Hello Alctel:
From everything that I have seen/read on the various SBO forums, and also my home copy of Don Casey's sailboat electrics book (see below for more info on this publication), Jim is spot on.
The back of my electrical panel looked much the same as yours when I bought my boat. But it did have a plastic back cover that had the appearance of being OEM. It wasn't in the best of shape, but I did manage to salvage it for continued use. The cover doesn't really need to be water-proof since the back of the panel is in a location that isn't typically subject to rain/seawater exposure. (And if either variety does get to our boats' back panel location, we have some really serious problems that will have taken out a lot more than our panels' ability to function!) You can make your own cover. Or take the measurements and head off to a big box store and see if you can find a storage box for instance that would fit over with lots of room to stay away from the connections. Use your imagination.
My "rats nest" is improved from what it looked like when I bought the boat. But compared to the elegant/precise work that I have often seen posted, I still have a rats nest...
... But it is now a much safer rats next. What I did was mark each wire, then I detached them from the copper bus bars on the back of the panel. With 600 grit sandpaper, I restored the surface of the bus bars to clean copper appearance. Then I cut off each wire's terminal. I removed about 3/8" of the insulation from the wire. Our boats were built before the ABYC began to specify only marine grade tinned copper wire. Ours are marine grade, but non-tinned. My boat has spent its life in a generally low humidity dry location. Because the condition of the metal strands remained good, I simply polished up the newly exposed surface of the wires and crimped on new terminals. I did this only for the low amp draw and short distance circuits. For the wiring to my bilge pumps, however I did replace with new marine grade wire.
My real exception however was for the heavy gauge high amp draw wiring wire from/to the batteries, to/from the A/B/Both switch, and to/from the panel and the engine starter. And the corresponding negative heavy gauge wires between the battery and the engine block and the panel. I found most of these original wires to be black tarnished underneath the terminals and also further up within the wire insulation. These wires then were replaced. (While on the subject of battery wiring, fusing at the positive battery terminal (or really close to it) is critical. Do a search on SBO for Maine Sail's tutorial on the subject.)
A couple of other quick things come to mind.
- My AC outlets were not GFIC protected. I replaced the first outlet in each of the port and starboard AC circuits with a GFIC outlet. When I did so, I was pleasantly surprised that the condition of the AC wires in/out of the first outlet were in excellent condition. This matches Jim's observation that the factory installed wiring between the deck and lining might be left in place.
- Regarding the interior cabin lights. My OEM fixtures looked dirty and yellow when I bought my boat. But before replacing, I decided to remove the clear plastic refracting lenses and bring them home for a deep cleaning. I used "Industrial Purple", a strong water-based de-greaser available in the janitorial supply section of Home Depot. (This stuff has lots of uses on a boat and home and car.) That and a tooth brush to get into all the micro recesses did retore the lenses to virtually new appearance. Big saving compared to replacing all of the fixtures with something new. Also on the cabin lights, consider replacing the incandescent bulbs with LED's. You can find exact fit replacements here on SBO. Doing the conversion serves two purposes. 1) LED's suck a fraction of the current draw. I think that I calculated that I can now switch on virtually every interior cabin light and use not much more amps than turning on just one of the old incandescent version. And 2), if the wiring might be a bit suspect, the lower current draw of LED bulbs is much less likely to cause wire overheating. Also when you have the lenses off, check the condition of the wiring (mine looked fine). And also at the switch contact points and at the bulb fittings, clean off any built up oxidation with some 400-600 sand paper.)
Regarding my reference to Don Casey's electrics book, it is one of six of his mini books that have been compiled into the "Complete Illustrated Sailboat Maintenance Manual" There are other similar publications, but I have found this to be the best for me. It's the equivalent of a bible for sailboat owners. The internet is great, but finding all the detail in one comprehensive well written location can be tricky. Here is the link to Amazon's offering of the book:
http://www.amazon.com/Caseys-Complete-Illustrated-Sailboat-Maintenance/dp/0071462848