Stepping up to a new battery box

We wanted to increase the battery capacity on Andiamo, our Wauquiez Pretorien 35, so we’d have the amp-hours to power our radar, autopilot, HF radio, refrigeration, instruments and lights. ds. Just fill in the table and add the loads up. Be sure to prepare two separate totals — one for passages and one for staying at anchor. It’s amazing how an amp here and a few watts there add up to a lot of amp-hours!

I wanted to be able to stay at anchor for several days without any charging sources. The worst case would be four cloudy, windless days without running the diesel. That meant that we would need about 80 amp-hours a day while at anchor and 92 during a passage. The conventional wisdom of using 35 percent of the total battery capacity available for normal charge-discharge cycles — with occasional deeper discharges — provides the best balance of usage and battery life. By my calculation, we needed 700 amp-hours!

Prior to preparing the spreadsheets, I had planned to add a couple of batteries under the aft cabin berth just forward of the factory-installed battery box and live with a slight list to port. A little research into the batteries we needed quickly convinced me that it would be more than a “little” list. We’d be swapping out our two 38-lb house batteries for six 62-lb batteries, a total of 372 lbs!

I searched for alternative locations, but a 35-foot boat doesn’t provide many options. Then one day when I was adjusting the valves on the engine, a promising location revealed itself. I dug out a plastic milk-carton box and fixed it in front of the engine compartment access panel that’s located under the companionway ladder. I then continued to adjust the valves and then I checked if I could easily service the raw-water impeller and the heat exchanger. I had already installed a remote oil filter at this location, and I plan to relocate the primary fuel filter there also. I decided I could build a box about the height of the ladder’s first step without having it encroach into any usable space. This box would hold four of the six required batteries. The other two go into the original box.

The location is perfect in that it’s right on the centerline and just aft of the beamiest section of boat. We repowered Andiamo several years ago, and the new engine weighs about 175 lbs less than the original. Adding 300 lbs just forward of the engine should very closely restore the boat to its lines.

The first thing was to build a corrugated cardboard mock-up of the box, tape it in place, and go for a weekend cruise with a couple of guests aboard. This was an empirical test to ascertain that the box indeed would not hinder movement around the galley and companionway area. The mock-up passed the test, so onto the next phase: designing the details.

Since the box is exposed, teak veneered plywood was the choice for material. I chose half-inch plywood as a good compromise between making the box as small as possible and as strong as required. The joints are glued with epoxy, and the inside corners all have thickened fillets forming a nice radius to facilitate cleaning. The entire exposed interior is coated with epoxy to have an acid-resistant finish. The top is 3/4-inch-thick plugged-core plywood with teak edging and Treadmaster covering to match the non-skid on the ladder rungs. The companionway ladder is cut off just above the lowest rung, and the box cover serves as the first step. The box is securely anchored with a combination of cleats and screws at the bottom and screws through the sides (in the upper third) of the box into bulkhead structures on two sides. It was secured so it could be completely removed if the engine ever needed to be pulled for major service or replacement. The batteries themselves are firmly secured inside the box with 2-inch-wide nylon straps. I needed to install several new cables linking the split battery bank, since you always want the cables to and from the bank to be at opposite ends of the bank.

This much “larger” first step has proven to be very user friendly and more convenient in accessing the companionway from the galley.

Lisa and John Caruso sail in the Pacific Northwest and are outfitting Andiamo, their Wauquiez Pretorien 35, for a 10-year circumnavigation starting in 2004.

By Ocean Navigator