Saturday, 1 October 2011

Leak free week and a pull test

So that's about a week now the thruster has been immersed in my "tank" without any water ingress. Every day I've spun the propeller for a few minutes before disconnecting the power and leaving it for another day.

So far so good :)

This morning I carried out a pull test with my new LiPo battery (BTW I'm quite wary of this battery as I know it can deliver a lot of energy very quickly if shorted out, hence the fuse!) and measured the supply voltage with my oscilloscope at the same time.

Below is a picture of the setup I used to measure the amount of thrust I can achieve with my thruster design.

The best figure I saw on the fish scale was 1.00kg.

Obviously as you can see from my mechanism there is a certain amount of mechanical advantage and it is actually twice the distance from the pivot point to the thruster as it is to the scale so the actual thrust is 0.5kg.

I don't think this is bad considering the water in the tank is circulating quite vigorously causing a decrease in actual thrust. I would guess without this factor the figure would be closer to 0.75kg. Also considering the amount of power going into the thruster, about 24VDC at about 3A and the propeller RPM is relatively low.

Keeping the propeller RPM as low as possible means that cavitation and damage to the propeller is less likely.

I can see some noise on the supply (oscilloscope trace) which may or may not affect the system as a whole further down the line, but we'll cross that bridge when we come to it ;)

My next job is to make a shroud/kort nozzle for my thruster and do another pull test to see what kind of difference it makes to the thrust.

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