English: Measuring distance through the water, before electronics, might have been easier if vessels made a constant speed. Drop a small floating object overboard near the bow and time its passage alongside to a measured distance aft, and you can figure it out. At 6 knots that floating object moves about 10 feet per second, and the rest is simple arithmetic. But vessels don’t move at constant speeds …

For long passages, with varying speeds, the answer came in the form of a log, a marine odometer of simple design. Sometimes called taffrail logs because the instrument was positioned way aft, at or near the taffrail, they consist of very simple parts:

- The register, or counter, is suspended at the taffrail and counts the turns of a long line trailing aft in the vessel’s wake. The forward face of the register is a dial with hands pointing to the distance run through the water in nautical miles.

- The logline, affixed to a small flywheel on the aft face of the register, leads aft and down into the water, to the rotator. It is braided line, treated to resist water intrusion and designed not to kink up as it is twisted by the rotator.

- The rotator is a streamlined “fish” with fore and aft fins set at a slight angle so that when it is pulled through the water it turns in relation to the speed. Ideally the rotator rides slightly below the surface. If it is visible at the surface it will not turn so fast and under-register distance. The solution is either to lengthen the line or add a sinker (which the line passes through) just ahead of the rotator.

The spinning rotator turns the logline, which in turn spins the flywheel on the register, and a train of gears inside the register housing moves one or more hands on the face of the register. Most log dials show tenths of miles and miles. Resetting the hands on the dial to zero for a new trip is performed simply by moving the hands with one’s finger. Now … why have a log on your boat today, with all the electronics at hand? It’s easy: back-up. By being purely mechanical, a log needs no power and very little attention. Oil the gears every 500 miles or so, and that’s all. When the power goes off you’ll have your log, and you’ll be very glad for it. Knowing how many nautical miles you have traveled through the water is critical information. Dividing distance run by the time gives average speed, another useful fact for the navigator. There are two models offered, both by Thos. Walker & Son, Ltd., the premier name in taffrail logs for a most of a century. Production stopped in the early 1980s.

The Excelsior is best suited for yachts with a normal speed range of 5-10 knots. The rotator is about 11’ long and at very low speeds will sink, reducing its accuracy and increasing the chances of encountering weed, etc. as it hangs well below the surface. The Excelsior register is cast bronze and has two hands on its dial. The hand on the smaller dial reads single nautical miles up to 10; the hand on the larger dial reads up to 100 nautical miles. The standard length of logline is 100 feet. This model was available in either Sling or Outrigger pattern (see below).

The Knotmaster is best suited for yachts with a normal speed range of up to 6 knots. The rotator is about 5” long, and because it is relatively light in weight it will rise to the surface at higher speeds, reducing its accuracy and necessitating either longer line or an additional sinker. The standard logline is 2 30’ lengths. The register has a very strong bakelite case. The dial has three hands and three dials: one reads tenths, another single miles up to 10, and the last miles up to 100 [robertwhite.com]