[Efshapo]

Originally Posted by VonHoffman

Interesting technique.

Thank you. :]

Originally Posted by VonHoffman

I use something similar. What you're doing is instead of measuring bearing angle change by rotating the periscope, you are letting the target pass the more accurate graduations on the scope itself to yield a more accurate measurement.

Indeed. Only later did I discover a historically accurate technique involving a moving periscope existed ("Auswanderungsverfahren", as indicated by our friend Stosstrupp).

Originally Posted by VonHoffman

The only issue I see is that your own ship needs to be stationary.

Technically, you don't have to be stationary. You just have to take into account you own lateral speed. You can either compute it, that would be own_speed * sin(relative_bearing), and add it to the result. Or you can maneuver to make sure it's negligible, either by stopping as you stated, or by going at any speed you want provided you're observing your target from a relative bearing of 0 or 180.

Originally Posted by VonHoffman

What I do is stop my boat, and time how long it takes for the entire visible length to pass the centerline of the scope. Regardless of AoB I know the target speed.

Example:

Target is directly abeam (90 degrees) and the target is 30 meters long, if it takes 10 seconds for the bow to stern distance to pass the centerline of my scope, then the target is travelling at ~ 6 kts.

If the same target is at relative bearing 120, and the AoB of the target is 30 degrees, how long does it take the ship to cross the centerline of the scope? IT STILL TAKES 10 SECONDS!

It appears to be moving slower, but the overall length of the ship still moves 30 meters in 10 seconds! As I'm measuring it crossing bow to stern across a fixed point, I can still get a good fix on its speed. Also, you can use any magnification you like to get a good view for the measurement as you only require the ship to pass a fixed mark - the center of the scope.

That's what people usually do, yes. I actually use this in a step of my second "refined" method, as a way to get an accurate AOB. You can see it in the video.

Originally Posted by VonHoffman

To get accurate AoB, I can then cheat:

If I know its range (let's say 10 hm), I know that a 30 m long ship should take 3 centiradians if it is at 90 degrees AoB. If I only see 1 centiradian (or 4 centiradians at 6x), its AoB is asin(1/3) = 20 degrees.

The method I propose here to measure AOB is more accurate because the "sensor" used is the stopwatch instead of the horizontal scale. I only use an exact number of horizontal divisions so I don't have to estimate horizontal subdivisions. And if I manage to get my measurement under a minute, the stopwatch displays 1/100th of seconds which gives a far better sensor resolution.

Originally Posted by VonHoffman

To parallel its course and remain in the same relative position:

180 - 120 = 60 off my stern
90 - 60 - 20 = 10 degree AoB less than I should see
I need to turn left 10 degrees and run at 6 kts to maintain relative position.

I'm not sure I'm following you here... If the target is at relative bearing 120 and its AOB is 20° left, then if you want to run parallel you should turn 180-120-20 = 40° left (and then observe the target at relative bearing 160).

Originally Posted by VonHoffman

If I was already parallel to his course and he's 60 degrees off my stern I should have an AoB of 90 - 60 = 30 degrees.

Again, I disagree. Running parallel to a target observed at relative bearing 120 means its AOB should be 60° left.