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A Ratemeter for only £6.99 ?
You can avoid spending zillions of pounds on commercially available ratemeters by following a few simple instructions and installing the ratemeter shown here.  Get down to Argos and get yourself a Raleigh 6 function computer for your bike, Cat.No. 333/1343 at an all up cost of £6.99 (Most other cycle computers where you can input the wheel size in mm will also work). Fix the magnet to the seat and the sensor to the boat so that the magnet moves past it. Fit the "rate meter" to the foot plate. (You may have to extend the length of the wire between sensor and meter depending on where you mount the meter). Set the mode to "D" give you the revs per minute of your cycle wheel at the bottom of the screen. The clever bit. At the top of the screen in "D" mode is the bikes speed in either MPH or KPH based on the size of the wheel. By working the ratios out you will see that if you set the wheel size to 17 inches and then set the read out to MPH the figure at the top of the screen is exactly half that of the revs per minute figure at the bottom of the screen and is therefore your actual stroke rate (ignoring the decimal point which appears on the display, but you rarely notice it anyway!)

Additional information:
David Raine points out that because the Raleigh meter will only allow you to set the wheel size at increments of 1 inch, 17" is the appropriate option to select. However, the correct wheel size should be 16.807 inches to give an accurate rate readout (the 17 inch wheel size gives a reading which is actually 1.15% too high).

With other cycle computers such as the Sigma Sport series, it is possible to set the wheel diameter more precisely, as you are not restricted to sizes in whole inches.

David comments:
I was interested to read the correspondence on the bike speedometer being used as a rate meter by fitting the supplied magnet to a boat seat with the sensor fixed to the deck so I did a check on the accuracy of the system.

Let the diameter of the bicycle wheel be D inches.

The circumference of the wheel is π.D inches, or π.D ¸ (36 x 1760) miles

For a stroke rate of R per minute, this is seen by the system as 2R per minute since the magnet on the seat passes the sensor twice per cycle or stroke.
i.e. 2R x 60 per hour.

The system sees the distance travelled in one hour, i.e. the speed in miles per hour, as
2R x 60 x π.D ¸ (36 x 1760)

For the device display to read the rate R, then
R = 2R x 60 x π.D ¸ (36 x 1760)

i.e. D = 3 x 176 ¸ π = 168.07 inches

i.e. If you set the bicycle wheel to 17 inches the device will read the rate as R/10

So that a rate of 34 strokes per minute will be shown as 3.4 m.p.h.  - This is about 1.15% too high.

If the device manufacturer allowed for the fact that a bicycle wheel of a given actual diameter actually has a different rolling diameter to take into account deformation, then the result will probably be less accurate.

So it is really quite a good approximation and well worth using if it gives a stable enough display.

You could make it display at multiples of the rate by putting several magnets in a line, ten giving the actual rate to one decimal place, although the required fabrication would be a disincentive.