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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

GPS or Knot meter

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Turk
Admiral

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USA
736 Posts

Initially Posted - 03/17/2007 : 21:16:09

This question may have been ask before but I have not found any search results for it. How accurate is a GPS for speed when sailing? Am I wrong in thinking that a GPS can be used for subtle sail changes while racing to watch for speed increases or decreases? Is it accurate enough to display tenths of a knot as these changes are made? My only experience has been with a knot meter on my old Cat 22 that had a meter from Standard Horizon with the thru the hull transducer with the small paddle wheel. It was very accurate in seeing very small changes in speed and gave me great feedback while racing.

I do not own a GPS and am thinking about buying one for this purpose.

Turk

www.turk.mysailboatblog.com
2003 C250 WK #663
Northeast Ohio
Mosquito Lake/Lake Erie

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saribella
Captain

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USA
286 Posts

Response Posted - 03/17/2007 : 21:27:59

I use both and find that my Magellan Sportrak Map (3 YO) has a +-2 second lag time in feedback depending on the number of satellites available, but is pretty accurate. Maybe other models are more accurate. I have it mounted on the inside of the transom. I am buying a Sportrak Marine in the near future which will hopefully be better for use overall. Sailing at night with the back-light on is nice and easier to read than the speedometer which is not back-lit. I hope this helps.

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OLarryR
Master Marine Consultant

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USA
3468 Posts

Response Posted - 03/18/2007 : 07:56:57

Good question - Which for more accuracy for sail trimming, etc - The GPS or the knotmeter.

I sail on the Potomac River, have an old knotmeter (analog dial type) mounted on the bulkhead and a handheld Magellan Marine GPS. Once is awhile, my knotmeter does not register but most times it does and checking it with the GPS, it is pretty accurate considering it probably was the original meter on my boat. I believe it does not read occasionally because of perhaps some debris that may get in the way but then clears. Not sure.

In any case, I do not know the exact lag time for the GPS but there is some as Saribella mentioned and believe there is basically no lag time with the knotmeters because the signal is being sent continually with each revolution of the small flywheel under the hull and that revolution is generally many times per second. My GPS is a handheld and so it is not as conveninet to use as compared to my bulkhead mounted knotmeter. So, those are some of the plusses on the knotmeter side.

On the plus side for the GPS is that it has no moving parts and basically subject to very little inaccuracy as compared to knotmeters that have flywheels and are sometimes subject to debris, turbulence or other water environmental effects that could throw it's accuracy off a bit.

The GPS provides true speed over distance, whereas, the knotmeter provides speed relative to the boat only and not the water environment that effects true speed. For example, where I sail I am effected by perhaps a 1knot current at times due to the tide shifts. Depending on which tack I am on, the knotmeter will be reading faster against the tide compared to true distance covered, whereas, the GPS will be dead-on with the speed.

I still like watching my knotmeter indicator dial for gaging sail trim even if my GPS is probably more accurate overall. Maybe it's the lag time on the GPS but probably more because I have a handheld that I do not rely on the GPS so much for speed readings.

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djn
Master Marine Consultant

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USA
1561 Posts

Response Posted - 03/18/2007 : 11:46:45

I find the lag time as well, both in the speed of the boat and the compass heading. My whiskey compass is much faster in showing heading than my GPS. I should say "my old boat". Cheers.

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Turk
Admiral

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USA
736 Posts

Response Posted - 03/18/2007 : 17:00:13

I just bought a hand held Foretrek from Garmin, fits on your forearm just to give it a try. $89 out the door. I've done quite a bit of reading since I posted the original question. It seems as if the GPS can have the needed accuracy, just a bit of lag over an analog knot meter. Considering my experience with the later, and others on the net, the lag problem is minor compared to the seemingly continuous problems with that little paddle wheel getting fouled. I've gone over the side many times with a brush to clear that problem up. It always seems to be fouled with something just as the race is ready to start. A GPS can have up to a 1/10 of a knot accuracy in the right conditions. There are issues with current, sideways drift, etc, but most who have compared both side by side, seem to agree that the accuracy can be very similar with the reliability of the GPS winning out.

Come to think of it, my old Standard Horizon knot meter had about a second to a second and a half lag. Maybe the newer ones are more continuous.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 03/18/2007 : 17:21:59

As mentioned, there is a lag--on my unit it's somewhat adjustable, since you can set the time period over which the GPS averages the speed. For general purposes this can reduce the wild flashing of numbers--for measuring trim effects, you would probably want to experiment with intervals to get the most useful response. Anyway, when trimming sails, you want to give the boat a chance to settle in on a new speed before you judge your trim--that'll be at least a few seconds, and can also be affected by sea-state.

Another thing the GPS can do for a racer is help determine the optimal course to a mark based on Velocity Made Good (VMG), which can tell you, for example, how far off the wind you should be on a dead-downwind leg to get to the mark most quickly. (If you're going faster but traveling further, are you gaining or losing relative to a more direct course...) Upwind it can help you determine a layline. (I never personally did very much of those things--I mostly used the GPS to get me from here to there.)

Edited by - Dave Bristle on 03/18/2007 17:24:31

Steve Siefken
1st Mate

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USA
81 Posts

Response Posted - 03/18/2007 : 19:36:10

It also depends on if you are in salt water or if you find yourself in a current. I was with a buddy heading into port as the tide was going out. The knot meter (I think it was a paddle wheel) showed us at about 6 knots. The GPS showed us at about 1 knot. Clearly the paddle wheel was spinning at a rate of 6 knots, but it was not an accurate reflection of the net forward speed.

Thanks,

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/18/2007 : 19:47:21

GPS will accurately measure Speed Over Ground (SOG). A knotmeter measures speed through the water.

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The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.