pic: 1726 NURC robot prototype

[falconmaster]

We did a little experiment. Modified Plane props, three blade reverse pitch 4 inch versus muffin fan 4 inch.
Plane 12v 4to 5 newtons at 8amps full throttle
Muffin 12 v 7-8 newtons at 8.5amps full throttle

We are building a second bot to assist the mother bot. We have not figured out what config. we are going with the little bot. On board batteries, off board??? we want a fast very maneuverable bot to help the lumbering mother bot.

[MrForbes]

quick answer for the current draw question...5 amps at 13v (with two 7.2v nicad packs in series)

[falconmaster]

Originally Posted by squirrel

quick answer for the current draw question...5 amps at 13v (with two 7.2v nicad packs in series)

Do you know how much thrust they put out? We are using the Mayfair cartridge bilge pumps 1000gph

You can get them at http://www.overtons.com/modperl/prod...ID=1009&r=view
In the pic it show a 750gph, they were lazy and used the same pic

[falconmaster]

Did you see the poster for this year, its going on the t-shirts! Look at the pics featured on the left.

[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.

[s_forbes]

Great tips Jon, thanks a lot!

This is the kind of feedback that we always hope for when we post our projects for everyone to see. The help gained from a massive community of clever people far outweighs any competative advantages gained by keeping it secret.

[GUI]

EDIT: beat to it...

[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?

[Jon Jack]

Originally Posted by falconmaster

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?

I did a search on Google for 'RC Boat Propellers' and I found this site. They have a variety of propellers in different sizes ranging from 30-60mm, which seems like the optimal size for this competition. They're also reasonably priced. I would start there.

If someone is interested, I also found a site that sells Kort Nozzels and Kort Nozzle Propeller combos. If you can't find an off the shelf kort nozzle, the least you could do is use some PVC tubing that'll fit closely around the outside of the propeller. A step up would be if you can find something that is tapered towards the prop so you can concentrate the water column increasing your bollard pull.

What are the rules in terms of what motors teams can use on their thrusters?

[MrForbes]

No rules....basically we can do whatever we want, we are limited to maximum total power and maximum voltage.

We are limited by our budget, though! which is one reason we started out using the $15 bilge pump motors. another reason is that they come with a shaft seal that works ok at the depths of the competition.

We are also limited in time/fabrication abilities/etc so a pre-packaged motor is a big plus. Boat props work best when attached to a relatively long drive shaft, so the motor is a ways away from the prop, not like our current design. That's one reason we wanted to go with a larger diameter prop. Also I tihnk that efficiency is not going to be our ultimate goal..instead, we'll be aiming for a propulsion system that can accomplish the mission objectives in the required time, and have enough battery reserve to get the robot(s) back to shore. Budget again comes into the equation....

We had found the boat props already, but not the nozzles...very interesting!

Thanks for your input, we'll keep working on it and let you know what we find!

[GUI]

Today we put some 6-4 model airplane props that we had lying around on it to see how they compare, and it's quite a difference. The bot was quite a bit faster, though they were spinning noticeably slower than the fan blades, indicating higher motor loads. I think with with some sort of Kort nozzle (we need prop guards anyway, why not make them aid propulsion?) these propellers will do what we need. We are planning on testing with larger fan blades and possibly with cut down airplane props.

EDIT: Here's a video of the teste with the airplane props ( the vertical thruster is still using a fan blade in the video) http://youtube.com/watch?v=DeYNCOiSV4Q

[tseres]

guys, that is awesome, and it's cool to see something like that other then the rov-in-a-box. i think i may have a summer project.....

[falconmaster]

Originally Posted by GUI

Today we put some 6-4 model airplane props that we had lying around on it to see how they compare, and it's quite a difference. The bot was quite a bit faster, though they were spinning noticeably slower than the fan blades, indicating higher motor loads. I think with with some sort of Kort nozzle (we need prop guards anyway, why not make them aid propulsion?) these propellers will do what we need. We are planning on testing with larger fan blades and possibly with cut down airplane props.

EDIT: Here's a video of the teste with the airplane props ( the vertical thruster is still using a fan blade in the video) http://youtube.com/watch?v=DeYNCOiSV4Q

Awesome guys! We see that same thing about the plane props. We have used three bladed reverse pitch 12 inch props that we cut down to 4 inches inside a abs coupler as a shroud. The girls team LEGIT is using this idea
http://legitrobotics.googlepages.com/frame2