IHW: Pigboats - Positioning

Positioning is the art of putting your submarine in the right place at the right time to intercept the target. If the target’s course is simple - a straight line - a simple successful Tactics check by the Assistant Approach Officer will tell you if it is possible or not, but only very fast ships take a straight course in wartime - there are submarines out there, after all! - and it will most likely be a fruitless chase. Usually, the target is running some sort of zig-zag pattern designed to make it difficult to get into position.
Simple Zig-Zag
A Simple Zig-Zag is a single symmetric zig followed by a mirror image zag, which repeats. This has -1 Small Modifier to solve. Roll 01-10 on d% for the enemy to be using this pattern.
Asymmetric Zig-Zag
An Asymmetric Zig-Zag is different on each side of the true course. This has a -2 Small Modifier to solve. Roll 11-30 on d% for the enemy to be using this pattern.
Double Zig-Zag
A Double Zig-Zag is symmetric, but composed of two repeating patterns either side of the base line. This has a -3 Small Modifier to solve. Roll 31-70 on d% for the enemy to be using this pattern.
Compound Zig-Zag
A Compound Zig-Zag is an asymmetric, multiple, irregular zig-zag. This has a -4 Small Modifier to solve. Roll 71-90 on d% for the enemy to be using this pattern
Complex Zig-Zag
A Complex Zig-Zag is an asymmetric zig-zag with a long, almost random, periodicity, at multiple angles to the baseline course. This has a -5 Small Modifier to solve. Roll 91-00 on d% for the enemy to be using this pattern.

Reducing Difficulty by Observation
You can reduce the penalties to figure out a zig-zag pattern by repeated periscope or other range-and-bearing observations over time. The observations need to be taken from different positions along the base course. The Approach Officer needs to roll a successful Observation check through either the periscope or other range-and-bearing equipment at each observation. Each successful observation - before rolling Tactics - up to the third reduces the modifier by one Small Modifier. Stating that the Assistant Approach officer is rolling a Tactics at any time puts the submarine at a firing point, and an unsuccessful roll means a blown attempt, far out of position.
Night Modifiers are -2 Large for dark nights, -1 Large for bright nights.
Avoiding Escorts
Generally, both merchant convoys and large warship groups are surrounded by escorts - destroyers or other Anti-Submarine Warfare (ASW) craft - patrolling various arcs around the main body, looking for you. There are several standard tactics to slip inside this ring of protection:
Hit ‘Em Where They Ain’t
By timing the escorts’ patterns, the submarine attempts to slip between two escorts at the most opportune time. The Approach Officer must make a successful Observation check with a penalty of 2 Small.
Dig Under the Fence
Using thermoclines - layers of cold water that tend to reflect SONAR pings - the submarine attempts to slip directly under an escort. The escort has -3 Small Modifiers to success in Acquiring a sub under the thermocline - if such a thermocline exists.
A thermocline can be located by the sub’s Signals Officer with a successful Acquisition check and 3 or more successes.
Using Geography
The submarine lies is wait just awash in shallow water at a headland on the coast or an island which the convoy must round. When the escorts all go to seaward to protect the convoy, the conviy lies open to the sub. Of course if anyone sees the sub, it will be helpless sitting there in shallow water. The Navigator may make a Tactics check to find such a place, with each success meaning a better ambush site.
Look! Behind You!
If working with another sub in a wolf pack or a duo, one sub attacks an escort on the other side of the convoy, drawing the other escorts from the unengaged side, clearing the way for the second submarine.
I’m One of You!
With a large number of escorts on a dark night, the submarine slips into the ring of escorts, acting like one of them, before cutting into the ring and attacking. Of course if the escorts aren’t fooled, the sub is a sitting duck.
The Rush
Going in at night on the surface, the submarine sinks one of the lead escorts and goes through the gap, right through the convoy from front to rear, raising chaos in its wake. If this works, it can shatter the convoy. If it doesn’t...

-clash