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Go Back   SailNet Community > On Board > Gear & Maintenance > GPS vs Knotmeter
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Thread: GPS vs Knotmeter Reply to Thread
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Topic Review (Newest First)
12-26-2011 07:13 PM
paulpangrazzi
But I care... ;)

As old as the original post was... I find myself in the same position with a 1988 Beneteau F235 (but new to me.) It's chock full of frozen, non-working parts and I'm grateful for this thread. Thanks.
02-25-2010 12:12 PM
negrini Good point SD. Considering the savings, he might be able by now to purchase his knotmeter ... **** I just jumped in too fast
02-25-2010 07:22 AM
sailingdog Well, usually within a year or two of deciding I need it... yeah...

Quote:
Originally Posted by 2Gringos View Post
youre one of those guys who just rushes out and buys something he needs the same year, ain't you...
02-25-2010 07:13 AM
2Gringos youre one of those guys who just rushes out and buys something he needs the same year, ain't you...
02-25-2010 06:52 AM
sailingdog 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.
02-24-2010 10:27 PM
negrini 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.
02-24-2010 09:43 PM
scraph 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.
02-24-2010 09:04 PM
chef2sail 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.
02-24-2010 08:44 PM
gskup
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?
04-28-2003 06:48 PM
petercra
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.
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