Backing pad/plate for skin fitting – why?

[Restoration man]

I think it also depends how tall your skin fitting is , on our old sealine the water stainer was about a foot tall , to take filter out you needed big spanner on the top to get the nut off , the sealine only had a very poor quality pad like about 9mm cheap plywood and the diameter was quite small, even when the top nut came off easily the hull flexed quite alarmingly, and when the nut became a bit tight over time , my arse would really twitch when I took the nut off , on my new boat I’m fitting fr4 /g10 backing pads whilst I’m doing the skin fittings for piece of mind , nothing was ever mentioned by the surveyor about the backing pads when buying or selling but they did report on dripping tap in the galley

 

[john_morris_uk]

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!

I’m not sure about the hull flexing bit as I’ve not seen or experienced it. I agree that ideally I would have preferred not to have those fittings all in a line but it was a bit of Hobson’s choice; there wasn’t anywhere else for them to go. However much or little the pads strengthened the hull, they did give me a nice flat surface to bolt the flanges of the seacocks down to. The pads were bedded down on the hull first with thickened epoxy, so I would suggest that that they did add some strength.
The pads were cut out of good quality marine ply with hole saws and a suitable router bit put a nice rounded edge on the outside. They then had two coats of epoxy. A few minutes work.

 

[Tranona]

I can see flex in large flat panel such as the Sealine on poat#22, but not in the location where they usually are in a sailing boat. Anyway the typical pads would just move the stress point further out, although I can't imagine any stresses on a seacock that would cause any flexing never mind failure of the GRP panel in normal use.. As I suggested earlier suspect the practice dates from wooden boat building days, maybe even to give the apprentices something to do between brewing the tea!

 

[Neeves]

Individuals, especially those with a pecuniary interest, making recommendations often have no idea of the actual installation environment, just think of so many threads on YBW whose OP omits to mention salient detail and does not think to take a photograph. Yet the OP wants detailed and expert advice. In the absence of detail - err on the side of caution, especially if the recommendation costs peanuts - longer term caution becomes set in concrete and is not necessarily wrong (even if unnecessary).

Jonathan

 

[Tranona]

I think the OP explained his situation rather well in posts#1,7 and 9. Not really asking for advice but for views on his reasoning. So far there has not been a convincing argument for backing pads except to level a curved surface or in a flat panel where flexing might be an issue. The former is quite common and I have seen many boats where the layup in the location of skin fittings has been built up and flattened specifically for this purpose. With the latter I have difficulty in seeing what extra stress is placed on a flexible panel by bolting through a fitting that would cause a failure, nor indeed how a backing pad if the size normally used would prevent such failure.

 

[doug748]

.................In the absence of detail - err on the side of caution, especially if the recommendation costs peanuts - longer term caution becomes set in concrete and is not necessarily wrong (even if unnecessary).
Jonathan

Indeed, the very definition of Good Practice.
If you insert a pin through an assembly you put a bit of grease on the end, not because it is always necessary but because it sometimes helps and costs little in time, energy or money.
If you drive an oval nail, you align it with the wood grain, same reasoning. Because it's Good Practice.

This case is a little different though, we have heard about 6 good reasons why backing pads are a sound idea. Additionally you might feel a little bit more comfortable when the lifting stops foul the flange or when the boat rests on it in it's cradle; they might just help prevent the whole fitting being punched into the hull, if striking a rock or floating container.

The hull has been weakened by drilling the hole, it is Good Practice to try and put some strength back.

.

 

[BabaYaga]

The hull has been weakened by drilling the hole, it is Good Practice to try and put some strength back.

That is indeed a good argument, from a principal point of view. And in line with what I stated in my post #7.
I'm not fully convinced though, that Good Practice is always best based on a principal approach.
As comparison, take the case of making holes and openings in aluminium spars. Every hole will weaken the structure to some degree. But drilling holes for rivets or making openings for various fittings is mostly unproblematic, because the weakening is well within the margins. Only when large openings are made close to other large openings (or possibly smaller holes are drilled in a row) the integrity is threatened. So Good Practice is much depending on the particular circumstances.

Despite of the above, I am not ruling out the possibility that I will make a backing plate for my skin fitting.

 

[Neeves]

That is indeed a good argument, from a principal point of view. And in line with what I stated in my post #7.
I'm not fully convinced though, that Good Practice is always best based on a principal approach.
As comparison, take the case of making holes and openings in aluminium spars. Every hole will weaken the structure to some degree. But drilling holes for rivets or making openings for various fittings is mostly unproblematic, because the weakening is well within the margins. Only when large openings are made close to other large openings (or possibly smaller holes are drilled in a row) the integrity is threatened. So Good Practice is much depending on the particular circumstances.

Despite of the above, I am not ruling out the possibility that I will make a backing plate for my skin fitting.

