Right: now to your model. I have absolutely no idea what power is required to move your hull through the water as what speed, so I have to take it from Model Slipway that a pair of Speed 600 Eco motors provides the right power.

I know that racers want to operate at peak power for a short time, but am assuming that scale modellers want to run for quite a while around the pond, showing off their handy work (I know I do). So we want to run at a greater efficiency than peak power, but probably at a better performance than peak efficiency. I’ve chosen half way up the torque curve to the peak power point as the best scale operating point. That’s 25% of peak torque and 75% of no-load speed.

(For the musical scale model tuners out there, that means operating on the pond three full notes below the no-load note.)

At the best scale speed we’re operating at 75% of peak power and 92% of maximum efficiency, so that’s a good place to be in my opinion. At that point the motor is running at 75% of 11,000 rpm = 8,250 rpm.

I put all my figures into one giant Excel spreadsheet and fiddled until I got reasonable values, so I won’t derive them here (message me with an email address if you want the spreadsheet). But here’s how the values I chose pan out.

With a 2:1 gearbox, the prop is turning at 4125 rpm.

With a 30mm prop, the circumference is pi x 30 = 94mm. Multiplying by 4125 rpm, gives a prop tip speed of 396,000 mm per minute, which is 396 metres per min. Dividing by 60 gives 6.6 metres/sec. Having analysed the Raboesch recommended max prop speeds, they equate to a max tip speed of 18 m/s, so 6.6 m/s will be well within acceptable limits.

So now to calculate boat speed. Again we are relying on Model Slipway’s advice here. But if the Speed 600 Eco is operating at a reasonable efficiency, and the power is well matched to the hull, then the prop slip must be less than 30%. I have taken 30% as a conservative value.

A standard prop has a 1:1 pitch ratio, which means that if the prop circumference is 94mm then it moves the water 94mm in one revolution. That makes the sums easier. If the prop slip is 30% then the boat moves 70% of the prop pitch per revolution, which is 66mm. 66mm at 4125 rpm gives a forward speed of 272,250mm per minute wich is 272 metres per minute. Dividing by 60 gives Multiplying by 60 gives 4.5 metres per second. If you can imagine your Tamar moving 14 feet every second, that’s a fair lick for a scale model I’d say! Multiplying 272 by 60 gives metres per hour, and dividing by 1820 gives nautical miles per hour, which is knots. Doing this sum gives a model speed of 9 knots. To adjust for scale, we multiply by the square root of scale (root 16 = 4) which gives 36 knots.

The prototype runs at 25 knots, so 36 knots scale speed is rather fast. But you don’t have to run flat out, and as Ashley Needham wisely points out, it is better to have a little extra in hand for getting out of trouble.

So that’s how I arrive at my performance expectations. Hope I haven’t gone on too much. Given the amount of research and knowledge scale modellers have of their subject, and since apart from modelling motor performance all this involves just multiplication and division, I’m hoping some of this will rub off on the community.

I’m sitting outside with my laptop at 10:30pm in front of a chiminea, burning logs and sipping a wee dram, so although I trust my thinking I know I make mistakes with the numbers. So if anyone has the inclination to check my numbers and let me know either way, I’d appreciate that.

(I hope you don’t need aspirin too much)