How much motor do I need?

[ElectricOnly]

Is there a rule of thumb to how many watts a motor needs to put out to weight or size of a boat. Also if different mention for two motor setup as well. In the 3D airplanes the rule of thumb is 150W/LB of aircraft.

Thanks

[Fluid]

There really isn't such a rule in FE. Too much depends on hull design and application. Instead of "rules of thumb" (which are often far too simplistic anyway) we rely on more empirical data - what works, what does not. Since watts is a function of voltage, the cell count matters a lot.

You can have two 1500 watt setups, one of which will not work well in a larger boat. 100 amps at 14.8 volts will move a large boat much better than 200 amps at 7.4 volts, yet both have the same "power" in watts.


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[Boaterguy]

A specific boat and other setup would be necessary, such as the boat, the batts, and the esc.

[steveo]

i guess you would have to consider design of the boat as well mono, cat, hydro- if given all 3 the same setup all would have different speeds

[ElectricOnly]

Fluid

I want to know how much motor I need for a MHZ Mystic 54" Cat on twins running 8, 10 or 12 cell setups. Want to go fast but would like to see some run time as well if possible.

There really isn't such a rule in FE. Too much depends on hull design and application. Instead of "rules of thumb" (which are often far too simplistic anyway) we rely on more empirical data - what works, what does not. Since watts is a function of voltage, the cell count matters a lot.

You can have two 1500 watt setups, one of which will not work well in a larger boat. 100 amps at 14.8 volts will move a large boat much better than 200 amps at 7.4 volts, yet both have the same "power" in watts.


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[Fluid]

Okay, now we know the hull. But "fast" and "some run time"...help us out here. Will you be happy with 45 mph? 55 mph? 65 mph? How much can you spend? 12S setups aren't cheap.

If you want a wattage value, then shoot for 5000 watts minimum. On 12S that's 115 amps average, so with 10,000 mAh battery capacity you would have four minutes of full-throttle run time, longer with less full throttle or with a smaller prop. Speed should be 45-50 mph depending on component quality and setup. You will want a large motor for that hull, something like a Neu 2230. Twins would not be out of the question, but that increases the cost considerably.


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[ElectricOnly]

Fluid I am very confused.

I can not figure this calculating thing out. I saw another 54" Mystic on here with the following. Leapoard 4082 x 2 each powered with a single 6S 5000mAh pack through Turnigy 180A ESCs spinning I believe X442 L/R props. The boat would do 80MPH for 2 minutes.

I thought if I went with bigger motors and higher cell count it would take less power because the motors are stronger and have a longer run time with the increase in cell count.

Okay, now we know the hull. But "fast" and "some run time"...help us out here. Will you be happy with 45 mph? 55 mph? 65 mph? How much can you spend? 12S setups aren't cheap.

If you want a wattage value, then shoot for 5000 watts minimum. On 12S that's 115 amps average, so with 10,000 mAh battery capacity you would have four minutes of full-throttle run time, longer with less full throttle or with a smaller prop. Speed should be 45-50 mph depending on component quality and setup. You will want a large motor for that hull, something like a Neu 2230. Twins would not be out of the question, but that increases the cost considerably.


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[Boaterguy]

Well, If you are putting 12S through them, you will be drawing A LOT of current, I would go with 200A esc's MINIMUM. if you are doing 6S on each motor, that's different, because the circuits are completely sepearate

[ElectricOnly]

I do not understand.

with airplanes and helicopters as the cell count increases the amp draw decreases so I can not figure why in the boats it is opposite to what is true in the air

[Fluid]

I thought if I went with bigger motors and higher cell count it would take less power because the motors are stronger and have a longer run time with the increase in cell count.

Motors have zero 'power', all 'power' is in the cells. Power is watts, not amps. Watts = amps x volts. Going to higher voltages with the same amp draw gives more power. Keeping the watts the same when increasing the voltage does mean lower amps which equals longer run times. Motor size has relatively little to do with amp draw except that larger motors will handle higher amps due to their mass. Larger motors are also needed to spin larger props for that huge hull.
This statement is wrong:

Well, If you are putting 12S through them, you will be drawing A LOT of current.

You will draw more amps only if you load the motors to draw more. Lots of newbies assume that you increase the voltage to increase the power. A better approach for most FE Boaters is to increase the voltage and reduce the amps. This reduces the stress on all components and gives longer run times. The problem is you cannot accurately assume an amp draw from a "rule of thumb". The software FECalc can give an estimate but it is obsolete in that it was designed for brushed motors and nickel cells, not LiPos. Sometimes it predicts well, other times it is not very close.

with airplanes and helicopters as the cell count increases the amp draw decreases...

This is wrong too. The amp draw will increase if all you do is increase the voltage/cell count. You have to reduce the load to keep the amps the same or less. This is what happens in planes, helis, cars and boats. The vehicle does not matter.

I know you want a simple answer to all this, but the reality is there is no simple answer. That is why so many FE boaters have trouble burning up their equipment, they have no idea what their amp draw is and all they want is higher speed. Eventually they realize that the power needed is a balance/compromise.



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[Boaterguy]

He is right, I must've been tired or worded wrong (i forget so it was probably tired )
anyways, you will want to go with a high rated (probably high voltage) esc, I don't know the specs on the swordfish ones but the 240 might take 12S, the high voltage will.

[siberianhusky]

No boaterguy it's called you don't know what you're talking about, you need to get a bit of experience and do some learning before you spew forth on every post.
You have been told on other forums, don't start out this way on this one!

[Boaterguy]

alright, thanks.

[ElectricOnly]

Fluid

If W=A*V and if the load remains the same producing say 4000 watts if you plug in the equation a 3S 11.1V pack and a 6S 22.2V pack and you calculate for amperage A=W/V and all things being equal the 3S at 360A is clearly a higher Amp than the 6S at 180A.

With my helicopters I know that if I want to run my 630mm blade at +/-13 pitch with the same motor rated for 4000 watts on a 6S pack I will need a 120A ESC but if I run it on a 12S pack I will only need a 80A ESC. So I am still confused as to what you say.

I understand what you say about a motor having no power. The power is makes is a direct relation to how much load you put on it. I am not looking or expect a simple answer but I do want to learn how to make sense of it with out trial and error. I have to think there is some way to figure out in the ball park range with out trial and error (not to mention $$$$) or going off another setup someone says works.