Chart Plotter Accuracy

[Vipersdad]

Can anyone comment on GPS accuracy.  I was hitting the high 5's in knots yesterday on a close reach with the genoa and a full main.  These readings were off of my new Garmin 547xs Chartplotter/Sounder.

Thanks,

V.

[wes]

Very accurate I would say, but remember they are measuring speed over ground, not speed through the water, so with a following current you may appear to be exceeding hull speed. Go ahead and congratulate yourself on brilliant sail trimming; your passengers are unlikely to know the truth  .

Wes

[Vipersdad]

No currents in the lake I was on but with the big genoa up and no big swells she was making good time.

V.

[HeaveToo]

That is very possible.  The hull speed is around 6 knots and most boats will make it to hull speed from time to time.  Also, a reach is a fast point of sail.  A beam reach or just aft of the beam can be really fast.

GPS is usually pretty accurate.  Where I sail there tends to be tidal current so I am always seeing different speeds.  Eventually you learn the boat enough where you can tell an approximate boat speed based on the way it is behaving, where the bow wave is, the way the dingy tows, or other things like that.

Remember that lakes can have current too.  If they are formed by damming a river, the current will flow towards the dam.  There is also wind driven current. 

[InertBert]

I've recorded a max speed well over 7kts on my gps in my CP16 and my ego requires that to be an accurate number. 

Actually, I have checked a chartplotter GPS system against resection triangulation on charts and done the math and it turns out to be about as accurate as my rounding errors.  I think GPS is as reliable as it gets for ground speed numbers, so if you want to know when you'll be pulling up to the dock, GPS is it.  However, water speed is a better indication of sail trim and when you go by GPS you can fool yourself easily and lose a bit of time in the long run.  Trust the GPS, verify with your compass.

[Koinonia]

I agree with speed, before our trip I carefully calibrated my knot meter with the gps since I was in still lake water and got the knot meter within a tenth of a knot.  On the trip I could tell what my currents were very well.  I was usually doing 6.2 knots through the water and at one point leaving key west we were doing 8.1 knots over ground. 
    On another note what I have is a Navman 5500 plotter mounted over my wheel and it was super accurate with its mapping in all but one place.  It was a saving grace tacking upwind through Key west NW channel late at night but in Pine Island Sound on the FL west coast it was off by about 100 ft in the ICW.  I couldnt tell you how many spoil areas and islands I motored through.  This just goes to show people still need to use some sense and paper charts are still needed!