Cab,

A free-air sub is self-sufficiently damped so it has different electrical and mechanical parameters. It will also have a higher resonance. A 10-inch free-air would optimumly operate in an infinite baffle of 3 cubic feet or larger. Just like any other sub a free-air requires complete front-to-rear acoustic isolation and a very rigid mounting surface.

In contrast, acoustic suspension subs are very dependent on a precise enclosure (sealed or bass-reflex) for damping and control.

Actually free-air subs can sound pretty good if they're properly executed. Unfortunately you rarely see the correct woofer put to task in the right manner. JL Audio makes the best free-air subs by a considerable margin.

Now that I better understand your sub mounting, here's what I would suggest:

Get a true free-air sub. If a sub manufacturer claims their product is a jack of all trades then you can assume it does all applications poorly. Seal up the floor hump the best you can. You're only concerned with openings that are in close proximity to the sub. The distant openings represent too long of a pathlength to be a problem. You want the sub to realize as much of the bilge cavity as possible.

Reinforce the thin fiberglass hump mounting surface with a carpeted thick KingStarboard overlay.

Properly tuned it will be a day and night difference from what you have now.

Another option is to close off the floor hump opening and install a sealed box over the top of the hump that extends toward the bow mold. There's plenty of room up there to fit a side-firing sealed box with .675 gross (before driver) internal displacement. Its a bit tighter up there in the last several year models of Moomba LSVs but it still easily works with 5/8" material thickness. This would be an ideal displacement for a variety of subs whether JL, Alpine, Polk, Kicker, etc.

And Sandm is right, a bass-reflex enclosure doesn't fit well with your scenario.

David
Earmark Marine