What thickness of backing plate should I use?

[Rivers & creeks]

I'm fitting a new winch to the aft cabin roof. Size 42 winch, genoa sheets but not a huge genoa, about 300 square feet and a 700 square feet chute.

It's going through solid laminate with no moulded reinforcement. The bad news is that it's replacing a vent so I'll have to put a hard wood plug into that and epoxy it in place. My plan was to then use a sandwich of 316 stainless disc above and below about 50mm wider than the diameter of the winch, sikaflexed in place, bolt through six holes at the edges and then bolt the winch through the middle. But the bolts will go through the plug so the load will all be transmitted and held by the top and backing plate. Is 3mm plate thick enough? There's a very slight curve in the glass fibre so I need to have plates that can take a tiny amount of curve. If I go up to 5mm will then bend very slightly? What thickness do you think is the minimum?

Thanks for the advice.

 

[prv]

Gut feel rather than rigorous engineering, but 3mm sounds plenty to me.

Pete

 

[Scotty_Tradewind]

Yes it's the hole plugged from the vent that weakens the structure as I see it.

My 'gut' says go thicker rather than thinner, just for peace of mind. So it's the 5mm for me.

Best of luck

S,

 

[bedouin]

I am not an expert - so please ignore this posting

I am sure 3mm is plenty. I think the size of the backing plate is more important than the thickness - I would go for a larger backing plate than you suggest to spread the load across more of the laminate

 

[Rivers & creeks]

Yes it's the hole plugged from the vent that weakens the structure as I see it.

My 'gut' says go thicker rather than thinner, just for peace of mind. So it's the 5mm for me.

Best of luck

S,

I am not an expert - so please ignore this posting

I am sure 3mm is plenty. I think the size of the backing plate is more important than the thickness - I would go for a larger backing plate than you suggest to spread the load across more of the laminate

Yes the plug is a real pain, it means the load is all being transmitted by the plates and the epoxy join between the plug and the deck. I could go up to 100mm diameter bigger than the winch base rather than 50mm bigger - does that add anything mechanically? I do worry about the 3mm thickness.

 

[Davegriff]

No expert, but I'd say the plate would be stronger than the glass either way, but 5mm less prone to buckling. I'd certainly go for the 100mm diameter though so as to spread the loading over the glassfibre better, with two rows of bolts, one to each edge sort of...

 

[charles_reed]

To read back my understanding. One or more of the winch fixings will go through a vent-hole in the solid laid cabin roof. This vent hole will be plugged with hardwood epoxied in place?
Your intention is to have a 3mm SS backing above the cabin-roof and another below the glass fibre section.

Whilst having a SS backing plate under the winch is a good idea, the thickness of the plug and its strength appears to me entirely inadequate.
I'd also hesitate to use an SS plate under the laminate, preferring a large peice of 8-12mm marine ply with rounded corners and extending well beyond the fixings and the plugged vent. This could, but doesn't necessarily need to be epoxied into position.
The reason for using ply instead of SS is that you'll avoid corner hard-spots cracking your laminate (which will happen with a 3mm backing plate).

 

[bedouin]

Yes the plug is a real pain, it means the load is all being transmitted by the plates and the epoxy join between the plug and the deck. I could go up to 100mm diameter bigger than the winch base rather than 50mm bigger - does that add anything mechanically? I do worry about the 3mm thickness.

I would keep the one on top fairly small and enlarge the one on the bottom - I would also tend to agree with Charles Reed and use plywood. There needs to be some flexibility in the backing plate to match that of the laminate otherwise you might cause some high loads at the edges of the plate (but once again I am not an engineer)

 

[Rivers & creeks]

To read back my understanding. One or more of the winch fixings will go through a vent-hole in the solid laid cabin roof. This vent hole will be plugged with hardwood epoxied in place?
Your intention is to have a 3mm SS backing above the cabin-roof and another below the glass fibre section.

Whilst having a SS backing plate under the winch is a good idea, the thickness of the plug and its strength appears to me entirely inadequate.
I'd also hesitate to use an SS plate under the laminate, preferring a large peice of 8-12mm marine ply with rounded corners and extending well beyond the fixings and the plugged vent. This could, but doesn't necessarily need to be epoxied into position.
The reason for using ply instead of SS is that you'll avoid corner hard-spots cracking your laminate (which will happen with a 3mm backing plate).

Largely accurate but as I said, the backing and top plates are round 316 plates for the reason you say. The winch is effectively being bolted onto the plates as I'm not attributing any strength to the plug.

 

[Rivers & creeks]

I would keep the one on top fairly small and enlarge the one on the bottom - I would also tend to agree with Charles Reed and use plywood. There needs to be some flexibility in the backing plate to match that of the laminate otherwise you might cause some high loads at the edges of the plate (but once again I am not an engineer)

I can see that now. Would a ply disc be stronger? What about ply with 316 SS on top/underneath it

 

[jwilson]

I'd aim to reinforce the laminate over a wider area before looking at pads etc. You can extend the new laminate across the vent hole (if I understand your situation correctly). To avoid stress cracking you need a tapered reinforcing - any GRP handbook will show the sort of thing - similar to repairing a hole.

