Deck Hatch Pick and Install

[opwan2002]

I'm going to install a lewmar size 10 ocean hatch on the foredeck of my 22' day sailer. The roof in the cabin below deck is the fiberglass material without a headliner. Should I buy a flanged or flat based Hatch?

[UNCIVILIZED]

Flat based tends to be easier to mount, or to remove for maintenance. While flanged based hatches tend to stay more watertight. This is due to the flange adding a bit of stiffness to the surrounding deck structure, so that the frame's seal to the boat doesn't leak. And that since the base flexes less, the hatch gasket tends to stay seated & thus sealed, better.

If the deck's cored I'd go with flanged. Though I'd also add significant reinforcements to the structure around the hatch's cutout. Both on deck, & below. And you can even go one step further by adding a breakwater around 3 sides of the hatch, it helps to stiffen the deck. As well as preventing green water from directly striking the edges (& thus gaskets) of the hatch.

On offshore boats many folks also build in a lip on the breakwater for storm covers, but I doubt that's needed in your case.

PS: Compass Marine (Maine Sail) has some good how to's on bedding things to the deck.