Log inaccuracy

[laika]

When I bought my boat the ST50 log didn't work and all that was available to replace it with was an st60. On the day before launch I realised that the through-hull tube for the new log was a different size, so a boatbuilder who'd been working on the boat hurriedly replaced the fitting.

Despite best efforts at calibration it has never performed well. It reads over a knot faster on one tack than the other but the difference varies with speed and conditions. It appears to be optimally placed (forward, as near the centreline as practical: this is undoubtedly as the manufacturer spec-ed it). It doesn't *look* wonkey.

As I enjoy non-GPS navigation I really want this to work (although I've now acquired a walker log for long trips). Has anyone experienced anything similar and cured it? I'd be happy to shell out for an ultrasonic log but I can't really see why it would help if placed in the same position.

 

[pvb]

I guess you've tried rotating it a few degrees in either direction? Water flow over the hull isn't entirely parallel with the fore-aft centreline.

 

[KenMcCulloch]

You might be sailing faster on one tack. Quite a few boats do that to some extent.

 

[laika]

I guess you've tried rotating it a few degrees in either direction? Water flow over the hull isn't entirely parallel with the fore-aft centreline.

Rotating in which direction though? The log has a notch which prevents rotating about an axis which passes vertically through the transducer without taking out and re-bedding the fitting. The little arrow seems to be pretty much straight forward. To rotate about any other axis would involve re-bedding the tube. Trial and error would seem to be a lengthy process with at least a year (normally 3 in my case) passing between opportunities to realign

You might be sailing faster on one tack. Quite a few boats do that to some extent.

It's a good point. My wind indicator used to be mis-aligned so until I began ignoring it and sailing to the *actual* wind I would tend to sail closer (and slower) on one tack than 'tother but this isn't the cause here: numerous cross checks with SOG tell me this is a real rather than imagined issue.

 

[johnalison]

It should be possible to calibrate an ST60 fairly well without difficulty from quite a wide range of error, though it can take time to get the best compromise. Mine under-reads at low speeds and high at fast, so I try to get it about right for around 6kn. It might be worth checking if the tacking error is always the same direction. My tacks are often at different speeds but this is almost always due to the wave pattern being aslant the wind.

 

[pvb]

Rotating in which direction though? The log has a notch which prevents rotating about an axis which passes vertically through the transducer without taking out and re-bedding the fitting. The little arrow seems to be pretty much straight forward. To rotate about any other axis would involve re-bedding the tube.

That might be the problem. The installation instructions say "Align the arrow on the flange of the housing to point forward toward the bow. If the sensor is not installed on the centerline of the boat, angle the housing slightly toward the centerline to align it with the water flow."

 

[laika]

It should be possible to calibrate an ST60 fairly well without difficulty from quite a wide range of error, though it can take time to get the best compromise.

I do have it at the best compromise. Unfortunately it still ends up wrong by a factor I was never able to calculate the formula for

That might be the problem. The installation instructions say "Align the arrow on the flange of the housing to point forward toward the bow. If the sensor is not installed on the centerline of the boat, angle the housing slightly toward the centerline to align it with the water flow."

Thanks for that. I did download the airmar installation leaflet but somehow didn't take notice of that part. It's certainly worth a go when I pull the boat out