Correct mounting of the log/speed sensor

lukeskysailor

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Before the start of this sailing season I have to replace the log/speed sensor. The new sensor is an Airmar tridata (depth, speed and temperature) sensor for a Raymarine I70s setup. Now I heard that you should not install the sensor parallel to the sailing direction but turned 3 degrees to the bow. This should be done to align the sensor to the direction of the water flowing underneed the hull alongside the keel. The keel (we have a short fin keel on our Hanse 341) would slightly deflect the water flow (water is not compressible) and that deflection would be 3 degrees. Does anyone know if this is correct? Is it indeed useful to compensate this slight deviation when installing the sensor or is it not worth the hastle?

Regards,

Luc
 

Buck Turgidson

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Installing
1. From outside the hull, push the housing into the mounting hole using a twisting motion to squeeze out excess marine sealant (Figure 3). Align the arrow on the flange of the housing to point forward toward the bow. If the sensor is not installed on the centerline of the boat, angle the housing slightly toward the centerline to align it with the water flow.

http://www.airmartechnology.com/uploads/InstallGuide/17-355-01.pdf
 

johnalison

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It may well be that angling the sensor in will provide the best accuracy, but short of trial and error this will only be an approximation. My last two sensors were both midline but I had one beside the keel on a previous boat. As far as I remember the readings were OK. An error in itself wouldn't matter unless it was different on each tack. Any known error can be re-calibrated, and I suspect that no logs are truly linear, so knowing your errors is always going to be needed.
 

lukeskysailor

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Thanks all for the reply. With my previous speed sensor I noted indeed a different speed depending on the tack. The difference was about 1 knot. I will mount the new sensor with the arrow pointing to the bow as the mounting hole is about 2 feet offset from the center-line next to the start of the keel.
 
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