Electrical, DC Main Circuit, Part 1: Constructing and Installing the Backplane

The back plane, dry-fitted into place
In the world of computer electronics a backplane is a foundation or an organizer of sorts - a board upon which the various components of a computer's system are built. In the world of marine electrical systems, Maine Sail, that ubiquitous contributor who refers to himself on many an online sailing forum by this name alone, uses the term backplane in a similar fashion, usually with regard to the various components of the DC electrical system, especially the main circuit. In the rewiring of Oystercatcher, my Ericson 25, I found it necessary to employ a backplane for the main circuit of my DC electrical system. Space considerations and accessibility were the motivating factors. In this article I describe my reasons for using a backplane and my construction and installation of this important foundational item for my own system.
If you've read my series of articles on the modifications I made to the lazarette, then you'll remember that I installed a hatch within my portside cockpit locker for the purpose of accessing the reserve battery bank for the DC electrical system.
If you've read my series of articles on the installation of a battery switch, a battery monitor, and other items, then you'll remember that I decided to locate the heart of the AC and DC electrical systems on the bulkhead on the portside of the galley.
Likewise, if you've read my article on the construction of a protective panel for the backside of these various components, then you'll remember that the aft side of this bulkhead would house many a cable and many a wire for these components.
Given that the space in this area was tight, and given that there were still numerous components of the DC main circuit that I needed to add to this area of the boat, I knew for a long time that I would need to make use of the lower end of this bulkhead, in other words, the end that was within the lazarette.
Before I had ever installed the access hatch, and before I had ever constructed the battery shelf in the lazarette, I had created a cardboard mock-up, just to see if I could actually fit all of the necessary components and all of the necessary cables in this relatively small space. It seemed that I could, so I moved forward with these major modifications to the lazarette. Now that they were complete, and now that I had installed the battery switch and other items in the galley, I could pull out the original cardboard mock-up and begin to get my mind around this part of the puzzle once again.
From the start, I had known that I wanted to use an automatic charging relay, or ACR for short, to charge both the house bank and the reserve bank at the same time. Note that I am using these terms to refer to the battery banks instead of the more common terms, house bank and starting bank. My terminology for these banks and my thinking on the use of the ACR and the wiring of the ACR was influenced by the many articles that I read by Maine Sail on various sailing forums and on his own website, Compass Marine (see my link to his site on the homepage of this site). Maine Sail argues repeatedly that it is preferable to use the house bank for house loads and for starting the engine, and it's better to leave what would have been the starting bank as the reserve bank, to be used only in the case of emergencies. I'm not going to get into the technicalities of this (which also involves a discussion of the battery switch), since Maine Sail himself is an expert on the subject and has stated his case in writing many a time, but the skinny of it is that by using the house bank both for house loads and for starting, you always have a reserve bank ready, just in case your house bank fails to provide the needed power for the cranking of your engine.
Maine Sail's argument impressed me, and despite the fact that his argument is geared towards those who have diesel engines, whereas my boat, Oystercatcher, has a Yamaha 9.9 horsepower outboard motor, I thought it wise to have that reserve bank ready for my motor in the event of an emergency. Sure, you can remove the cowl of the Yamaha 9.9, and after some fiddling around you can crank the motor with a pull-start cord by hand. Nevertheless, do you really want to have to resort to these measures in a true emergency, when a simple turn of the battery switch can immediately provide the power you need?
In an earlier article I described my initial installation of a Victron brand battery monitor. My decision to use a battery monitor in the first place was based on my reading of Don Casey, This Old Boat, 2nd Edition. My decision, after further research and some shopping around, to settle on the Victron brand monitor was based on my readings of Maine Sail's work. In the picture below, we see the face or the display of the monitor.
Next, we see an important component of the Victron battery monitor - the shunt. This device is the workhorse of the system. It sends the data that it collects from the batteries to the display. The shunt is the small device (lower right) with the brass screws / terminals.
I bought the various items that you see pictured above at different times, as my budget allowed. I got a good deal on the ACR at West Marine, back when they still did price-matching. The Victron battery monitor I got from Jamestown Distributors, back before Maine Sail starting selling them on his own website. The Blue Sea Systems bus bars I ordered from Defender during one of their big sales. Likewise, I ordered the backplane itself from Defender.

