A Photo & Discussion Forum for Wharram Design Enthusiasts
I have 2 Honda 20 hp outboards on my Pahi 42, but can only get 4.5 at 3000rpm, my preferred motor sailing revs. I have tried high thrust props to get more speed, but only get 4kts, I assume because of the reduced pitch (the lager diameter obviously does not offset this).
I am now considering Yamaha 9.9 high thrust motors with the much larger prop and higher engine gearing (2.9 to 1).
my cruising weight is 6500 kgs.
Does anyone on the forum have any experience in this area of engine performance and any advice to offer. I have wasted many $ on the so called high thrust props. To my logic, reduced pitch means reduced thrust, despite the diameter and blade area increase.
Hello , I have 2 parsun 9,9 on my pahi 42 and the boat is betwen 6 and7 kn with the 2 motors and 5 with one .
Hi John, just remember the greater the pitch then the more load on the engine and you might end up working the engine too hard, i.e. like trying to take off in 5th gear in a car as all of the power is generated in the high end of the rev spectrum.
I am certainly no expert in all of this its just how my understanding has developed from talking to lots of people, especially outboard mechanics, and trying to work it out logically.
I absolutely agree that electric motors are the way to go, with big fat course pitch that revolve slowly utilising all of the torque electric motors can supply but i don't think the technology is advanced or cheap enough for me yet. Ive been following the advances by Tesla and Elon Musk as this tech seems the most likely to be spun off to boats. His battery tech looks cool but probably still too expensive but we'll see over the next year or so. The system i like the most so far is Bertrand's hybrid system on Grand Pha, but is not an easy retrofit if you have outboards. The other option i like is the diesel outboards but finding them is like finding rocking horse poo...
Good luck with the experiments, i'm sure everyone is looking forward to real world comparative tests..
John Wilkie said:
Thanks for that Marty, if only electric motors were further developed. I am realising that to get more speed at the same revs (3000), I need a prop with more pitch, given that pitch is the distance in inches that the prop travels forward in 1 revolution. No wonder the "high thrust prop of 7" pitch was worse than my standard 10 pitch prop. It sounds like a marketing strategy to sell more props.
If I can find an 11: pitch prop I'll let you know how it goes.