Prop diameter vs pitch.

[imported_John o]

I am currently running a 15.25x15 aluminum three blade yamaha prop on my yamaha 175hp Vmax hpdi outboard. The boat is a Manitou 24' Osprey Pro pontoon with the SHP package. (three tubes, center one is 25" and set 5" lower then the two 23" outer tubes)


Iboats Boat and Motor Info
1. Year, make and model of boat

2007 Manitou Osprey Pro.

2. Length, width and base weight of boat, look for boat decal on back of boat

24' x 8'6". Weight 4000 (including boat, motor, gas, batteries, incidentals)

2a.What is the recommended HP for your boat

200hp max

3. Number of people and gallons of gas normally on boat

Two people (for these calculations) - about 20 gals gas


4. What do you use the boat for

fishing, cruising, tubing. General purpose do everything boat.

5. Is it a Deep Vee and if so how many degrees of deadrise

Pontoon

6. Year, make and model of motor

2006 (might be a 2007) Yamaha HPDI VMAX 175 Hp.

7. HP and gear ratio of motor IMPORTANT

175 HP - 1.86

7A If you have a mercruiser and don?t know the ratio get your serial off the outdrive and go here http://www.sterndrives.com/mercruisersterndrives.html
8. Manufacturer?s recommended Wide Open Throttle (WOT) range

5500 is normally stated as preferred WOT. Will run up to 6000 with the rev limiter set at 6200.

9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches

I believe I am set 1 inch above bottom of center tube

10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs

no

12. Make, model, diameter, pitch and whether SS or aluminum prop

Yamaha 3 blade 15.25 x 15 aluminum


13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50

5500, upwards of 20 gals (tank is 28 gals when full) 1 person. Speed was 37.6GPS

14. Are you at sea level or a higher elevation, give us the elevation in feet

500feet? (south central Indiana)

15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the forgoing could be the reason your prop is not attaining full RPM

Everything seems to be OK. Was last serviced end of last year.


16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop

Hit a rock with the earlier prop (Michigan wheel 15.25 x 15 three blade aluminum. Roughly same performance). Bought this one as replacement. Got that one fixed for spare. I've used this one for about a year and a half now. Performance pretty constant.


17. Does the prop show any damage that you can see

Nope. Missing some paint on the tips but metal is all good. (It was reworked last year after hitting another rock. The prop shop likes me)

18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations


With a 15 pitch my theoretical maximum speed is 42mph. So I'm showing about 11% slip. I am wondering if I can drop the diameter and increase the pitch to get more speed.

I'd really like to get over 40MPH.

Question is what will that do to my hole shot (I now hit 15MPH in about 10 sec and 30 MPH in about 20 seconds) and how much do I need for tubing.

Likewise how would it effect cruising (3500RPM) efficiency


I've searched the web and can't seem to find a relation between pitch and diameter. If I go to a 17 pitch my theoretical top speed goes to 47mph which (if slip remains the same) should get me to 42 MPH. But I don't know if dropping the diameter will drop increase the RPM as much as increaseing the pitch will drop them.

Any ideas?

 

[hwsiii]

Re: Prop diameter vs pitch.

John, that was an excellent answer to all my questions, as good as it gets. And I can tell that you have spent a lot of time working and thinking about this.
I show you having a 10% prop slip with the present MWC prop you are running on the boat, and that is very good.
I am not going to say you can't get over 40 MPH at WOT, but what I will say is that to find the correct prop to do that will probably require prop testing 3 or 4 props. There is a prop shop I know who is willing to do that and I think charges about $ 35 for every prop change you test. I do think it can be done, but it will require time effort and money. There is a possibility that by raising the motor up one or two notches and 1 prop change could accomplish this. The problem with this is that a lot of pontoon boats tend to break the prop loose and ventilate as they run into waves from other boats and this can cause the wrist pins in the motor to come loose, I don't know whether your boat does this or not I just want to mention the potential problem and if it does do it now forget totally about raising the motor.
BUT, i think the possibility of finding a prop that will increase the speed to what you want and also try to keep your low end performance are not compatible with each other.
By dropping Diameter and increasing Pitch we may could find a prop that will keep the RPM lose to where they are but I believe the prop slip will increase in most cases and thus you won't gain but a small amount of speed. And when you want to ski or pull a tube it might not put too much pressure on the prop and increase the slip much because of the weight mass you have pulling it.
With the props made today changing Diameter does not have the same connotation it did 25 years ago, they are designed in most cases suh that the pitch designates what diameter the prop is in any make and model of prop. There is at least one manufacturer who does make certain pitch props in the same make and model of prop in a couple of different diameters just for raising the RPM on the motor. We might luck out and be able to match that with your boat and motor. But I am not sure that the prop slip won't increase to eat most of that gain up.

