Securing battery box/cabinet to hull - SailNet Community

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Old 10-27-2011
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Securing battery box/cabinet to hull

Hi all. I've got a Mirage 24 with an outboard and there is some space behind the companionway steps where the factory ice box used to be. A previous owner ripped out the ice box and replaced it with a shoddy set of drawers, the housing of which is made with untreated plywood, the cheap stuff to boot. Needless to say, it's falling apart and I want to replace it. I am also rewiring the boat over the winter so the thought is to move the batteries (2 x 12v) from their current location, one under either settee, into this area (weight in the middle down low)... but I don't want them to sit in bilge water. I want to glass in a battery box or somehow fasten it so it does not move around. Any tips or advice on how best to do this? The idea now is to build a structure out of plywood and seal it with epoxy or resin (?) and then bond it to the hull. Is this a bad idea? How would I bond it to the hull, would I have to rough up the hull first? The structure would actually be a cabinet with a drawer on top, the batteries would sit in the bottom of the cabinet so that that they are accessible. The structure would span the bilge. Any advice, tips or ideas much appreciated.
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I'd recommend glassing in something out of plywood that is large enough to accommodate the batteries and the cheap plastic battery boxes (without the plastic lids) that are sold at Walmart. The plywood would be to keep the batteries secure. Be sure to build lids (to keep metal tools from shorting out the batteries when working nearby) that allow for plenty of ventilation. I would further recommend using multiple strips of woven roving saturated with epoxy when tabbing the wood to your fiberglass hull.
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Thanks. That's what I was thinking and in fact I already have the plastic battery boxes. I'm just not clear on the tabbing part. How do I prep the hull?
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"I want to glass in a battery box or somehow fasten it so it does not move around. Any tips or advice on how best to do this?"
Epoxy and fiberglass, either direct or over plywood. The West Systems (epoxy) web site has plenty of detailed files on just how to do that so you get a solid bond.
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Old 10-30-2011
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I just used Bondo (from Lowes) and fiberglass to make a shelf to hold my AC unit in a lazarett. The instructions are on the bondo can. I used glass mat but I would suggest roving as its more substantial. I also used disposable foam brushes for applying the epoxy so I could just toss them after each use. After glassing the box to the hull, I sanded and painted it.
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Automotive bondo tends to absorb moisture if it's not sealed and is generally not a great idea unless it's just for fairing that is going to be somehow sealed.

The pics above show the tabbing idea nicely, but I'd be doing more aggressive prep on the hull - the paint/gelcoat should be ground off to bare glass for best bonding, and the tabbing looks kind of light.

In the OPs case, if the hull is flat enough the base of the box could be set on the hull in a thickened epoxy bed, 'leveled' and then tabbed all around. Alternatively you could mold a fiberglass box the right size and avoid any potential future rot altogether. Then you could also avoid using the cheap boxes too. It would be easy to fashion a restraining strap of some sort for security and a lid for protections as suggested above.
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I would not use mat as it has little strength. Roving of some type is better, biaxial 1708 being a good choice. It is a stitched roving with epoxy compatible mat on one side.
Most mat is not compatible with epoxy as the binders that hold it together will only dissolve in the styrene in polyester resin - epoxy is free of styrene. This is necessary for the fibers to absorb the resin as they have zero strength otherwise. The only mat that should be used with epoxy specifically states that it is epoxy compatible.
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The forum member posted his question 2 days ago and nobody gave him a reply. Much as I am a beginner at glass/epoxy work, I at least wanted to provide a starting point to give the person a seed of an idea so he will come up with his own solution. I did suggest he use roving, not mat. I am thankful for forums such as these because of the exchange of ideas which allow an individual to select the perceived best advice. In my years of banging my head into the wall trying to do my own DIY projects, there was no internet available and you had to ask whomever was available on the dock or in the marina. At least I didn't suggest he screw the box into the hull with long screws.
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I did this same project following the guidelines in Nigel Calder's book-tabbed the plywood box, (fiberglassed on both sides), to the inside hull with two layers of tabbing, the second extending out past the first. I used woven fiberglass & epoxy thickened with the West System thickener they recommend for adhesion, (I'd have to look at the products on the shelf to see which one).

I prepped the inside hull with Interlux fiberglass cleaner/?boat wash, (wear a respirator-in close quarters it'll get you high), then coarsely wire brushed the hull to roughen it up, then used the cleaner again.

I could probably lift the boat with a hook through the battery box-it's a very secure installation.
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