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Old 01-10-2010, 16:35   #1
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Backing Plates

I had a shipwright tell me that backing plates for deck fittings weren't really necessary these days with the thickness of fiberglass decks. Seriously? If you were taking a boat coastal cruising or off shore would you just use a few washers and a locking nut on a bolt? Even if the deck was solid fiberglass?
Just wondering!
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Old 01-10-2010, 16:50   #2
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If the deck is solid fiberglass and thick enough (sell the boat) then backing plates may not be needed. Look how existing fittings are surviving. But the deck is probably cored, and the fiberglass thin, especially on the inside. So you need backing plates. Mechanically, aluminum has got a slim lead in plate stiffness to weight but the corrosion issues push me towards stainless steel.

Large (fender) washers are only effective for lightly loaded cosmetic-like items.
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Old 01-10-2010, 16:55   #3
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If someone were to make a deck without a core, the keel would have to be huge to make the motion correct!!!
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Old 01-10-2010, 17:06   #4
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Quote:
Originally Posted by ShayW View Post
I had a shipwright tell me that backing plates for deck fittings weren't really necessary these days with the thickness of fiberglass decks. Seriously? If you were taking a boat coastal cruising or off shore would you just use a few washers and a locking nut on a bolt? Even if the deck was solid fiberglass?
Just wondering!
While your shipwright "might" be correct, I prefer backing plates on all stress related hardware.

I'm currently adding SS straps 4" X 18" to attach my anchor bridel using 3/8" material and will be thru bolted with (4) 1/2" bolts and 6" X 18" X 1/4" SS backing plates. Overkill, maybe.......
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Old 01-10-2010, 17:26   #5
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The boat in question had the teak decks removed and fiberglass laid down. I have not taken a fitting off to examine the deck but noticed the 5 washers per fitting and no backing plates. Looked messy so my red flag rose quickly raised. Looks like I'll have to finagle a looksee. The owner is willing to pay a shipwright to install backing plates.
I think I'll show up for that project and see what happens.
Thanks!
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Old 01-10-2010, 17:31   #6
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Quote:
Originally Posted by Sailmonkey View Post
If someone were to make a deck without a core, the keel would have to be huge to make the motion correct!!!
Do What.... ????..... why... are you suggesting that a fibreglass deck with no core would make her top heavy...???
Bang in the backing plates... forget the shipwright... its the real world out there not Bayliner country....
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Old 01-10-2010, 17:54   #7
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Go swing on the lifelines and Stanchions.... hook a halyard up to the fittings and yank on it really hard.... If any of them make creaking or fiberglass breaking noises you'll need backing plates.....if they are rock solid and don't move... you're good. I still think backing plates spread the load and will increase the load any fitting will take before ripping it out of the deck... Better safe than holly out at sea in a storm? ( Wholey... holey? Shikers..... you know like swiss cheese? )
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Old 02-10-2010, 07:26   #8
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There are backing plates and there are backing plates done right. There is a backing plate under this mount:

Click image for larger version

Name:	Davit mount.JPG
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ID:	19754

The backing plate needs to match its job, and obviously this one does not. Water is also getting in through the bolt holes and/or the gel coat cracks and the core is wet a good distance from this mount.

My backing plate philosophy is more along the lines of DRSanbo, however for highly stressed deck hardware like cleats, I start with marine plywood epoxied to the underside of the deck. This provides a large, flat bearing surface that really spreads out the load, and the epoxy conforms to the underside of the deck perfectly so there are no hard spots.

I also (am in the process of) drill out an oversize hole and fill it with epoxy to increase strength and prevent the two layers of fiberglass from flexing independently, which could break the sealant's bond between the fastener and deck. Also, with the epoxy plug if water does get in, it comes into the cabin instead of getting into the core. That is what my recent thread about drilling parallel holes was about - getting the holes drilled parallel so the bolt holes like up with the backing plates.

Overkill? Not on a cruising boat.

Aluminum backing plates are ok for dry areas. Here is a badly corroded aluminum backing plate in the chain locker for a cleat. BTW, cosmetically, this is a beautiful boat.

Click image for larger version

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ID:	19755

Also, note the stress cracks in the fiberglass at the three bolts for the locker door hinge - a backing plate is in order here!

