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Motors for a BIG boat?

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Old 03-19-2018, 11:25 PM
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OutputCoupler
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Default Motors for a BIG boat?

As the title says, I'm working on a very large RC boat. A 16' catamaran, that should weigh around 400 pounds. Right now I'm looking at propulsion, and have some questions about RC motors I was hoping somebody could help me with.

If I've done my math correctly, then to push my boat at ~10 mph top speed then I need to spin a pair of ~6"x6" props at ~3000 RPM, which should take ~5 HP. My power is coming from a pair of very large deep cycle batteries, so nominal voltage is only 24V. So, a 125 kv motor that can produce ~1500W of power at 24V should do the trick, right?

Motors marketed for RC boats seem to mostly be aimed at much higher RPM, so I've been looking at plane motors, and it looks like something like the Turnigy Aerodrive SK3 149kv is pretty close (can't post a link, sorry).

149kv should produce around 3500 RPM on 24V. Giving it 24V of the expected 44V should result in ~55% max power, for around 1,250W, right?

Am I completely off in my math, here? Or, better yet, does somebody have a recommendation for a pair of motors that could meet my requirements?
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Old 03-20-2018, 01:50 AM
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At 100% efficiency, 1HP = 745Watts. A more likely efficiency figure would be around 75% (a bit better with brushless), so 2000W would probably be nearer. Consider gearing to match the motor to the prop, this will increase the possibilities of a workable match. At that size, with present day technology, internal combustion is probably a better bet.
Fuel motors quote power out, electric ones quote power in.
Big heavy lead batteries are not generally good for sustained high output, but are good for ballasting a boat down.
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Old 03-20-2018, 03:14 AM
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Why do I need gearing if the motor is already turning the prop at the speed I need? Seems there are plenty of motors that advertise about the right speed at about the right power.

Gas motors are out. Maximizing runtime is not my concern, reliability is. Also, the batteries have plenty of juice, they're 120 Ah each and can handle 900A sustained current draw. I have no intention of running the boat at full speed unless there's a very good reason to do so, like needing to outrun a storm to get back to shore. Otherwise, this will be plodding along at ~3 mph, which the batteries should be able to handle for much longer than I care to operate the boat at any one time.

Revisiting the math, I don't think it's worthwhile to bother with RC motors. Optimistically it would cost ~$500 for a pair after all is said and done, and give a top end speed of 10 mph. A pair of 24V trolling motors with high pitch props is about $300, and gives a top speed of ~7 mph. Using trolling motors is also going to be way simpler than using RC motors. The efficiency gain of brushless motors would be nice, but I'm not so sure extending a ~100km range to ~110km is all that meaningful, considering the added cost and complexity.

Last edited by OutputCoupler; 03-20-2018 at 03:42 AM.
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Old 03-20-2018, 06:01 AM
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There is more to motor performance than watts and rpm. The OP will likely never find an R/C motor capable of turning a 6” x 6” prop at 3000 rpm, certainly the noted Turnigy motors won’t do it as they lack the necessary torque. The recommendation of gears tries to address this by increasing available torque through gear reduction.

The OP’s needs are in the realm of trolling motors.


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Old 03-20-2018, 07:34 AM
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Originally Posted by Got RPM View Post

The OPs needs are in the realm of trolling .


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Could have stopped there.
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Old 03-20-2018, 10:14 AM
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Old 03-20-2018, 05:27 PM
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Originally Posted by rgburrill View Post
Could have stopped there.
I assure you, I'm being sincere. Just evaluating all my options for propulsion before I start buying parts. Given that these motors are in the same ballpark of size as a trolling motor engine, it seemed reasonable to evaluate their use.

Seems it was probably unwise to ask you guys for information so far outside your wheelhouse. I'll take my questions to a different forum, where they work with larger vessels.
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Old 03-20-2018, 08:43 PM
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First thing that came to mind was to use trolling motors - rather than reinvent the wheel. 10mph is surely within capabilities.
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Old 03-21-2018, 09:31 AM
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OP has probably gone now, but just thinking that 10mph might be a bit near hull speed on a 16' displacement hull, and two of the batteries described will be about 180 lb. Add the weight of the boat to carry a load like that, it becomes a lot of water to shift to get the speed, even if they are two very narrow hulls.
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