I assembled a prototype long shaft electric outboard for testing on Peter's 4t proa.  This is intended as a proof of concept and will only be used a few times so it was assembled from bits and pieces I had at hand. 

I have created a folder for electric drive and placed a short video of bench tests and on the boat.  There is a link but I am not sure if these work.

The exercise was worthwhile from a learning point of view.  Some details;

1.  The shaft tube is 2m long.  The shaft is only supported at the ends and hits critical speed around the highest RPM it would run at pushing the boat.  So the tink tink sound is the shaft hitting the steel tube because of the whirling and I guess a slight misalignment.  The shaft needs intermediate bushes to prevent this.

2.  The initial idea of this leg was essentially a thruster for pushing the boat in any direction so the prop is deep enough to have its jet pass under the hulls.  The mounting had to accommodate rotation and our lashings were really not secure enough to hold the top in place even long enough for me to get a good clip.  You can see it begins to turn and move sideways during the clip.

3.  This single unit with the large overpitched prop should have been capable of getting above 5kts at the rated torque of the motor.  The speed that Peter relays is 5kph from my bike GPS not knots.  So it was well below expectations.  Main reason was the hull fouling.  The opportunity to get it clean did not present itself in the time we had.  At this speed and on the motor rated torque the thrust was about 450N and power demand was about 1220W.  

4.  Despite the lower than predicted performance the comparison with the 65HP outboard he has installed was not too bad.  Even in the light sea conditions the boat's speed was limited to about 8kph due to the motor ventilating violently.  The motor slides down into the water about amidships and the prop would rise out of the water as each wave passed.  In the crests the water level was at the cowling.  So it needs a really long shaft to be effective in its current position.

5.  The drive leg could make way at about 3kph into the wind on the motor rated torque.  Wind strength was 10 to 15kts.  The motor is mounted beside the lw hull and it did not have enough way to turn the large cabin on the ww hull through the wind.  We just crabbed sideways about 20 degrees off the wind.  It would have been better to have the thrust behind the ww hull.

The plans for the next test are to lock the leg to the in-line position and improve the bracing with solid connections rather than lashings.  Peter will make an attachment point for the trailing stay off the side of the lw hull so it has a shallower angle.  We will also be able to raise the prop as it will not be required to thrust below the hull.  Most important will be to get the growth off the bottom.

I am also considering making a metal prop that can use more of the motor power.  The motor is rated at 4kW on 48V.  The battery for the test was 24V so that limits to about 2kW.  Exiting prop is a $14 Hobbycity special having a 30" diameter and 20" pitch.  The pitch is too much to get the full potential of the motor with the 2:1 ratio box.  I was also concerned about breaking the blades off.

Long term intention is to mount two thrusters near the notional stern of each hull.  One as is now close to the lw hull and the other close to the ww hull.  This then provides ability to spin the boat similar to a cat.  The legs will be able to tilt up when sailing.  

In looking for battery options to get up to 48V I came across this site:

http://www.ecoboats.com.au/productrange_bellmann.php

There are some interesting options coming on the market.    

The leg as you see it weighs around 20kg.  The final units will probably use a larger gearbox so somewhat heavier.

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