Jet Boat question

[bigquestion]

I need to make a impeller for a model jet boat. The diameter of the impeller is 53mm and the diameter of the nozzle is 28mm, what pitch of the impeller blades would give the ultimate performance? Also is a stator necessary in this size of jet unit? If somebody could awnser my questions i would be very grateful

[Ralph]

I was halfway through typing a big long answer but I lost it, ouch!

Anyhow, I found a pdf that gives a sort of outline on waterjet theory which you can get here. My notes from school are currently in a different country to me but I can check them early next year, but I'm sure there is no easy answer. My initial thought is that you're closing up the outlet quite a bit. A waterjet works like a propeller in a tube rather than a water squirter. The efficiency gains come primarily from almost eliminating any tip losses from the impeller, effectively doubling the aspect ratio of the blades.

A stator would give some efficiency gain in converting some rotation of the water into forward thrust, but personally I think you're worrying to much about the small stuff. If your unit was 10 times bigger, this stuff would be worth worrying about. These things don't scale down too well because of major factors such as the boundary layer, which doesn't scale at all!

If this were my project (and I have something alarmingly similar in the pipeline) I'd follow the words of a semi-well known yacht designer: if it looks good to the eye, it'll look good to the water. Gather info on what the professionals do with the full sized stuff and copy it. They do it for a good reason so you won't be too far wrong.

I'm sorry if this answer wasn't what you were looking for, without going into Kt/Kq diagrams and a lot of maths, there isn't an easy answer. I hope it goes well for you and if you do some sort of web site, I'd really like to see your jet in build and in action, let me know! Also, if there's anything else I can half-answer, just shout.

[bigquestion]

Thanx alot Ralph, loadsa help. Cheers!!