Need a prop for a wee bit more speed.

[towtruck1]

Hello

I need help in eking an extra 1 to 2 mph out of my rig at 5000'. The speed at sea level is fine for me, and for some reason I don't seem to lose a lot of rpm at 5000 ft, but the speed is less. Sorry for the long post, but I wanted to provide as much info as possible.

Boat:
Calibogie 15. 15', 1200 to 1250 lbs with driver, spotter, & recreational skier in the boat.

Engine:
91 Tohatsu M50D. Max RPM: 5700, gear ratio: 1.85

Current propeller:
5 or 6 yr old Tohatsu Al 11 x 13.5

Current performance (WOT), sea level, salt water:
5200 to 5300 RPM, 33-34 mph INDICATED.

In theory, the engine should produce maximum power at the MAX rpm (5700), so it seems I need a prop that will give me 5700 rpm at the loading conditions I specified and run efficiently at that condition. I should get a higher rpm by a) dropping the pitch; b) dropping diameter, or c) finding a more efficient prop.

1.
I had a chat with someone at Turning Point propellers, and he wanted me in an aluminum 10.75 x 12. I did the math, and to match the same speed I get now I need around 5900 rpm, assuming the same slip. What was he thinking?

2.
I also had a chat with someone at a Tohatsu dealer and he wanted me in a SS 10.5 x 14 (Tohatsu Prop). This seems more reasonable as far as achieving the higher speed, but I am concerned that with a higher pitch prop I won?t achieve the RPM I need.

So?
What should I make of the conflicting information? I am inclined to follow the advice of #2. Will I get enough RPM boost from the SS due to a possibly more efficient design plus a reduction in diameter? Will a reduction I diameter adversely affect slip?

If I go with a SS, these brands/sizes are in play:
Toahtsu 10.5 x 14
Michigan Wheel Apollo 10.75 x 14 or 10.75 x 13
Quicksilver Silverado 10 1/2 x 14 or 10 3/8 x 13
TurningPoint Express 10 1/2 x 14 or 10-1/2 x 13
Solas New Saturn 11 1/8 x 13
Does any one have any recent experience w/these or preferences? What about aluminum?


Final Notes:
I do not need a particularly good hole shot, as long as the boat still has the same ability to get a skier out of the water. I have also considered Piranha, but I don?t think they have the right size.

Any help/suggestions/advice that can be provided will be greatly appreciated.

Thank you!
Andy

 

[steelespike]

Re: Need a prop for a wee bit more speed.

What is the rpm and speed at 5,000 feet.Not a pro but your present sea level performance is darn good.Your certainly not likely to gain any speed at 5,000,perhaps maintain speed.Have you fine tuned your setup?If you can raise the motor to a point just before venting becomes a problem you might gain a mph or more.Just be sure it pumps water.If the present prop is aluminum the same size in stainless should improve performance.It will also aid in fine tuning the height.

 

[crb478]

Re: Need a prop for a wee bit more speed.

How are you measuring your speed at 5000'? If it is with a pivot tube you may already be at nearly the same performance. You will need both speeds verified by gps to have any true meaning. It does sound like you have it running good at sea level.

 

[bhammer]

Re: Need a prop for a wee bit more speed.

Not every prop is made the same as in cup and rake and those will have an impact in your speed and RPM. I have found that my TP propeller at the same pitch runs about 300 RPM less but produces the same speed as anoter prop that is not a TP.

Are you in a position to try a couple? I think TP has an exchange program and they offer them here on iBoats too.

 

[towtruck1]

Re: Need a prop for a wee bit more speed.

Thank you all!

Steelspike:at 5000'
Sorry for not being more clear. I'm not trying to increase speed over what I get now at sea level. I'm trying to increase speed at 5000' to over what I get now at 5000'. If I match what I'm getting at sea level I'll be more than happy. I've thought of raising the motor, but as it is now I'm venting in the turns, and the driver usually trims down just a bit in the turns to prevent it. As I can't get exactly the same prop size in SS, would you suggest the 13 or 14 pitch? If I got a TP prop as per bhammer, I'm gessing the 13 would be the way to go. Get the rpm up a bit (or not let it drop any) while still getting good speed.

