Coxmate HC impeller readings

HC GPS calibrationWe got a question from Bjørn Johansen about his Coxmate HC impeller

 
 I have a problem with sudden changes in the response of the impeller.  In one case I observed a change of about 6 % compared with GPS (no wind, and very little current). Is this a known problem?

And so we consulted the manufacturer, Peter Hodson and this is his reply.

I have actually seen this happen on my scull – so I believe you! It is very strange, and the only explanation I could come up with was there is some transitional change in flow around impeller. I suspect it might be related to the alignment of impeller, but I have never been able to make it happen.

I have since bought a new scull and impeller is mounted nearer to the bow and I have never seen problem again. I was talking to a cox of an eight and he observed similar effect with an NK impeller on an eight when the impeller was mounted ~12 metres from bow.

I don't have a certain fix for problem, but I strongly suspect it is relate to position or alignment. My suggestion is to move impeller further forward – Mine is mounted under seat. If my theory is right, it may only require a small shift to keep it out of the transitional zone.

Apologies not to be more helpful, and please forward any details which might help understand causes better.

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