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Old 24-07-2007, 17:34   #1
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Speed transducer placement - Oops?!

So I finally got my new speed/log and depth sounder up and running.

They are both Nasa brand, Clipper range. I bought them brand new, in the box, on eBay for a fraction of the cost from the local agent. I have the displays fitted in a box on the cabin top.

The depth sounder transducer is fitted to, not through, the inside of the hull, which I was a bit worried whether the signal would get through thehull ok because the hull is not just GRP, but kevlar & GRP. But it seems to work reasonably well... actually, it goes a bit "loopy" (i.e. intermittent) in deeper water (say 50' or more), but it works fine in shallow water, which is where it matters, so I am happy.

The speed log transducer is a standard paddle wheel type with the through-hull positioned about 15" forward of the frone of the keel and about 12" starboard of the centreline. This location was chosen for it's accessibility - I only use the log for racing, and remove the transducer when not in use. The speed displays fine and having calibrated it, seems pretty accurate under motor. The problem is that under sail, when sailing to windward, it reads about 10% high on one tack and 10% low on the other tack. It seems to me that this must be a result of differences in water flow above and below the keel when the boat is heeled, and, if this is the case, there is probably nothing I can do about it.

It is particularly frustrating because I can get my speed reasonably accurately from the GPS, but I like having "thru-water speed" to monitor the effectivness of sail trim and other small adjustments.

Any thoughts / suggestions (or you can just laugh at me)?
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Old 24-07-2007, 18:38   #2
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Quote:
It is particularly frustrating because I can get my speed reasonably accurately from the GPS, but I like having "thru-water speed" to monitor the effectivness of sail trim and other small adjustments.
Over ground and through the water are not the same thing. So to compare the two isn't all that practical. If it really were the same then the GPS would always win. On my boat the sensor is port forward. I don't see how you could do it different other than starboard. I feel forward is better as there is less turbulence.
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Old 24-07-2007, 18:58   #3
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You could get another transducer and locate on the other side of the CL and use a mercury switch to select one at a time when heeled... and then calibrate each to read correctly.

jef
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Old 24-07-2007, 18:58   #4
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Originally Posted by Weyalan
The problem is that under sail, when sailing to windward, it reads about 10% high on one tack and 10% low on the other tack. It seems to me that this must be a result of differences in water flow above and below the keel when the boat is heeled, and, if this is the case, there is probably nothing I can do about it.
Yes, that is one reason for the difference but you also have to consider current set and drift as well as leeway. But when making adjustments, you are looking for the small changes in speed as a result of your trimming, and not really concerned about the absolute accuracy of the the indicated speed.

Eric
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Old 24-07-2007, 19:07   #5
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Originally Posted by fairbank56
Yes, that is one reason for the difference but you also have to consider current set and drift as well as leeway. But when making adjustments, you are looking for the small changes in speed as a result of your trimming, and not really concerned about the absolute accuracy of the the indicated speed.

Eric
Sure thing Eric, I understand that, and it is precisely the process highlighted above that justifies the need for a thru-water speed rather than the GPS speed over ground (although speed over ground is also important, of course )but it is frustrating seeing 5.8 knots on starboard tack and 4.4 knots on port tack when you know that it isn't really the case... it offends my sense of "rightness"
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Old 24-07-2007, 19:08   #6
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... and then calibrate each to read correctly.
The Clipper Log does not have individual calibration settings for port/starboard transducers.

Eric
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Old 24-07-2007, 19:19   #7
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Quote:
Originally Posted by Weyalan
The speed log transducer is a standard paddle wheel type with the through-hull positioned about 15" forward of the frone of the keel and about 12" starboard of the centreline. This location was chosen for it's accessibility - I only use the log for racing, and remove the transducer when not in use. The speed displays fine and having calibrated it, seems pretty accurate under motor. The problem is that under sail, when sailing to windward, it reads about 10% high on one tack and 10% low on the other tack. It seems to me that this must be a result of differences in water flow above and below the keel when the boat is heeled, and, if this is the case, there is probably nothing I can do about it.
When your on a stbd tack with the paddle wheel on the high side it could be introducing some air or froth. In that case it would most likely slow when catching air.

See if leveling the boat out without loosing speed (trim perfect) if it speeds up a bit?
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Old 24-07-2007, 19:55   #8
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Worth a try Del,
We were actually trying to race with only 5 crew, one of whom is 13yrs old, in 15+ knots of true wind, with plenty of rag up so we didn't exactly have the rail stacked.

Hopefully with full crew we will sit a bit more upright.
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Old 25-07-2007, 04:17   #9
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You don't need separate calibration with two transducers. You need the same one corrected. It should be the same for port and starboard tacks. The display will only read one transducer at a time because of the mercury switch.

jef
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Old 25-07-2007, 10:16   #10
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Del's got it right about the froth etc.

I finally installed another transducer and a mercury switch on my previous boat. It solved the problem. Exactly same reading on both tacks.
Of course, if you just got yourself a multihull...



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Old 25-07-2007, 16:26   #11
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Originally Posted by defjef
You don't need separate calibration with two transducers. You need the same one corrected. It should be the same for port and starboard tacks. The display will only read one transducer at a time because of the mercury switch.
High end instruments like B&G and Ockam allow you to calibrate port and starboard transducers individually.

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