Composite chainplates: The ideal solution for composite hulls

Sometimes, old ways of doing things survive simply because "that's how it's always been".

From time to time, though, it is useful to look at the old ways to see if, in the context of modern knowledge, they still make sense. If they don't, perhaps they should be changed. In today's article, I'd like to take a look at a key component of many sailing yachts- the chainplate. Current chainplate designs date back to before the Industrial Revolution, and I think they're overdue for a redesign.

Back when boats were planked up piece-by-piece out of wood, it was difficult to find any one point that was strong enough to attach the standing rigging. The solution- a very elegant one for the time- was to use straps of iron or bronze to spread the loads from the rigging over many planks. For a planked-up wooden boat, even today, there is still no better way to make chainplates than to drill many bolt-holes in straps of bronze.

When fibreglass came on the scene, it was originally treated as a direct replacement for wood. Early fibreglass boats had hefty structural grids resembling those of their wooden counterparts, hull skins as thick as their wooden counterparts, and hardware (such as chainplates) that was virtually identical to what had gone before.

As the art of engineering these materials developed, composite hulls became lighter, more efficient and cheaper to build. (One could argue that some of them are too light and too cheap, but that's a story for another day.) Some of the old ways stuck around, though, and even today you routinely see stainless steel chainplates that are laminated into the hull sides, or bolted to bulkheads with part of them sticking up through the deck, or even elaborate contraptions involving a deck clevis through-bolted to an interior bracket linked to a tension rod linked to a stringer clevis fibreglassed to the structural grid in the bilges.

A major part of the appeal of fibreglass is that it lasts nearly forever and is all but indestructible in normal use. If it's properly engineered and properly built, a composite hull will never fatigue, corrode, rot, or otherwise deteriorate with age. A metal chainplate, though, has a finite life. If it pokes through the deck, it'll corrode inside the water seal; if it's laminated into the hull, it'll corrode along the bonds as water seeps in; if it's bolted to the outside of the hull, the bolts will eventually rust through. Preventive maintenance is therefore mandatory- which was fine when we had to replace rotted wood planks all the time, but not OK now that we're using a hull material that is effectively maintenance-free.

Fibre-reinforced plastic composites have an interesting trait: If you orient all the fibres in the same direction, they are obscenely strong in that direction- even the cheapest unidirectional fibreglass is, relative to its weight, stronger in tension than most metals. Chainplates are loaded mainly in tension, and that load comes entirely from one direction. Do you see a solution to our chainplate problem?

The answer is yes: Make the chainplates out of the same material as the hull, i.e. fibre-reinforced plastic. Some specialized boats already do this, and it has many advantages:

  • No potential for corrosion.
  • No leaks. (The chainplate is an integral part of the hull, with no joints or holes.)
  • No maintenance.
  • No stress concentrations that could lead to premature failure.

Let's assume we're looking at a typical fibreglass hull on a modern sailboat. Above the waterline, it has a thick outer skin and a thin inner skin, separated by a structural core material. We drape many layers of unidirectional fibre over a mould, and lay it into the hull so that the bundle of unidirectional fibre replaces the core in that region.

At the gunwale, we're left with something that looks a bit like this. All the fibre in the green bundle runs in one direction, over the hole and down the hull side. You could use a metal toggle and pin, or lashings of high-modulus line, to attach some stays and shrouds to these eyes. It's worth noting that you can form them in almost any shape you need- even splitting them into double clevises, if that works better with your hardware.

How do we know it won't rip out, though?

To answer that, remember that we're dealing with long rope-like bundles of dry, unidirectional fibres. They're obscenely strong in one direction, and we can orient them however we like. The logical thing to do is to run those bundles all the way down the hull side, fanning them out as we go, cutting them off somewhere in the solid fibreglass laminate below the boat's waterline. You have to reinforce the hull in a similar way for metal chainplates, too, so we're not looking at any extra work or cost.

