice based on this recap?

 

[Silvertip]

Re: Yamaha F70LA prop advice?

If 6300 is the manufacturers receommended WOT rpm, why are you shooting for 6000? You shoot for the top or very near it for less stress on the motor, best hole, shot, and speed will be whatever that combination of motor, prop, and boat will provide. 70 HP on a 20 foot boat with 8 people on board is a big load. What your expecting. When doing your testing are you doing it with the same load as you are finding in the test reports? And stop trying to compare anything other than than wide open throttle. You will NEVER match the test data 100%.

 

[phmatt]

Re: Yamaha F70LA prop advice?

I got the 4 blade 13.25 x 13 Solas in yesterday and it is the prop.

WOT got to 6200/6300 adjusting the tilt and 22.4 MPH.

So the 2 13 pitch props are running 1400 or so more RPM's than the 2 15 pitch props.

the 4 blade 15 max RPM was 4800 and the 13 is 6300.
The 3 blade 15 max RPM was 5200 and the 13 was 6800.

Blows the 200 RPM per inch of pitch. I guess the pontoon boat skews the deal.

I still have the question on the 3 blade 13 pitch at 6800 RPM. Is that bad for the
motor with the 5300 to 6300 RPM range?

 

[steelespike]

Re: Yamaha F70LA prop advice?

Yes its bad for the motor to routinrely run 6800 I don't think I've seen any suggestion to run any of the 4 strokes beyond their rating. Whether outboard or I/O.
I don't know if I have ever seen a 2" pitch change same brand produce a 1400 rpm change.
Its nice to see the Solas do the job again. I do wonder if the tach may be off a bit as rpm goes up.
Its hard to believe on a pontoon that that you could have a 1300 or 1400 rpm change without some underlying
"problem".