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Which Electric Motor?

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Which electric motor? How to choose the right one.

There is a huge selection of electric motors available today in model shops. Perhaps inevitably, these cover a very wide range of applications and prices, but when selecting a motor for use in any of the hulls featured on this website, the choice can usually be narrowed down to two or three basic types. These motors are designated 380 or 540 within the trade, and even the those at the lower end of the price range will give excellent service, if treated with a little respect. For most hulls around thirty inches (760mm) in length, the 380 types should be adequate, while above that size and for heavier or deeper displacement hulls like tugs, the 540 size would usually be a more appropriate choice.

This Schnellboot S-9 performs well with two 385 motors, one on each of the outer shafts, the central one being an unpowered dummy.

You may be confused to see both 380 and 385 motors on sale, as well as 540 and 545, but the explanation is simple. The 380 and 540s have three poles, or armature segments, while the 385 and 545 types are five pole. So which one should you choose? In my experience, a good 5 pole is usually better than a good 3 pole motor, at least for most scale type models. Five pole motors are smoother running, and this is can be a great advantage, as it will tend to be more controllable at low revs, especially with a good electronic speed controller. As well as this, a 5 pole will usually draw slightly less current. Three pole motors can perform better in higher speed models, as they should produce higher revs at any given voltage. There is also a third possibility, and if you want to construct a tug that might be used in towing competitions for instance, then the additional power of this third choice, the 550, might be more suitable, though current drain can be a good bit higher.

As this Boston Blenheim trawler is a heavy model that sits quite deep in the water, it needs a 545 or 540 motor to propel it at realistic speed.

At the time of writing, all these motors can be found on sale at around £5, or even less, and they are perfectly adequate for most of the hulls within the Kingston range. Rather more exotic motors of the same type are available for use in ‘buggies’ or ‘fast electric’ boats, but for our purposes they don’t really justify the considerable extra expense, and in any case the extra power of these motors will shorten running time appreciably by consuming more of the battery output.

This Brede lifeboat was originally powered by two high power Tamiya ex-buggy 540s, which gave high speed but limited duration. Later re-engined with a pair of Decaperms, it is still quite fast, but with much longer runs on the same battery pack.

When selecting an electric motor, do remember that if used with direct drive, the 380 motors will be reluctant to turn a prop of more than about 30mm, while the 540 type are usually good for up to 40mm props. This is because electric motors are essentially high-speed devices. Propellers above these dimensions will tend to overload the motor, which as a result may burn out. That said, there are products on the market, which allow the motor to be geared down. By gearing down, torque is vastly improved and the motor works at a higher, but more natural speed. Termed ‘Olympus drives’ these devices come with two toothed drive wheels connected by a matching drive belt. The motors are mounted on the appropriate ‘Olympus’ unit and the smaller toothed wheel is fitted on the output shaft of the motor. The belt is then connected to the larger wheel of the module, which in turn drives the prop shaft. The unit is designed so that the motor can be moved up or down to apply the correct tension to the drive belt.

In some ways, gearing down an electric motor can be likened to driving a car, which if driven in too high a gear can judder and perhaps stall. If a lower gear is selected the car will pull away quite happily with no detriment to engine or transmission. Having covered the most economic method of gearing down a motor there is a further option, and this is to purchase motors that have gearboxes already attached. These are of German origin for the most part, and come from the Marx ‘Monoperm’, ‘Hectaperm’ and ‘Decaperm’ range. The gearboxes are of the epicyclic type, and by altering various gear wheels within the box. different ratios can be selected. Although approaching the ideal, they are rather expensive. These gearboxes can be removed, and the motor used instead as a direct drive unit if required.

Although only 760mm long, this Eskgarth tug has a geared Decaperm motor, which makes it a really powerful model for tug towing competitions. A 550 would be a much cheaper alternative that would probably perform almost as well.

Other manufacturers such as Robbe offer motors with a step down gearbox and prop shaft, all incorporated in one single unit, but like the Marx motors, these are a fairly costly solution to the problem when compared to the ‘Olympus’ drives. Although compact and very efficient the disadvantage with these all in one units is that the propeller shaft is a fixed length, which might not always suit the intended application. There are also a number of other motors on the market from companies like Graupner, many of which follow a numbering sequence, 600, 700 etc. The higher the number, the higher the power output, and also of course the price. Where high speed is a requirement, then obviously the higher rated motors should be chosen.

Although small paddle wheels are not the most efficient means of model propulsion, a single Monoperm driving each wheel independently provides this Director paddle tug with more than enough power.

Another source of good electric motors suitable for powering model boats can often be from redundant electrical equipment or cars. These can often be really high quality powerful low drain motors for not much money. Living as I do quite close to what we would once have called a scrapyard, but which now prefers to call itself a ‘vehicle dismantling specialist’, I quite often receive visitors who have driven to the area, just to pick up something like a 1986 Ford Escort heater fan motor (though I made up that particular example, so please don’t quote me on this) or some such item for their latest model boat. It should of course be remembered that most of the electric motors used in cars are 12 volts, although in most cases they will run quite happily on six volts. If non-boating items like these are used, then a few experiments with running time and current drain will be required.

Even though these Windermere launch hulls are 1040mm long, they are so fine-lined that an ordinary 385 will be powerful enough for well beyond scale speed.

Some electric motors when purchased from model shops, will have built in motor mounts. Others will require some form of suitable mounting to be fabricated to fit motor and model. There are also a number of different size ready-made motor mounts available, but these are mostly aimed at the standard 380 and 540 type cylindrical case motors that this feature opened with. It is essential that any electric motor whether used as a direct drive or with a gearbox, should be connected to the prop shaft through some kind of universal joint or coupling. It is almost impossible to line up a motor 100% accurately, but a properly installed good quality universal joint will alleviate most alignment problems. Do take time to align the motor and prop shaft as accurately as you possibly can before fitting the coupling, as this is always time well spent. Never expect any universal joints to make up for any substantial incorrect alignment between motor and shaft though, as the energy absorbed by such misalignment can have a disastrous effect on power consumption, as well as overloading, heating up, and thereby possibly damaging the motor.

With twin prop shafts and a pair of 540 motors, this 1420mm Fairmile motor launch has to be throttled well back for realistic performance on the water.

Try not to use too small a motor for your model, it’s always better to have too much power rather than not enough, you don’t have to run at full power all the time, and speed controllers are so good these days that it’s a simple matter to throttle back.

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