LET'S MAKE MUSIC
OK, let's try to sound like a real bass. If right-handed, stand behind your tub, left foot behind and right foot on the side edge. (If wrong-handed, reverse position). This will allow you to keep your balance and hold the tub on the ground as you pull back on the handle with your left hand. Different notes are made by changing the tension on the string. While holding a constant tension, snap the string like a bow and arrow to achieve a crisp note. Play the same note several times to eliminate any variation, then change the tension and try a different note. The objective is to snap sharply from one note to another without the dreaded rubber band sound. Most volume can be acquired by pulling the string away from the handle at the center. Moving up and down the string will not change the note. It depends entirely on how tight or loose the string is when you snap it.
Once you have learned how to make clean, sharp notes, practice playing with music. Tape it to hear what you sound like. Sometimes what you hear in your head is not what other people hear. Try not to give the washtub bass a bad reputation. Start simple with down beats only, then go to alternating notes. Listen to other bass players to develop your technique.
Remember, if you play it right, you will be welcomed into many jam sessions.