Prop Tuning Needed

Hello All,
I am looking for recommendations of a good prop shop that specializes in performance tuning. We have just launched a new 24' prototype catamaran that is designed to cruise as efficiently as possible at 22 knots. This design is a proof of concept boat to push the limits of efficient power boat design. It is a displacement hull with twin 20hp Yamaha four strokes (and hydrofoil assist). Low end torque is not really necessarily on this boat because it has very low resistance until it gets up to around 15 knots (it does not need to get on plane). What were are looking for is a gain in efficiency at cruising speed...and right now an optimized prop is the lowest hanging fruit. Currently we are using the stock aluminum Yamaha props that came with the engines to test the boat.

Any help or suggestions are much appreciated.

Sincerely,
Brandon Davis
Turn Point Design

 

I suspect that no off the shelf prop is really designed to go 22 kts with 20 hp. This is an outboard? I think that the Yamaha OB's turn up to about 6000 RPM and have a 2.08 gear in the lower unit, maybe a 9.25" diameter prop?

The key will be to find out where the engine is most efficient and pick a prop pitch that lets you run at (or near) that RPM.

In order to really look at this in any detail, you would need to provide resistance vs speed or hull parameters and dispalcement. And then also any data about how you are operating now and the current prop numbers (diamXpitch) would be useful.

 

you can buy adjustable pitch props

 

John,
Attached is a performance prediction graph of the boat. The boat was launched 2 days ago and we have tested it for 5 hours without hydrofoils. You are right about the Yamaha specs. Redline is 6000 rpm and the stock prop is 9 1/4" x 10"... not sure about the gear ratio. The boat reached 20 knots at 6000 rpm with quite a bit of throttle to spare... so I am waiting for a pair of 12" pitch props to show up before taking it out again. That is the highest pitch prop that Yamaha caries for the motor. Hopefully that will get the pitch close enough to get a prop shop involved.

In regards to where the engine is most efficient. From testing a Yamaha 20hp on another planning skiff and some misc. research-- the engine is its most efficient from 50% to 75% throttle. It is carburated, and is not so efficient above 80% throttle. I wish I could have put fuel injected engines on the boat... but no one bothers to make one in this size and weight.

Thanks for your comments,
Brandon

 

Adjustable pitch would be great while trying to find the right pitch for the boat... I have never seen these for small outboards.

Most likely though, any off the shelf prop will be an all purpose prop and/or designed for a planning hull that needs a lot of low end torque to get onto plane.

We are looking to squeak as much efficiency out of this boat as possible.

 

Deffinitly adjustable props that adjust themselves .

http://www.propellerpages.com/?c=manufacturers&f=propulse

http://www.piranha.com/kits.php

 

Fuel injection would give you less improvement in high rpm economy than you might think, imo. 10" to 12" is quite a jump so you'd best try those before drawing too many conclusions. ARe there stainless props for that motor, they are usually cupped and effectively greater pitch ?

 

If you use stainless props you'll have more freedom to experiment with engine height as well.

 

If you got to 20 kts and had reached the RPM limit before the HP limits without the foils, it seems that your actual resistance curve is probably much lower than the prediction. Going from 10 to 12 inches of pitch is a huge change. At the same speed and RPM you should see almost 2X the thrust. So the question is how much faster will the boat go. The good news is that you probably will have bracketed the solution with these two props. The basic parameters seem to indicate that the propeller should be operating quite efficiently, in the 70+% range, so there is not much more that could be done with the propeller design.

The only question remaining is if there is a standard propeller that will provide what you need. There are many companies that props for this market, so once you have the best pitch for your boat, you may need to try a few. In general, look for low blade area, since this is lightly loaded, you won't need much and avoid heavily cupped designs as these are not as efficient (they are designed to keep producing thrust even when overloaded).

 

Thanks for the replies.
I took a look at those composite props and they look very interesting. Does anyone have experience with their efficiency compared to a good stainless prop?

I'll give the new pitched props a try this coming weekend and then post the reports.

 

I put on the new props this weekend. 9 1/4" x 12". With these the boat went 22 knots at 5800 rpm WOT. Thats with one person on board. With three people on board the boat reaches 20 knots at 5500 rpm WOT. The new props are also the standard off the shelf Yamaha aluminum props. They have blunt/ almost square leading edge and trailing edge. Now that I'm close I'm planning on sending these to a performance prop shop to change the pitch to 11 1/2, and shape the edges. Any input would be appreciated.

 

Those numbers sound reasonable. 1/2" of pitch reduction should get you about 200 RPM increase at 22 kts. You probably won't gain any speed from that change. Grooming the edges, particularly the leading edge will help, but once again, I suspect that the difference will not be huge, but I guess every bit helps. If the tip speed were higher or there was a heavy loading situation, you would see more gain from a better shaped edge, but not so much in this case. Be careful with the trailing edge, they may have some cup or some shape to keep them from singing. A good prop shop should be able to tweak them appropriately.