Sizing the system

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What steps should you go through to size the different parts of an electric boat before attempting to put it all together?

The major modules that need to be 'sized' are:

  1. the electric motor - the key parameters being power output, voltage and rpm.
  2. the propeller - diameter, pitch and perhaps material.
  3. the drive train (if the motor rpm are such that the propellor cannot be directly driven).
  4. the electronic controller - how to choose its rating and type.
  5. the battery pack - construction, capacity and voltage.
  6. the interconnecting wires - their diameters need to suit the currents being carried.
  7. the solar array - if your boat area can justify one and your budget can cover the cost.

1. Sizing the motor

I am going to follow the approach set out by Cedric Lynch in his paper Design of Electric Drives for Boats (needs Adobe Acrobat), since I think that his idea of measuring the drag of the hull is the most reliable way to establish the motor power needed for a given cruising speed. And that really is the crux of the matter - in order to know what size of motor you need you have to know your cruising speed and the drag of your hullform at that speed. Another view of this topic can be found at Solar Navigator

Cedric suggests that it is a relatively straightforward matter to arrange to tow the target hull using a spring balance to measure the pound-force or kilogram-force required at different hull speeds. This needs to be done in still waters, ideally in low- or no-wind conditions, and a constant speed should be maintained for the duration of each run. The speed would be most accurately measured by recording the elapsed time over a measured distance (two points at least 10 metres apart). Between 3 and 6 speeds above and below the target cruising speed would be enough to produce a usable chart. If you are concerned about the effects of wind and current, then take the average of two trips, one in each direction (an alternative description of this type of trial is to be found here). You can see the results of just such an excercise in this extract from Cedric's paper: Drag curve for 4 typical hull shapes

It is worth noting at this point that if your intended hull is similar to one of those in the chart, you could base your sizing on those results, perhaps allowing a margin for error. In case anyone needs to follow the calculations through from the beginning, they are as follows:

	The spring balance readings are taken in either 
	kilograms or pounds and are converted to Newtons by
		1kg = 9.81 Newtons
		1lb = 4.45 Newtons (1kg = 2.2 lbs)

	Newtons are converted to power by the speed of the hull:
		Power (watts) = Newtons * metres/sec

What you now know is how much power your hull needs to achieve cruising speed.
Essential though this is, it is far from the complete story, and several other factors need to be taken into account if you are to get an accurate estimate of the design motor power.

Before doing that, it is worth pointing out the very low powers needed (0.5 to 1 hp) to achieve the cruising speeds typical of an electrically-powered craft. I.C. engine zealots tend to be disbelieving that such low powers could be of any use at all in driving a boat. What they forget is that the power required increases approximately as the cube of the hull speed, so that very small speed increases need major power increases, as the boat speed approaches its hull speed. Other significant loss factors for an I.C. engine are in the propellors used (small diameter at high revs.) and in the gear box and drive train. When these factors are combined, it isn't suprising that motors of 20 hp or more are often fitted to quite small boats (see Lynch .pdf above).

2. Optimising the propeller

In this section, I am going to cheat. Having gone to the trouble of getting hold of David Gerr's 'Propeller Handbook' (Nautical Books - A & C Black, ISBN 0 7136 5751 0), and contacting P.J.M. Bollen the retired chief designer of Bruntons Propellers, with the intention of learning enough on the subject to give some advice, I have discovered a site that does far more justice to this topic than I can hope to in any sensible timescale.

So if you want to know how to estimate the propellor dimensions for your particular boat, you probably can't do better than extract the wealth of detail on the subject to be found at SurfProp. The site is laid out with a precise logic; each concept is isolated and discussed on its own and then all the topics are brought together with an Excel spreadsheet to allow you to enter your own particular boat parameters.

However, what I will do, for those builders who only want an estimate of the likely size of propeller for a proposed design, is to list the results of the SurfProp calculations for each of the three sizes of electric craft described on another page

One other link that may be of value is at the Pedal Power site, where Phillip Thiel (a Naval Architect) describes how to create a wooden propeller from ply. laminates.

< To be continued >


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Created: July 2002.