Absorb energy, the hatch lets the energy pass thru to be reflected and trapped in the cabin, the cockpit because the right angles and such reflect and distort the energy and the cushions in there if there are any would attenuate the energy as well.
I just lost a huge chunk of writing cause I was off at flikr looking at images, and I got logged out.
Here is a great example of reading the SI, courtesy of Marine Electronics posting on flikr,
http://www.flickr.com/photos/21719579@N ... 691455670/
See how a relitively shallow but abrupt change in depth on the left, and return, shows up on the SI display on the right? A depth change of 3 feet on that creek channel widened that black bar that amount. so now you can start estimating things like depth changes based off of what you see on the black center section.