[jergofish]

Another way to find a crack or leak is to put your boat on the trailer and put the garden hose inside. Prop the trailer way up to check the back of the hull and then put the tongue on the ground to check the front of the hull.

WARNING. WATER IS HEAVY. DON'T FILL YOUR BOAT UP. FIFTY GALLONS WEIGHES ABOUT 400 POUNDS. THAT'S ABOUT THE MAXIMUM YOU SHOULD PUT IN THE BOAT. IF YOU DON'T SEE WATER COMING FROM THE HULL THE CRACK HAS TO BE HIGHER ON THE HULL or getting in some other way.

I've used this technique twice and found leaks in seams of welded aluminum boats that would have been impossible to diagnose otherwise. The seams looked perfect. One of the leaks was almost two feet in length.