After I had slowly acquired the various pieces, I sat down and began to arrange them according to the design I had originally sketched on the cardboard mock-up.
The backplane was Starboard, which is simply a UV-resistant sheet of HDPE (high density polyethylene). HDPE, in its less-expensive non-UV-resistant form, is the same stuff that they use for making kitchen cutting boards. Just as the Starboard promotional literature promised, it was easy to cut and shape this material with common woodworking tools.
Needing one end of the backplane to conform to the shape of the hull, I scribed an arc on one side using a flexible scrap piece of luan plywood.
It might seem cheap or unprofessional, but I've used this scrap piece of luan many times in the refitting of this boat . . .
. . . with good results.
Out in the boat, I thought the backplane fit quite well in the space.
Satisfied, it was now time for me to layout the ACR, the shunt, and the three bus bars to get an idea of how they would all fit with the weld studs. The bus bar on the far left would serve the house bank. The bus bar adjacent to it would serve the reserve. Finally, the bus bar on the far right would collect all the negative cables and wires of the DC system.
To mount the backplane I would use weld studs. These I ordered from McMaster-Carr in Atlanta, Georgia. The term that McMaster-Carr uses for these is "perforated base studs." Some manufacturers call them "bonding studs." Whatever you choose to call them, I had learned about them from various postings on the Cruisers Forum. These studs are helpful when it's not possible or not desirable to use wood screws or through-bolts. I wanted to be able to install and remove this backplane by myself, without having a helper holding a wrench on a nut for a hex-head bolt in the cabinet area of the galley. That's why I chose these studs.
Also on the Cruisers Forum I had learned about the mounting of these weld studs using various 5 minute epoxies, you know, the kind that come in dual-syringe dispensers.
Some people mentioned Devcon brand 5 minute epoxy. I found some at a local hardware store.
When you dispense the material, you end up with a small clump that is both white and blue.
As you mix the clump with the included stir stick, the blue slowly disappears. This indicates that the two parts are thoroughly mixed and ready to be applied.
It didn't take but a few minutes for the epoxy to grab the first two weld studs and hold them securely in place. Pay no attention to the base of the stud on the far right (just above the small hole). I had installed this stud for a panel that covers the entrance to lazarette. I did not install this one with Devcon 5 minute epoxy. Instead, I secured it with small wood screws (through the perforations).
Back inside the house, with some extra weld studs as guides, I drilled the necessary holes for the installation of the bus bars and other items.

Here's how it looked after I had installed all the items.
Back in the boat, I temporarily installed the backplane on the first two weld studs (far right). Then, I inserted a pencil into the two holes on the left side of the backplane and marked the bulkhead. This would indicate to me where I needed to install the next two weld studs.
I again followed the same procedures with the Devcon 5 minute epoxy. Again, although I didn't mention it the first time, I thoroughly cleaned the bulkhead and the weld studs with acetone to remove any impurities that might prevent a good bond.
Then I installed those next two weld studs.
Despite my best efforts, the weld studs did not line up perfectly with the backplane. Therefore, I had to drill out the holes to a larger diameter. This was a good thing, however, since the Starboard expands and contracts slightly under different temperatures.
This extra room in the mounting holes made for a much easier installation of the backplane.
After I had tightened down the nuts, I installed a cutting wheel on my Dremel and removed the excess threaded portions of the studs.
I was quite pleased with the results.
Satisfied, I moved on to the creation and installation of the many different cables. In this part of the process, as you have seen, I removed and then reinstalled the backplane numerous times. When at last I reached the point where it appeared I would not need to remove the backplane again, I tightened the nuts firmly on the studs.
For the next two or three weeks I focused on some of the branch circuits in other parts of the boat. When I returned to main circuit to tie up some loose ends, I discovered that the backplane was loose on the outboard side (the left side in this picture). The nuts were still firmly screwed to the studs, so it became clear to me that sometime in that two to three week period the studs had broken free from the bulkhead. I had experienced something similar in the past with a weld stud I had installed on the fiberglass hull liner in the head. On that occasion, however, the stud popped free as I was tightening the nut down with the ratchet wrench. My guess is that these two attachments for the backplane failed for two reasons. First, I might have over-torqued the nuts. They were hard to reach, and the pressure I exerted on the ratchet wrench with my extended arm was probably different than it was from the pressure I exerted on the nuts that were inboard. Secondly, despite the fact that I had made the mounting holes considerably larger than they needed to be, the fit was still a bit snug on the outboard side of the backplane. Starboard, as I said, expands and contracts with changes in the weather. This sheering action might have been the source of the problem, or at least a contributor.
Despite the fact that the attachments on the outboard side had failed, the backplane was in no danger of falling off the bulkhead, even with most of the cables still loosely hanging here and there. There were just too many different things holding it in place. Nevertheless, I did not like the idea of the backplane not being snug against the bulkhead. I considered removing the backplane and then reattaching the weld studs with small wood screws through the perforations in the base of the studs. This, though, would have required me to remove many of the cables, and I doubted whether I could install the studs in the fashion described. They were just too far away, and it was almost impossible to get more than one hand in that area of the lazarette.
Ultimately, I opted to drill two holes through the backplane and through the bulkhead against which it sat. Through these, I inserted two hex bolts. Employing the aid of the Admiral, I held the hex head with a wrench, while she used the ratchet wrench on the galley side. In the picture below you can see the two hex heads. One is top left (between the original fastener and the bus bar); the other is to right of the ACR, just visible between the cables). This was a quick and easy solution, one that made the backplane very snug.
Fortunately, I had not yet reinstalled the stove or the cabinets in this part of the galley, so it was easy for her to access the fasteners. 
The threads of the hex bolts extended too far into the cabinet space for my liking, so I opted to cut them down to size with the Dremel. 
Pay no attention to the chipped paint on the bulkhead. I address that story elsewhere. The Dremel did a good job at removing the unwanted portions of the hex head bolts. If I could do it all over again, I would not have tightened the nuts on the weld studs too much, and I would have made the mounting holes even larger than I had the first time. Of course, in lieu of using Devcon 5 minute epoxy, I could have used Weld-Mount, a 5 minute epoxy that Maine Sail touts highly. I considered Weld-Mount, but it was just too expensive for my limited number of non load-bearing applications in the refitting of this boat.
In the end, my decision to use the backplane and the weld studs was well worth the time, effort, and money. It allowed me to organize some of the major components of the main circuit, and it allowed me the mobility to work on this main circuit without being confined to the cramped corners of the lazarette.
This ends this posting on my construction and installation of the backplane for the DC main circuit on Oystercatcher, my Ericson 25. 

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