This map, with integrated odometer, may not put an end to all those discussions over how far you actually travelled, but will sure elevate the discussions to new levels.

At my rowing club here in Hamburg, we enter every trip we make into the computerised logbook of the club. At any time, you can extract the total distance travelled by any member, and, naturally, it is fun to beat your friends by one or two kilometers. (That should really be nautical miles, but rowers are hopeless landlubbers. Those here in Hamburg anyway.)

After a number of (not altogether unpleasant) discussions over the actual distance covered, I manufactured this gadget. I am perfectly aware of the existence of GPS and other technical marvels that would have solved the problem more accurately, but accuracy (alone) was not the only design goal considered here.

The solution that I finally chose does work nicely when the number of possible routes are restricted, as is the case with the canals of Hamburg (within 1 hour rowing distance from our club).

I hope you will forgive me, but I did not take any pictures during the actual manufacture. Don't worry. Everything is quite obvious to build, once you have seen the close-ups of the finished thing.

Step 1: Functional Principle: Colour-coded Chord Is Wrapped Around Nails

The basic design idea is quit simple. You take a big piece of plywood, glue a map on it, hammer some 200 nails into it, and then add a colour-coded chord that can be wrapped around the nails.

In my case, the route is restricted to the waterways of Hamburg. The nails are placed at strategic points: where the canal turns, at good landmarks such as bridges, at other watersport clubs.

Step 2: Manufacture of the Nails

As you may have noticed in the previous "step", I did not use actual nails.

I did not want my pins to have heads since that would catch the chord, making it more annoying to unwrap it. Apart from that, I suppose the chord would wear faster if pulled repeatedly from nails with heads.

Since I couldn't figure out who would be interested in completely headless nails, or where one can buy them, and since I just happened to have several metres of shiny, stainless steel 2.5mm rod, and a good cutter for it, I rolled my own.

Since I needed about 200 of them, I put some effort into figuring out how to reduce the time spent on each to a minimum. The pictures below show how I pimped my cutter in order to be able to cut the rod quickly to pieces of (about) the same length.

One end of every nail was then ground flat, with bevelled edges. Once again, this was with the longevity of the chord in mind, but also so that my rowing friends wouldn't hurt themselves every time they come to close to the board.

At first, I thought I would have to drill a hole for every pin, and then hammer them in place with some glue, but then I found out that it was possible to force them in place like a nail, even though they have rather blunt points. I used a small sledge hammer, and the pins did not keep track as well as real nails, but I saved loads of time!

Step 3: Close-up of Fairlead Eye

The eye that brings the chord smoothly to the front (and back again), is standard sailing dinghy equipment. This model is heavy-duty with a stainless steel reinforcement. It looked somewhat rough, so I filed and polished a bit in order to reduce the wear on the chord.

Everything has to be pretty heavy duty, since we row A LOT. (We are talking about more than 100Mm per year. One lap around the earth is just 40Mm, and Hamburg isn't California exactly...)

Step 4: Pull Out Block

In order to keep the chord from disappearing completely into the hole, and to facilitate the pulling out of the chord, the chord was threaded through a small ball bearing block.

The pieces of coloured band was not just added to make things more chic, it also serves as a nice handle to grab when pulling out the chord.

Step 5: Final Touches on the Front

The map is only held in place with wallpaper glue, so I thought it was a good idea to protect its edges with an aluminium frame. Before the frame was screwed in place, the explanatory notes were glued in place. It looks professional, and their edges have to protected too.

Step 6: Secret Magic at the Back

On the flip side of the board all the magic that "sucks" that chord away after use, is hidden.

Gravity needs no batteries, so I opted for a simple tackle with 4-fold advantage and some ballast. It had to be as much as 4-fold, since I did not want the counterweight to appear below the lower edge of the map when at rest. Furthermore, I wanted to be pretty sure that only the most insane of my rowing friends would ever exhaust the capacity of the system. With the current design, you would have to endure 30 km at the oars (sculls) to pull off that trick.

Step 7: Counterweight

For a positive restoring action for the chord -- despite the fact that several ball bearing blocks were used in the system -- 1/2 kg ballast had to be used. I happened to have the necessary amount of lead at hand. If you don't happen to spend some of your time using up diabolo pellets, suitable lead weights might be had at your local angler's shop.

I would really have preferred a small bag over the plastic box shown in the picture, but I needed to pick a container that did not hang down too much. I wanted to maximise the amount of chord the system could swallow with the 4:1 purchase.

More blocks would lead to more friction, so I was reluctant to increase their number.

Step 8: Zero Adjustment

The fixed part of the chord isn't tied down directly. A few loops of string between the chord and the mounting point makes the zero point of the system easier to trim accurately.

Step 9: Block Leading the Chord Through the Reinforced Eye.

This was cheaper than buying a special block. Most of the construction are made of stuff I already had at home, so every piece of extra luxury would have increased the cost considerably. At least in relation to the total cost :-)

The below construction was originally meant to hold a normal single-sheave block. Being very small, lacking ball bearing, and at the point of the largest transverse forces in the system, it proved to be a major source of friction. In order to reduce the overall friction of the system to acceptable levels, I finally had to replace this block by the sheave - with integrated ball bearing - shown in the picture.
Amazing stuff!! As a non-nautical cartographer I am deeply impressed.
Very Interesting: cheap, simple and useful.

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