Hi Jim, hope this is an answer for you, quite happy to have any other comments.
When a ship goes through the water it creates a wave as the water is moved out of the way and there is drag on the hull. Making waves absorbs energy from the engines pushing the ship along. Bigger waves, then less power available from the engines so speed is slightly reduced.
The bulb below the waterline is calculated to create a wave of its own that tries to counteract the wave from the hull, it sort of smooths it out.
The shape of the bulb will be 'tuned' to have the best effect at the cruising speed of the boat.
In essence it reduces the bow wave and subsequent waves created by the passage of the ship, this releases more power for the boat to go faster or using less power for the same speed.
hope that helps,
Roy