And you never know what someone one day will use the winch for, so just calculating sheet loads might not give enough strength. On a racing boat one day the owner was telling me to put more tension on a main halliard winch, and he kept saying "... more, more". I thought he was completely wrong and a bit of a pillock, so I really wound it up (I was pretty strong in those days). Big bang, winch flew into sea narrowly missing my head.....

 

[sailorman]

Yes the plug is a real pain, it means the load is all being transmitted by the plates and the epoxy join between the plug and the deck. I could go up to 100mm diameter bigger than the winch base rather than 50mm bigger - does that add anything mechanically? I do worry about the 3mm thickness.

the loads on the fixings are in sheer a plats 50m/m larger should be fine. why not a sq plate if its under a head lining

 

[Rivers & creeks]

To avoid corner cracking. Yes, all loads are sheer with a little compression because the sheet comes up to the winch, which I think makes the plug less of an issue

 

[bedouin]

I can see that now. Would a ply disc be stronger? What about ply with 316 SS on top/underneath it

It is not really about strength but about evenly transferring the load from the winch to the hull (i.e the laminate). Instinct suggests that is best done with something that has about the same amount of give in as the hull. My fear about putting a rigid plate onto flexible laminate is that the forces would end up concentrated at the edges of the plate - rather like patching a nylon spinnaker with a dacron patch. But I am not an engineer so don't place any reliance on what I say.

 

[prv]

Going beyond your suggestion, what I'd probably do if I was worried about the strength would be to fill the hole according to the classic taper method, and then install a large plywood pad underneath with a thick layer of epoxy and microfibres in between. Push the pad up as far as it will go, so quite a lot of epoxy will squidge out, but any irregularities and curve will be filled in. Once it's set, drill the holes for the winch bolts as normal, and install with large washers under the nuts.

On the other hand, I was surprised to find that Ariam's sheet winches were bolted straight through the cabin top with no particular reinforcing and no backing plate, just ordinary washers under the nuts. They've withstood 16 years of hard charter use with no sign of movement or damage. So the scheme above is probably massive overkill, but it'll give you peace of mind.

Pete

 

[JumbleDuck]

It is not really about strength but about evenly transferring the load from the winch to the hull (i.e the laminate). Instinct suggests that is best done with something that has about the same amount of give in as the hull. My fear about putting a rigid plate onto flexible laminate is that the forces would end up concentrated at the edges of the plate - rather like patching a nylon spinnaker with a dacron patch. But I am not an engineer so don't place any reliance on what I say.

I think the reduction in forces as the patch spreads out probably achieves what you're after. The rule of thump I have used is that a metal backing plate for anything bolted down should be twice the area of the base of the thing and half as thick as the bolt diameter. A wooden backing plate should be twice the area and twice as thick as the bolt diameter. Ish.

 

[Rivers & creeks]

On the other hand, I was surprised to find that Ariam's sheet winches were bolted straight through the cabin top with no particular reinforcing and no backing plate, just ordinary washers under the nuts. They've withstood 16 years of hard charter use with no sign of movement or damage. So the scheme above is probably massive overkill, but it'll give you peace of mind.

Pete

Actually now you say that I remember the central Genoa winch on my HT didn't even have washers and showed no sign of trouble (I then put large washers on for luck).

 

[Spirit of Glenans]

I 'm no engineer, but I think your original idea is probably overkill. I would go down the road suggested by Jwilson(#11) and PRV (#15). I think blocking the original vent hole with a hardwood plug is not a good idea and would prefer to just taper the edges and patch it with laminate. I don't thing a top plate is necessary , a ply backing plate underneath is the tried and trusted method.

 

[LadyInBed]

I can see that now. Would a ply disc be stronger? What about ply with 316 SS on top/underneath it

When I put in my centre cleats I used a 12mm marine ply pad plus a SS plate under the deck.
With your job, I would assume that the plug will have no strength, so the more the under deck overlap the better.
Another thought, my winches are mounted on 12mm (may be 14) hardwood chamfered disks above deck. They look good and would give you a good plug overlap above deck.

 

[PuffTheMagicDragon]

I don't like the idea of the hardwood plug and would suggest that closing the vent hole with laminates of fibreglass, adding thickness if you think that this is necessary, would be much better. My genoa at 34m2 is more or less the same size as yours. My ancient winches, Barient 26, are held down with M8 bolts that go through 20mm Iroko and the GRP top of the coaming. Underneath they just have large and thick stainless washers bedded down onto sealant, normal penny washers and then Nylock nuts. The boat was built in 1973 and I restored her in 2013. If the method was good enough for Wauquiez it is good enough for me.