pic: 1726 NURC robot prototype

[falconmaster]

Originally Posted by Jon Jack

First off I'd like to compliment you guys on a nice, clean design. I think you guys are off to a great start. I'm interested to know if you guys have done any testing yet as I have a couple of concerns about your thrusters:

1) Your use of computer fan blades
From my testing at SeaBotix, I can say that fan blades are horribly in efficient. Especially in water. The blades tend to flex too much which cause for your performance to decrease, especially when you're moving at a higher speed. May I suggest using an RC/Model boat propeller. One advantage to this design is that you seem to have a lot of clearance to use a larger diameter propeller. Using a larger diameter propeller means you'll be able to move more surface feet of water per revolution, which means you can run the propeller at a lower RPM therefore cutting down your power consumption and making those batteries last longer. Also, there are a couple modifications you can make to an off the shelf RC/Model boat propeller to improve its performance and efficiency.

For example, make sure that you've gotten rid of ALL mold reliefs on the surface of the propeller. You may want to steal a page from the America's Cup boats and sand and gloss the propeller (spray paint gloss works just fine) then sand the gloss finish with a 2000 or finer grit sand paper. The gloss will cut down on the friction and the small grooves from the sand paper make sure that the water does not stick to the surface of the blade. If you do decide to make these modifications be very careful not to change the shape of the blades as that will effect the propeller as well. If done correctly you could increase your thrust by about 5-10%.

2) Water flow around the thruster
From some of the pictures I've seen, it seems like 80-90% of the propeller is being blocked by the thruster body. Remember that you want to keep as much of the area behind the propeller free from obstruction, increasing the flow of water and improving your performance. This will also help cut down on cavitation which will give the vehicle a much smoother ride. Cavitation not only sucks the performance out of a thruster, it also causes spikes in current draw (imagine running it in air/in water over and over again).

You can fix this by going with a larger diameter propeller. The more blade that is not being blocked by the thruster, the better.

3) The Kort Nozzle around the propeller
I found a picture in your gallery that had a very good picture of the thruster. One of the things I noticed was that your kort nozzle isn't wrapped very tightly around the propeller. You want this wrapped as tightly around the blades (with out actually touching them) as possible so you can concentrate and control the flow of water as much as possible. If there is a gap left between the nozzle and the blade the kort nozzle loses some of it purpose.

Anyways, I hope you find this as useful. Good luck to you and all of the NURC teams.

You are so right about the fan blades, We have had a very hard time finding boat props the size we need. they are either too small or too big. Do you have any ideas where to get them. We find that plane props work, buy you need higher rpms. Ye sthey are not efficient, but they are better than none. We also found that if you use large plane props and cut the tips off they work even better. Find the highest pitch you can. I like the muffin fan idea and it is possible to find ones with blades that are not that flexible. I was even thinking of doing a bollard test with every other blade cut out. Just to see what happens.

Thanks for posting your experience! Its right on. Please let us know where to get props. Do you think Seabotix might sell us just the props?