Electric Jet Drive

Okay. You may be asking yourself, "Why the hell would you want an electric jet drive?" Well, I'll tell you. I'm a senior in mechanical engineering at Louisiana State University. For my senior project, my team will be taking part in Solar Splash 2007. This is a nationwide competition, where teams build solar powered boats and compete in a number of races. You can check it out at www.solarsplash.com.

So, one of our first goals is to look at propulsion systems. We have to consider jet drives and conventional prop drive systems. It would be greatly appreciated if anyone with something of a knowledge of jet drives could help us understand the efficiencies and design of these systems. Thanks beforehand for your help.

 

Hi LSU,
In general, jet drives have pretty poor efficiency at low speeds. Where they excel is in craft that operate over 20-25 knots or so.
In a solar powered craft of any kind, efficiency is paramount. So is weight, by the way. A jet unit is a lot heavier than a conventional prop driven system.
So yes, look into the jet option, but I'm pretty sure if you do all the math, you'll end up with the largest diameter propeller you can fit.

 

Jet drives also allow the engine to run at optimum revs with the boat speed controlled by the bucket when necessary. But an electric motor develops full torque at all revs so is a better match to a standard propellor anyway.

I think just comparing water jets with props is a waste of time. I think the disadvantages for you of a water jet can be taken as a 'given'. You might be better off using your research time figuring out how to get the cleanest flow of water to your prop. There is some development afoot into hubless props in ducts, but you also need to investigate conventional shrouds, or putting the prop at the bow, or....

 

Thanks a lot for the replies. That pretty much coincides with the information that I've found online. In our case, low speed performance will be critical. Acceleration is obviously important in any kind race. I do think we will end up giong with a shrouded prop design, which means we get design our own lower unit. That should be fun.