Speed number--figuring out ships speed

[Leoz]

(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.

[Leoz]

This may be of use to have next to your periscope and UZO things...






Time it takes for this 78 meter long ship to cover it's own length....

[Zosimus]

Do you really do that? Does it work?

I mean, normally when I am chasing a ship, I already know it's speed because it's part of a convoy that I found when the game announced a nearby convoy complete with its speed.

If not, I'm probably just shooting at it with my deck gun.

Otherwise, if I think it's big enough to justify a salvo, I'll just chase it for 32 minutes. A ship going 7 knots will cover 7 km in 32.4 minutes. That gives me lots of time to set up a shot. You'll know its course spot on, its speed, and be able to set up for an easy shot with time to spare.

[3catcircus]

Originally Posted by Zosimus

Do you really do that? Does it work?

I mean, normally when I am chasing a ship, I already know it's speed because it's part of a convoy that I found when the game announced a nearby convoy complete with its speed.

If not, I'm probably just shooting at it with my deck gun.

Otherwise, if I think it's big enough to justify a salvo, I'll just chase it for 32 minutes. A ship going 7 knots will cover 7 km in 32.4 minutes. That gives me lots of time to set up a shot. You'll know its course spot on, its speed, and be able to set up for an easy shot with time to spare.

You're joking, right? Chasing it for 32 mins? One can easily calculate speed. Take a range/bearing and plot it. Do it again 3 minutes and 15 seconds later. Measure the distance it has traveled and divide by 100. That's its speed in knots. You can also calculate based on start/stop of the stopwatch as its bow and stern cross the wire, knowing ship length.

[Zosimus]

Originally Posted by 3catcircus

You're joking, right? Chasing it for 32 mins? One can easily calculate speed. Take a range/bearing and plot it. Do it again 3 minutes and 15 seconds later. Measure the distance it has traveled and divide by 100. That's its speed in knots. You can also calculate based on start/stop of the stopwatch as its bow and stern cross the wire, knowing ship length.

No, I'm not joking.

Oh, and I think the number you're looking for 3 minutes 14.4 seconds. That is exactly 3.24 minutes. In fact, if you multiply that number by 10, you'll get the 32.4 minutes that I mentioned above.

But no, I don't measure ship speed over 3m15s because it isn't accurate. You see, even if you have map contacts on, it will surely take you at least a half a second to click the x on the screen and then turn the stopwatch on. Even then, you'll notice that that x is not exactly in the middle of that box. And the problem only gets worse if you zoom in or out. Suddenly the x isn't even near the box.

In fact, a lot of times you'll watch the ship for 6m29s only to find that it appears to have covered 1.3 km. Really? The ship speed is 6.5 knots? I find that hard to believe. Maybe we should watch it out to 9m43s to be certain whether it's speed is closer to 6 or to 7. I suppose you propose to do all that underwater, scope up while the ship is moving away from you. Brilliant.

Or what if you're hunting by hydrophone? How exactly do you determine the ship length from 25m under water at 14 km away? Do you use your psychic abilities?

Or are you one of the types who fires a few torpedoes off and, if they miss, you just reload your last save and try again?

[3catcircus]

Originally Posted by Zosimus

No, I'm not joking.

Oh, and I think the number you're looking for 3 minutes 14.4 seconds. That is exactly 3.24 minutes. In fact, if you multiply that number by 10, you'll get the 32.4 minutes that I mentioned above.

But no, I don't measure ship speed over 3m15s because it isn't accurate. You see, even if you have map contacts on, it will surely take you at least a half a second to click the x on the screen and then turn the stopwatch on. Even then, you'll notice that that x is not exactly in the middle of that box. And the problem only gets worse if you zoom in or out. Suddenly the x isn't even near the box.

In fact, a lot of times you'll watch the ship for 6m29s only to find that it appears to have covered 1.3 km. Really? The ship speed is 6.5 knots? I find that hard to believe. Maybe we should watch it out to 9m43s to be certain whether it's speed is closer to 6 or to 7. I suppose you propose to do all that underwater, scope up while the ship is moving away from you. Brilliant.

Or what if you're hunting by hydrophone? How exactly do you determine the ship length from 25m under water at 14 km away? Do you use your psychic abilities?

Or are you one of the types who fires a few torpedoes off and, if they miss, you just reload your last save and try again?

I'll assume you meant no offense since tone can't be implied from your text.

I've done real time/bearing and geoplots as part of a Section Tracking Party in a former life. For purposes of the game, the pause key is the equivalent to that Section Tracking Party made up of multiple people. The minimal amount of tine necessary to switch to the nav map and hit pause is actually a lot less than people doing a real plot. How *I* do it in the game with map contacts off is get a range, note the bearing, go to nav map, pause, plot a mark, go back, hit the stopwatch, and wait. Get the new range/bearing at 3:15, go back, pause plot, get the course and speef from the plot, and do it again. Do that over the span of the 10 minutes spent on your final leg and you've got an adequate solution. I don't bother with the plot if I'm doing an overhaul maneuver once I've established his course.

[Pisces]

From the originating post

Originally Posted by Leoz

...
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.

Sorry, I know this thread had enough fire in it already. But I still feel this needs to be corrected. I know part of the arguments were about the non-need for accuracy. And that has merit. But if a method is to be explained it should have the right values from the start. Afterwards one can cut corners and round numbers of to your heart's content, yet deal with the consequences of inaccuracy.

Leoz, the proper conversion factor between meters/second and knots is actually 1.94. You see, it goes from meters/second to knots, or nautical mile/hour. 1.94 is 3600 seconds divided by 1852 meters. If it has to be done easy rather than accurate 'length times 2', divided by seconds, would be good also.