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-   -   GPS vs Knotmeter (http://www.sailnet.com/forums/gear-maintenance/6843-gps-vs-knotmeter.html)

skyamsen 04-27-2003 09:34 PM

GPS vs Knotmeter
 
I have an old signet knotmeter that has finally given up. I am tight on cash right know and am looking at replacing it with either a Gps or a Raymarine Knotmeter. I just have a couple of questions. I can''t afford both so. Is the Gps as accurate as the knot meter? I have heard that it doesn''t update very quickly as far as speed, Is that true. Is it possible to us the gps to trim for speed? I''m looking at the Garmin GPSmap76 or RayMarine knotmeter. Any help would be great. If I get the Gps I would plan on getting the Knotmeter in the future.

Silmaril 04-28-2003 12:28 PM

GPS vs Knotmeter
 
While a GPS will give you an average speed reading, it is over the bottom speed and not through the water speed. This is most significant when sailing in an area with a swift measurable current, such as a river, an area subject to tidal currents, or the ocean currents. It is totally unacceptable as a method to trim for speed, as you suggested, as it cannot react quickly enough and may be off-set by currents.

Same can be said of substituting a GPS for a helms compass. Be aware that you may be reading "True" courses, rather than magnetic courses. And also be aware that is is showing you your "Reading" or course to a particular nav point, not the direction you are pointing!

GPS are wonderful devices, but you have to use them for what they were intended for, a navigational aid. And then as a supplement to other forms of navigation. A person who relies exclusively on his "GPS/Map-color Model 4554" for navigation will have a rude awakening the first time the power is down on the batteries.

Can a GPS be used as a temporary fill-in while you save for your knot meter? Of course it can, but remember it''s limitation on showing it''s specific type of data.

geohan 04-28-2003 12:38 PM

GPS vs Knotmeter
 
The speed function of the standard GPS is not accurate enough to assist sail trim. The VMG (velocity made good) however has some strategic usefullness.

sedelange 04-28-2003 05:03 PM

GPS vs Knotmeter
 
What model Signet Knotmeter do you have? I am trying to resurrect mine. I am looking for parts. Contact me @ SEDeLange@aol.com

petercra 04-28-2003 05:48 PM

GPS vs Knotmeter
 
A GPS and a knotmeter are reading two completely different things. The former measures speed over the bottom, the latter speed through the water. The former is useful for navigation, the latter for optimizing performance. One can be calculated from the other if the current strength and direction (called set and drift) are known. Tide and current tables may be able to tell you the set and drift predicted at a particular time.

A GPS is subject to random fluctuations of as much as a knot depending on the time over which velocity is averaged and whether it''s a differential or WAAS model. There is no calibration adjustment. A knotmeter needs to be calibrated, but once done should be much more accurate than a GPS over the short term (assuming that one is interested in speed through the water).

Over 100 miles, a GPS will average within .05% of the average speed over the ground for the journey. That''s because the GPS knows the starting and ending points, and the time taken, with an incredible degree of accuracy. A knotmeter will never do as well over a long distance: .05% of 6 knots is .003 knots, no knotmeter I know of reads to a resolution of less than .01 knots, and the accuracy is unlikely to be anything close to .01 knots.

gskup 02-24-2010 07:44 PM

Velocitek Speedpuck option
 
I've been researching Velocitek's Speedpuck. In comparing it to the Garmin marine GPS handhelds it looks like the Garmin marine GPS handhelds have a lot more features than the Speedpuck for less money. But, I like the large display and simplicity of the Speedpuck. I have a small keelboat and do not like the idea of adding a throughhole impeller a real knotmeter. Too easy to damage with trailer and prone to clogging.
The Speedpuck seams like the perfect substitute for an impeller knotmeter for people in my situation. However, I am hung up on the price/feature ratio. I don't need all the other features of the handheld GPS. Has anyone tried the speedpuck as as substitute for an impeller knotmeter? Is the Speedpuck a good choice over the handheld GPS? Are there other similar products to the Speedpuck at a more reasonable price point?

chef2sail 02-24-2010 08:04 PM

We use the knotmeter for sail trim. GPS is more for position of the boat and relative speed vs tides, currents, ect. Two different uses.

scraph 02-24-2010 08:43 PM

I'd still recommend a paddlewheel-style knotmeter. You can go to Airmar and purchase such an instrument in a retractable model. The thru-hull will be included ... and features an internal valve to limit in-flooding of water when the knotmeter is removed. You can remove it while in the water to clear any possible clogging and can remove it prior to trailering to prevent damage. In its place you insert a flush ended plug.

You could purchase an Airmar ST800 (NMEA 2000 compatible Waterspeed/Temperature instrument) for around $80 and get a Lowrance LMF-400 (NMEA 2000 multifunction display gauge) for around $200. Or, any other NMEA 2000 compatible display.

Once you move to NMEA 2000 intrumentation ... you won't be getting yourself into a situation where you have to purchase yet another dedicated display for a new parameter. You simply purchase a new instrument and the new data will be displayed on whichever display you own.

I just purchased a Garmin GMI-10. It is a multifunction display of the same size as standard marine instrumentation. It can be configured to display all sorts of data (wind, water, GPS, navigation, engine, tank levels, etc). Right now I have it displaying a graphical compass in the background with bearing on top. Below that is water speed and depth.

In summary, I'd recommend looking at NMEA 2000 multifunction displays and seperate instruments before you start buying dedicated displays/instruments for each parameter you'd like to see. In the end it will save you money.

negrini 02-24-2010 09:27 PM

Skyamsen, if you plan to have both, buy GPS first. It will assist you in much more activities and information regarding navigation. Although limited for trim use, it can provide lots of useful information like VMG to a point. Then, when budget allow, you improve and fine tune your trim by using a knotmeter. These unities are good for relative speed in short term, as those paddle trying to measure your velocity is simply unacurate for absolute readings.

sailingdog 02-25-2010 05:52 AM

I doubt the OP much cares, as his post is SEVEN FREAKING YEARS OLD... If he hasn't gotten a GPS or a Knotmeter by now, he's probably dead.


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