Galvanised vs stainless as backing plate

[srah1953]

I'm thinking of putting a better backing plate under my bow cleats. Am I correct in saying that if the plate is to be laminated in, it should be galvanised rather than stainless. I've thiss vague notion that stainless needs access to air. Yes/no?
Thanks

 

[vyv_cox]

No. Don't use any carbon steel on a boat, except for anchor chain.

 

[srah1953]

Hi Vyv. Thanks for this.

 

[DanBurrill]

My preference as a surveyor would be not to laminate stainless or galvanised steel into the boat's structure. I'd normally recommend backing up deck fittings with plywood laminated to the GRP, and then use stainless penny washers or a plate washer to spread the load from the bolts.

 

[Tranona]

Not sure why you would want to use metal in this application. As already suggested, the normal way to laminate a backing plate is to use ply. sometimes aluminium is used if the intention is to tap for the fastenings rather than use through bolts. The purpose of backing plates is to spread the load over a wider area.

 

[Avocet]

The thing with stainless needing air, is true, up to a point, depending on the grade, but it won't be a problem in the application you're thinking of. However, as has been said, you're really looking for something "stiff" rather than "strong". The stainless (unless you get an almighty thick piece, is likely to bend ever so slightly. As soon as it does this, all the stress is back in the area around the bolts. Wood has the advantage that the resin, when you glass it in, will soak into it and fill the gaps between it and the underside of the deck nicely, to spread the load. You can always put your stainless plate underneath the whole lot to stop the nuts crushing the ply locally. If the fasteners passing through it are stainless, you're best using stainless for anything that touches them - otherwise you're likely to get galvanic corrosion.

 

[William_H]

Backing plate

You might consider buying some carbon fibre cloth and laminating that onto the fibreglass under deck. It is very easy to use with epoxy and will strengthen the existing fibreglass at minimum intrusion into the space below. It may flex a bit but it will transfer the loads to the gunwhales very effectively. Of course if the intrusion into the space below is not a concern then a piece of angle Ali or SS or even mild steel will give more strength and resistance to bending. good luck olewill

 

[srah1953]

Thanks for all of these.
PLywood has many attractions, not least of which is that I can cut it again if I don't get the measurements quite right.
If using plywood, what is the best way of transferring some load to the gunwhales?

 

[VicS]

Thanks for all of these.
PLywood has many attractions, not least of which is that I can cut it again if I don't get the measurements quite right.
If using plywood, what is the best way of transferring some load to the gunwhales?

Additional layers of glass and resin under the deck as William H suggests, I would think, before fitting the backing pad which could also be glassed in if you feel it's necessary.

Picking up on your point about "stainless requiring air" Be sure that any stainless fastenings through the deck are well sealed in so that water cannot get into any tiny gaps and cause crevice corrosion.

 

[srah1953]

Additional layers of glass and resin under the deck as William H suggests, I would think, before fitting the backing pad which could also be glassed in if you feel it's necessary.

.

What's necessary is the big question, to which I have no idea of the answer. At the moment the 2 bolts coming through from the cleats have smallish squares of metal under each bolt without any obvious strengthening otherwise. So it just seems to me to be inadequate if the cleats were put under extreme stress, eg moored in very unfavourable conditions.

 

[alahol2]

What's necessary is the big question, to which I have no idea of the answer. At the moment the 2 bolts coming through from the cleats have smallish squares of metal under each bolt without any obvious strengthening otherwise. So it just seems to me to be inadequate if the cleats were put under extreme stress, eg moored in very unfavourable conditions.

From the sound of it, any additional strengthening would be better. I have put (replaced) 12mm ply backing pads under my forward cleats. The bigger the better, within reason. Round off the corners. Coat with epoxy. Bed on mastic. Use the metal squares or penny washers to prevent crushing the ply.

 

[alan_d]

What's necessary is the big question, to which I have no idea of the answer. At the moment the 2 bolts coming through from the cleats have smallish squares of metal under each bolt without any obvious strengthening otherwise. So it just seems to me to be inadequate if the cleats were put under extreme stress, eg moored in very unfavourable conditions.

