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Old 25-03-2023, 05:57   #1
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Backing plate size

Hello all,

I am going to upsize the cabin top winches on my boat to Lewmar Ocean 16ST winches. I am going to make backing plates out of 1/4" G10, but am not sure what diameter to make them. I've read everything I could fond on the internet and the closest I came to an answer was a diameter of 10x de bolt diameter. That seems more for washers than backing plates however.

The diameter of the base (center of bolt hole) is 3 3/4". Does anyone know what are the guidelines in this case?

Thanks.
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Old 25-03-2023, 06:29   #2
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Re: Backing plate size

The backing plate should be at least as large as the surface of the winch in contact with the top side of the deck. If there's room to go a bit bigger than that, I would, as it's cheap insurance for a stronger install. There's probably a point of diminishing returns once you go too much larger than the winch base, however.
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Old 25-03-2023, 06:37   #3
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Re: Backing plate size

Quote:
Originally Posted by rslifkin View Post
The backing plate should be at least as large as the surface of the winch in contact with the top side of the deck. If there's room to go a bit bigger than that, I would, as it's cheap insurance for a stronger install. There's probably a point of diminishing returns once you go too much larger than the winch base, however.
This right here ^^^

Making the backing block 3/4 of an inch to one inch larger than the base of the winch would be the way to go, will make a very stout install.

Fair wind,
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Old 25-03-2023, 07:28   #4
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Re: Backing plate size

The failure is bolts pulling up through the deck (the 2-3 bolts opposite the load). I've seen it happen--a prior owning installed a winch on cored deck with only bolting washers.


Thus, the 10x bolt diameter is the cure, and a plate just slightly larger than the winch does this for every bolt.



Also remember to remove core in each hole, fill with epoxy, and re-drill. If not, you will crush core when tightening the bolts, and water will leak in an rot the core(or if foam core there is still frost and crush damamge).


And bedding compound. That will start a fight. I would used butyl for this because you might need to remove it someday for service you cannot do from the top, and polyurethane will make this horrible. Silicone works too, but someone will bring up that it is a scourge. Butyl tape is better.
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Old 25-03-2023, 08:24   #5
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Re: Backing plate size

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Originally Posted by thinwater View Post
Also remember to remove core in each hole, fill with epoxy, and re-drill. If not, you will crush core when tightening the bolts, and water will leak in an rot the core(or if foam core there is still frost and crush damamge).
I'd say this is the minimum - straight epoxy to core then fill with mixture of high density thickener. Another option, a better option, is to drill a larger hole (through one skin) and bed a G10 or SS hollow rod (tube) in there.
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Old 25-03-2023, 10:48   #6
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Re: Backing plate size

Thanks everyone for the info. Not to worry, my decks are cored with plywood, so that's good. In any case there are already winches there, just non ST and small in one case. I'm going to oversize the holes and fill them with epoxy to protect the core. I'll probably bed the backing plate with thickened epoxy as well to get a flat surface. I just came back from the boat, that's on the hard and winterized and as it turns out, the starboard winch couldn't have a backing plate much more that 6" anyways, as it's closer to the companionway. I'm going with 5 1/2".
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Old 25-03-2023, 19:46   #7
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Re: Backing plate size

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Originally Posted by Laskadog View Post
I'll probably bed the backing plate with thickened epoxy as well to get a flat surface.
Tape up a piece of thin plastic sheet, (like a piece of a plastic trash bag,) to the overhead and then use your well thickened epoxy on the backing plate.
You'll only need a little screw or two from the outside to hold the plate in position until the epoxy kicks.
Easy to use a couple of the existing holes before they get filled.
Now you can have a perfectly fitted plate without having a permanently glued to the overhead disc of epoxy.
Use "Dolphinite" for sealing-up all the new holes/bolts/winch/backing plate.
Easy future removal without semi-destruction and colorful language.
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Old 26-03-2023, 04:43   #8
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Re: Backing plate size

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Originally Posted by Bowdrie View Post
Tape up a piece of thin plastic sheet, (like a piece of a plastic trash bag,) to the overhead and then use your well thickened epoxy on the backing plate.
You'll only need a little screw or two from the outside to hold the plate in position until the epoxy kicks.
Easy to use a couple of the existing holes before they get filled.
Now you can have a perfectly fitted plate without having a permanently glued to the overhead disc of epoxy.
Use "Dolphinite" for sealing-up all the new holes/bolts/winch/backing plate.
Easy future removal without semi-destruction and colorful language.

I don't understand why being able to remove the plate is an advantage.



Also, if the plate is bonded it will be several times stiffer and stronger than if it is free-floating; do the math, laminating works that way. In fact, the most efficient way to do this, in terms of materials and weight, is to prep the surface and lay up 4-6 layers of 1708 with a staggered edge, wrapping around the curves a bit. It will be thinner, better fitted, lighter, and will not introduce a hard spot at the edges. Then just use bolting washers. This is how high $$ race boats are built.


The "right" way, if you think about it, is to use solid glass where there are high loads, make it thick enough, and skip the backing plates altogether. Obvious. This is how the factory winches were mounted on my PDQ. This is how chain plates are usually (properly) mounted.
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Old 26-03-2023, 06:39   #9
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Re: Backing plate size

Quote:
Originally Posted by thinwater View Post
The failure is bolts pulling up through the deck (the 2-3 bolts opposite the load). I've seen it happen--a prior owning installed a winch on cored deck with only bolting washers.


Thus, the 10x bolt diameter is the cure, and a plate just slightly larger than the winch does this

I really like the 10x rule of thumb and as an engineer that doesnít deal with engineering anymore I need rules of thumb. However can we develop a slightly more detailed rule of thumb about shear strength of fiberglass? What if the application is a large cleat or drogue attachment points? Would the 10x be the right rule to use?

Can we use something like 4,000 psi (27 MPa) for shear strength of fiberglass to reduce the assumptions associated with the 10x rule?

I donít really know what the rule would be and Iím hoping that the more technical engineers floating around the forum can shed some light on it for me.
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Old 26-03-2023, 07:22   #10
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Re: Backing plate size

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Originally Posted by thinwater View Post
I don't understand why being able to remove the plate is an advantage.
At some point the OP or a future owner may want to change or move things around, (or simply clean-up the overhead).
Using chisels and grinders overhead to remove now unneeded/unwanted epoxy is a mess.
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Old 26-03-2023, 20:34   #11
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Re: Backing plate size

In my opinion, a large backing plate is not necessary for this application (winch mounting). They are more appropriate for joints that are primarily in tension (like the keel bolts in the other recent thread). Winch bolts, on the other hand, are 'primarily' in shear, which needs to be transferred into the deck (usually through the skin(s), but the OP's plywood core should help too). The 'secondary' load is the overturning moment that is resisted by a couple bolts in tension on the opposite side from the line coming in. This load is easily handled using fender washers (not regular flat washers), which probably are around the 10x recommendation in size. A large backing plate has no effect (or nearly none) on the shear strength of the joint.

I have no idea what assumptions the winch manufacturer used in determining the number / size of bolts. But I believe they are compatible with typical deck construction. If your deck has thin skins, though, they may not provide enough shear bearing area with only five or six bolts. That's where the sleeves I referenced above can help - the bearing width changes from the bolt diameter to the sleeve diameter, something like 3x larger.
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