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      A Few Simple Rules   05/22/2017

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rmdanko

Backing Plates

26 posts in this topic

Installing some backing plates behind winches and clam cleats

is it best to bed with thickened epoxy or 3M 5200

Opinions ???

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G10, bedded with thickened epoxy. You want the backing plates to act as part of the structure. 5200 isn't very strong, and will creep. 

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Why do you need backing plates if they are through bolts in locations already fattened up to take load, large washers will do the job? If not DDW suggestion of epoxy and G10 next best thing....5200 has no place on a boat.

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Can someone explain why you would need any adhesive at all? All of these backing plates will be held in by large bolts.

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Any backing needs a true matting surface to properly spread the load if that backing is say larger than a washer.

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If the surface need leveling to fit the backing plate then you should follow a different procedure IMO.

Cover the backing plate with packing tape, put a layer of thickened epoxy on the "top" side of it and press it into place - hold it there with tape or whatever until the epoxy kicks. When cured, remove the backer, peel off the packing tape - et voila - a perfectly mated surface.

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17 minutes ago, SloopJonB said:

If the surface need leveling to fit the backing plate then you should follow a different procedure IMO.

Cover the backing plate with packing tape, put a layer of thickened epoxy on the "top" side of it and press it into place - hold it there with tape or whatever until the epoxy kicks. When cured, remove the backer, peel off the packing tape - et voila - a perfectly mated surface.

Yes ^^^ that. And how I do it when the surface conditions are difficult is this: Cover the backing plate with tape...or release agent. Fasten it in place with the bolts, or a temporary screw, or whatever. Shim as necessary to make it level. Mix a "small" amount of thickened epoxy. Apply as only a minimal bead under the edge. Make a nice fillet...all pretty. Don't run it over the edge of the plate...only under. Use gloved finger. Cure. Remove the backing plate. Mix more thickened epoxy and fill in the interior. The little edge bead is your guide. Use a steel trowel to level it to the edge bead. Voila! No waste. Minimal epoxy. Perfect fit. No grinding.

This can be done on the deck side too if a level pad is needed. In some cases the backing plate is not needed as the thickened epoxy may be good enough to support large washers.

Not my idea. I am not that clever.

Related note: Those thin fender wasters sold by most shops are worthless for spreading load. They simply bend...have you noticed. With a little more effort one can buy "heavy" fender washers. Doubling thin washers doesn't really work, either.

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Daddle is right about creating an "edge dam" if you have to create a pad in a heavily contoured surface. I did exactly that when fitting a small hatch on deck - covered the hatch flange with packing tape, fixed it in place with two screws on the centerline to hold it level then pushed thickened epoxy under it from both sides.

If you create a nice bevel with fingertip you'll have a nearly finished edge. When cured, lift the hatch off, remove the tape, fill in the voids in the epoxy flange you just created and you're nearly done. If you work neatly very little finish sanding is needed.

The little forehatch in the pic is the one I did this way. The pic doesn't really show it but it looked like it came out of the mould that way.

 

deck (3).jpg

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If the inner surface isn't true to the outer, I've used the bolts as a guide to get the backing plate square.

Run some nuts all the way up on the bolts used to affix the piece.  Adjust the nuts so the tip of the bolts is just barely proud of the inner surface when the bots are placed into the mounting holes.  Make the length under the nut the same for each bolt.  Put some tape or weight on the heads so they don't push out easily.  Now if the backing plate is pressed up against the the tips of the bolts, it will be square to the outer mounting surface.

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My reasons for bedding with epoxy:

1) The back side is very rarely completely flat, that means the backing plate will be bearing on 3 points until something crushes enough to gain some bearing.

2) If the backing plate is not bonded, you have not taken full advantage of the material. Two surfaces held together with bolt pressure act as a leaf spring, each contributing its own innate stiffness. Lets assume for simplicity they are the same thickness. Unbonded the two together will have twice the strength and twice the stiffness. Bonded they will have 4 times the strength and 8 times the stiffness. Why would you not want that?

