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Old 26-01-2011, 08:04   #1
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Material for Windlass Backing Plate ?

What material should I make the new windlass backing plate out of ?

SS, epoxy-soaked plywood, ... ?

Thanks,



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Old 26-01-2011, 08:26   #2
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I used a cutting board for a anchor roller...The stuff is very hard but workable. I use very large washers.

However, if it is a very big windlass I would have a welding shop cut me one out of aluminium.
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Old 26-01-2011, 08:35   #3
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I used 4 sections of 1/8" G10 material epoxied together to take the shape of the curve of the under-deck. A 1/2" section would not have flexed enough.
Once it was cured in place I cut the pattern for the winch.

e.g. G10 Fiberglass Board

I have a local distributor.

I use to use SS backing plates until I found this stuff. At twice the thickness of SS it seems to be just as strong with less weight and no corrosion.
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Old 26-01-2011, 09:54   #4
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I have used 3/8" thick pre-made Fiberglass sheet with fender washers, but Aluminum or SS work great! In my business... What I do when installing a REALLY powerful windlass, Is pre-mount the windlass with its backing plate only, 100% bedded in thickened epoxy first. I use a release agent on the bolts, and mask off anything I don't want nasty. Then I remove the windlass and clean up any oozes or epoxy goobers. Then I caulk down the windlass for good, unless it has a hard rubber base, in which case I caulk only the bolts.

The advantage of this is that the ceiling under a typical windlass, is usually an arc, and a very lumpy one at that. A 1' square backing plate might have only 10% of its surface actually touching the underside of the deck, and it will usually wobble as well. The epoxy bedding creates 100% surface contact!

If it is a smaller size windlass, and there is a good solid core fiberglass deck, (like > 1/2" thick), Then the next best thing is stack a couple of the largest fender washers and one of the next size down. (like a pyramid) This keeps them from bending under a load, and given a strong enough "solid" deck, with a small load, is better than a full backing plate that is NOT epoxy bedded, and only touches in a few odd places.
Hope this helps.

Photos attached... The unbelievably powerful SS "Lighthouse" windlass, came with an optional SS backing plate, so it got the full treatment, and I epoxy bedded the SS plate first, for 100% contact. It is an easy extra step...

The other photos were where I replaced the boat's entire raised deck at the bow, as it had all become rotten inside. In this case, since the windlass and its bolts, were going through about 6" of deck, the bolt load was almost totally in sheer. In this case, the thick fiberglass sheet backing plate, that I had glued in to start building the deck back up, was amply strong, when combined with the mounting bolt's pyramid of fender washers.

Mark
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Old 26-01-2011, 10:05   #5
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Quote:
Originally Posted by Mark Johnson View Post
I have used 3/8" thick pre-made Fiberglass sheet with fender washers, but Aluminum or SS work great! In my business... What I do when installing a REALLY powerful windlass, Is pre-mount the windlass with its backing plate only, 100% bedded in thickened epoxy first. I use a release agent on the bolts, and mask off anything I don't want nasty. Then I remove the windlass and clean up any oozes or epoxy goobers. Then I caulk down the windlass for good, unless it has a hard rubber base, in which case I caulk only the bolts.
+1 - that's the way to do it.

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Old 26-01-2011, 10:22   #6
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i have a full foredeck backing plate making my deck 3 inches thick. i have lofrans windlass with aluminum backing plate which i will glass in , as it is a bit old. the aluminum plate came with windlass when was installed. i had to replace the teak mounting pad with a cypress one, as the idiot who took it off the boat was aggressive with a coal chisel to remove it and broke it in 4 pieces, so isnt worthy of use anymore.. was teak. dang.
equate ceiling in a boat to walls in your house. overhead is what tall folks bang heads on..... DA CEILINGS IS WALLS/HULL......
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Old 26-01-2011, 13:28   #7
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Thanks for all the replies and suggestions.

Either layered G10 or epoxy bedded SS (or aluminum) sounds like the ticket.



