Calibrate speed log on land

[Pethautwp]

Last season I noticed that there was a definite (incorrect) difference between my speed through the water and that displayed by my gps. on inspection the log was heavily fouled (as was the whole hull!!) and I put the differences down to that effect. since hauling out I have cleaned the log and it appears to be spinning freely, but just wondered if anyone had any methods of checking/calibrating the log whilst on the hard just for peace of mind.

Also, am I right in thinking that the info from the log in used for the computation of the apparent wind? if so this would explain some inexplicable (?) displays too!

many thanks

PT

[Waka Tiki]

Hi PT,
At the risk of being a bit too obvious... And apologies if I am... Your GPS will be showing your speed over the ground, ie how fast you are travelling over the sea bed as measured by American satellites. Your log will be measuring your speed through the water. So, for example, if you are stationary on a mooring the gps will register no speed as you are not moving in relation to the ground under your hull. However the log may measure a speed through the water if the water is moving in relation to your stationary hull due to the tide or perhaps a river current if in an estuary.

For what it's worth... I clean and anti-foul my sum log impeller religiously before I launch. I check it is spinning freely and if I am lucky it will continue to give a read out for a few days after launch... But that is usually about it!
john

[Jolly Roger]

PT

There is only one way to calibrate a log, over a measured distance in both diections. So to answer your question, no there is no way to check a log out of the water.

[philipstevens]

Also, am I right in thinking that the info from the log in used for the computation of the apparent wind? if so this would explain some inexplicable (?) displays too!

many thanks

PT

Yes. The old AP-Philips GPS I had on my Duo would give these readings - apparent and actual wind speed, using the log input, and using the wind direction would give the apparent and actual wind direction.

Clever piece of early GPS electronics.

[Pethautwp]

Hi Waka Tiki, thanks for the response, appreciated. The issue I had was that the log would read zero whether running with the tide at 7kts over or sailing into a tide at 3kts over ground, which made me think I might have an issue.
Jolly Roger, will have to use your method on slack water.

Cheers all