Erratic speed log.

[ga1135]

Tide really does affect the impeller reading, even on the way.
Try keep same fuel level and go up & down stream see how the readings behave.
Go for a measured distance up & down the stream also give you two different trip distance readings.
This 2 trip figures are also be used in the calculation when we re-calibrate the log setting for accuracy reading.

Using only SOG for relax cruising is fine. But having both log speed & SOG are very important when racing.
It helps you to decide whether to go closer to shore to reduce tide effect or go into the channel to follow the tide,
It also helps you to select the more favourable tack on the race course.

 

[nigelmercier]

The log shows boat speed through the water pure and simple...

Quite, it displays the relative speed between the boat and the water.

I think we are talking at cross-purposes. My statements are all in response to this:

The tide has no effect on the impeller speed whatsoever.

This statement seems to imply that the tidal flow component does not affect the impeller, which it clearly does.

The only way you would keep SOG constant in a changing tidal pattern would be to deliberately alter boat speed to keep SOG constant.

Doesn't everybody do this when motoring? Look at SOG, we are going a bit slow, so speed up?

 

[GrahamM376]

Tide really does affect the impeller reading, even on the way.
Try keep same fuel level and go up & down stream see how the readings behave.
Go for a measured distance up & down the stream also give you two different trip distance readings.
This 2 trip figures are also be used in the calculation when we re-calibrate the log setting for accuracy reading.

Don't agree with your reasoning. For the sake of argument - no sails, no wind, just engine and some tide running. Fix the revs and whichever direction you motor, the speed of the hull through the water surrounding it will remain the same, therefore the log paddle is spinning at the same speed.

When calibrating the log, as you say, it's necessary to time a passage over a measured distance. If there's no tide running, it's easy to see if the log is reading speed and distance correctly but, if tide is running, time taken over the ground needs to be timed over the measured distance into and then down tide and distance/time added or lost by tide effect, taken into account.

 

[Pye_End]

Tide really does affect the impeller reading, even on the way.
Try keep same fuel level and go up & down stream see how the readings behave.
Go for a measured distance up & down the stream also give you two different trip distance readings.
This 2 trip figures are also be used in the calculation when we re-calibrate the log setting for accuracy reading.

If you are observing an effect it will be from another external change, eg wind effect or a change in sea state. Otherwise, if there was a simple effect of tide on boat speed you will have discovered a whole new set of physics laws, and you would be arguing that heading east or west effects the logged speed due to the rotation of the earth.

 

[Pye_End]

Quite, it displays the relative speed between the boat and the water.

I think we are talking at cross-purposes. My statements are all in response to this:



This statement seems to imply that the tidal flow component does not affect the impeller, which it clearly does.



Doesn't everybody do this when motoring? Look at SOG, we are going a bit slow, so speed up?

Are these responses a wind-up?

 

[nigelmercier]

Are these responses a wind-up?

Not at all, I said we were talking at cross-purposes, and are both correct.

 

[Sneaky Pete]

It’s a bit like being on a travelator. If the travelator moves at 4Kn then your speed in relation to the surroundings is 4Kn. If you decide to walk on this travelator in the same direction at 4Kn then your combined speed is 8Kn in relation to the surroundings. However if you walk in the opposite direction at 4Kn then your speed in relation to the surroundings is zero. An impellor works in a similar way moving through water. There are two separate entities the boat speed through the water which the log will read and the speed of the water over the ground tidal flow. As stated if the tide is on the beam it is negligible.