DRSanbo - who is drilling the holes through that stainless?
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Old 02-10-2010, 09:50   #9
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Quote:
Originally Posted by boatman61 View Post
Do What.... ????..... why... are you suggesting that a fibreglass deck with no core would make her top heavy...???
Bang in the backing plates... forget the shipwright... its the real world out there not Bayliner country....
Fiberglass as a single panel isn't at its best under a compression load. On a smaller boat or smaller pieces its beefed up with stringers.and that is okay. As the boat gets bigger the loads become tremendous. The deck is doing a lot more then keeping the water out its supporting the hull. Cored or laminated material do much better with strength to weight ratios especially in this application. When the sails fill the mast is pushed down against the keel trying to push the bottom down the stays are pulling inward and the hull if it didn't have the deck would flex inward so the bottom could flex the hull and the keel would get deeper the rails narrower so to speak..So the decks are cored to improve this compression strength.. The weight would be huge if we had solid glass decks. Maybe that explained it kinda
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Old 02-10-2010, 16:45   #10
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Quote:
Originally Posted by ShipShape View Post
There are backing plates and there are backing plates done right. There is a backing plate under this mount:

Attachment 19754

The backing plate needs to match its job, and obviously this one does not. Water is also getting in through the bolt holes and/or the gel coat cracks and the core is wet a good distance from this mount.

My backing plate philosophy is more along the lines of DRSanbo, however for highly stressed deck hardware like cleats, I start with marine plywood epoxied to the underside of the deck. This provides a large, flat bearing surface that really spreads out the load, and the epoxy conforms to the underside of the deck perfectly so there are no hard spots.

I also (am in the process of) drill out an oversize hole and fill it with epoxy to increase strength and prevent the two layers of fiberglass from flexing independently, which could break the sealant's bond between the fastener and deck. Also, with the epoxy plug if water does get in, it comes into the cabin instead of getting into the core. That is what my recent thread about drilling parallel holes was about - getting the holes drilled parallel so the bolt holes like up with the backing plates.

Overkill? Not on a cruising boat.

Aluminum backing plates are ok for dry areas. Here is a badly corroded aluminum backing plate in the chain locker for a cleat. BTW, cosmetically, this is a beautiful boat.

Attachment 19755

Also, note the stress cracks in the fiberglass at the three bolts for the locker door hinge - a backing plate is in order here!

DRSanbo - who is drilling the holes through that stainless?

Ship Shape, great responce with pics' to make your points.

Note also the 3 different fastners used on that leg not to mention the weld job

Great shot of how a backing plate should not be done (anchor locker pic) and how improper bedding or lack of will eat your fastners.

Thru bolting core material does require the removal of core replaced with epoxy to negate any compression of the glass skins when tourquing thru bolts, as well as negating intrusion into core materials.

"Who will be drilling my 3/8" SS"? LOL not me and my Dewalt, I do have a great machine shop owned by a life long boater here local.

It's all good..
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Old 02-10-2010, 17:17   #11
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I love my steel decks for this very reason. It sucks when you find a patch of corrosion around a porthole, but it makes everything else so much more peaceful when the loads are up.
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Old 02-10-2010, 17:19   #12
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I love my steel decks for this very reason. It sucks when you find a patch of corrosion around a porthole, but it makes everything else so much more peaceful when the loads are up.

Cheater..........
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Old 04-10-2010, 07:05   #13
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If the load on a fastner were shear only, i.e. parallel to the deck across the fastner; and, the deck was of sufficient thickness, the compressive load of the faster over it's length through the deck on the deck itself might be sufficient to eliminate the need for a backing plate (note that cored decks are notoriously poor in this case however as they provide great flexural strength but have poor compressive strength parallel to the deck). If the fitting is arranged in such a fashion that any of the fasterners will be subjected to tension (which must be resisted by vertical shear through the deck), such as would be the case of a load applied to the top of a stanchion casuing a prying action on the base, a backing plate is advisable as the plate provides a much greater shear area to distribute the load through the deck. The determinant is the types of loads that will be applied to the fitting and in what direction. For a lay person not able to calculate loading or the statics of the fitting, a backing plate is the safest course. Ideally it should be somewhat larger then the base of the fitting it supports and sufficiently thick to ensure a reasonable measure of stiffness.

FWIW...
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