Boat speed indicator

[Terence Carleton]

I recently purchased a ‘Sailproof’ android tablet, at great expense from Belgium, so that I could use the ‘Navionics’ chart plotting software. The tablet is designed for yachting. It is rugged, waterproof, with built-in GPS chip and an exceptionally bright 10″ screen that can be viewed in full sunlight. On first use two days ago, I noticed that the indicated boat speed, in knots on the tablet, differed greatly from the speed indicated on my Mac26M built-in depth/speed indicator. I was travelling on a calm day, very little wind; old Johnson 90hp motor at 2000rpm.The tablet showed my speed at about 50% greater than the built-in device of 3kts. My intuition told me that the tablet, running ‘Navionics’ was correct but I have no way of confirming this impression. Any similar experiences from others please?
Cheers,
Terry Carleton

[Alan & Belinda]

What sort of speed sensor is built in? The tablet will be GPS speed over ground, the sensor may be reading speed thru the water.

[Terence Carleton]

Yes. The 26M speed indicator is indeed reading water speed, independent of the gps-based chartplotter on board. In the Salish sea – I am based on the Saanich Peninsula – tidal currents through the various channels will affect water-based speed indicators. Nevertheless, I would expect more consistency between speed estimation methods. My tablet covers three different satellite-based position indicating services, including Galileo, the latest and most accurate at a claimed 1m resolution. Clearly more research is needed as different software will affect the interpretation of satellite-based methods. Topography/altitude, however, is not a confusing issue at sea level. I will try testing different software programs at constant speed with the tablet.
Cheers,
Terry Carleton

[Richard Robinson]

Terry – what is your ” Mac26M built-in depth/speed indicator” connected to? Who makes the device, and what type of a sensor does it use?

On my fishing boat, I have an older Humminbird unit and the speed is determined by a little water-wheel device that sits in the water right next to the transducer and the default reading is rarely accurate, and it gives STW (Speed Through Water). In the Humminbird master settings, there is a +/- setting that allows you to adjust to “Actual” speed which can be measured by Navionics (which uses GPS “SOG” – Speed over Ground) when you are in a no-current environment. Hope this helps.

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