Steelspike & crb478
Don't know what the speed is at 5000' feet. I just know its down a bit from what I get at SL. I can definitely feel the difference when I ski. As for RPM, I don't know for sure either, but I'm sure I'm below max, especially if I'm below max at SL. Interesting thing is, I'm remember checking one time and pretty sure it's close to what I get at SL. Could the salt water at SL be slowing the prop down a bit at SL? Or maybe the boat was lighter at 5000'. In any case, I've got margin. The numbers I gave you are all I got until I go back up to the lake in a month; so they are the best I have, but I think they provide a decent reference.

bhammer
Thanks for the perspective on the TP prop. very helpful. BTW, is your TP prop SS or AL? I'll look into the exchange program, because it really would be helpful to try a few props.

Thanks again for all the help! Really appreciated!
Andy

 

[Tail_Gunner]

Re: Need a prop for a wee bit more speed.

http://boatpropellers.iboats.com/To...r=1991&**********=849819017&*******=429829100

 

[steelespike]

Re: Need a prop for a wee bit more speed.

Keeping in mind I'm not an expert::
If you go to a 13 stainless it should eliminate your venting problem and perhaps allow a little more height.Its almost a sure thing it will get you some speed.

 

[towtruck1]

Re: Need a prop for a wee bit more speed.

Thank you Steelspike
That makes sense to me.

tailgunner:
Thank you also for your link. I understand the hustler aluminum are good props. But 10.5 pitch? And 12" diameter? Even if slip went to zero, I'd never get the speed I want according to the prop equation. I'm pretty confused how you got there? Could you explain? I'd love to save over the cost of a SS. BTW, it is interesting to me that iBoats only lists Hustler props for my application. That speaks volumes for the brand.

Thanks again to all.
Andy

 

[bhammer]

Re: Need a prop for a wee bit more speed.

Thank you all!
bhammer
Thanks for the perspective on the TP prop. very helpful. BTW, is your TP prop SS or AL? I'll look into the exchange program, because it really would be helpful to try a few props.

Thanks again for all the help! Really appreciated!
Andy

You are welcome. All my props are AL. You may look at SS and get another MPH or two as SS typically performs a little better.

 

[Tail_Gunner]

Re: Need a prop for a wee bit more speed.

Thank you Steelspike
That makes sense to me.

tailgunner:
Thank you also for your link. I understand the hustler aluminum are good props. But 10.5 pitch? And 12" diameter? Even if slip went to zero, I'd never get the speed I want according to the prop equation. I'm pretty confused how you got there? Could you explain? I'd love to save over the cost of a SS. BTW, it is interesting to me that iBoats only lists Hustler props for my application. That speaks volumes for the brand.

Thanks again to all.
Andy

You prop or gear for top rpm on your motor. People with a far better understanding have posted as much.....And in hindsight it makes perfect sense.

That is your not taking care of A WALL FLOWER..... its a motor...designed to run under stress so stress it to it's limitation's...they are very conservative.

 

[towtruck1]

Re: Need a prop for a wee bit more speed.

Thanks again Tail_Gunner!

I understand that you prop for max rpm. But as I understand it, a faster prop will do that too.

Starting point:
Max rpm of 5700
Current 11x13.5 is giving me around 5300 at WOT (at 5000').

Rules as I know them:
gain 200 rpm for every inch of pitch lost
gain 100 rpm for every 1/4" of dia.

So, compared to my current prop, I would lose 400 for the 1" dia change, and gain 600 for the loss in pitch. OK so that works. I'm at 5500 now. Good enough.

But I could also put on a 10.5 x 13, and compared to my current prop I would gain 200 from the diameter drop, gain 100 from the pitch drop, so now I'm at 5600 rpm and going a heckuvalot faster than the 10.5 pitch. Or no?

I'm not trying to be contradictory here, but I'm really confused why you picked the 12x10.5 What am I missing?

 

[steelespike]

Re: Need a prop for a wee bit more speed.

personally I wouldn't depend on a diameter change to change my rpm.
Select your prop by material (stainless,aluminum,composite)and design/pitch
diameter will follow.Diameter can change rpm but depends heavily on the design.It wouldn,t be a surprise if diameter didn't change anything unless large difference.

 

[Tail_Gunner]

Re: Need a prop for a wee bit more speed.

gain 100 rpm for every 1/4" of dia.



nope

 

[NoKlu]

Re: Need a prop for a wee bit more speed.

Just a thought. I have no experience with boats but with quads and sleds, when we go higher in altitude we re jet the carbs to get the performance back. Would it not be the same with the boats?

