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[Colin Bishop]

There has been quite a lot of recent discussion about motor and propeller sizes and how to get the correct combination for particular boats. Key factors quoted have included prop diameter, pitch, number of blades and blade area while on the motor side it has been about torque, number of windings, size and quoted power output.

Something equally important and which has been almost entirely overlooked is the driveline arrangement. I would guess that 95% of our models rely on direct drive so that the propeller rotates at the same speed as the motor and that is in fact where a lot of the mismatching between the two arises. For example, the suggestion that if you have a large diameter high pitch prop then you need a big torquey motor to drive it. That is not in fact the case, you could use a much lower torque motor geared down which will work equally well and use a lot less power in the process.

If you look at full size practice then with the exception of reciprocating steam engines and low speed marine diesels a common characteristic is that the prime mover, engine, turbine or motor rotates much more rapidly than the output device, propeller or wheel as it may be. The same holds true for a pedal bike! This is because the prime mover is most efficient at high speed whilst the output needs to be a lot slower for efficiency, prop slip, wheelies etc. So, in our boats if you can have the motor rotating faster than the prop then you are gaining a lot of efficiency and torque for free and can have smaller batteries or longer running times.

I know a lot of people don’t much like the idea of gearing as they think it complicates the driveline, is tricky to set up and can be noisy but none of this need apply and there are advantages too in where you locate the motor and it can be anywhere around the inner end of the propshaft, to the side, above in front or behind. In one of my boats the motor is suspended from the underside of the deck!

Only a large and a small pinion is needed for gears and plastic ones if carefully mounted so that they can be adjusted will make no more noise than a conventional coupling. A pulley drive can be almost silent and toothed belts or simple ‘O’ rings can also be easily tensioned to give the least friction. This can be measured by running the motor on a low voltage so it is just ticking over and including an ammeter in the circuit. Then adjust the tension to get the motor running as fast as possible without it slipping when you place your finger against the prop.

How much to gear down? Well, the usual range for a brushed motor is 2:1 – 2.5:1 but with a brushless motor that likes high revs then something a bit higher may be needed. But any reduction of speed between motor and shaft will bring about a worthwhile efficiency gain.Gearing down is always worth considering for the benefits it can bring.

Colin

[Subculture]

Brushless motors work at low RPM just as well as brushed motors. You may get some cogging on a sensorless  set-up at extremely low rpm, but this rarely causes issues as a propeller acts like a fluid clutch. A sensored system behaves just like a mechanically commutated motor in my experience.

Gearing will always cost you efficiency, chain and toothed belt are most efficient (if correctly tensioned), gears are next with friction belts being lowest efficiency. With low power levels used in most scale models, it's not much to worry about.
Very low revving motors with heavy windings are also less efficient owing to the inductive resistance in their windings. This is where a single stage of gearing can bring about improvements in efficiency, but there is a fine line between the two.

Sometimes gearing is necessary owing to packaging/space restrictions.

Many model boat builders choose motors far larger than models require, especially when it comes to brushless motors.

[Tug Fanatic]

Only a large and a small pinion is needed for gears and plastic ones if carefully mounted so that they can be adjusted will make no more noise than a conventional coupling. A pulley drive can be almost silent and toothed belts or simple ‘O’ rings can also be easily tensioned to give the least friction. This can be measured by running the motor on a low voltage so it is just ticking over and including an ammeter in the circuit. Then adjust the tension to get the motor running as fast as possible without it slipping when you place your finger against the prop.

How much to gear down? Well, the usual range for a brushed motor is 2:1 – 2.5:1 but with a brushless motor that likes high revs then something a bit higher may be needed. But any reduction of speed between motor and shaft will bring about a worthwhile efficiency gain.Gearing down is always worth considering for the benefits it can bring.

Colin

I agree that gearing definitely has its place and that belt drive & toothed pulleys are very flexible methods of connecting motors & propshafts even if reduction is not required.

I have always avoided gearing because of the meshing requirement whereas belts & O rings have a little more flexibility. How many of us have tried home made gear transmission? I suspect that the "if carefully mounted" and "easily tensioned" parts of the quote that I have highlighted above might require a few examples and explanation! Personally I have always found an O ring to fit my pulley spacing rather than adjust the pulley spacing to fit an O ring.

[raflaunches]

I have used belt drive in two models, one set up was on a 3 prop layout where the port motor powers the centre shaft. On my current build of HMS Invincible I have used two motors to power four props by two belt drives. They have proven to be virtually silent when running, and the 3 prop-er has been used almost continually for 14 years without mishap. As long as you set the shafts up to accept the drive belts and gears they will run smoothly- too much tightness and it’ll drain your battery very quickly and vise-versa too loose and they’ll slip and will not drive the shaft. It’s a fine balance but if used and built properly it’s works brilliantly well.

[NickelBelter]

A lot of people are used to the gears in various cheap toy vehicles that make an excellent impression of a gas turbine as they squeal away. 

Personally the cost is what keeps me away as a good gearbox for a 500-size motor costs several times what the motor itself does, and getting tiny toothed belts and pulleys from a place like SDP-SI is not any cheaper. 

[Colin Bishop]

You don't need a commercial gearbox for a simple single moderate ratio reduction with just two gearwheels but I do agree that getting the meshing exactly right to avoid frictional losses is more tricky than with belts as the tolerances are much smaller. With belts, it is easy to fine tune the tension. If the motor mount is above the propshaft then all that is needed is a pair of runners each side into which the mount can be screwed and its height adjusted by spacers which might be thin pieces of wood or simply washers. If the motor is mounted to one side then it can be screwed to a flat or angled horizontal plywood plate mounted on th bottom/side of the hull. Hold the motor in position with the belts attached and run at slow speed, moving the motor in and out until you get to the sweet spot then mark this through the motor mount holes and screw the motor down. Even finer adjustment can be achieved if you open up the motor mount baseplate holes so that the screws are a loose fit and this gives you a bit more wriggle room before tightening down. More sophisticated methods entailing clamping the motor mount to slots in the hull can also be devised.

One other advantage with both gears and belts it that you only have to make the adjustment in one plane (dimension) you just move the motor away or towards the shaft, that's all.

But if you are using a conventional direct coupling then it has to be accurately lined up with the shaft at all angles and this is often what causes people grief and introduces noise and inefficiency.

I have heard concern expressed that the pull from a belt could wear the upper shaft bearing unevenly but in practice this is only likely to happen if you have excessive tension on the belt. I have never experienced any problems myself and I suspect you would have more likelihood of a badly adjusted conventional coupling opening out the bearing from not being accurately centered.

I am not saying that belts or gears should be used in every circumstance but they do offer useful alternatives to more conventional setups in many respects in scale models.

Colin

[Tug Fanatic]

It is nearly too obvious to say but the trick with using O rings is to get the profile of the pulley groove to match the profile of the O ring so that the two are in maximum contact. Ideally you don't want a polished pulley surface but rather one that wants to hold the O ring rather than slip past it.

On my heater motor tugs I have used up to three O rings in parallel . I have both purchased ready made O rings (preferred) or made up my own from cord & glued them with super glue.

For small motors (385 etc) I have seen ordinary elastic bands used which seem to work just fine