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Author Topic: Power / Torque calculator.  (Read 1239 times)

nivapilot

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Power / Torque calculator.
« on: August 02, 2015, 08:22:34 AM »

Probably been covered before, but here goes.
I am a complete noobie to boats, and was wondering, where in gods name do you start?
Is there a calculator, or guide sheet that will give at least a start point, for determining the Motor and prop sizes?

I'm retired and don't want to build a boat, thinking of a fairy Hunter or Swordsman, which I know are very well documented and sizes already worked out.
But, if I were to build, say a 36" ply constructed luxury fast motor cruiser, where do I start to find out what and how much power, without trial and error. I can't afford to get it wrong too many times before I break the bank....or worse, lose faith completely.


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inertia

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Re: Power / Torque calculator.
« Reply #1 on: August 02, 2015, 10:02:07 AM »

How long is a piece of string? What you're asking for isn't available because the factors involved are a lot more than just boat length and hull type. The only thing more or less constant is the density of water.
The place to start is with another boat of the same or very similar lines and performance. As long as you avoid silly things like fitting lead-acid batteries into a hydroplane (too heavy and not powerful enough to deliver high current) or a 50mm 4-blade brass prop on a 385 motor then installing the same components should give a similar performance. From there you can tweak things like prop diameter and pitch, weight distribution, ballast etc until you get exactly what you want.
Sorry but there's no Holy Grail here either!
For a 36" fast planing hull you might start with something like this motor http://www.componentshop.co.uk/lc3536-8t-960kv-leopard-outrunner-brushless-motor.html running from a 3S LiPo or 10 cell NiMH battery pack with a 60A speed controller and a 40mm 2-blade prop. You can increase the speed by upping the supply voltage or changing the prop - or vice versa. The radio will be the same whatever motor/prop/ESC combination you chose, as will such items as a charger. Others might read this and suggest a better alternative - they're welcome to do that, because that's what the forum is for.
There is a rough guide for brushed motors at the end of this article http://www.modelboats.co.uk/news/article/but-i-don%27t-understand-electronics/18054 but I must point out that it is only a practical suggestion and by no means The Only Way.
Dave M
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malcolmfrary

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Re: Power / Torque calculator.
« Reply #2 on: August 02, 2015, 03:59:29 PM »

A thing to bear in mind is that when a full size boat is made, very often the prospective owner will specify the motor to go into the hull that he is buying.  For any given hull, a wide range of motive power could be fitted, so there can only be a quite wide range of motors that would "suit" that hull.  If making a model of a particular boat, it helps to know the actual HP of the original.  This does scale very well as long as you remember that IC engine figures give power out - the figures that you find for model electric motors are invariably power IN in Watts, so due allowance needs to be made for conversion inefficiencies and whatever the shaft and prop do for, or at, you.
Long established rules of thumb that work say that the prop diameter should not exceed the motor can diameter, and the motor should have more poles than the prop has blades.  This generally works for direct coupled brush motors, the coin is still up and spinning regarding brushless motors.
Back in the olde days, when there was a range of maybe 4-5 motors from the shops, they were noted as being suited for a range of hulls, and generally sort of worked.  Some still do.  Now that there is a vast amount of choice, there is much more scope for mistakes.


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"Trying is the first step towards failure" - Homer (Simpson)

nivapilot

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Re: Power / Torque calculator.
« Reply #3 on: August 02, 2015, 06:51:42 PM »

Many thanks. Bit more research then.
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Time Bandit

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Re: Power / Torque calculator.
« Reply #4 on: August 02, 2015, 07:19:25 PM »

Itīs pretty easy to calculate power requirements for slow boats.

Displacing hull needs around 1.5W per lb weight to reach its hull speed https://en.wikipedia.org/wiki/Hull_speed.
Half gliding hulls need around 8-12W per lb weight to reach around 2.5 times their hull speed.

Anything above these speeds is difficult.
While a full planing scale yacht needs maybe 25W per lb to reach scale speed, high powered racing boats sometimes use 1kW per lb.

A rule of thumb is to divide the original power / scale^3 to get the power to reach scale speed. This is completely incorrect since the speed is left out of that calculation (should be scale^3.5) but gives pretty good values, probabaly because of lower efficiency of our smaller drives.
Best plan I believe is to follow others, if a drivetrain on a similar model works, you should just copy it.
Youtube videos .... help a lot here.

Another way is to try to calculate at least the intended speed, which is pretty easy to avoid the biggest errors.

Speed [mph] = Motor rpm [1/min] * prop pitch [m] / 60 *2.23 (to get mph out of m/s) * 0.7 (prop slip in the water).

So if you calculate a speed of 60mph with the motor you want to use its wrong unless you are buildig a SAW boat  {-)





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regards

Tobias

inertia

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Re: Power / Torque calculator.
« Reply #5 on: August 02, 2015, 07:35:47 PM »

"The place to start is with another boat of the same or very similar lines and performance".

"Best plan I believe is to follow others, if a drivetrain on a similar model works, you should just copy it".

See? It's not just great minds that think alike........................... 8)
DM
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