Here, on YBW, I would always recommend using a backing plate as we will never know exactly what the location, actual situation might be. Commonly we all overlook some aspect when we describe something and it simply seems sensible to take the belt and braces cautious approach. The seacock maybe does not need a backing plate, the layup is good - but when the seacock seizes in 3 years time the layup may not be sufficiently robust to accept the extra grunt needed to close or open the seacock. In an OPs description he may omit to mentioned a slight curve in the layup, the fact his hole is not drilled perpendicular to the interior, the interior is a bit rough, he may not realise the layup includes foam in this location but was solid glass where he worked previously. - I'd simply be cautious. - and use a backing plate every time.

When I'm doing any glass fibre work I keep the offcuts, I may make up extra, and make small sheets of resined glass and we carry a few on the boat 'in case'. We have some of the offcuts kept handy and I can drill holes in them with a hole saw when needed. Now the yacht has been ours for years it is not so necessary but when new I was constantly adding backing plates, pad eyes, stanchion bases, new hull fittings (second log), wind gen pylon etc etc.

I just wish people, OPs, used their phones with a bit more gay abandon.

I note with interest that here, Oz, at least the new Samsung phone is being marketed more for its features as a camera than anything else - some obviously use their cameras (on their phones) as second nature - this has not seeped through here yet. The PBO section should be full of pictures - its not.

A picture is still worth 1,000 words.

I'm fully aware that a picture or video can be fudged - but that needs some thought, a couple of snap shots is all that is needed.

Jonathan

 

[Rappey]

I just wish people, OPs, used their phones with a bit more gay abandon.

I think everyone would if it were that easy, but it's not as the pic needs resizing

 

[Plum]

I think everyone would if it were that easy, but it's not as the pic needs resizing

Agree, having to re-size pictures to less than a Mb is a pain and often puts me off posting photos. In my opinion it is the top issue with this otherwise excellent forum system. The resizing should be built-in to the photo posting process.

Www.solocoastalsailing.co.uk

 

[Restoration man]

One other thought, do the people who think it’s ok to not use a backing pad also. Have the same thoughts towards deck fittings ie cleats anchor winches ,stanchions ??, I know someone mentioned it just moved the load when fitting a backing pad , but I thought the whole point in fitting backing pad to deck fitting was to spread the load surly this would apply to hull fitting Same as a deck fitting?? Or have I got this all wrong ???

 

[Tranona]

One other thought, do the people who think it’s ok to not use a backing pad also. Have the same thoughts towards deck fittings ie cleats anchor winches ,stanchions ??, I know someone mentioned it just moved the load when fitting a backing pad , but I thought the whole point in fitting backing pad to deck fitting was to spread the load surly this would apply to hull fitting Same as a deck fitting?? Or have I got this all wrong ???

Yes. Unlike deck fittings such as cleats there is zero load on a skin fitting!

 

[Restoration man]

Yes. Unlike deck fittings such as cleats there is zero load on a skin fitting!

Not if you stand on it or fall on by mistake

 

[Tranona]

If you look at post#24 on the sprayhood thread lower down (sorry failed to find a link) there is a youtube of an HR50. At 3.10 there is a shot of the toilet outlet. This shows modern practice. The laminate is ramped down to provide a flat base for the skin fitting. Under the locking nut is a thin washer, the same diameter as the outer flange.

Nothing like the sort of backing plate advocated by many here but exactly like I have just used for a new water intake using a ball valve. Note also that the ball valve is plated plain brass.

There is a lot talked about "best practice" as if everybody "knows" what this means. Best practice is evidence based and there is no evidence in this thread (nor anywhere else) that fitting a seacock through a GRP hull weakens the structure to point where it may fail, nor that adding a backing pad would reduce the chance of a failure. I cannot see from what people have posted any sign of "6 good reasons" why they should be fitted as claimed it post#26 The closest is a suggestion that GRP panels may flex, but the only example of that is a flat panel on a powerboat that flexed when a 1' high intake water filter stack was pulled.

BTW I would love to see the empirical evidence that having a lifting strop catch a skin fitting flange could punch a hole in the boat - or that having a backing pad inside would prevent that happening!

There are tens of thousands of production GRP boats in use that do not have backing pad under their seacocks (as many as 10 on some boats!)s. If it was such bad practice why do builders ignore it? and more importantly where is the rash of boats sinking because of lack of backing pads?

 

[Tranona]

Not if you stand on it or fall on by mistake

So don't do such a stupid thing - and in most sensible installations it would (or should) not be possible to do that. Even if you were stupid enough to do that I very much doubt you would pull the fitting out of hull.

 

[john_morris_uk]

If you look at post#24 on the sprayhood thread lower down (sorry failed to find a link) there is a youtube of an HR50. At 3.10 there is a shot of the toilet outlet. This shows modern practice. The laminate is ramped down to provide a flat base for the skin fitting. Under the locking nut is a thin washer, the same diameter as the outer flange.

Nothing like the sort of backing plate advocated by many here but exactly like I have just used for a new water intake using a ball valve. Note also that the ball valve is plated plain brass.

So HR build in their backing pads….

 

[Neeves]

I think everyone would if it were that easy, but it's not as the pic needs resizing

Re-sizing

If you are an OP then if you want good and well informed answers then make the effort, re-size, and your investment will be rewarded with better advice.

