How do I light my water heater?

That makes absolutely no sense. 45 minutes of good flame through an unobstructed tube should result in very hot water in even a 10 gallon tank. The only things I can possibly think of right now are that the tube is obstructed somewhere with a wasp nest or cobwebs or water is constantly flowing through your tank and then out to... I dunno, the fresh water tank????

Is it possible that the inside of the heating tube (submerged in the water) has built up so much sediment that it is insulating it and preventing proper heating of the water??