Hi Mark,

Thank you for an interesting post. It’s quite a coincidence actually as there is a possibility that there will be another Special Issue of Model Boats next year and I was only discussing yesterday the possibility of putting in something about choosing motors and props!

You talk about ‘starting with scale displacement hulls’ but that is in fact the most difficult area. With competition models there are class rules which generally determine the size and shape of the hulls and power options. There is usually only one prop and it is intended to push the boat as fast as possible in one direction only. As the models are competing against each other, the best power drive configurations sort themselves out and there is plenty of evidence as to what setup works best which beginners can tap into.

When it comes to scale hulls you are in an entirely different ball game as the number of variables needed to establish a satisfactory power configuration increases exponentially.

Firstly there is the overall size of the model expressed in both terms of dimensions and displacement. Then there is the shape of the hull itself, a model tug will need more power to push it along than a destroyer of similar weight. But then within the range of scales we build to the destroyer would be expected to actually go faster than the tug to give a realistic appearance on the pond. As far as props are concerned, these are available in a wide range of diameters, blade configurations and pitch, all of which affect the power generated to propel the model. On top of this, your choice of prop may be constrained by what is scale in appearance rather than being able to opt for the most ‘efficient’ one which can require more or less than optimum power to be provided. Some props give good forward thrust but are pretty useless in reverse and a scale boat needs to be able to stop and manoeuvre effectively. Many models have two props and motors (some three or four!) which obviously need a different power input to that of a single screw vessel – but you can bet it won’t be half the power of the single for each of the twin motors!

Electric motors usually benefit from being geared down but gearboxes at our model sizes tend to be both inefficient and noisy. A satisfactory alternative, which I use, is to fit a belt drive. Others prefer not to bother with this extra complication and plump for a larger motor/smaller prop combination which usually works fine in practice if possibly at the expense of higher power consumption – but if you are using lead acid batteries as ballast this doesn’t matter.

Trying to calculate all these parameters for a particular boat would be way beyond the majority of us modellers so, being practical types, we cast about for empirical solutions and usually the most effective is to find another boat of around the same size, shape and dimensions which performs well and use that as a guide. Sometimes this doesn’t always work though and two apparently similar models can perform very differently. This is probably due to the detailed hull form, particularly at the stern, and the way in which the prop interacts with the water flowing around the hull.

So, in practical terms I rather feel that the best we could hope for is to produce a table of commonly built models together with the motor/prop/battery combinations that work for them and people building other models could use these as a base reference for fitting out their own boats. As Tony says above, the kit manufacturers usually give practical guidelines for their models based on experience and these are also a good starting point. However, these are my own personal views and I’m sure that other members will be happy to pitch in with their own which may be different.

Colin