ALL DEPENDS on proven functionality of the speed transducer- first things first, remove transducer and test…
Update 04/11/2011: The speed log transducer was removed from the boat, drilled out, and a new reed switch wired and seated, and the whole unit was then re-potted and placed back in the hull.
It seems the "Teensy" may be the best board for the jobs. The board retains a USB connector, so will be easy to update/troubleshoot.
Display Unit: the main unit should have two or three RS232 NMEA0183 inputs. First function button should scroll through those inputs, a long hold should change the number of inputs on the display (i.e. one input, two inputs side by side, two inputs vertical, three inputs, etc.). Ability to set input type (depth, speed, GPS location, GPS Speed, others?).
Speed sender: leads to the reed switch in the sender are the simplest to extend, so a teensy should be enclosed along with power toggle, and calibration momentary switches. Output of the teensy should be NMEA0183 serial over RS232.
Depth unit: should be an “intelliducer” type, which outputs NMEA0183 sentences.
Possible displays:
Light on dark 4-line Serial with Bell (expensive at $60)
Large Graphic, Serial, E-ink type operation (expensive at $80)
OLED two-line with Large Fonts (also expensive at $50) – I just read the OLED specs, +10C to +35C with no direct sunlight… not so good for this application.
Someone else's Arduino based speed log: http://www.holdentechnology.com/component/content/article/36-sailing-projects/58-speedo
Older thoughts:
Best I can get this described in ASCII:
D = distance in nautical miles
T = time in hours
S = speed in knots
D/T = S
X = Number of clicks counted while traveling a calibration mile
Y = Number of clicks counted in a single sample period
Z = Sample period in Seconds
D = Y/X * 1 n.m.
T = Z/3600
So: (Y/X * 1 n.m.) / (Z/3600) = Speed in knots for the sample period Z
E.G. If we calibrate 1 n.m. at 2500 clicks (X=2500) and our sample/update period is 2 seconds (Z=2), and over that period we count 10 clicks (Y=10), then calculated speed would be (10/2500)/(2/3600) = S = 0.004 / 0.0006 = 6.67 knots.
Hopefully we get faster turn rates that 5 rotations per second, otherwise it won't be very precise.
http://store.seetron.com/bpp420l-4x20-serial-lcd-with-led-bac420420.html
http://www.seetron.com/bpp440_1.htm - $140
http://www.seetron.com/bpp420_1.htm - not black on white, $59, $12 bezel kit - large numbers with punctuation
http://www.seetron.com/pdf/bpp423.pdf
→ http://www.matrixorbital.com/lk20425gw-p-11.html - blue on white, $89 for extended temp (-20C - +70C) and 9-15v range, large numbers - could be good. manual here: http://www.matrixorbital.ca/manuals/LK_series/LK204-25/LK204-25.pdf
http://www.matrixorbital.com/lcd-character-lcds-c-1_2.html
http://www.matrixorbital.com/lcd2041gw-p-71.html - black on white, $69, includes large numbers
http://www.matrixorbital.ca/manuals/LK_series/LK204-25/view?searchterm=None
http://moderndevice.com/LCD.shtml
More expensive, but viable: http://www.crystalfontz.com/products/index-ser.html
Some decent info: http://reefprojects.com/wiki/Main_Page
Sparkfun has some interesting displays, this one isn't serial, but nice looking and cheap: http://www.sparkfun.com/commerce/product_info.php?products_id=710