You're doing something to encourage this high failure rate.
The ship's "Draft" is found in the Ship Recognition Manual. If you click on the word "Ship" on the notepad in the upper right, while you have the target locked in your crosshairs, the Ship Recognition Manual will automatically open to that type of ship. Draft is listed on that page.
"M" or Magnetic triggers are designed to be fired UNDER the target and are triggered by the metal hull while the torpedo passes under the ship. If you hit the hull, as you've noticed, nothing usually happens. If firing with this type of trigger, set the torpedo's depth to .5-1 meter below the Draft of the ship (i.e. 6.5-7 meters if the Draft is 6 meters). Any higher and you risk hitting the hull. Much lower and you may pass too far under the ship to activate the trigger. This gets much trickier when the seas are rough as the ship is bouncing up and down in the waves, but in the torpedo tutorial everythings pretty calm. Main advantage is that you can hit the ship from any angle.
"I" or Impact triggers are the ones designed to go off if you hit the hull. If firing this type of trigger, set the torpedo depth to about 1/2-2/3 of the draft (i.e. 3 or 4 meters if the Draft is 6 meters). If you aim much lower you risk bouncing off the rounded part of the hull (Ship's hulls round off as you near the hull's bottom). If you aim too high, the hole you open may not be as effective (It spends time out of the water as the ship rocks back and forth and doesn't flood the ship as fast, or even worse, be above the waterline entirely). Main disadvantage is that the torpedo must strike at an angle no more than 30 degrees horizontally as well (The closer to 0 degrees the better) or it will probably glance off the ship. Main advantage is that there is a better chance of hitting something critical, like the engine room or a fuel bunker (But you can worry about aiming specifically for those later).