Calibrating speed in tidal conditions

[npf1]

Any tips on the easiest way to do it?

 

[Vara]

Borrow a Stowe trailing log, (the one with the big battery) and use that as your standard, no need to compensate for tide/current then.

I'm sure somebody will be along shortly with the "book" method.

 

[Poignard]

Not a million miles from you, in Southampton Water, is a measured mile with transit posts on the western shore with yellow crosses. Run up in one direction and record the distance logged and times, turn round and repeat in the other direction and average the two readings. The only problem is likely to be ferries in the way at the critical moment!

I roughly checked my log there last week on the way to Marchwood, although admittedly at slack water so I only checked it one way.

 

[johnalison]

Actually it's never very easy whatever you do. A quick alternative is to motor at constant speed both directions and split the difference of the two GPS speeds. Non-tidal water is very hard to find; even Dutch canals tend to have a current. An additional problem can occur in shallow water. In less than about 5 metres my speed-log overreads while my ground speed drops, presumably by the hull/keel reacting to the bottom.

 

[ebbtide]

Have used the measured distance in Southampton Water for years. (What happened to the one off Lee-on-the=Solent? I see the East pair of leading marks are still standing . . .) Suggest you have a dummy run first as those markers are not that obvious, particularly into the afternoon sun, and do have someone with you: setting/reading log and stopwatch leaves little for keeping a lookout! However you can keep off the beaten track by steering just outside the main channel buoys.

(I'm sure someone will tell us how to calibrate a trailing log before using it as a bench mark . . . )

 

[reeac]

[ QUOTE ]
Actually it's never very easy whatever you do. A quick alternative is to motor at constant speed both directions and split the difference of the two GPS speeds. Non-tidal water is very hard to find; even Dutch canals tend to have a current. An additional problem can occur in shallow water. In less than about 5 metres my speed-log overreads while my ground speed drops, presumably by the hull/keel reacting to the bottom.

[/ QUOTE ]

I support the use of the GPS as it's an exceedingly accurate instrument and you only need to travel say 0.1 NM each way which makes the whole operation very quick thus reducing errors due to changing conditions and failure to keep to a constant speed.

 

[jb2006]

I agree with the use of the GPS - with it now being accurate to a few tens of feet you can set up your own measured distance accurately between a couple of waypoints.

I need to calibrate our log - except in very strong tides SOG always seems to be higher than log speed.

 

[Bav34]

This has worked for me: sit alongside a bouy with a strong tidal stream ie off Lepe Beach in The Solent.

If you hold the boat there AGAINST the tide then your log will read (say) 3.0 knots and the GPS will show 0.0.

Then knock the engine into neutral. Your LOG will drop to 0.0 but your GPS will show the real speed of the tide as you will now be travelling over the ground at tide speed.

If the GPS readout is 3.0 knots then your log is reading correctly.

/forums/images/graemlins/ooo.gif

 

[ebbtide]

I don't calibrate instantaneous speed , which is not a reliably consistent reading, but distance over the ground as recorded over a measured mile(half-mile in the Emsworth Channel.) Adjust your log and speed correction follows.

Comparatively tedious and time-consuming maybe, but accurate.

I'll certainly keep a lookout for boats drifting backwards off Beaulieu entrance in future!

 

[Bav34]

Tee hee! ALWAYS check it in the middle of Cowes week!! /forums/images/graemlins/ooo.gif

 

[npf1]

Thanks for all the responses. Most helpful.