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Prop and hull size recommendation

Old 04-05-2016, 08:09 AM
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Aeronautic13
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Default Prop and hull size recommendation

I've never built an airboat before, but I have a project that requires me to carry 1/2 pound of cargo in the water, so I decided to go ahead and build one. I arbitrarily chose to use a hull of approximately 20 inches long and 10 inches wide. Does this seem like a reasonable hull size for carrying the 1/2 pound cargo as fast as possible without being too big?

After much research, I decided to run a Turnigy XK2845-3650KV Brushless Inrunner motor using a 40A Brushless Boat ESC off a Turnigy 1300 mAh 4S battery. This is about all I could afford for my price range, and I'm not 100% sure these choices will be best for the airboat, so I'm open to suggestions.

Based on this information, what would be a fairly cheap prop that I could use to maximize speed? Thanks!
Old 04-05-2016, 10:33 AM
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What is the cargo? Why is the cargo? Why no directional control for 30 seconds?
Old 04-05-2016, 11:19 AM
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The cargo is just a medium sized orange, so from 1/4 pound up to 1/2 pound. This is for a university project where we are racing to carry an orange to the finish line. There is no directional control because the boat needs to be autonomous, so I'm just going to turn the rudder to a fixed angle so that it turns in a very rough circle shape.

Last edited by Aeronautic13; 04-06-2016 at 09:23 AM. Reason: Typo and clarification
Old 04-06-2016, 08:26 AM
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14.8 v x 40 amps = 563.2 watts
I don't think you need to bother with a water cooled esc in an airboat.
I would use a 4 x 1 (or longer) ratio on length for directional stability and home depot foam for the hull 3' x 4' x 2" (white or wood glue works well on this)
speed will be drag limited by cross sectional frontal area (mostly)
I don't think you need or will benefit from a high RPM motor. Something in the Outrunner 1200 to 1400 kv will due nice with the right prop.
Props are where your going to need to experiment.
For the prop think balance, it needs to be low enough in pitch to get it moving quickly from a dead stop. High enough in pitch to give you good top speed.
A starting point might be 10 x 10 e. So make it easy to change props quickly and have 1 of each in a large range :
7x14 7x12 8x12 8x10 10x12 11x8 11x7 12x7 these are just quick ideas as example of how your purchase should go.

Hopefully some more experienced help will chime in.

Last edited by kmeyers; 04-06-2016 at 08:42 AM.
Old 04-06-2016, 09:48 AM
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Aeronautic13
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Wow, thanks for all of that good information. I'm having trouble finding parts online that I can get shipped to me within 2 or 3 days so I have time to build and test. Plus, I only have about $60 to buy the motor, ESC, battery, servo tester, and prop (I don't have to factor shipping cost into my budget). Do you know which parts I could get that would fit into that budget and still get about 500W of power with around a 1200 to 1400 KV motor? Also, which RC sites have pretty fast shipping time (to the US)?

The airboat will be turning in a large circle (approximately 50 to 75 feet in radius in case that matters). I was planning on just using a fixed angle air rudder behind the prop, is that feasible for this situation? Also, the length of my boat is restricted, and I don't want to restrict the width of the boat too much to accomodate a good size propeller. So, given the same motor, ESC, and battery, would you recommend a 8 inch x 30 inch boat with an 8 inch diameter or a 10 inch x 30 inch boat with a 10 inch propeller? Considering front cross sectional area and prop size (and the turn radius if that matters at all).

Thanks for all your help!

Last edited by Aeronautic13; 04-06-2016 at 09:54 AM.
Old 04-06-2016, 10:06 AM
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Also, for $60 would I be better off with a brushless motor, ESC, battery, and servo tester (for the ESC); or a brushed motor and a battery? And looking at a 4 to 1 ratio of length to width just seems unnaturally long. I should think a 2 or 2.5 to 1 ratio would be more correct, but what do I know? Doesn't a 30 inch long boat seem a little overkill for just carrying an orange?

Last edited by Aeronautic13; 04-06-2016 at 11:27 AM.

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