3) Most deck hardware is loaded at least partially in shear. Unbonded, the backing plate will slide along for the ride as the fasteners shear through the skins. The unbonded backing plate does not improve the installation. Bonded, they have also to shear the backing plate, or break the bond, improving the installation.

If you need frequently to remove the backing plate, well then OK. 

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I love threads like these. 

The best answer I can give is, "It depends. Tell me more about the application."

What kind of boat? What fitting? Where do you plan to install it?

if I don't know the boat I will need to be told about the laminates and possibly discuss the sails and uses so I can understand the loads. 

Generally:

If the backing plate is necessary because washers would be catastrophically inadequate the backing plate answer is much different from, "Nobody else with a boat like mine uses backing plates but I want to add a layer of material just to be sure to be sure."

if you are adding a thin backing plate on the back side of a thin flexible laminate and the backing plate will be squeezed tightly to the laminate at the fasteners, slow cure 5200 may be the very best bedding material on the planet.

 

i don't have enough information to consider your question and certainly cannot offer an answer. 

 

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Ive used 1/4 or 1/2 thick star-board along with a large washer as a backing plate. Use a hole saw drill bit about 1-2  inch in diameter then put the washer over that. round over the edges and it looks fine.

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I adore the stuff for trim and nonstructural purposes, but StarBoard is far too flexible to be a good backing plate and is pretty expensive for that purpose too. Also, it creeps under pressure. G10, stainless, aluminum, plywood, and slabs of hull from knackered old boats are the materials of choice depending on cost/cosmetic/machining/finishing appetite. Some people swear they like to lay up 6-8 layers of their own fiberglass cloth and epoxy but I can't see how that is a good use of time or cosmetic effort.

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Something else to put in the arsenal, is double sided sticky foam tape as a spacer that compresses a little...

If you have a 1/4 inch or so difference between each side of the backing plate from camber changes, you can stick a 1/2 inch long piece of foam tape in the middle of the plate, butter it up and put it into place and the foam will keep the bolts from pulling it all the way into place.  I pull a clean square edge, and then a half hour or so later pull a finger fillet.

Cheers,

Zach

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Is there a rule of thumb for how thick the G10 needs to be in the backing plate application above?  I looked the stuff up, found also G9 and FR4, see that everything is rather expensive, and now am a bit more confused than before.  It will come clear with a bit more research....

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How thick the G10? I would look at what is in the structure already. How much laminate on the nut (inside) side of the deck. And look at what kind of deformation has occurred with the old fasteners. In my case, for example, for the anchor windlass, there was balsa core and maybe 2mm of fiberglass layup. The single thin fender washers had crushed the laminate just slightly. The silly-thin washers were ruined...cupped. No sign of lateral tearing. But overall the structure had withstood the test of hard use over considerable time. My conclusion was that any little improvement would be sufficient. So I bedded in a sheet of 3mm G10 and used proper heavy s/s fender washers...about 2mm themselves.

If the deck had been warped or had been oil canning when using the windlass I would have put in a considerably heavier and larger sheet of G10...or perhaps fully laminated plywood to save cost and weight.

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Here are the data for G10 from Norva Plastics in Norfolk, VA who will ship and it's reasonably priced (I paid $19 for a 1'x1'x1/4" piece). http://www.norvaplastics.com/phenolic-g10-fr4.asp. I used to use aluminum plate for backing, but now it's G10 as it's very strong, no corrosion, weighs about the same as Al, and reasonably priced.

No link with this vendor, just a local company I use.

cheers, Greg

 

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On 5/15/2017 at 10:57 AM, SloopJonB said:

If the surface need leveling to fit the backing plate then you should follow a different procedure IMO.

Cover the backing plate with packing tape, put a layer of thickened epoxy on the "top" side of it and press it into place - hold it there with tape or whatever until the epoxy kicks. When cured, remove the backer, peel off the packing tape - et voila - a perfectly mated surface.

