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Catalina/Capri 25/250 Sailor's Forums

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Which reads Faster: GPS or Speed Meter?

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ssteakley
Captain

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USA
467 Posts

Initially Posted - 09/12/2003 : 20:39:53

OK,
I am trying to calibrate my Standard Horizon paddle Wheel transducer type speed meter. The instructions say to have a calibrated boat motor along side and match their speed on the meter...well I do not have such a luxury so I want to use my Hand Held GPS to get it close enough. I know that the GPS measures speed across the face of the earth and the paddle wheel measures speed through the water, but which would read faster??
thanks,
Steve

<img src="http://im1.shutterfly.com/procserv/47b3dc32b3127cce94d57c8cff3b0000001010" border=0> Moon Chaser #385WK <img src="http://www.catalina25-250.org/forum/forum1.gif" border=0>

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 09/12/2003 : 20:58:14

The wheel is essentially instantaneous, and the GPS uses an averaging algorithm that, on most units, updates every 3-5 seconds based on the position readings it takes over 10-15-second periods. Since you're on a closed lake, you can expect the GPS to be fairly accurate through the water. Make your comparisons when you've reached an apparently constant speed.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - Dave Bristle on 09/12/2003 21:01:32

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ssteakley
Captain

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USA
467 Posts

Response Posted - 09/12/2003 : 21:09:13

Dave,
does that mean the GPS and the speed meter should match if the speed remains constant?
Steve

<img src="http://im1.shutterfly.com/procserv/47b3dc32b3127cce94d57c8cff3b0000001010" border=0> Moon Chaser #385WK <img src="http://www.catalina25-250.org/forum/forum1.gif" border=0>

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Oscar
Master Marine Consultant

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USA
2030 Posts

Response Posted - 09/12/2003 : 21:26:20

Yes, unless the lake is moving......

In tidal water or rivers with current, the boat speed through the water is different from the speed over the ground (SOG), because the water is moving. Impeller measures the first, GPS the latter. (If you have trouble envisioning that, picture a boat traveling at 4 knots, going directly into a 2 knot current . Boat speed is 4, but because of 2 knot "head" current , the SOG is 4-2=2, hence GPS would read 2)

There is another way to calibrate. If you can locate prominent landmarks with a known distance between them, you can measure the time it takes to go from abeam one to the other and figure out your speed. (This would be average speed, so attempt to travel at a constant speed, ie. constant rpm.)

Oscar
250WB#618 Lady Kay on the Chesapeake
<img src="http://www.woodenshoemusic.com/Images/familypics/Forumshots/sunglitter.JPG" border=0>

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 09/12/2003 : 22:40:25

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
...does that mean the GPS and the speed meter should match if the speed remains constant?
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
Yes, as long as you aren't in currents (as Oscar said). But variations will show up on the speed log some seconds before they do on the GPS, and the speed log may show shorter spikes and dips (such as when you accelerate on the front of boat wake or decelerate when you run into one).

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

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Bryan Beamer
Master Marine Consultant

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USA
1038 Posts

Response Posted - 09/12/2003 : 22:44:18

Hi Steve,

I use a GPS to calibrate my knot meter. I do it under motor. It is much easier to get a constant speed for a long period of time.

Bryan Beamer
<img src="http://im1.shutterfly.com/procserv/47b3d607b3127cce8d07b033366e0000003180" border=0>
Daylight Again
C250wk #495
2003 National Champion

Edited by - on

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 09/13/2003 : 00:19:43

although they should read the same (with no current) my GPS always reads faster (ex. 6 knots on the GPS 5.5 on the knotmeter).

<img src="http://www.indiscipline.org/cat25/pictures/icon.jpg" border=0>Indiscipline 1978 FK #398

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 09/13/2003 : 08:42:49

Actually... a gps unit has quite a significant error. Giving that the earths rotation propells us at about 1,041 mph in a rotation direction.... and the earths orbit propels us at some rather significant speed around the sun... and the solar system is traveling thru space at some astronomical figure... and the gps consellation is traveling at these same rates so unable to provide the correct figure... the error is pretty substantial....<img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle>

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.cox-internet.com/arlynstewart/"]Arlyn's C250 Mods n Cruisin Stories[/url]

Edited by - arlyn stewart on 09/13/2003 08:45:28

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Doug
Captain

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USA
457 Posts

Response Posted - 09/13/2003 : 12:51:05

I don't know why or how this lodged in my mind right now, but with special thanks to Python:

...Just - re-member that you're standing on a planet that's evolving
and revolving at 900 miles an hour,
It's orbiting at 19 miles a second, so it's reckoned,
the sun that is the source of all our power.
The Sun and you and me, and all the stars that we can see,
are moving at a million miles a day,
In the outer spiral arm, at 40,000 miles an hour,
of the Galaxy we call the Milky Way....

...The universe itself keeps on expanding and expanding,
in all of the directions it can whizz,
As fast as it can go, at the speed of light you know,
twelve million miles a minute, and that's the fastest speed there is.
So remember, when you're feeling very small and insecure,
how amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
because there's bugger all down here on Earth.<img src=icon_smile.gif border=0 align=middle>

Doug - #1913 Noeta

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 09/13/2003 : 23:05:28

When I left sailing 10 years ago there were no end user GPS products. What is the appropriate on for a lake sailor? All I want is speed and log, other than that its pretty pointless for a lake sailor. Which one should I get?

Frank and Martha in Wichita KS. Lake Cheney

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 09/13/2003 : 23:19:09

I really miss Monty Python!

Frank,

I recently installed a Raymarine bidata system (depth, temperature, speed and log), and have been thoroughly satisfied. In my opinion, anything else on a lake, excepting a Great Lake, is overkill. Although, many racers on our lake use waypoints set to the shorelines, especially during our annual 100 mile/24 hour race. However, I night sail weekly and don't miss it at all. As a matter of fact, I'd probably run aground looking at and trying to manipulate the damn thing. Of course, when I eventually make it to blue water I intend to have a dozen on board. <img src=icon_smile.gif border=0 align=middle>

J.B. Manley
<img src="http://im1.shutterfly.com/procserv/47b3d609b3127cce8d2e212441c60000002010" border=0>
Antares '85 FK/SR #4849
36°29'58" -94°59'59"

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Oscar
Master Marine Consultant

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USA
2030 Posts

Response Posted - 09/15/2003 : 11:44:44

I sail the Chesapeake with a chart, binoculars, hand held compass and a hand held waterproof Garmin GPS 76 (no map, $200 or so)......It works for me.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>And pray that there's intelligent life somewhere up in space,<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Of course there is......it's the motherplanet. Where the ones we descend from came from when they shipwrecked here on earth....<img src=icon_smile_shock.gif border=0 align=middle>

Oscar
250WB#618 Lady Kay on the Chesapeake
<img src="http://www.woodenshoemusic.com/Images/familypics/Forumshots/sunglitter.JPG" border=0>

Edited by - Oscar on 09/15/2003 11:47:51

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deastburn
Captain

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USA
334 Posts

Response Posted - 09/15/2003 : 12:26:02

Monty Python to the contrary notwithstanding...

My Horizon Speed/Log reads (always) about two knots faster on starboard tack than it does on port tack, where it is exactly calibrated to the GPS. Basically, I take my reading from whichever tells me I am going fastest. In the Cape Cod Canal I have often topped 11 knots... but only on the GPS

Dave on "Wood Duck" (#2616)

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