Have a think about what you would want to happen under "extreme stress". Would you prefer the cleats to pull out of the deck, or the deck to pull out of the boat?

 

[srah1953]

Have a think about what you would want to happen under "extreme stress". Would you prefer the cleats to pull out of the deck, or the deck to pull out of the boat?

This is an interesting perspective which hadn't occurred to me but I'm not sure it's really an either/or situation.
A better way of looking at it might be to ask what would be regarded as best practice for supporting cleats and to try to replicate that as much as possible.

 

[Tranona]

Don't think there is any "best practic" - rather many different ways of doing the job. My Bavaria uses aluminium plates bonded into the hull and tapped for the fastenings. In general the deck area around the cleats is solid and has extra laminations compared with the rest of the deck. Other builders re-inforce the area with bonded in play pads and through bolt. Traditional boats often used solid posts attached to structural members rather than just bolting to the deck.

All of these methods sem to be effective as cleats failing does not seem to be a big issue. In other words the builder will have made them strong enough for the expected loads. In practice doubt cleats are often subject to severe loads, except perhaps when towing. There are ways in which snatch loads can be reduced - I always use a nylon strop and a chain hook for the anchor and nylon warps or a snubber for mooring lines if there is a possibility of high loads.

 

[boatmike]

I'm thinking of putting a better backing plate under my bow cleats. Am I correct in saying that if the plate is to be laminated in, it should be galvanised rather than stainless. I've thiss vague notion that stainless needs access to air. Yes/no?
Thanks

There might be a little confusion here regarding "laminating in" Assuming this is an existing boat with bow cleats already fitted the backing plate does not need laminating in at all. I agree that for a closed laminated structure plywood should be used but if you are trying to give additional strength to existing cleats from the inside it is fine to fit either an aluminium or stainless steel backing plate on the inside of the skin without laminating over it. A good plan is to use a plate liberally bedded on Sikaflex to ensure the load is spread evenly. This does not need to be laminated over. In fact an even better plan is to add several layers of chopped strand matt over a larger area than the backing plate first, to strengthen the inner skin over a wide area and avoid hard spots at the plate edges. Some would say to then land the plate on wet glass but I would prefer to let it cure and bed the plate on sikaflex still. Hope this helps.

 

[srah1953]

There might be a little confusion here regarding "laminating in" Assuming this is an existing boat with bow cleats already fitted the backing plate does not need laminating in at all. I agree that for a closed laminated structure plywood should be used but if you are trying to give additional strength to existing cleats from the inside it is fine to fit either an aluminium or stainless steel backing plate on the inside of the skin without laminating over it. A good plan is to use a plate liberally bedded on Sikaflex to ensure the load is spread evenly. This does not need to be laminated over. In fact an even better plan is to add several layers of chopped strand matt over a larger area than the backing plate first, to strengthen the inner skin over a wide area and avoid hard spots at the plate edges. Some would say to then land the plate on wet glass but I would prefer to let it cure and bed the plate on sikaflex still. Hope this helps.

Excellent. Thank you. Would it be possible or acceptable to use your suggested method but then using plywood rather than metal plate?

 

[boatmike]

Excellent. Thank you. Would it be possible or acceptable to use your suggested method but then using plywood rather than metal plate?

The point of using a metal plate is to stop the bolts pulling through the skin by acting as a load spreader. Think of it like a very big washer. Plywood would not have the same effect as it is not really any good in compression and bolts would pull through this probably more easily than the GRP underneath. Pywood is often used as a local core material because it is better in compression than foam core. That's what previous posts were telling you, but for a backing plate I would prefer stainless bedded on Sikaflex.

 

[srah1953]

The point of using a metal plate is to stop the bolts pulling through the skin by acting as a load spreader. Think of it like a very big washer. Plywood would not have the same effect as it is not really any good in compression and bolts would pull through this probably more easily than the GRP underneath. Pywood is often used as a local core material because it is better in compression than foam core. That's what previous posts were telling you, but for a backing plate I would prefer stainless bedded on Sikaflex.

OK. Thank you