Power Selection for an Older Hydro

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Jeff Carter

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Joined
Jun 3, 2024
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Hello all, I am new to the electric powered RC world but have already managed to cook a couple of motors and ESC,s. So I humbly ask for your help, is there a formula for power selection ie Boat type x size = power required = Motor size = Battery size = ESC size = Prop size. I understand it probably won't be quite that simple. The boat I am fitting out is a Dumas - Miss Unlimited Hydro - she is 36'' long and should weigh 7.4lbs with all hardware installed. She has a solid prop shaft and the prop will be fully submerged. Thanks Jeff
 
This article is a little out of date but gives you an idea on the calculations.

Lohring Miller
 

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Thanks Lohring, I think I am starting to get my head around this, to maintain same revs- more battery with smaller KV = more torque - less battery with larger KV = less torque. Larger boat needs more torque, larger prop needs more torque.
 
I read that a Hydro hull needs about 30,000 RPM, I presume that is for a surface piercing prop, would that be the same for a fully submerged prop?
 
The safe solution is to pick a KV that gives around the same rpm as the original IC engine. On 4S that would be a KV of around 1800 for about 20,000 rpm. Pick a watt input that's similar and decide on a battery voltage. For your boat around 1500 watts is similar to the best you could get from a 30 nitro engine.

With a 4S pack that would mean around 100 amps. Use that current to pick an ESC with a 120 to 150 amp rating. Selecting the prop is harder. It's what sets the current draw. Start small (40 mm diameter) and work up. The best way to measure current draw is with an ESC that has data logging.

As long as the motor, battery, and ESC isn't hot (150 degrees F) after a couple minutes at full throttle the current is safe. An estimate of current draw is the mAH put in the battery after a full throttle run converted to ampere hours (divide by 1000) divided by the run time in hours.

Lohring Miller
 
There's actually a double loss factor in that calculation. First; batteries charge to 4.2 volts per cell or around 16 volts for a 4S pack. Second, I assume the loaded rpm will be around 80% of the nominal KV. See some actual data on my sport hydro below. The rated KV of a TenShock CZ11240 is 2680 so the rpm at 80% of 14.8 would be 31,730. Since it's actually closer to 40,000 rpm at full throttle the 80% correction is overkill.

Lohring Miller

Black Pearl 6 24.jpg
 
here's how I look at it. I convert everything to watts then find out what battery can output by testing the internal resistance and inputting that into the RC explained calculator. good batteries can put out 160A at storage voltage. so you takes the voltage and multiply it by the amps

under laod @ 3.6v per cell
4s x 160 = 2300w
6s x 160 = 3550w
8s x 160 = 4600w

you will size a motor accordingly based on its watt rating and choose a KV for 25,000 RPM.

watts will then translate directly into speed. drag coefficient, weight, setup, and water conditions being variables. lets very roughly say 3000w = 60mph, 4500w = 75mph, 2300w = 55mph. grossly speaking. That is how you will pick some props out with the speed calculator. it will at least get you in the ball park to not damage things. It also kinda takes the mystic out of electric when you can see on your data logs the watts and top speed relationship.

For your boat you will run 6s with a 4092 or a TP 4070 (4070 = 40107). pick a KV between 1250-1400 and get a WYE wind motor. The TP 4060 (= 4092) 1350kv is a good motor for that hull.
 
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