jaykchan wrote:
those boxy structures are totally useless

If they're not water tight and foam filled, their purpose may still be
to add strength to the hull and, in particular, the transom. I know
several boat models that have designed in stength with similar means.
Never having seen yours, I of couse have no clue. But you might find a
boat builder to take a look at them and get an opinion.
Anyhow, that's just my 2 cents. Be careful.

Rick