Raymarine ST70 log calibration

[NPMR]

We have an under-reading log. Raymarine ST70. Have checked the paddle wheel and it is fine, so I need to 'calibrate' it.

So far, so good. It seems very complicated, for me anyway. Instructions are to get into the Transducer Setup menu, select Speed then Calibration.

Then ! "calibrate each speed as follows, starting with the lowest"(!)

Not having done it before, before I get into it, has anybody any clue as what the speed options are? It is a Jeanneau SO33i, so sailing speeds is all I can imagine. But what are the options? or do you have to do it for every option, such as 1 knot, 2 knots, 3 knots etc etc. ??

Confused.

 

[earlybird]

Paddlewheel transducers working close to the hull of a sailing boat can't be very accurate over a range. Mine under-reads at lowish speed and over-reads at higher speeds, probably typical.
Best to decide what suits you as a typical speed and calibrate to that. You may still well find that it varies depending on which tack the boat is on reach or close-hauled, motoring or sailing etc. etc.
Mine's set to be about right between 5 & 6 knts.

 

[johnalison]

My ST60 is only calibrated at one speed, which I normally do at about 6kn. You need to motor in flat water otherwise the speed will be jumping around. I would not advise adjusting to the GPS since there are so many causes of error, such as unnoticed current or shallow water causing misreading.

My usual procedure is to determine roughly what the error is, say under-reading about half a knot, do a run and adjust upwards about 3/4 of that amount. I then leave it alone and reassess in different conditions before trying to readjust. I usually aim to have the log overreading by 0.1 knots at about 6.5 knots. This is in case I need to use the distance function with GPS failure since an overreading log is safer.

The whole process is a bit hit and miss, so you need to take a dynamic view of the process.

 

[Pye_End]

Is it a DST800 transducer?

Have been playing with one of these for a month or two, and as you say you set it over several set points (maybe 5). I tried to set them across the range that I can motor at so that I could complete the calibration.

Didn't find the manual that helpful, but took 13 year old son out for an hour to do it, and it all seemed straightforward in the end. Would have been harder without him! Think there are a couple of ways of doing it, and can adjust after the initial calibration. Pick as much as a tideless area as you can, and go across the usual stream, just in case, and compare with gps. After we did it, the calibration factors for the different dpeeds were not that far different, but results so far are very encoraging - seems better at the extremes than the old ST50 that it replaced.

 

[GrahamD]

I had a look at this recently. I think the calibration speeds were 2, 4 and 8 knots. Does seem to be a fairly complex process.