i've heard alot of people use the 3 inch flexwatt. my question is how?

for example let's say the tub is 33x16x5 if i had a rack, how would i place the flexwatt? the length of the tub is 33 inches, so i dont understand how 3 inch of flex watt (run the width) will provide enough heat, because what about the other 30 inches? or is the tape being run 1/3rd the length of the tub? if so, how are all the flexwatt wired in one level of tubs

i'm pretty sure i'm missing something here