(Important: This idea is not new or original. Others have provided similar lists/charts. It is just a reminder for new people (like myself). Anything that makes producing a firing solution quicker is good.)
This is useful if you don't happen to have the wonderful Ujag timepiece on your boat.
Scenario: calculating a ships speed by how many seconds it takes to travel its own length.
Typically it is done by ships length in meters, divided by the seconds you observe, then multiply by 1.85 to get a ship's speed in knots.
The list below takes out an extra step.
Pick the "speed number"* (made up the term) for the length of ship you are tracking. For example: "144" for a ship that is 78 meters long. Divide it by the time in seconds it takes for the ship to travel its own length. For example: 30 seconds and you come up with the following.
144/30=4.8 knots.** Your ship that is 78 meters long, is moving at a speed of 4.8 knots.
Ship length: 78 meters long. Speed number: 144
Ship length: 94 meters long. Speed number: 174
Ship length: 140 meters long. Speed number: 259
Ship length: 150 meters long. Speed number: 278
Ship length: 160 meters long. Speed number: 296
Ship length: 170 meters long. Speed number: 315
Ship length: 180 meters long. Speed number: 333
Ship length: 190 meters long. Speed number: 352
*Rounded to nearest number.
** Note: as I rounded out the speed number for simplicity, the actual speed number for a 78 meter long ship is 144.3. Thus the ship would be moving 4.81 knots for this scenario.
