Without such a hatch, should you ever need to quickly get into the engine room from the cockpit, you would need to scramble down the companionway, go through the galley, through the tunnel, into the aft head, to the door to the engine room. With this addition, you can open the hatch and drop right in. If you fit that is. I better not gain any more weight or else this may be useless.
I got the idea from Dave on the Westsail 42, Elysium, but took a slight different approach to his otherwise perfect installation.
The flange that supports the hatch and provides the pathways for the water drainage was built from Coosa (instead of molded from fiberglass, as was Dave's method). The CNC machine was used heavily to provide the template for the cutout and for cutting the pieces of the flange. Edges were rounded over on the "manual" router table, and the whole thing assembled in place and glued in with thickened epoxy and thru-bolts.
Final sanding, trimming and paint, as well as choice of latch mechanism, will be done when the deck is refinished (not this year).
For completeness, here is the slide show...