Backing pad/plate for skin fitting – why?

[BabaYaga]

I intend to replace the cooling water intake on my sailing boat this spring, the brass hardware has been in service since I installed a new engine in 2006.
The bore is 19mm and the GRP hull is single skin. The intake is positioned a bit down into the 'sump part' of the long keel, that is on a fairly flat GRP area, below the turn of the bilge. Hull thickness here is (from memory) something like 13mm.
I am leaning towards using brass again, this time DZR throughout.
It is sometimes suggested that a backing pad/plate bonded to the inside of the hull should be used for skin fittings of the threaded type. I can see the point of that if the hull is curved at the location, as the nut of the skin fitting should have a flat surface to mate to. However, it is also claimed that the backing pad/plate should be in place to serve as a reinforcement.
My questions are: What is it supposed to reinforce, the hull or the skin fitting and, in this particular case, is it really necessary?
I would be grateful for your views on this.

 

[Boathook]

The backing pad reduces the amount that the hull can flex in that location thereby protecting the fitting and stopping it from breaking. GRP can flex a lot before it actually 'fails'.
I suspect that there is a optimum size for a backing pad but do make sure that the outside edge is tapered, possibly at 45 degrees.

 

[Tranona]

Think it is one of those "old wives tales" dating from wooden boat days. However we used a thin composite washer about twice the diameter of the fitting under the nit. The installer suggests it helps spread the sealant across a wider area. Only cost pennies and looks neat.

 

[Daydream believer]

One might check how far up the fitting it is before the thread starts. It may be that the nut will not engage on the GRP because it will not screw down enough. Depends on the fitting, of course, but some do not get threaded all the way, as it is not necessary & stronger.

 

[doug748]

It's not great practice to omit a backing pad.

Drilling a hole weakens a structure even, perhaps especially, a GRP monocoque. The backing pad helps redress the balance as well as spreading the compressive forces - just like a washer on a nut and bolt.
.

 

[superheat6k]

I always fit a pad to spread the load, but have you considered Forespar Marelon.

 

[BabaYaga]

Thanks for the replies so far.

Undeniably a structure with a 28mm hole in it will be less strong than the same structure without that hole. I just not convinced that it makes any significant difference in this particular case.
My 9 m boat has an encapsulated lead keel of 1.6 tonnes and the part of the hull that forms the keel pocket is dimensioned to hold this internal ballast. There are transverse vertical GRP floors (?) laminated across the bilges that should counter any flexing far more efficiently than a backing plate, I would think. One of these floors is less than 100mm from the intake.
So I find it hard to believe that there is any flexing going on in that location, hole or no hole.

 

[john_morris_uk]

How much effort and time and materials are you saving in not fitting a backing pad.? What happens if and when you sell the boat (and despite your protestations), the surveyor picks up no backing pad as a fault?
Personally I always fit one.

 

[BabaYaga]

How much effort and time and materials are you saving in not fitting a backing pad.? What happens if and when you sell the boat (and despite your protestations), the surveyor picks up no backing pad as a fault?
Personally I always fit one.

That is another consideration all together. I was more curious about the technical reasons for fitting one.
NB, I am not saying that backing plates for skin fittings never have a place, for instance to counter flexing of the hull. Only thought that it might depend a bit on circumstances such as location, size of hole, hull thickness etc, rather than being a strict norm.
Would you say that backing pads for skin fittings is a norm or standard today on new boats from the major producers?

 

[emmalina]

I think you are talking about bronze not brass ! Big difference

 

[Spirit (of Glenans)]

When you can over-engineer something at little or no extra cost and with no negative consequences for the structure, why wouldn't you?

 

[john_morris_uk]

Blue Peter moment.
Here’s some I made earlier.

(I was fitting a heads to our daughters little Pandora 700)

 

[William_H]

If the hull is 13mm thick and the hole is 19mm then I can't see much point in a backing pad. Hull thickness for GRP boat is often dictated by need for stiffness of the structure as in bending under stay tension fore and aft and sideways or in your case bending loads of the keel. I think you would find that the whole structure is vastly over engineered with thick GRP so your hole and no backing plate will make no difference. ol'will

 

[KevinV]

Blue Peter moment.
Here’s some I made earlier.

But did you use double-sided sticky tape for speed?

 

[Plum]

Would you say that backing pads for skin fittings is a norm or standard today on new boats from the major producers?

In my experience of Beneteaus they do dot fit backing pads for skin fittings.

 

[john_morris_uk]

But did you use double-sided sticky tape for speed?

I missed that trick. (I used double sided Sikaflex..)

 

[vyv_cox]

I think you are talking about bronze not brass ! Big difference

Why is that then? There may well be a difference in corrosion resistance but not much else that affects a backing pad.

 

[BabaYaga]

When you can over-engineer something at little or no extra cost and with no negative consequences for the structure, why wouldn't you?

I suppose that is one way to look at it.

 

[BabaYaga]

If the hull is 13mm thick and the hole is 19mm then I can't see much point in a backing pad. Hull thickness for GRP boat is often dictated by need for stiffness of the structure as in bending under stay tension fore and aft and sideways or in your case bending loads of the keel. I think you would find that the whole structure is vastly over engineered with thick GRP so your hole and no backing plate will make no difference. ol'will

It should be noted, that the hole for the 3/4" (19mm bore) skin fitting would be about 28mm. Your conclusion may still be valid, though.

 

[BabaYaga]

Blue Peter moment.
Here’s some I made earlier.
View attachment 150517
(I was fitting a heads to our daughters little Pandora 700)

Neat! I can see the benefit of these pads, as they provide a flat mating surface for the seacock flanges.
But, if making a hole through the hull significantly increases the potential for GRP flexing in that area (as suggested by some replies upthread), the photo raises two question for me:
1) Isn't that risk increased further by making three holes so close together?
2) And if there indeed is such an increased potential for flexing because of the holes, would these small plywood pads stuck on with Sikaflex make much of a difference?

Thanks to all for your replies, BTW!