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Discussion Starter #1
Hello,

Made a weekend project out of servicing the spinnaker winches on my boat. When I bought the boat I noticed that there were no backing plates on these winches. I hadn’t done anything about it, mainly cause we haven’t had a chance to play with the spinnaker yet, and figured I’d add a backing plate when I took them off to service them.

The question I have is... what is the rule of thumb for backing plates? I've tried to research the topic, and haven’t found all that much out there (I didn’t have too much time to research so forgive me if I've missed some obvious resources).

The winches measure about 5.75" in diameter (Barient #18). I was thinking about a 6.25" piece of 3/16” or ¼” aluminum? Does this sound right? Would Stainless be better?

Thanks for your comments!
 

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You are going to get two trains of thought here. While both agree backing plates are needed, how you get there is another!

All of my winches have BIG fender washers under them vs one big plate with smallish washers. The fender washers are to me what is really helping to hold the nuts from getting ripped thru the deck. This is not to say a big piece of metal or plywood would not help! BUT, you winches also are more of a linear pull vs an upward pull on a sheave/pulley of some sort, so IMHO, a backing plate is not needed as bad.

Of course, this is not saying a winch will not pull out or off of a boat.......

I like BIG washers along with backing plates, or big washers alone depending upon the item being secured.

marty
 

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Discussion Starter #3
Thanks for the reply!

I was going to put washers on no matter what. The reason I was leaning towards a plate instead of individual washers was that when I removed the winches the washers that came off were all bent and deformed. They were small washers so I'm sure using a bigger fender washer would have a better result, but it made me think that perhaps a plate might be better...
 

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If you're going to go with a big backing plate, make sure that the plate is in full contact with the underside of the deck... If there are any gaps or space, you really need to fill it with thickened epoxy, so the strain is spread properly by the backing plate.
 

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Discussion Starter #5
Thanks for the heads up. I eyeballed it when I took them out, and it seems pretty flat. I'll definitely take a better look at it when I got to remount them.

Any thoughts on Stainless vs. Aluminum? I'm leaning towards Aluminum from a cost perspective (I'm really not racing so weight is not a big deal). If I go that way, what can I do to avoid dissimilar metal corrosion between the plate and the bolts/nuts/washers?
 

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Aluminum is much easier to work with, and far less likely to cause hard edges along the fiberglass, which can cause fatigue and flexing... If you're worried about dissimilar metals reacting, use polyethylene washers cut from a plastic milk bottle to isolate the aluminum from the stainless steel and use lanocote or tefgel on the bolts to isolate them from the plate.
 

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I made my backing plates the same dia as the base of the winches.
Then as Sailingdog says, I installed them in a solid bed of thickened resin, polyester with micro balloons, in my case.

Your bent washers may be an indication that they're not backed solidly.
 

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Discussion Starter #8 (Edited)
Sounds like Aluminum is a better way to go!

Thanks for the tip on the plastic washers. I've read a lot of your posts (saildog) about lanocote, so I figured that'd be the way to go with the bolts.


Does the size seem appropriate? If I go with 6.25" it leaves about a half inch worth of overhang from the bolt holes (IE from the middle of the bolt hole to the edge of the plate), is that enough bite?
 

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Discussion Starter #9
kaluvic: Sounds like I might be leaning towards the overkill side of things! Better safer then sorry I guess, good to know I can get away with a plate that is the same diameter as the winch...
 

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You could probably get away with "rings" like giant washer with holes around it.
Some winches have the possibility of adding an electric motor to them, on mine this is done by inserting the motors drive into the center of the base.
Using a ring may enable this addition later with out changing the backing plate.
 

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If you're going to go with a big backing plate, make sure that the plate is in full contact with the underside of the deck... If there are any gaps or space, you really need to fill it with thickened epoxy, so the strain is spread properly by the backing plate.
I agree with this. I use a rule of thumb of "slightly bigger than the base", because I want enough plate "outside" of the five or six mounting bolts.

When you remove the bases, take the opportunity to seal the core, if applicable, with the usual "epoxy donut" method you can see half-a-dozen places in the archives.