Why not bond the plate? You didn't offer a reason. It seems to me that if that portion of the boat is under strength, making it stronger, permanently, makes sense. I can't see why I would ever want to remove it.

And everything DDW said.

The other thing I have done , when the area is very curvy, is simply add laminate, tapered off. Less is needed.

Why reinforce an area the builder should have already reinforced for a winch? I my case I dramatically increased the head sail size and the "reinforced" deck was cracking. The fender washers were cones. The deck was solid, but just too thin.

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Ok, thank you daddle.  That makes eminent sense.  I need to assess before purchase.  Brand new old boat, so I haven't gotten all the way around to listing every upgrade yet.  The cost (ref. gcutter) is reasonable for the sizes we are using (but $80 per board foot nevertheless!), even after currency exchange etc.

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13 hours ago, groundhog said:

Is there a rule of thumb for how thick the G10 needs to be in the backing plate application above?  I looked the stuff up, found also G9 and FR4, see that everything is rather expensive, and now am a bit more confused than before.  It will come clear with a bit more research....

I did a bunch of testing for Practical Sailor, bolting to cored laminates with many backing plate materials. Subscribe, it is in the archives.

A safe value is about 1 bolt diameter, or about 60% if bonded. The minimum extension beyond the bolts should be 3-5 bolt diameters. This was based on balsa core and a 34-ounce skin. With a weaker core or skin, increase the extension and thickness.

And yes, thin fender washers failed in flames. A complete waste of money that they shouldn't stock. Extra heavy washers can work.

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G10/FR4 are often used interchangeably in this context. FR4 is an electronics industry term (circuit boards). G9 is usually something using melamine for the bonding material. I would use G10/FR4.

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2 hours ago, thinwater said:

Why not bond the plate? You didn't offer a reason. It seems to me that if that portion of the boat is under strength, making it stronger, permanently, makes sense. I can't see why I would ever want to remove it.

And everything DDW said.

The other thing I have done , when the area is very curvy, is simply add laminate, tapered off. Less is needed.

Why reinforce an area the builder should have already reinforced for a winch? I my case I dramatically increased the head sail size and the "reinforced" deck was cracking. The fender washers were cones. The deck was solid, but just too thin.

Personal preference - I don't like metal permanently bonded to or laminated in glass.

What happens if the backing plate corrodes or otherwise gets damaged?

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Thanks for all the great replies, I may need to bed some of the backing plates I have installed. :-/

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On 5/18/2017 at 0:02 PM, SloopJonB said:

Personal preference - I don't like metal permanently bonded to or laminated in glass.

What happens if the backing plate corrodes or otherwise gets damaged?

Yeah, with metal I get you. Particularly aluminum. I was thinking fiberglass plate.

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I have similar problem I need to solve at some point.

I would like to put new In-hauler set up. My plan is to use Antal R3 X 14 Deck organizer as a strong point. Possibly have dyneema core attached to mast foot, to a LF Ring/sheet, to Antal Organizer to turning block at mast foot and then single line back to jammer&winch. Sheet will be pretty short, maybe 1,5' and I need to deflect it up to 45 degrees. Jib is 485 sqf.

16509-9890679.jpg

What sort of backing plate I should have. It needs to be strong as the loads will be pretty high. The under deck has some stiffeners and surface is uneven with these.

 

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I would try a G10 backing plat using the guidelines mentioned above (1x bolt diameter thick, 5x - 10x bolt diameters in width). Bed underneath with thickened epoxy. If the unevenness under the deck is extreme, build something up out of G10 or grind a thicker piece to approximately match first. 

An additional problem you may have with this type of hardware is the deck surface might be cambered and the organizer will rock on it. My solution to this is to CNC machine a custom part that matches the deck - but I suppose not everyone has a CNC machine shop at their disposal ^_^. If not, then I would use a G10 pad matching the outline of the part, carving the underside by grinding and sanding to fit the deck. Paint it a suitable color and it will look good. 

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