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Post by marmalade11 on Sept 14, 2011 21:09:02 GMT
Please can anyone confirm the best location for a speed transducer on a swing keel Jeanneau 29.2.
currently my speed transducer has been fitted just in front of the stub keel but the faster the boat sails the less accurate the speedo, reads.
it seems that the water flow increases ahead of the keel has this been encountered by anybody ? if so how is this resolved ?
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Post by sitara on Sept 14, 2011 21:29:16 GMT
Hi marmalade,
I have the same problem with the factory fitted speed transducer in my 29.2 (fixed keel). I get errors of up to 1 knot especially with the boat heeling, and more so on one tack than the other, compared to the GPS. I fitted a Raymarine autopilot and use it to display SOG at the helm.
Cheers Sitara
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Post by mkremedy on Sept 19, 2011 2:48:36 GMT
Hull speed and speed over ground will differ under normal conditions, when yo are sailing against the tide, the hull spedd will be faster than the SOG. When sailing in the same direction as the tide, the SOG will be faster than hull speed. The only time they will be the same is when the tide is not moving.
Hopefully this will answer your question.
Happy Sailing,
MKREMEDY
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Post by rene460 on Sept 21, 2011 7:40:39 GMT
Hi Marmalade, We have a SO30 with swing keel, so basically the same configuration as you. The transducer is just in front of the keel and to one side about 150 mm. I don't know what instrument you have - we have a Raymarine ST70 which allows calibration at 6 different speeds to get over the non-linear characteristic of the paddle wheel transducer. Our previous boat had ST-60 which only allows one calibration factor, so is only correct at the speed it was calibrated.
The paddle wheel is very dependant on the direction of flow over the hull in the area of the transducer and will always vary from one tack to the other due to both hull shape and angle of heel. It is best a rough estimate. On the ST-60 I calculated the calibration factor using the log reading on a straight course at typical speed over a distance that I could measure on a chart. Then the total distance is roughly correct so the average speed is about correct despite the difference on different tacks. Error in instrument calculated true speed and direction is unavoidable but hopefully not too serious.
ST-70 does not allow manual calibration factor and I had to borrow a compatible GPS to calibrate - in still water with no tide, under motor, a slow procedure but the result seems to be holding, but it is frustrating not being able to tweak it. However speed comparison with GPS under sail deviates considerably from tack to tack even with no current. I do not think there is any way of avoiding this. I regularly compare the log with a GPS trip meter. It is surprising how well they correlate over a few hundred miles, but speed difference between GPS and instrument between tacks varies by up to a knot or possibly more.
rene460
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Post by marmalade11 on Sept 21, 2011 21:46:05 GMT
Hi Rene many thanks for your comment i have a Raymarine ST60 and you are correct i can only make one single calibration.
it is interesting that your speed log is off centre was that deliberate or was there something situated in front of the keel?
it appears that a pressure wave develops forward of the keel and at 5.5 knots plus this water flow significantly increases the.
The short term answer is to calibrate the log at 6 knots and make mental allowances for this when below this speed.
Normally this would not be of any concern but as i intend to race the boat boat speed is quite important when sail tweaking.
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Post by rene460 on Sept 22, 2011 12:19:03 GMT
Hi Marmalade,
Transducer location is as supplied by Jeanneau. I do not know if there is any reason other than it was probably installed with speed on one side and depth the other when the ST-60 was standard. The ST-70 was supplied with a single transducer for speed and depth. I am not sure about your pressure wave theory as water is essentially not compressible. I think it is more about the localised direction of flow around the hull and keel. You can see how this affects calibration when going straight in still water by angling the transducer slightly say towards the centreline. Unfortunately it also increases the difference between tacks when sailing. When the flow is at an angle to the plane of the paddle wheel, the wheel sees a lower speed, but the local speed is also increased by the need to go around the hull, and flow around the hull changes direction with tack so overall effect is not easy to guess. The GPS is better in showing the difference between tacks in still water but any current makes it quite confusing especially if it is not in line with your course. For sail tweaking, it hardly matters whether the speed shown is accurate, only that it changes, however remember that 0.1 knots is 608 ft/min, so any sail adjustment that achieves that is more than a tweak. That is why two boat testing is so often recommended. One drawing ahead even slowly is a very sensitive signal. I sail on a lake system and hence I find that wind direction and strength vary continually, making it very hard to correlate any adjustment to a change in speed. Sailing with another boat means we both get similar changes (at least while we are close) so helps eliminate this problem. I will be interested to hear how you go though.
rene460
PS I had intended to say ....calculated true WIND speed and direction... The wind instrument is really great.
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