Best way to measure speed?

[lukewhiting]

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?

[GJ0KYZ]

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.

[lukewhiting]

Originally Posted by GJ0KYZ

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

[captnjack]

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.

[Brendan Townsend]

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

[Nos4r2]

Garmin Foretrex of some description. Waterproof, wristwatch type GPS.

[Cookee]

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!

[lightning]

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....

[lukewhiting]

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

[Lee argyle]

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+

[Mollers]

Little boats always feel fast until a big one goes sailing past. ;-)

[lukewhiting]

Still think 36mph is reasonable? The second video is the race class that mine is tuned. You can tell its a lot faster than 36mph and more like the one in the video

https://www.youtube.com/watch?v=RhMF_1_QZ9g

https://www.youtube.com/watch?v=xoyfcQE3PU4

[Mollers]

Originally Posted by lukewhiting

Still think 36mph is reasonable? The second video is the race class that mine is tuned. You can tell its a lot faster than 36mph and more like the one in the video

]

. I'll race ya. :-D Had 47.2knts and climbing with three big blokes and a heap of camping gear coming back from the Scillies at the weekend.

[Cookee]

Try speedbox, max speed, speedometer or GPS speed amd let them get going for a while, check them against another GPS or a car and you'll be certain that he phone is accurate.

It might have a surf prop on which will focus on acceleration and not top speed - is it running out of revs? You can get something called tiny tach (Tinytach website) or something similar which is useful for setting one of those things up.

[lukewhiting]

Originally Posted by Cookee

Try speedbox, max speed, speedometer or GPS speed amd let them get going for a while, check them against another GPS or a car and you'll be certain that he phone is accurate.

It might have a surf prop on which will focus on acceleration and not top speed - is it running out of revs? You can get something called tiny tach (Tinytach website) or something similar which is useful for setting one of those things up.

Just tested it against the car and the phone was 16mph under when I drove at 30mph!! It's not a surf prop but I'm not sure what pitch it is because it came with the boat & all the serial numbers have worn down.

I used to have one of then but I managed to get water in it.

Ill just borrow a GPS & see what happens but either way I'm not too fussed just interested

[treerat]

Originally Posted by lightning

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

Have you checked the settings? I use a Venturer and it has a marine setting, lat and long rather than grid and knots

[lakelandterrier]

True speed....borrow a proper old fashioned trailing Walker log & measure v. accurate time over set distance. Might be a bit tricky on a Zapcat, but I'd rate proven low tech over electronic gizmos. And if you can calculate the impact of the drag of the towed log even better.

Not a serious suggestion, but might be an interesting experiment.