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Question about boat motor+gearbox

Old 12-01-2015, 05:27 AM
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malteser2
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Default Question about boat motor+gearbox

Hi, I am new to scale model building.

I've built a 1m scale ship using balsa wood. I will be using one motor with a 50:1 gearbox. It does around 320rpm at 12v and 160rpm at 6v. Would the ship be very slow with this motor and gearbox combination ? I can use 30mm or 40mm propellers.
Old 12-01-2015, 09:55 AM
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mfr02
 
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It will be incredibly slow, deeply frustrating and boring. That kind of RPM is OK for paddlers, but not for props.
A 1M boat with a 40mm prop needs something like a direct drive 545 motor running on 6 volts if it is a displacement model, more volts and a smaller prop if it is going to be a fast boat. A model needs a faster rotating prop than the real thing to give the appropriate looking speed. (multiply RPM by the square root of the scale).
Old 12-01-2015, 10:04 AM
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malteser2
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I don't need my model ship to be fast, since it is not a speed boat of course. Any idea how much RPM would be needed to make it look appropriate please?
Old 12-02-2015, 02:24 AM
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mfr02
 
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In about 30 years of boating, I've never had a thought about putting numbers on prop RPM, but oddly, matching the motor, hull, prop and power supply has never been a problem. Without knowing what your model is of, and its scale, any reply is pure guesswork, but -
A 1:100 destroyer, real props turning at @300, model props need to turn at least 3000. Small prop, turns fast, long narrow water column,
A 1:100 freighter of the same length might have a prop that turns at @80, model turns at 800. Large prop, moves a lot of water each turn, but not far.
For a boat to move forward by its waterline length in a set length of time, the prop needs to move water equal to the submerged volume of the hull back by the hull length in that time (at least, add a large percentage for slippage). A long way for a fast hull, but not much volume, not as far for the freighter, but a lot more volume to shift.
OK you are not making a speedboat, but I have to assume that you would like to get it back against a light breeze. Most of us do. A light breeze for a full size person is a strong wind for a model.
What is the model of? What is your proposed motor+gearbox?
Old 12-02-2015, 02:34 AM
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malteser2
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The model is 1m long and it is made of balsa wood. Would also have a 1.3kg battery on it, plus the motor and some electronics. I think that you are right, a 50:1 gearbox (max 320 RPM) on a 545 motor would be too slow. I am now considering a 545 motor with a 6:1 gearbox, with max. 2660 RPM.

I am more interested in the ship's maneuvers rather than speed, but as you pointed out, it must overcome a light breeze, while I don't want it to be boringly slow. Do you think the 6:1 ratio would be better for this application ?
Old 12-03-2015, 01:43 AM
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That is what a model supplier over here recommended for use in a Revell Snowberry a few years ago (although he might have used a 540), so it should be just fine on yours (roughly the same size and weight). I used one on direct drive and about half its rated volts in mine, also a 385 with very similar results. It has always performed well, but having a good controller and remembering that the stick is a linear control that does not need to be flung against its end stops all of the time and not a switch allows very precise control when maneuvering. Remember that a propeller is a part of the transmission system, and acts as a slipping clutch most of the time, compensating for a lot of mismatches further back.
The big trick is to avoid over-thinking yourself into problems that didn't exist before.

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