 

[Randybeall]

Re: Need a prop for a wee bit more speed.

Change the Aluminum prop for a SS of the same dimensions and let the efficency get what you want. You could drop an inch to two in pitch to bring up the rpm to peak. Also, in this discussion we should mention that at 5000' you are loosing considerable power. The reduced air density will keep you from making the power you are used to at sea level. Rejetting should be considered to keep the efficency as close to factory spec as possible. I would suggest the engine literature in the factory shop manual will guide you in this.

 

[NoKlu]

Re: Need a prop for a wee bit more speed.

There is a jet called "dial a jet". It allows you to change the jetting by the turn of a knob and can be done in seconds once you learn the settings for the altitude your at. I'd go that direction [jets] before I started changing props several times and never being able to recreate your performance at sea level.

 

[towtruck1]

Re: Need a prop for a wee bit more speed.

Hi everyone.

Thanks for being so helpful. This thread has gone on very nicely.

Re: Prop dia efffect on RPM. I know dia is much less important than pitch, and maybe the rule I heard about 100rpm/quarter" diameter is not very reliable, but all other things being the same, diameter change HAS TO have an effect on RPM for two reasons:

1. Tip speed. The higher the diameter for a given RPM, the higher the tip speed of the prop. The higher the tip speed, the more drag (resistance) that the engine has to overcome.

2. Prop Area: Diameter essentially increases blade area, assuming blade geometry is the same. More area means the prop can absorb more energy, which of course has to be produced by the engine.

I have been seriously considering re-jetting, but its a lot of work for one week of play. I can do the work myself, it's just a matter of time. I'll take a good look at the engine and manual this weekend to see how difficult it is. The boat has been up there (the 5000' lake) many times before and runs quite nicely. Its just that I know I can get more power out of it by raising the rpm by about 400 with a prop change alone, and that may be enough for me.

At this point I'm leaning towards a Turning Point HUSTLER. I understand that a lot of the efficiency benefits of a SS prop (which I was seriously considering) are simply due to design, and from what I've been hearing and reading, TP has managed to incorporate many of those design benefits into their Al HUSTLER props.

The real problem is simply that not all props of the same description (dia x pitch) are created equal, so I may have to try at least 2 props. Staying with Al at this point makes it a little easier to project what pitch I need, as I'm not changing material AND manufacturer AND diameter all at the same time.

Most likely I'll go with the 10.5 x 12 recommended by the turning point tech. It seemed crazy to me at first, but the more I read, listened, and thought about it, the more sense it made.

Anyway, I haven't bought it yet, so I'm still open to all and any suggestions. Thanks again to all. You've been awesome. I promise to share the results with you when I have them.

 

[Tail_Gunner]

Re: Need a prop for a wee bit more speed.

Engine:
91 Tohatsu M50D. Max RPM: 5700, gear ratio: 1.85

Current propeller:
5 or 6 yr old Tohatsu Al 11 x 13.5

Current performance (WOT), sea level, salt water:
5200 to 5300 RPM, 33-34 mph INDICATED.
==============================================================

Ok the above are your own figure's

Engine's recomended Wot rpm .. 5700

5200 with 13.5.. 5400 with a 12.5.......5600 with a 11.5............5800 with a 10.5.........

 

[towtruck1]

Re: Need a prop for a wee bit more speed.

You are probably right Tail_Gunner

I'm starting with 5300 rpm (top end of what I observed), so a 12 pitch should give me 5600. Close enough, considering that it may hit 5700* with only one person in the boat. Also, the 12" is slightly smaller in diameter than my current prop, in theory giving more rpm.

Also, in theory, the 12" prop at 5700 rpm will give me the speed I get now only if I assume zero slip. So if I get perfect rpm, but not the speed I want, the whole exercise is pointless.

The wild card here is differences in design between the Tohatsu prop & the TP Hustler. In theory the Hustler should be more efficient. Anyway, I'm just gonna have to put one on and see what I get. If I'm wrong, I can pay $20 to exchange for the right one, or pay about $55.00 for a new prop only (I'll already have the hub). Yeah, and about $9.00 more for shipping.

*BTW, I don't remember where I got this, but I am assuming that 5700 is max, as in redline, as in don't exceed! I checked the Tohatsu manual and can't find anything saying so. It just says 'full throttle' operating range 5000-5700.