Paul wrote:

You may have trouble finding a zener diode rated at enough amps,

For just a depth sounder, knotmeter, portable GPS, and radio is that likely
to be a problem?

Since I'm only concerned when the starter is engaged, how about wiring in a
bypass with a NC relay that opens whenever the engine key is turned on?
When the alternator is going, low voltage shouldn't be a problem and I would
then have no voltage drop when sailing and drawing on the batteries. Since
I have a diesel, the engine is stopped before I turn the key off.

I'll confess though, I'm trying to figure out how to work a starting battery
into the engine room. It would have to be aircraft or motorcycle size but
that should get my little diesel going. People have started them with a
handful of flashlight batteries taped together.

--
Roger Long