Backing Pads

[Roach1948]

I am adding a self-draining cockpit and an electric bilge pump and therefore several new holes will be appearing in the hull.

My question is regarding the necessity to add backing pads to skin fittings. I know its proper to do so, but what wood (Plywood/Mahogany) and how big? Some of the holes will be near the reverse tuck in the stern of the yacht, meaning that fashioning a backing pad will be quite a challenge in carving. Other holes will be near bulkheads meaning that the pad can only be of limited size. Should backing pads stretch over several planks to spread load? Another thing is how to fix - a gloop of epoxy or screw them in? Your thoughts, as always, much appreciated...

 

[JesseLoynes]

Hi,

I've used both ply and normal hard- or softwood for backing pads. The ply works well because it offers streength in all directions and in a particularly curvy bit of hull, you can use a couple of thinner pieces bent around the curve rather than than doing a lot of carving in a solid piece. I usually find it depends on the inside of the hull, ply offers a stronger spread if you've only got space for a small pad relative to te size of the whole.
There shouldn't really be any extra load placed in the area unless you have very tight plumbing, so you are simply returning strength to the area after weakening the plank. If you are putting, for example, a 2" fitting through a plank only 3" wide, then obviously you need to link the pad across the planks on either side. If the plank is 8" wide, then a pad with a couple of inches all round will be fine. Remember that if you are not using ply you may want to extend the length of the pad along the grain for more strength.
Pads are usually screwed down on mastic so they link the plank together around the hole. Remember to chamfer the top surface, or at least run it down to one side so that water running down the inside of the planking cannot pool above the pad.

Anything else? I'm sure there will be some more suggestions or other methods..

Have fun!

 

[ccscott49]

No, I agree with BSJ in this, just make sure if you use ply, to seal all the edges/surfaces with epoxy, no matter what they say about marine ply, water will capillary up the grain.
Personally I wont use ply, but that's a personal thing, I think ply is an abomination!

 

[Blueboatman]

imho . I like ply because it is less likely to split if wet or overtightening on the skin fitting-imho. It is quite difficult in my experience to get a piece of marine ply that is exposed all round to actually rot through,though others will disagree.
Re shaping the reverse curve-If you make a template of the profile required and then use a 4inch 40grit grizzly disk in an angle grinder tilted slightly it is quite quick and easy!

 

[ccscott49]

I agree with the grizzly disc, brill arent they!