GPS vs Knot Meter

[sandy daugherty]

First I have to check the back of a few trucks leaving the Airmar factory.

[svHyLyte]

Reliable knotmeter with no holes in the boat, batteries not included:

Knotstick Sailboat Speedometers

$45 USD

[Celestialsailor]

This has turned into a great topic. I forgot that radar can calculate speed also. In a sense, radar can also be a back up for a failed GPS. I'm so low tech that I had never heard of doppler units. I probably could not afford one anyways. I also wonder why the manufacturers of knot meters do not build them to be calibrated at low speed (say 2 knots) and a higher speed (say 6+ knots). This would certainly correct the problem I experienced. On Faiaoahe we were offshore of the Ca. coast heading south towards tropical depression "Boris" this summer. With 50% of the Yankee only, the log read 6-7 knots. We were probably doing 8+. The vessel was doing fine but if I had known, I would have switched to the staysail.

[Nauticatarcher]

Removed our 20yr old speed log and used hole to install our forward looking sounder, GPS speed is fine by me, we're cruisers not racers

[Weyalan]

Provided that your log is reasonably accurately calibrated (and that is not a difficult exercise, in general), I think that having both is a good thing.

[Celestialsailor]

If you read my first post, you'll see that the log I had was not able to be calibrated over the whole range of speed. I wonder if everyone checked theirs at 2, 4, and 6 knots with their GPS if they would find the same error that i did...25%.

[Pelagic]

Originally Posted by Celestialsailor

If you read my first post, you'll see that the log I had was not able to be calibrated over the whole range of speed. I wonder if everyone checked theirs at 2, 4, and 6 knots with their GPS if they would find the same error that i did...25%.

Along with the Doppler, we had a complete B&G system that was used more for a convenience monitoring solution in the crew lounge, officer’s cabins, my office and guest areas.

I did note that the speed log when compared to the Doppler in water reading mode, was always off by at least 5% and up to 30% in a Seaway.