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Old 31 July 2013, 17:50   #1
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Best way to measure speed?

On my thundercat I don't have any way to measure the speed apart from the GPS on my phone. However the highest I have ever got it is 36mph?! & when your over taking taking boats with 135hp+ on the back that are clearly trying to keep up, you know your doing a lot more than 36mph more like 50+

Are handheld GPS better than speedo apps on phones? Or what other ways could I find out the speed?
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Old 31 July 2013, 18:00   #2
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Ask one of your mates to sail alongside you at the same speed. Then compare GPS readouts. If they're the same then I'd say your original GPS reading was accurate.
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Old 31 July 2013, 18:02   #3
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Quote:
Originally Posted by GJ0KYZ View Post
Ask one of your mates to sail alongside you at the same speed. Then compare GPS readouts. If they're the same then I'd say your original GPS reading is accurate and that you would have to ditch your current outboard and buy an Etec to improve your performance.
Etec on a thundercat?! They wouldn't last a minute haha
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Old 31 July 2013, 18:10   #4
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At least around here, many charts have measured miles marked on them. E.g. between 2 landmarks. This is all pre-GPS stuff, so you could determine your speed over ground at a given RPM with a stop watch. A reference you'd write down and use for dead reckoning purposes. Currents would need to be accounted for obviously.
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Old 31 July 2013, 18:37   #5
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I'm new to both RibNet and boating, but I do work in the electronics industry with interests in physics.

GPS devices all work from the same basic principle, though readings could vary - based on the way they attempt to integrate correlated satellite readings in software to a meaningful display (e.g. mean-average, median, peak-hold, etc). Only if the speed were sufficiently constant should all methods read the same, though I'd guess bouncing around on water could have significant impact in the result.

If it were me I would simply borrow a few devices (phones/software or other) and carry them on the boat, comparing readings at given speeds.

As for absolute speed, the relationship between power and speed isn't linear of-course, and there's power-to-weight to consider. Sounds like you're really motoring though, whatever the actual pace


All the best,

Brendan
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Old 31 July 2013, 22:42   #6
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Garmin Foretrex of some description. Waterproof, wristwatch type GPS.
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Old 01 August 2013, 03:11   #7
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Try your phone GPS in your car and you'll find it's probably reading ok (the car will read a bit higher - they all do) - It's a disappointment but real speeds are pretty much always slower than you think they are!
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Old 01 August 2013, 04:16   #8
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I have a phone app for speed (speedview pro) and it does work but keeps freezing so I gave up with it.
I bought a cheap GPS device designed for hikers from ebay second hand for 40 (Garmin Gecko I think it is called) which has a speed readout and this works fine.

It doesn't do knots of course, but MPH is OK for me.

An added advantage is that it plots your route. Useful the other week on Loch Sween where we went the wrong way coming back from Tayvallich to Achnamara and got lost....
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Old 01 August 2013, 04:23   #9
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Ill try borrow a GPS from a mate. I've gone up against zapcats before & have never been beaten by one in top speed so unless they all went 30mph then it's the app that's rubbish
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Old 01 August 2013, 04:30   #10
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My brother-in-law runs a zap cat / 50 tohatsu, running alongside and using my GPS he tops out early 40's - normally I then open my craft up and leave him for dead...........70+
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