Just wanted to give a shout out to a little hidden gem of a game called Tonnage War by Leonoard Heinz. It's originally designed to be a Print and Play game but there is a VASSAL module that is very well done, all for free.
It is a solitaire game with the player taking the role of a convoy escort commander, in charge over several days of engagement against U-boats that are controlled with the game system.
You have to decide where to place the escorts, whether to use convoy zig-zags, illumination via snowflake, HFDF, and even airborne patrol aircraft.
The rules are a bit tough to work through at first but there is a very well done tutorial included and once you get the sequence down the game flows pretty quickly. It's a great game to play a turn or two and come back to later in the day, slowly watching your convoys get hunted down.
Since it's free and it's obvious that the designer has put a lot of time and dedication into the game I think it's well worth submarine fans giving it a go.
the module and rules can be found here:
http://www.vassalengine.org/wiki/Mod..._War_Solitaire
