Which would you trust

KREW2

KREW2

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I have a hand held garmin XL12 gps mounted in the cockpit, and an over complex micrologic admiral down below. Today on a run with just full genoa and no main and 18/20knts of wind the garmin showed 5.9knts the micrologic 4.9knts there was a discrepancy of approx 3/4 to 1 knott all day
 
Neither!

Get another one (beg, steal or borrow - your choice) and run a comparison then return the duffer to the makers.

Are you sure both were reading NM and not one reading Statute Miles and the other NM - was the speed reading the same or different. Both setup the same way? DGPS/EGNOS etc. - Just a thought.
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3/4 to a knot - not worth worrying about.
Do you have 'averageing' on?
If so are they averageing over the same period of time?
The best way to get an indication of speed is to look back in the memory over any previous hour and see what distance has been run, assuming you have been heading in a straight line /forums/images/graemlins/smirk.gif
 
thanks for your advice I did't know you could have different set ups I just looked at the speed on both, the micro logic has so too many facilities I only use it for getting a position and SOG. With todays technology I thought they would both give Identical readings I will go out again friday and play with the settings and see if lat and long differ
 
the garmin 12 reads in mile per hour so when you work it out if the other reads knots they work out exact
 
thanks I'll check it out, because I use it on a boat you automatically think Knots. Perhaps I should do a lateral thinking course! the reason I for asking is I want to go to Alderney next wednesday and I wanted to know which is the more accurate reading for SOG I find the only use I can get out of a GPS is a position. I think I will try and find a course that will instuct me how to get more out of a GPS. Dead reckoning is dead boring
 
If you are heading to Alderney I would suggest that COG is more important than SOG.
Set your WP then in the last couple of hours of your passage if COG is the same as CTW (Course to WP) then, not withstanding tide changes, you are on track for your destination /forums/images/graemlins/wink.gif
 
They all have, I understand, the facility to set and alter the Units of Measurement - really, the Units of Display. These options/facilities are buried deep down in the layers of pages, and one needs the relevant Owner's Manual ( from maker's website ? ) or a handy schoolboy.

GPS receivers calculate SOG from the difference in a series of sequential ground positions - sometimes over quite a short time-interval - so this gives a short-term speed which can fluctuate quite rapidly - despite the 'damping' algorithm. It's worth doing the occasional 'conventional' calculation of SOG, by plotting a series of positions at 12 minute intervals, then multiplying the distance between any two in sequence by 5, to get distance run in 60 minutes i.e. Nautical miles per hour....

It's also easy to see changes in trend due to tide-change, eddies or other factors.
 
If your h/h Garmin works same as mine...

...then unit of measurement is set by going to the 'main menu' page, scrolling down to 'setup menu', entering that, scrolling to 'navigation', entering that, and scrolling to 'units'. This will show one of three choices, 'nautical', 'metric', or 'statute'. Change between them by presing the down part of the cursor button. You want nautical for knots and NMs.
 
Statute vs Nautical ...

5.9 statute miles is approx. 5.1 Nautical miles.

Check settings for display units .....

also

check damping or averaging ..... as you may have one set to little or no damping / averaging and other to high level. This is obvious by watching them ... if one fluctuates at very short intervals and widely on speed / course - then its averaging is near off ..... this is a function that tries to smooth out the variations in speed / course made good because position calcaulation accuracy varies and it jumps around .....
 
Re: If your h/h Garmin works same as mine...

Thanks very much for all your help I've just followed your instuctions and reset it. (it was set to mph) I can handle ropes but I must learn to use a GPS other than just for position, do they do a book GPS for dummies? thanks again
Keith
 
Re: If your h/h Garmin works same as mine...

On the subject of calibration..... Advice sought please!

I have a Navamn GPS/plotter and Navman log. I have tried to calibrate the log from the GPS by motoring in light winds along a straight, 1.5 mile stretch of non-tidal water. Despite using the maximum correction on the log, I can still only get it to read about 0.5 knots less than the GPS (they are both set to Knots not miles). Any thoughts of what to try or do next. Also I note that the difference can vary up to 0.75 knots at different speeds! The log sender if fitted directly infront of the keel of the boat - could this be a problem?
 
Re: If your h/h Garmin works same as mine...

Water speed logs are never 100% accurate at all speeds. I set mine to be accurate at around my normal crusing speed under sail.

If adjustment is out of range as yours is, then it is likely the impellor is mounted in an area of 'dead' water on the hull. Just ahead of the keel there may be an appreciable bow wave effect which is reducing the water flow speed over the impellor, and which will, as you have noted, give different effects as speed varies.

Impellors need to be located in a clean water flow - no significant protrusions forward of it, or within around .3 metre to the side or aft.

One boat I had the impellor was mounted between the bilge keel and ballast stub. Accurate at 3 knots, it was nearly a knot over at a true speed of 5kts, due to a tunnel effect between the keels! Adjusted to 5 Knots it grossly under read at 3....
 
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