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Electric propulsion is ideal for use on inland waterways, where speeds are restricted. It is quiet, clean, efficient and cheap. However, the point which many people find baffling is how much power to use.

Page Contents

• Efficiency
• Power required
• Measuring drag factor of a boat
• Motor and Controller ratings
• Recharging
• Solar power

Efficiency

Electric boating is all about efficient use of power. Efficiency in a boat comes from 2 areas:

• Hull design
• Propeller

You cannot do much about the hull design of the boat you are going to convert. However, for new boats, hull design is all important. There are many sites on the ww showing you how to calculate the hull drag factor.

For new, and conversions, you can (and should) pay attention to the propeller. Good propellers for electric boats tend to be larger than otherwise and operate slower to increase drive efficiency. Your propeller manufacturer should be able to supply details of its performance.

However, with the correct boat and propeller, results can be excellent. A 30 foot boat can cruise quite comfortably at 4 mph on a power of 500 watts.

Power required

There are two basic ways to find the power required:

• Calculation.
  There are several pages on the www telling you how to calculate the drag factor for a boat.
• Measurement

Measuring drag factor of a boat

The empirical method is very easy: simply tow the boat to be measured behind another boat, and use a spring balance in the tow rope to measure the force required.

You will end up with a graph something like that below (note: this is not a graph for any particular boat, simply to illustrate the expected shape.)

1. Decide on the maximum speed you want: Vmax (perhaps, 5 knots).
2. At this speed, if you have measured a force F (in kilograms - perhaps 100kG), then you can calculate the power that this represents since
   • 1 knot is 0,51444 metres/second
   • As far as the spring balance is concerned, a 'weight' of 1kg is a force of 9.98 Newtons
   • Power is force x speed: precisely 1 Watt = 1 Newton x Metre / Second
3. So the power here (100kg at 5 knots) is F x Vmax x 9.98
   or 100 x 5 x 0.51444 x 9.98 = 2567 watts.

In practise, you would allow a significant safety margin on this power, to allow for winds, tides and other unforeseens. So multiply this power by, perhaps, a factor of 2.

In our example, we would choose 5kW for motor and controller.

Power is volts times amps. Remember, that is motor volts times motor amps. If you gear the motor appropriately, then motor voltage will peak at battery voltage when motor current will equal battery current.

Controllers are nearly 100% efficient: typical controller losses are only around a few percent of the power they are handling. The controller acts as a transformer, feeding almost all power it takes from the battery through to the motor. So battery power is approximately equal to motor power.

Motor and Controller ratings

Most applications for electric drives are for land use, where sustained power is not required. High power is usually only needed for instance, to climb a hill. Boats are different: here we are only concerned with long term ability, say 15 minutes or more.

The motor

The controller

Recharging

You may like to think of alternative energy charging sources such as solar power, or wind power. It may also occur that you can use the propeller to recharge when under sail. The link is to a separate page on the subject!

Solar Power

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