The bottom line is I am not sure, and if anybody can tell you what is the right prop to do that it is Dhadley and it will be a stainless prop. Maybe he will chime in and give you a recommendation.


H

 

[imported_John o]

Re: Prop diameter vs pitch.

There is a possibility that by raising the motor up one or two notches and 1 prop change could accomplish this. The problem with this is that a lot of pontoon boats tend to break the prop loose and ventilate as they run into waves from other boats and this can cause the wrist pins in the motor to come loose, I don't know whether your boat does this or not I just want to mention the potential problem and if it does do it now forget totally about raising the motor.

I don't ventilate when hitting waves but do in sharp turns. Have to trim down to regain grip. (The cool thing about the Manitou SHP is that it banks into the turns and handles about like a bowrider at speed)

BUT, i think the possibility of finding a prop that will increase the speed to what you want and also try to keep your low end performance are not compatible with each other.
By dropping Diameter and increasing Pitch we may could find a prop that will keep the RPM lose to where they are but I believe the prop slip will increase in most cases and thus you won't gain but a small amount of speed. And when you want to ski or pull a tube it might not put too much pressure on the prop and increase the slip much because of the weight mass you have pulling it.

This is somewhat what I was afraid of (and expected)

What about cupping. I think I've come to an understanding of pitch but cup is beyond me at the moment.

One problem I have is my lake is very stumpy. The marina maintenance guy tells me that he replaces several lower units due to stainless steel props hitting trees and rocks and such. So I greatly prefer to stay with aluminum even though I know that SS normally gives better performnance.

The bottom line is I am not sure, and if anybody can tell you what is the right prop to do that it is Dhadley and it will be a stainless prop. Maybe he will chime in and give you a recommendation.

I'm open to all inputs.

Thank you.


H[/QUOTE]

 

[hwsiii]

Re: Prop diameter vs pitch.

If you want to stay with aluminum props your chances of reaching that goal are almost nonexistent. Aluminum props don't normally have enough blade geometry.
Cupping can be done in two areas of a prop.
1. Blade tip cup increases the rake of a prop and helps to pick the bow up.
2. Trailing edge cup increases the stern lifting qualities of a prop and increases the theoretical pitch of a prop and lowers RPM and increases speed.


H

 

[imported_John o]

Re: Prop diameter vs pitch.

If I open it to stainless steel props what should I look at first?

Thanks

 

[hwsiii]

Re: Prop diameter vs pitch.

Probably an Enertia, but Dhadley an answer that question muh better than me.


H

 

[imported_John o]

Re: Prop diameter vs pitch.

OK. I've just about got myself sold on getting a stainless prop. I know of only one cove on my lake with an abundance of stumps and I tend to stay away from the shore so I should be pretty safe.

I talked to a guy with a similar boat and a 150 Mercury. He's running a 15 x15 enertia and getting 40mph so I should get a bit more than that.


Is the enertia the Mercury pontoon specific prop?

What effect will it have on gas mileage at cruise? (My daughter's grandma doesn't like me going too fast with her grandbaby on board so if I can sell it as a mileage saver it will keep her content)

 

[hwsiii]

Re: Prop diameter vs pitch.

John I am going to ask you to talk to Ken at www.propgods.com/ and tell him I sent you. He is better than I am at the boat you have and he also is the one who will let you trade props until you get what you want for a small fee.


H