J109 Airmar DST810 Speedo Calibration

[jkalucki]

Here's what I've come up with. I'm hoping that others will post their calibrations, so we can compare notes. Very little J/109 on J/109 racing going on here in San Francisco, so no rivalries to protect!

I aim for boat speed to be correct within |0.1| knots at >= 3.0 knots boat speed. Transducer is mounted starboard side, in what I think is the original opening, just a few inches outboard from the spine. All calibrations in reasonably flat San Francisco Bay waters, at both near slack and with a current running and corrections taken against SOG. Tweaks to this table were made over the last two years, always within a day or two of a bottom clean. I aim for repeated rotational position alignment with a marked stick, so the arrow is pointing to within a few millimeters of the same spot each time. Paddlewheel is stored in fresh water between sessions -- I find dry storing the wheel causes many tens of minutes of slowness. I want to leave the dock, verify the numbers immediately, then go racing. I start each calibration session with a number of reciprocal motor runs at 0 degree heel in a quiet, mostly slack, channel, verify those numbers, then go sailing to induce heel.

Numbers seem pretty good when eyeballing against SOG, to within 0.1 to maybe 0.2 knots, between 3 and 8 knots boat speed, up to about 25 degrees of heel, but seem OK enough at 30 degrees too. Heeled 1.5 knot numbers are just guesses.

Note the Airmar system has some common floating point representational issues. Put in 0.9, get 0.89 back.

Code: Select all

kn,0.0,1.5,3.0,4.0,5.0,6.5,7.5,9.0
20.0°P,0.00,0.60,0.80,0.99,1.11,1.30,1.40,1.59
10.0°P,0.00,0.60,0.70,0.89,0.99,1.11,1.21,1.30
0,0.00,0.51,0.60,0.80,0.80,0.89,0.99,1.11
10.0°S,0.00,0.51,0.51,0.70,0.70,0.70,0.89,1.11
20.0°S,0.00,0.41,0.41,0.60,0.60,0.51,0.70,0.89

[Vento Solare]

John - that is a noteworthy effort you've done on calibration. The first year I owned Vento Solare I moved the paddlewheel to the centerline as you saw in this old post. Since then I have updated the paddlewheel to use the Gen 2 Airmar analog version that has a larger/flatter paddlewheel that fits in the original housing. There is a notch in the insert so the paddlewheel is always inserted the same every time. I consider this a design flaw in the DST810 as you must properly align the sensor every time it is inserted, rather than relying on a fixed position.

My calibration is done in an H5000 Hercules processor. I'll make a separate post that has the calibration values as I need to get it from the boat. I pull the sensor most every time the boat returns to the dock and occasionally spray it with Windex to kill any potential growth. To reduce nuisance water I put a section of 4" PVC pipe that is caulked to the hull around the transducer. It has a hole on the starboard side aft so water flows into a rag every time I swap the xducer with the blank plug.

pvc-pipe-xdcr.jpg (219.49 KiB) Viewed 163 times

[jkalucki]

Yes, moving the sensor is the dream! Also the water catchment. And the keyed housing.

There's a guy in Richmond CA who does these sorts of reinforcement and glass jobs -- it's common enough to move the sensor. Besides the expense, there's some layup time and there always seems to be a calendar issue, and, well, I always have bigger fish to fry with racing improvements than the speedo. I hope what I currently have is good enough?