Our electric boats are intended as displacement boats, that is they push their way through the water rather than planing (or skipping over the surface). This is the most energy efficient way of boating but limits the available speed. The theoretical hull speed (in knots) of a displacement boat with water line length L (feet) is calculated by the equation Speed = 1.3 x square root of L - for a 17ft boat, about 5.4kt and for a 20ft boat about 5.8kt. In fact once the speed goes above about 90% of this the bow wave increases, the boats starts to try to plane and the power required climbs rapidly. Economical cruising speed is generally  approximately equal to the square root of L unless the boat is heavy or poorly shaped when it may be lower.

Each of our boats is subjected to a performance test. The boat is loaded to its average load condition and is run in still conditions at a series of speeds up to the maximum. When the boat is moving consistently (as measured by GPS) the current being drawn from the batteries is measured. Since Power (W) = Current (Amps) x  Volts, we can draw a graph of power vs speed for that particular boat and drive system. This takes account of the effects of the hull shape, size and weight of the boat, and the efficiency of the drive system.  
To find more about these tests download Electric Boat Considerations

Typical results are shown below show clearly that speed consumes energy! Roughly speaking a 50% increase in speed leads to a doubling of the energy consumption! As the theoretical hull speed is reached the power required rises very steeply so a big increase in energy consumption gives only a very small increase in speed.

The Boats
Tahbilk           A 9m 2 motor ferry - punt hull shape makes for an inefficient boat using a lot of power to achieve modest speed. 2 ETEK inboards.
Duffy 21         A 6m launch. Relatively heavy for this size boat and the transom starts below water level adding to drag. Old style electric inboard
Woolwich 20   A 6m launch based on sailboat design; easily pushed with little power. various motor options.
Greenwich 17 A 5m launch based on an old motor launch design. PowerPod motor.
Dangar Is.      A 1940's timber launch similar to Greenwich but narrower. ETEK inboard.

The Drive Systems
Duffy                 Traditional style electric motor and shaft seal, belt drive, 15"x15" propeller.
ETEK Inboard  Permanent Magnet motor (Lynch design) with belt reduction gears, Packless Shaft Seal, and 15"x15" propeller.
B&S Outboard A traditional style electric outboard using ETEK motor mounted above bevel gears to turn drive 90 degrees..
PowerPod          A submersible electric motor mounted beneath the boat (similar to sail drive).
Whisper XT      Modern outboard motor with same motor as PowerPod.  

Comments on Results

• A bigger heavier and less easily driven hull such as the Tahbilk ferry (basically an aluminium punt) needs considerably more power to reach a given speed than an easily driven boat. The Tahbilk requires over 4kW to reach 3 knots. The more easily driven smaller boats consume  less than 1kW at this speed .
• The Duffy 21 is heavier than the Woolwich 20 and drags its transom so it needs more power to reach equivalent speed (1.7kW against 1.3kW for 4 kt).
• The overall efficiency of the drive system is also significant. At 4kt the Woolwich 20 hull needs 1.5kW with the B&S outboard, 1.3kW with the inboard and under 1kW with the PowerPod submersible motor. Obviously the bevel gears of the B&S consume more power than the belt drive, shaft seal and bearings of the inboard, which in turn consume more than the water cooled direct drive of the powerPod.
• The performance of the 2 small 17ft boats is identical up to about 3.7kt. Above that, the narrower old timber boat (Dangar Island)  is clearly more easily driven (the wider Greenwich  is observed to be making more bow wave, a sure indicator of energy waste!)