Such a structure is limited by two factors:

  • The tensile strength of the fibre/plastic composite, which can be calculated with high accuracy and can be made far stronger than necessary with very little penalty in cost or weight.
  • The tendency of the hull sides to buckle inwards due to the lateral component of the load applied by the rigging. This applies to all chainplates, and is managed with careful placement of structural bulkheads and deck beams.

If you have inboard chainplates, they can easily be integrated into the bulkheads in a similar fashion. (Note that such a design should never rely on the peel strength of the bulkhead skins.)

The composite chainplate protrudes through a cutout in the deck. As there is no potential for corrosion, no joint movement and no differential thermal expansion, a virtually perfect seal is easy to achieve here- and even if water did get in, we're using materials that don't deteriorate when they get wet. These chainplates will still be able to lift the boat's entire weight many decades later when all the systems have clapped out, the gelcoat's faded to chalk, and the metal bits have corroded away to nothing.

The best part of it? There's no reason why, in 2012, composite chainplates should cost any more than metal ones. Twenty years ago, when fibreglass boat hulls were laid up wet by hand, this would have been a labour-intensive thing to build- you'd have needed extra staff, and they'd be racing against the clock to get it all put together before the resin kicked off.

These days, we don't wet-lay production boats by hand; we lay up the entire part dry and then infuse the resin under vacuum in a single shot. Once you've made the tooling to support the head of the chainplate during the infusion, the work involved is minimal: simply drape a bundle of fibre over the hole mandrel, clamp the mould around it, clamp the chainplate mould to the hull mould, and feather the fibres out in a suitable fan pattern. One worker can take his sweet time to put the fibres right where they need to be, without any stinky sticky stuff getting in the way.

For an ultra-high-strength application, some boats even use carbon fibre in the chainplate head; only a kilogram or two of carbon would be needed and the extra materials cost would be small. (As Kurt Hughes has noted, though, it is quite easy to achieve far more than the required strength in cheap fibreglass alone.) You can easily modify the technique to produce aft-facing chainplates for a drogue, or any other kind of hardpoint you need.

The issue of chainplate design has come up a few times in discussions over the Adventure 40 ocean cruising yacht that's taking shape over at AAC; I am very much convinced that fully integrated composite chainplates will be the strongest, longest-lived and most economical way to do it.

TL;DR: For a composite hull, composite chainplates are superior in every way to metal chainplates, and once the tooling for them has been fabricated, they shouldn't cost any more than their ancient counterparts.




Retrofitted chainplates

Matthew's picture

Hi Dena,

Composite chainplates, if done properly, are an integral part of the component they're attached to. This is much harder to achieve as a retrofit than it would be in a new build.

Simply gluing a composite chainplate in to replace an existing metal one is not an ideal solution. The point of a composite chainplate is that the unidirectional fibres that form the chainplate are part of the same matrix as the hullside or bulkhead they are joined to; the loads can therefore be spread out through the entire laminate. A glued-on chainplate would put all the load on the bond with the outermost layer of fibreglass; if the inter-layer bonds in the laminate are weaker than the adhesive (a very real possibility), it'll simply rip the inner skin of the hull off.

Retrofit recommendations are always on a case-by-case basis, but more often than not, I'll recommend replacing a metal chainplate with another metal chainplate. The replacement, though, would be through-bolted to the surface in the traditional fashion (embedding metal parts in a composite laminate is difficult to do well). And I almost universally recommend bronze alloys, not stainless steel, for metal chainplates and their bolts. Good bronzes are not particularly susceptible to the crevice corrosion problems that attack stainless steel in this situation.

If the geometry is suitable for it, excellent results in a retrofit job should be possible with the technique used by the Scrumble Project team to build their bulkhead-mounted composite chainplates. This design spreads the loads well enough, and does not rely on the peel strength of the adhesive or the skin. (If the forces on yours would tend to peel it away from its surface, this technique would not be appropriate.)

Add new comment | M.B. Marsh Design

Terrific post but I was wondering if you could write
a litte more on this subject? I'd be very thankful if you could elaborate a little bit more. Many thanks!

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