[C320-list] Speed log problems

[Graeme Clark]

Our original fitted instrumentation (#303, 1996) was ray marine ST50+ Tridata and two years ago we fitted a replacement Ray transducer paddlewheel. This is located directly beneath the two smaller drawers in the forepeak cabin, pretty much on the centreline, with the depth transducer next to it.

I am having a real problem calibrating the instrument. earlier in the year we set a calibration factor of around 0.80 and now it has had to be change to 2.00 to get anywhere near a sensible reading.

I regularly withdraw the transducer and clean it for any fouling and other marine life and even pushed out the axle pin, to enable a really thorough cleaning and gave everything a squirt of silicone spray lubricant before reassembly, but still the calibration changes

I can only come up with two theories: (1) the instrument or transducer is faulty or (2) other fouling and marine growth on the hull in front of the transducer is affecting the water flow over the paddle wheel as the season progresses.

I'd be grateful for any other suggestions and also for an efficient way to trouble shoot without simply replacing every single item piece by piece!  I guess a haul out and scrub would initially show whether the hull fouling was a factor, but (where I am) that is the bets part of $500. A new transducer is about the same and a replacement  instrument (ST60+ is nearest available type that is compatible) would be $3-400 I think. If I end up doing all three that is and expensive trouble shooting exercise!

Graeme
(Simia, #303, Falmouth, England)