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Old 31-01-2011, 15:59   #8
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We recently installed an anchor windlass. The deck where it was installed was foam cored, so we drilled large (3/4")holes through the top skin and foam (with a spade bit), gouged out more foam (maybe to 1.5" diameter) and filled them in with eopxy resin, filler and gluefiber mix, which we then drilled through to suit the bolts: this prevents the core from being compressed by the compression on the bolts. We also glued in a timber plank, approx 10" wide by 3/4" thick (hardwood, coated with timber preserver and then resin coated) that ran fore and aft, which was fiberglass tabbed in to the frames immediately fore and aft of the windlass - this was to help transfer the windlass loads onto the frames. We used very large diameter stainless washers instead of a backing plate, because the hardwood board was effectively a backing plate anyway, and was plenty tough enough,
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Old 31-01-2011, 16:00   #9
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My vote is for G-10.
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Old 31-01-2011, 18:26   #10
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Stress and Women in high heels.

I say you can make it out of any thing you want. Plastic, pine, bronze, stainless steel, teak---you name it. The job of a backing plate is to distribute load into the boat structure. So what is the load you wish to distribute and what is the strength of the boat structure?

i.e., if you have a 1600 pound load pulling on a perfectly inflexible backing plate of 4" x 4", then the perimeter of the backing plate (4" x 4" = 16") will transfer 100 pounds per inch to the surrounding structure. If the surrounding structure is that strong then all is fine. For fibreglass, 100 pounds per inch is not much load--not much more than a lady in high heals standing on the deck--one of my favourite stress analysis metrics.

I said a "perfectly inflexible backing plate", which of course does not exist. So with softer materials you have to make it thicker or bigger. But used properly, the load is not as much as you might think.

Maybe you were just looking for "how big is big enough" and not a technical analysis. In that case, I'd say just about anything will do. A single quarter inch bolt will hold 1600 pounds and that's about all a beefy anchor windlass will pull. You should only expect an anchor windlass to be able to pull the weight of the anchor and all its chain suspended vertically, typically 1500 pounds or less. Anchor windlass' are not rated for and DO NOT hold the anchor chain in high winds. That's the job of a anchor chain snubber line on a Sampson post and I use 10,000 pounds as my minimum load.

In high winds an anchor windlass is useless. Wind loads at 35-45 knots will equal the pulling capacity of most common anchor windlass, not to mention the weight of the chain and anchor. In that case you either row out more anchors or cut loose and go.
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Old 31-01-2011, 18:45   #11
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G10 works great for me after removing the rotten glassed over plywood that was there. Aluminum with SS bolts in the wet chain locker environement seem like a recipe for corrossion.
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Old 31-01-2011, 19:37   #12
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Good analysis on the loads of a backing plate. One needs to consider the loads, the thickness and stiffness of the material you are bolting to and the area of the object which will distribute the loads, the washer and bolt head size as well.

The loads on a windlass for a 40' sailboat are not terribly great. What is it actually "doing" and what are the loads?

It running out some chain with little tension on it and a bit more when you retrieve the anchor.

Stronger is better but overkill makes no sense, cost money and adds little.
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Old 31-01-2011, 20:03   #13
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G-10 is an exceptionally durable material. You could write volumes about it for its use in environmental test (vibration and thermal cycling) chambers. However, one aspect of back-up blocks has always bothered me. That is the difference between the Coefficient of Themal Expansion for the stainless steel bolt versus the stack of fiberglass, wood, and metal materials that make up the deck, plus the mounting base, plus the back-up block. Wood in a back-up block has a certain amount of resilence, so you can torque the bolt and pull a washer into a wood block so that you can maintain a pre-load on the bolt. That way you don't have a bolt that is loose some of the time. You won't get that in G10, its too stiff. Of course, the wood itself can have up to 2% change in cross-grain thickness depending on the moisture. It makes me think that putting a wood block on the other side of the G10 might be a rather good idea. If the boat is in the tropics it probably doesn't make any difference, but it may in the northern latitudes.
Any thoughts on that??
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Old 31-01-2011, 20:31   #14
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I inspect my G10, windlass fasteners, etc. regularly and not seen any sign of problems in 3 years of cruising PNW to Australia.

Don't know if the thermal expansion issue is valid considering the temperature ranges involved, but I'd consider a thin layer of Starboard before wood in a wet chain locker.
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Old 31-01-2011, 20:37   #15
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I am of the opinion that all hardware that uses a backing plate should be inspected and retorqued every few years. This may even involve dismounting to inspect for damage. Sadly too many boats had hardware installed that could allow the core to be subject to water and thus rot.

Perhaps I am too cautious.
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