Given our advice is free than faffing about for no return, usually not only no thanks but complaints that the post is 'boring'. certainly does not encourage.

But, I am not the first to agree that re-sizing is a pain. Some nerd will tell me I'm wrong, or there is a better way, but I simply send them to myself as an email and reduce the image size by choosing 'medium' for the size I wish to send. I'm working with a Mac, might not have the same facility for other hardware/software. Once I receive the email, which takes seconds, just an image or images I simply drag to the post. I find I send many of the same images again and again and I store the reduced images in a folder/album and delete the email. I do try to vary the images - to fend off the ungrateful.

I'll be happy if someone tells me I'm wasting time etc - but show, tell, me a better way. Its easy to be critical - let's have some positive support. Life is part about accepting new experiences - and none of us know everything about everything - unless, of course, you are a grandparent.

Jonathan

 

[john_morris_uk]

Re-sizing

If you are an OP then if you want good and well informed answers then make the effort, re-size, and your investment will be rewarded with better advice.

Given our advice is free than faffing about for no return, usually not only no thanks but complaints that the post is 'boring'. certainly does not encourage.

But, I am not the first to agree that re-sizing is a pain. Some nerd will tell me I'm wrong, or there is a better way, but I simply send them to myself as an email and reduce the image size by choosing 'medium' for the size I wish to send. I'm working with a Mac, might not have the same facility for other hardware/software. Once I receive the email, which takes seconds, just an image or images I simply drag to the post. I find I send many of the same images again and again and I store the reduced images in a folder/album and delete the email. I do try to vary the images - to fend off the ungrateful.

I'll be happy if someone tells me I'm wasting time etc - but show, tell, me a better way. Its easy to be critical - let's have some positive support. Life is part about accepting new experiences - and none of us know everything about everything - unless, of course, you are a grandparent.

Jonathan

I load pictures straight off my iPhone. No need to resize as when you choose a picture from your photo album it gives you the option (click on the size) to choose a smaller resolution. Less than 1mb is fine.
Zero faffing

 

[Neeves]

If you look at post#24 on the sprayhood thread lower down (sorry failed to find a link) there is a youtube of an HR50. At 3.10 there is a shot of the toilet outlet. This shows modern practice. The laminate is ramped down to provide a flat base for the skin fitting. Under the locking nut is a thin washer, the same diameter as the outer flange.

Nothing like the sort of backing plate advocated by many here but exactly like I have just used for a new water intake using a ball valve. Note also that the ball valve is plated plain brass.

There is a lot talked about "best practice" as if everybody "knows" what this means. Best practice is evidence based and there is no evidence in this thread (nor anywhere else) that fitting a seacock through a GRP hull weakens the structure to point where it may fail, nor that adding a backing pad would reduce the chance of a failure. I cannot see from what people have posted any sign of "6 good reasons" why they should be fitted as claimed it post#26 The closest is a suggestion that GRP panels may flex, but the only example of that is a flat panel on a powerboat that flexed when a 1' high intake water filter stack was pulled.

BTW I would love to see the empirical evidence that having a lifting strop catch a skin fitting flange could punch a hole in the boat - or that having a backing pad inside would prevent that happening!

There are tens of thousands of production GRP boats in use that do not have backing pad under their seacocks (as many as 10 on some boats!)s. If it was such bad practice why do builders ignore it? and more importantly where is the rash of boats sinking because of lack of backing pads?

Tranona, I value you comments and often read your posts, simply because they are educational and amazingly polite (I have much to learn).

There may not be evidence that backing plates have value but I'll guarantee that many here carry flares and have no evidence they are needed, have lifejackets and have never used them in anger, have a manual bilge pump in the cockpit they have never used and have 20 year old rigging or replaced at 15 years and never had a rigging failure etc etc. Lots of things we do we do because it is 'standard practice' or because it seems sensible. Of the thousands of life rafts sold - how many are actually used in anger (my father used one). We have carried flares for 40 years and never used one in anger, have had transom mounted bilge pumps, never used them, emergency steering never used it, jackstays under the bridge deck - you guessed correct - never used them. We have a roll of signal flags, never used them, not even as decoration. I qualified for a boat licence and a VHF (plus SSB) licence, never been asked for sight of same

Adding a backing plate seems sensible, whether it is needed is a subject for a forum debate (as a replacement for an anchor thread) - and if asked I will continue to suggest (that like carrying flares, LJs, jackstays, emergency steering etc) it seems sensible. Unlike a liferaft, or new rigging at 7 years, adding a backing plate costs...... nothing, seems harmless and might in general be beneficial and in some circumstances - essential ( but I stand to be corrected). My inability to be flexible, if asked 'why', is that I have a verbal description of the installation and though the absence of the backing plate might neither here nor there - I don't actually know the actual location - and I'm cautious. Yes - the horizon looks flat but it is actually a curve and most hulls, for example, are curved to a great or lessor degree - and some surfaces, described as flat, are actually curves......

Jonathan

 

[vyv_cox]

I think everyone would if it were that easy, but it's not as the pic needs resizing

Yes, I was unable to post pics when sailing last year as my phone seems to be unable to edit photo resolution.