I am starting from the beginning. Here we have the first german U-boat, the SM U-1, built in 1906. There were a few war subs built before this, but this is the first official U. It was 238 tons, 139 ft long, and powered by twin kerosene engines. Max surfaced speed was 10 knots (about 12 mph), submerged was 8 knots. They used kerosene because it was less likely to blow up, however, the engines could only run wide open. This meant that the props had to have a variable pitch to control speed, since the engine couldnt slow down. It must have been a kerosene thing, because the next generation of u-boats used diesel, and could run at variable speed. The ship was deemed obsolete by WWI and used for training, until it was hit by another ship in 1919, at which point it was sold to a museum. It never saw active duty. There were several more of this type of U-boat built before and during WWI, going all the way up to U-17, all kerosene powered.
There are so many variants of WWI U-boats it will blow your mind, so I wont attempt to cover them all. I will give out a few highlights though. This next picture is of U-20, a diesel powered boat that scored the first WWI kill. It famously sank the Lusitania, a beautiful ocean liner based out of Liverpool. It was on its way from New York to Liverpool.
Here the Lusitania in NYC. The captain of the U-20 had a target book that classified the ship as an acceptable target, even though it was not an official war ship. This was because it was capable of carrying troops to england, which some of the actual passengers were. However, it was such a horrific sinking that both sides considered it a bad move. Anyway, more subs next post.