Prop for Colorado at 5760'

[Dana D]

Hey guys! I need help with my prop.

1) Performance issue you are trying to correct.
A need for more speed

2) Current prop manufacturer, model, aluminum or stainless as a minimum.
SMC 344? Aluminum

3) Current prop diameter and pitch (required).
17P 13" in diameter?

4) Wide open throttle RPM and speed with an average load (very helpful)
5000 rpm WOT 25mph with 4 people in the boat

5) Engine/drive make, model, year, and HP
Volvo Penta 230 1992 120-130HP (sorry, old owner did not have much info)

6) Boat make model, year, length and weight
Glastron SSV 175 1993 17.5' not sure on weight

We are not sure the boat was ever run in Colorado. We do know it was run at Lake Havasu which is a much lower elevation. Someone recommended a 13P prop but it really did not do well so we switched back to the 17P and it does much better. Does anyone have recommendations? My husband and his brother water ski with it.

 

[jestor68]

Re: Prop for Colorado at 5760'

I haven't seen volvo props in that small of diameter(13"). The pitch seems right, since you're turning 5000 rpm(max is 4800 rpm).

It seems like you would benefit by going up in diameter to a 14.5/15" X 17. That would bring the rpm down to acceptable limits and provide better thrust for acceleration onto a plane.

 

[Dana D]

Re: Prop for Colorado at 5760'

I haven't seen volvo props in that small of diameter(13"). The pitch seems right, since you're turning 5000 rpm(max is 4800 rpm).

It seems like you would benefit by going up in diameter to a 14.5/15" X 17. That would bring the rpm down to acceptable limits and provide better thrust for acceleration onto a plane.

Thanks Jestor, I will look into it.

 

[Dana D]

Re: Prop for Colorado at 5760'

So, I spoke with a local boat guy who deals with Volvo Penta and he said a 14x17 or a 15x15 because running at that high of an rpm is bad for our motor. Which would be better to keep our good hole shot? We have a good hole shot now but we are definitely running too small of a prop. The prop guy said the 15x15 would kill our hole shot and that Volvo Penta 230 runs high rpm anyway. Thoughts anyone? These two are contradicting each other it would seem.

Thanks

 

[jestor68]

Re: Prop for Colorado at 5760'

I'd go with the 14 X 17 prop. The inch of diameter has a similar effect on WOT rpm as an inch of pitch; it'll bring it down a couple hundred.

The inch of diameter increase equals more blade area, which equals more static thrust( the push you feel when you first give it the throttle). The greater thrust upon initial acceleration should help the "hole shot", as that is what pushes the boat onto a plane; and pulls up a skier.

Think of it like this; small diameter props are for speed because they have less drag at high speed. Larger diameter props are considered "power" props, generating higher thrust initially and at medium speeds.

The larger blades create more drag at high speed, and usually can't generate as much top speed, but where they shine is in their greater initial thrust. They make good ski props for that reason.

But in your case, I'm guessing you might actually see a slight increase in top speed because of the fairly low gear ratio that your drive uses being better suited to more diameter.

 

[Dana D]

Re: Prop for Colorado at 5760'

Thanks for the info jestor, this is my husband's 2nd boat (the other is a 66 Larson his dad bought 30 years ago and still runs) and my first. I am learning a LOT about boats!

 

[moosehead]

Re: Prop for Colorado at 5760'

Unfortunately, when getting to higher elevation from lower altitude, there are severe limitations to gaining more WOT speed as no matter what you do there is roughly a 3% power loss for every 1k' of elevation gain. You're then somewhat stuck with having to lighten the load and trim optimally for more speed. You can try a more aggressive prop, but your RPM's will drop below optimum, WOT speed gained will be somewhat nominal, and holeshot will be weaker.

The more common objective at altitude is to prop down to counter reduced engine efficiency, allowing the boat to run at higher/optimum RPM, and get a better holeshot. As Jestor noted, sometimes gearing is changed for running at higher altitude, but that is a bigger proposition.

Maybe try asking Prop Doctor's in Denver if they'll let you try out a few different props, otherwise, they have used props that can be a bit more reasonably priced. Good luck.

 

[Dana D]

Re: Prop for Colorado at 5760'

Thanks for the info moosehead!

 

[Dana D]

Re: Prop for Colorado at 5760'

We put a 14x17 on and it was perfect. Thanks for all the help.