Glenn Ashmore wrote:
...
How do I figure out which spring to use to just maintain the arm slightly
above level? ...

There are a couple of other things to consider here, if you haven't
already. One is damping. With bearings at the corners, your mount will
be very free and in constant motion. One good bump and your camera will
be bobbing like one of those silly car rear window statues. You'll need
some form of (adjustable) friction.

Another is accelerations. Given the size of your boat, they will be
pretty small, I'd guess. So you won't have a problem with your mount
bottoming out. But the mount might be too stiff for the small
accelerations that you will have. You could calculate the deflections
that you will have, given the geometry and Don's spring constant, but I
don't know what numbers you'd use for the accelerations.

Hmm - it just occurred to me that the way to think about your mount is
as a high frequency filter. Of the spectrum of frequencies that the
boat is moving at, you want to camera to only move at low frequencies.
The camera/mount frequency is mostly a matter of the spring constant:
high spring constant equals high frequency. A lower spring constant
means more extension, means a longer horizontal arm. The gotcha, of
course, is *how* low the frequency needs to be and how to calculate the
spring constant and geometry from that. I'm sure that there's an ME
reading this that can help.

Don't you just hate it when the problem becomes *much* more complicated
that you thought it was. Sorry about that.

Bob