Page 38 – Water Speed Measurement

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Water speed? Easy! Use a mobile phone app!

Sadly, they are not very good. It may get better as time goes by, but there are basic issues.

They are designed to give instantaneous speed. In a two second refresh interval, the boat has probably moved 15 feet? The sat nav coordinates are not changed much. The timestamps may be very accurate, but the figures on the screen flicker too much.

We need to know whether we are doing 4.3 mph or 4.4 mph.
Readings that are not steady and flicker by 0.2 mph don’t inspire confidence.

There are practical problems.
Motorways are well served with mobile phone mast coverage. Rivers are not.

The river also flows – so upstream speeds are different to downstream speeds.
Head/tail winds have the same effect.
And another factor – rivers are not straight. Far from it.

But how do boats measure their speeds? Most river boats don’t.

Commercial boat speed meters use paddle wheels and are intended for far higher speeds.

I have even calibrated a water flow meter paddle wheel with an arduino and towed it
alongside Stirling craft. It worked, but I have found a better way.

When the Thames authorities instituted a speed limit on the Thames, they had the same
challenge. The placed transit posts at various places along the river. The posts were
spaced so that if you passed them more quickly than one minute, then you were exceeding
the 5 mph speed limit.

So you need still water, no wind, and a decent straight run.

My solution is a timer on my wrist watch and a notebook.

My favourite place is upstream of a lock that has a long, relatively straight, lock cut.
When the boats exit the lock, going upstream, there is no water flow.
Both banks are tree clad. Wind is low.
When exiting the lock gates, I start the timer. When I reach the end of the lock cut,
I note the time.
Bridges are equally good markers. Or if there is a large boat moorings near you, they
are often still water and you can pick a straight stretch.

Back home, I use Google maps to measure the distance.
The “right button” gives a menu to measure distance. You can trace a curve to follow the river bends. 2,184 feet in 5 mins 36 secs = 4.43 mph