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Old 24 December 2007, 14:53   #31
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I don't think self tappers are designed for ply and therefore not suitable. .
I think they are eminently suitable when used across the ply because the plies fit reasonably into the threads and make for a good hold. The problem arises when;
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Originally Posted by Rogue Wave
the nobber thats installing eveything gives it too much arm and breaks the thread in the ply/glass.
Part of the problem is the poor quality of most of the marine ply. It is possible to source good stuff but I've not found it readily available. In good ply, a stainless selftapper will snap before the ply thread strips.

I agree with Linda.
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that is just a terrible piece of workmanship.
And, six self tappers just isn't going to do it for that application.
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Old 24 December 2007, 19:27   #32
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I just bought a load of ply from these guys after hunting round for something more specific than generic "marine ply" of unknown origin.
3 Choices of BS1088 ply depending on your requirements. Gaboon, Mahogany or Sapele.
http://www.robbins.co.uk/Pdf%20Files...wood_guide.pdf
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Old 03 January 2008, 14:00   #33
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Check your seat mounting points

http://www.ybw.com/auto/newsdesk/200...bogeneral.html

(Sorry if this has already been posted.)
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Old 03 January 2008, 15:48   #34
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I've always felt a bit nervous with consoles and seats that are simply screwed to the deck. I've only had one boat built that way, and couldn't wait to replace screws with decent bolts through the deck - not easy to get access, but at least you feel a bit more confident when the boat starts bouncing about a bit.
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Old 03 January 2008, 16:11   #35
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I've always felt a bit nervous with consoles and seats that are simply screwed to the deck. I've only had one boat built that way, and couldn't wait to replace screws with decent bolts through the deck - not easy to get access, but at least you feel a bit more confident when the boat starts bouncing about a bit.
I have never felt nervous in Humbers which i have driven many, because they use plenty of decent sized screws.

James
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Old 03 January 2008, 16:16   #36
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In my experience self-tapping S/S screws are more than up to the job, BUT you must use sufficient to ensure security. Humber have used this method without problems for as long as I can remember, spacing the scrwes approximately 10-15 cm apart thus using some 20+ screws for each seat. Hence the seats remain in place!!
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Old 03 January 2008, 17:09   #37
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(Sorry if this has already been posted.)
That's OK, it was only a month or so ago!

I've merged the threads now . . .

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Old 03 January 2008, 19:32   #38
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Part of the problem is the poor quality of most of the marine ply. It is possible to source good stuff but I've not found it readily available. In good ply, a stainless selftapper will snap before the ply thread strips.
I always though that self tappers were designed for sheet metal but If your saying they were designed for plywood then that's good enough for me.


If I've snapped my self tapper, in my good ply, then have I still got a bad fastening?

Given a few years when the ply becomes a little moist and the screw a little rusty what gives first ?
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Old 03 January 2008, 20:35   #39
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I always though that self tappers were designed for sheet metal but If your saying they were designed for plywood then that's good enough for me.
Stu, I think you are right, real self tappers are designed for cutting into sheet metal but they are made from very tough, hard steel. Stainless selftappers are not much use for that because they are too soft and also they tend to pick up and tear. I think they are pretty much just a style of screw which has good holding, in the same way as twinfast wood screws have just about superseded normal wood screws.


Quote:
If I've snapped my self tapper, in my good ply, then have I still got a bad fastening?
I wasn't meaning that. It was just an illustration of how strong and durable a good quality plywood can be. It's not specifically a marine ply problem, most of the commonly available plywoods are softish in nature. A piece of quality birch ply is very strong.

I've got a couple of offcuts of good quality marine ply which came from a Zodiac and a piece which was cut from my Humber. I'll take a snap of them to show the difference just to sorta complete this thread. Having said that though, the bits of Humber have been lying out in all weathers for the best part of 4 years and they're still ok, so the resistance to weathering of the wood and adhesive must be alright. On my previous Humber, all the fittings were held to the deck using selftappers and silicon, including a 50 gallon stainless tank, and none of them came adrift, the deck didn't leak water either. On my present Humber, the trimtabs are held to the transom with selftappers and they must take some battering but they seem fine so I think it is mainly down to how well it's done. Lots of silicon as a sealer and the fattest screws you can lay your hands on. Thickness is better than length but both is the best....
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Old 03 January 2008, 21:04   #40
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Good post JW. problem is that most folks ( including boatbuilders) do not know how to do it properly. D'ont have time to write it up at the mo. However, it is not as simple as drilling a hole and whacking in a screw with a Black and Decker, as I know you know.
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