From ZX Spectrum instructions
As the commander of a submarine, it's your job to destroy four
convoys, each consisting of a freighter and two destroyers.
Having selected the difficulty level (Captain is easiest, Admiral
is hardest) and the weather conditions (press F if you want fog
to appear), press ENTER to start the game.
You start the game docked on a friendly island. The top third of
the screen shows the view through the periscope, while the bottom
two thirds are taken up with four windows showing various pieces
of information.
(..)
Your first task will be to locate an enemy convoy, indicated on
the map screen. As your sub only has a top speed of 22 mph,
you'll need to make use of the accelerated time feature (which
speeds up the game by about 30 times) to cross the large map area
quickly. Having sighted a convoy, you need to be within 1.5 miles
to be able to hit them with a torpedo. The torpedoes travel twice
as fast as the sub. Alternatively, you could fire a wire-guided
missile, but this is rather harder to control.
Once you've attacked, of course, the destroyers will be alerted.
If you're on or near the surface they'll shell you; if you dive,
they'll depth-charge you. Sitting on the sea bed with your
engines off is a good way of fooling them into thinking you've
left the area. However, you need to have enough compressed air
on board to be able to blow the ballast tanks and return to the
surface; if you run out of compressed air, you're stuck.
Compressed air is produced by a compressor attached to the sub's
diesel engine (which also recharges the batteries), but this can
only be run on the surface. Underwater, you can only use battery
power.
You only have a limited amount of ammunition and fuel on board,
so you'll need to return to a dock to rearm and refuel. The map
gives the location and co-ordinates of your two docks. Docking
is quite tricky and needs some practice! (Hint: going in slowly
is a good idea...). You also have a time limit before the four
enemy convoys cross the map and reach their destination.