Chain plates

[pmagowan]

How big are yours!

I have been looking at chain plates recently and wondering about the best designs. It seems that a lot of the modern production boats have the shrouds coming down well inside the hull and terminating on the deck. The chain plates are therefore simply attached to the deck. They are normally stainless steel from what I can see. On my boat, which is only 28 feet, they are simply a heavy flat plate that runs down the inside of the hull and are through bolted to the hull. They are also stainless steel.

I have seen, in some design forums, the use of carbon fibre to make chain plates. In this method the carbon fibre is part of an epoxy composite. Where it breaks through the deck it is made into a loop that looks like a carbon fibre rope (coated in epoxy). Under the deck the fibres are splayed out and epoxied to the hull so that the load is widely distributed.

Since these are a big factor in the strength of the rig I am interested in what you have on your boat or any knowledge you might have on the subject. I am always considering things with regards to a potential circa 40 foot blue water cruiser.

 

[prv]

It seems that a lot of the modern production boats have the shrouds coming down well inside the hull and terminating on the deck. The chain plates are therefore simply attached to the deck.

Except perhaps on a metal boat, they are highly unlikely to just be attached to the deck. There will be a web inside between the deck and hull, which the chainplate is bolted through. Ours on Ariam are done this way. Inboard shroud bases allow a narrower sheeting angle for the jib, at the cost of higher loads in the shrouds. They are notorious as a source of leaks as the chainplate and deck move differently under load, although touch wood ours seem ok.

My fantasy blue-water designs are both metal, so strong chainplates are a simple matter of welding. On a GRP or wood-epoxy boat I think I'd favour the traditional metal strap down the outside or inside, with several through-bolts and appropriate large washers or backing plates. Another pair of these right aft, rotated 90 degrees with the securing point protruding just past the transom, would make an ideal fixing point for a Jordan series drogue if you buy into that approach (I do).

I've also wondered about the possibility of using chainplates as lifting points, for flexibility with industrial cranes in out of the way places. They should certainly be strong enough (the rule of thumb for design is that the shroud chainplates on one side should be strong enough to lift the loaded boat) but it's a question of fore-and-aft balance. I've also considered incorporating them into leg arrangements instead of the normal through-hull bolt.

Pete

 

[duncan99210]

The chain plates on our Bavaria come down well inside the deck, about a foot or more in from the gunwale. Beneath the deck a rod then transfers the load to the framing of the hull via a massive set of plates which are bolted to the frames. I could foresee a design made in steel where a rib extended up from the keel to the deck level under the chain plate instead of the rod to transfer the load. The important thing is that the deck itself is not taking the load at all, it's just sandwiched between the upper and lower elements of the chain plate.

 

[Tranona]

Your choice of chain plates may well be determined by your rig. As duncan says it is common with a fractional rig and an overlapping genoa to have inboard chain plates with a tie rod to take the loads to internal framing. On the other hand my newer Bavaria (and some other recent designs) have smaller genoas (110%) so you can achieve narrow sheeting angles with the chain plates strapped to the hull with internally stiffening to take the loads down to the keel. If you have mast head rig you will need more chainplates as you will need fore and aft lower shrouds as well as cap shrouds (and even runners if a cutter), but depending on the beam you could have inboard or outboard shrouds. Inboard would go through the deck to either a bulkhead or to knees. This type can give trouble because of the need to seal them to the deck, usually with a plate bedded in sealant.

With your proposed construction method it would make sense to have ring frames to take the loads so that the hull skin does little other than keep the water out.

 

[William_H]

As you can imagine wider spaced chain plates (on the gunwhale) will give better support for the forces trying to push the mast top sideways. However as said often it is desirable to sheet an overlapping jib closer to centre line but outside the shrouds so chain plates in board are used. In this case chain plates can pass through the deck to attach to bulkhead etc however this can leave movement relative to the deck and possible leaks. An alternative is to attach to the deck then reinforce the deck from below. A simple example being 2 saddles or U bolts one above and one below deck. A wire and turnscrew go down to attachments on the hull.
Interesting concept using carbon fibre as part of the deck and attached by resin to it. I would suggest that kevlar would be far better material to make a loop for attachment of shrouds. Of course the deck still has to be very stiff or reinforced from below. In the end most of us end up with whatever the boat builder fitted. But sometimes you can do better. good luck olewill

 

[vyv_cox]

Photos of mine at https://coxengineering.sharepoint.com/Pages/chainplates.aspx

 

[lw395]

The actual plate is just one link in the path.
Most of the force needs to get to the mast foot, via bulkheads, hull skin and/or whatever.

Not much point in thinking about the plates in isolation.
If there is a massive bulkhead handy, the plate only needs to be a bit stronger than the shrouds.
If the boat is built light, the plate is part of a system to carry the loads where they need to go.
My last boat had more complex chainplates, because they were a few inches aft of the bulkhead, so they needed to cantilever the load into in and resist bending, rather than just being in tension.
Way more cross-section area in them than the 3 shrouds that connected to each.

 

[pmagowan]

Wow, thanks guys. Loads more to think about. I love the idea of thinking about the circular path of the forces a bit like an electric circuit. The shrouds need to be connected back to the mast at the keel through adequate structures.

I do like the simplicity of a Bermuda rig but I am moving towards a Cutter rig due to the versatility. I know this affects the shroud positioning and is probably a bit more awkward. The cutter rig is probably less sensitive to wide shrouds.

My design would allow for ring frames or strong bulkheads for support. All bulkheads in my design so far are going to be composite watertight bulkheads. I will just have to think about positioning.

 

[William_H]

Ring frames reminds me of a story of an Oz yacht (Called I think Australia III?) a huge (j class?) yacht built for Americas cup challenge some years back.
It had a 12t bulb keel on the end of a very long fin keel. The rest of the boat only weighed another 3 t all in carbon fibre.
They built a relatively massive ring frame to carry the shroud and keel step loads.
Unfortunately the whole hull was not strengthened adequately in the fore and aft plane. The tow out to race site was very choppy producing a lot of pitching of the hull with huge strain from the pendulum effect of the bulb keel, then a failure of a jib winch caused them to use a running back stay winch for jib such that suddenly the hull parted in the middle. She sank in seconds in very deep water. Just another lesson for the design engineers. Fortunately no one was lost. olewill

 

[knuterikt]

Over om sailnet there is a long thread (362 posts so far) called "Bob Perry's Carbon Cutters" http://www.sailnet.com/forums/sailboat-design-construction/223386-bob-perrys-carbon-cutters-2.html

In this thread there is lot of interesting details on the construction details - also chain plate details.

 

[NormanS]

Photos of mine at https://coxengineering.sharepoint.com/Pages/chainplates.aspx

Your link mentions "17mm bolts". That's an odd size. Maybe you mean 12mm, whose heads, and nuts, are 17mm AF?

 

[vyv_cox]

Your link mentions "17mm bolts". That's an odd size. Maybe you mean 12mm, whose heads, and nuts, are 17mm AF?

Yes.

 

[macd]

Your link mentions "17mm bolts". That's an odd size. Maybe you mean 12mm, whose heads, and nuts, are 17mm AF?

Surely 10mm has 17mm AF? M12 is normally 19mm AF.

 

[NormanS]

Surely 10mm has 17mm AF? M12 is normally 19mm AF.

Yes, quite right.

 

[wklein]

My boat originally had u bolts through the deck unsupported underneath. (deck was 22mm solid glass) which started to hogg slightly after 50 years so I did something similar I hadn't finished filling old holes