Hey, thanks very much for the response!
As you can see from the schematic, this is a very (very) simple design. I selected the particular mosfet, IRF1501, because it's spec'd VGS(th) Gate Threshold Voltage is a maximum of 4.0V, and as you can see I'm driving it with 4.9V... so, I should be operating it within spec, and I think I should see the spec'd Rds(on) of 0.055 ohms...
So... am I reading the spec wrong?
I can add an NPN transistor to level shift the PIC controller TTL level output to the ship power rail... but, in theory, it shouldn't be needed, and in an aircraft I've learned to avoid things that could fail but aren't really needed..
I'm using an LM338 for current limiting... but I'm looking at various buck topologies... that isn't an easy reach because in my installation the regulator has to be a fair distance from the load.
Anyway, here's the schematic... would much appreciate your thoughts!