A 400 owner sent this repair in which shows the construction of the deck to hull joint on the 400. This is not a problem common to just Mainships, but can be found on most any boat. The pictures here show a good cross sectional view of the rubrail, and what to do if a leak should develop in your 400.
Following is Chris' repair.
"I found when I removed the forward port quadrant of the rub rail that there was a gap of about 6 to 10 inches at the deck to hull flag where the sealant was apparent left out during original construction. The 4 screws in this area were also loose."
Here is a close up of the gap.
"I resealed this gap with 5200 and also installed 3 additional screws."
"During this operation I noticed that the rub rail extrusion formed a channel where water could collect if the lower side was tight to the hull as is intended and flood the area of the flagged joint. I drilled 3, 1/4 inch holes in the underside of the rub rail rubber at low points to allow any accumulation of water to drain and I also did this to the starboard side as a precaution.
I think this has fixed my leak but will keep a close eye on the area."
I especially like what Chris did to relieve any accumulation of water from the rubrail. I once had a boat with a similar leaking problem. The rubber railrail would hold and channel water through the deck joint, which had many gaps in it. Without removing and resealing the joint, I opened the joint underneath, at the lowest point on the rubrail to let water out, and this cured all my leaks as the rubrail acted like a gutter and it just ran to the lowest point and drained.