How do you secure heavy batteries to GRP?

JohnGC

JohnGC

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I'm changing the layout of my starter and service battery locker.

The existing fix down straps are retained by machine screws into the GRP. I presume a backing plate and retained nut or threaded hole where fitted when the boat was constructed or first fitted out.

Unfortunately the changes I'm making mean that the existing fixing points are now in the wrong place and I need to create new ones.

Given that I can't get behind the GRP to add a new backing plate, how should I go about creating a fixing point that is strong enough to retain the batteries in a knock down? Are there fixings made for the purpose, do I need to fabricate something or is major surgery needed? It seems to me that significant load spreading is required for the GRP to withstand the forces of inverted batteries.
 
If you have a bit of vertical room to spare, you could epoxy/GRP wooden batten/battens to lie on the GRP underneath the batteries. A housing underneath the batten will allow a webbing strop to pass through and around the battery/battery box.

That's what I did. Sewn loops in the webbing can be lashed tight with turns of small stuff. Simple, no metal bolts or catches needed, no drilling of the structure and costs very little.
 
Definitely glass in some wooden batters to screw into.

I also just glassed up 2 new battery boxes.

We made a section of ply, glassed it in and fitted a battery box. I then glassed around the edge of this to create a 'lip' before screwing it to the ply.

We also used screws to attach the new section to a beam within the structure and glassed over these.
 
Make a battery box out of substantial plywood ( I used 18mm) and either screw down to battens previously glassed in (preferred), or glass in the box base.

Don't forget to fit a lid to the box but drill some holes at the top of the sides for ventilation.

Fit clips to hold the lid down
 
Without knowing the layout of your battery box, this may be of little use... Don't forget that you can put a solid bar above the batterries to keep them secure. My own batteries sit on the flat GRP base of the compartment under the navigator's seat. A batten of suitable thickness then screws to the seat frame above them, preventing them from rising and a few blocks glassed around them stop them from sliding around. If necessary, an adjustable pressure pad could be mounted from a beam with bolts to wind it down to bear on the top of the battery.

Rob.
 
>a fixing point that is strong enough to retain the batteries in a knock down? Are there fixings made for the purpose, do I need to fabricate something or is major surgery needed? It seems to me that significant load spreading is required for the GRP to withstand the forces of inverted batteries.

Our batteries were on wooden raised platforms so you could put ties around the platform and batteries. If there is space would it be possible to glass in (very thick) a metal plate with welded metal struts with a platform secured on top. That's what we had but the boat was steel so the struts were welded to the hull.
 
rob2 said:
Without knowing the layout of your battery box, this may be of little use... Don't forget that you can put a solid bar above the batterries to keep them secure. My own batteries sit on the flat GRP base of the compartment under the navigator's seat. A batten of suitable thickness then screws to the seat frame above them, preventing them from rising and a few blocks glassed around them stop them from sliding around. If necessary, an adjustable pressure pad could be mounted from a beam with bolts to wind it down to bear on the top of the battery.

Rob.
Click to expand...

I've been wondering about a version of this. My battery compartment is formed by the hull moulding (base and one side). The salon seat/locker and lid (cushion on the top) form the others. Currently all of the battery hold down is into the hull GRP. Next time I'm at the boat I'll assess the strength of the seat/locker structure. If it's strong enough I may be able to adapt the locker lid to clamp the batteries down. It would be relatively easy to add restraints to prevent the batteries moving in the other two plains.

If that proves to be unfeasible then I do have spare height which would allow me to glass in some battens. The shape of the hull moulding means that I have to span a void anyway because the new batteries are shorter than the originals.

The boat is an ETAP. ETAPs have a double skinned hull which is filled with foam. I had assumed that the shapes in the hull (on which the batteries sit) are simply full of foam, but it is possible that there are some reinforcing beams within the moulding. I'm tempted to drill a couple of small holes and explore that possibility.

You've given me food for thought - roll on the weekend.
 
If you don't have head room for wooden battens then stainless steel plate can be glassed onto the floor or sides of the box. A lip on the plate can have a hole to fit a bent rod through like a lot of cars. Drill large holes in the palte so that resin and glass on top can join to existing GRP underneath. Of course the larger the plate the stronger it will be.
Plates attached in this way will have less "peel" strength than stength in shear. ie pull sideways. So try to get the attachment to the plate nearer the middle of the plate rather than one edge. good luck olewill
 
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