Const. Bearing: Divergent Spread

[KlassenT]

After some time ago reading RR's theory on how O'Kane may have feasibly combined a constant bearing solution with maximum divergent spread [Post (http://www.subsim.com/radioroom/showpost.php?p=995114&postcount=3)], I've been considering the merits of this attack method, or more precisely, when it's prudent to use. Common sense says that this technique is most justified in clear visibility conditions when your target has a better chance of torpedo detection, and thus evasion. Does it hold true, though, when the chances of that happening are more limited?

My first approach to a constant bearing solution for Aft-Mid-Bow was to use it primarily at long range against well-guarded TFs, when the fish are going to have a nice, long run-up during which they could be detected. I don't know, though, whether to chalk my particular problem up to poor acquisition speed or distance of approach: about 25% of the time, the spread would be wide enough that given an early detection, mid-sized vessels would have enough maneuverability to turn into the spread and cut between the volley. The likelihood is obviously reduced when you use more fish, but it also means you stand to sacrifice more ahead-of-target misses if they turn early, in spite of the fact they end up taking a torp that was intended for aft/MOT right up the nose.

At this point, the theory and execution of a divergent spread using constant bearing doesn't leave much unexplained, but the real crux is when it's wise to give it a try. For point of reference, my 'long-range' attacks range anywhere from 2.5-4k yds, and of course only when heavy escort cover prohibits inching any closer. Less maneuverable targets like a nice juicy Kongo are still pretty susceptible, so is my big flaw really just in trying to take out the accompanying cruisers from this range, or is there something else underlying my tactics that needs improvement?

[vanjast]

I think I inadvertantly use DK's method without knowing it :har:.

My method of obtaining target speed is :
1) ship AOB + scope bearing = 90 :- adjust sub course so this is so.
2) pick a target at scope bearing of 60, 45, or 30.
3) adjust sub speed to either 1, 2 knots when submerged, higher speeds when on the surface.
4) without locking the scope monitor the ships relative bearing.
Simple mental trigonometry can be applied here to give you a ball park speed.

So we have speed, next is to setup the TDC so torps strike at about 5Degs.
Easy as pie, turn scope to bearing of 10-15 degrees off dead ahead and punch in the numbers - Wait for ship to cross-the-line.

[Munchausen]

:cool: The standard O'Kane method doesn't use a spread, as such. Instead, you determine where to set your crosshairs and then let loose with a torpedo as "juicy" parts of the ship cross the line. But, at 4,000 yards, the accuracy is far more dependent on your scope adjustment. If you're half a degree off at 800 yards ... no big deal. But, at longer ranges, it can mean the difference between a hit and a miss.

Depending on which mods you're using, you can sneak in a lot closer ... as long as you keep your speed at or below 3 knots and only use the periscope for quick peeks. Some skippers will swear you also need to stay deep until the escorts pass by but I've made approaches at periscope depth and gotten inside 1,000 yards of the big boys without being detected. At that range, it's hard to miss.

[Rockin Robbins]

The advantage of the Dick O'Kane method is that you are not using a calculated offset which could be a half degree to one degree off. You are letting the TDC calculate the correct lead angle and then are able to sight exactly where you want the torpedo to hit and press the fire button.

This is much more accurate than a calculated lead angle or an interpolated lead angle from tables. However, the lead angle isn't the true limiting factor of the technique.

The true monkey wrench in the works is the determination of target speed. And similar to the growth in significance of a lead angle error with range, the speed error also becomes proportionally more important with long range.

Because of this, and because torpedoes with stated ranges of 4500 yards tend to poop out at 3000, I limit my shots to 2000 yards, regardless of the type of shooting I'm performing.

Yeah, I've seen the movies of multiple hits from 4000 yards, and they're mighty impressive, but how many misses do you have to accumulate for one perfect video? I also don't find it difficult to make a periscope depth approach to within 2000 yards for the high percentage shot. I've never done the duck under the layer trick approaching a convoy.

Yes, it is possible to avoid a divergent spread. It is just less likely than the longitudinal spread. I try to take long distance shots at an angle that will allow misses to have an opportunity to hit other targets, and a divergent spread helps there too.