I use 1/4" aluminum as backing plates and it's never been too little.
 

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should work nicely. :)
Sounds like Aluminum is a better way to go!

Thanks for the tip on the plastic washers. I've read a lot of your posts (saildog) about lanocote, so I figured that'd be the way to go with the bolts.


Does the size seem appropriate? If I go with 6.25" it leaves about a half inch worth of overhang from the bolt holes (IE from the middle of the bolt hole to the edge of the plate), is that enough bite?
 

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Discussion Starter #14
I've seen Maine Sailing's "How To" site that gives a great tutorial on how to seal the core. I am going to do this for sure.

Sounds like I have the right idea with the aluminum and the sizing. Thanks for the tips! This has been VERY helpful.
 

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The goal of a backing plate is to spread the load. In order to do this, stiffness counts. Aluminum is stiffer than stainless and much stiffer than glass/resin. When your washers bend, they do so because all the load is on the center and they do not have the stiffness to transfer the load to the rest of the area they cover. The result is that all the load is concentrated in the small area at the center that has flexed. In this situation a washer is almost no better that just a nut.
The moral is that the thicker the better and aluminum is good. The only drawback to aluminum is the aluminum/stainless problem, but if you own a modern boat you already know how to deal with that.
 

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If going with aluminum backing plates, using 1 & 1/2 D. of the bolt size as an edge distance will eliminate cracking from the bolt hole outward to the edge of the plate. De-burring the holes with a countersink helps as well.
 

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Discussion Starter #17
Thanks fullkeel7. I just ordered a pair of 6.25" x 6.25" 1/4" aluminum plates. This should give me the 1.5D that your talking about.

In case anybody is looking for a place to get small peices of metal (custom cut to size), you should check out OnlineMetals.com. Lucky for me they are local to Seattle so I can swing by after work and pick up my order, but they ship anywhere. I was trying to figure out where I was going to be able to get such a small amount, and cut it to size and my roomate sugested that place. We'll see what the finished product looks like later tonight, but so far I've been pretty happy with them!
 

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From an engineering perspective, you really don't need particularly large backing plates on the mounting bolts on a sheet winch as the loads are essentially horizontal shear loads, transmitted into the deck with frictional loading due to to tension on the anchoring bolts and, to a much lesser extent, by compression across the faces of the bolts as they penetrate the decking. As a practical matter, on your Cal 31 a couple of over-sized fender washers on each bolt, followed by a properly sized cut washer and then a lock washer will be more than enough. A big backing plat will add little but cost.

FWIW...

s/v HyLyte
 

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I probably was not real clear either, BUT, while backing plates are nice, ALL of my current 5 winches, from 2 genoa 40's, to three 14, 15 and a 16 on my cabin top have no backing plates, just BIG fender washers on the smaller ones. I have not taken my 40's off, but they have standard sizes washers under them. Some day I will put some fenders under them. As I have redone all of my deck gear, again, most had only smaller standard sized washers, as I replaced deck gear, on went fender washers, and in some instances, with more upward shear, I would put a backing plate.

Marty
 

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From an engineering perspective, you really don't need particularly large backing plates on the mounting bolts on a sheet winch as the loads are essentially horizontal shear loads, transmitted into the deck with frictional loading due to to tension on the anchoring bolts and, to a much lesser extent, by compression across the faces of the bolts as they penetrate the decking. As a practical matter, on your Cal 31 a couple of over-sized fender washers on each bolt, followed by a properly sized cut washer and then a lock washer will be more than enough. A big backing plat will add little but cost.

FWIW...

s/v HyLyte
I guess this shows how a physicist's intuition can be lead astray in engineering situations. Can you give me a reference from an engineering perspective about how this situation is analyzed?

From a physicists point of view, a 1000 lb side thrust exerted 8 in. above deck level results in 8000 in-lb. of torque. This is resisted by pivoting around the forward edge of the winch base which is separated from the rear mounting screws by about 8 in. This will result in a vertical pull on those bolts of 1000 lb. I realize that the example geometry I have used for this example is over-simplified, but where does it go wrong?
 
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