Chainplates: Do Yours Leak?

by Mark DeSchane, Marine Surveyor, Walker, MN

The second year that my wife and I owned Pyxis (our 1978 O'Day 23) I read an article by someone in one of the sailing publications about the potential for leak and rot problems by not maintaining a water-tight seal at the shroud chainplate penetrations of the side decks. This attachment point for the shrouds is subject to the highest tension loads of any place on the vessel. Along with the tension loads created by the force of the wind heeling the vessel , there is shock loading concentrated at this location by pounding through the seas on a brisk beat to weather.

In many instances, the upper & lower shrouds are connected to a single chainplate of bar stock stainless steel which is through bolted to the main cabin bulkhead. If the vessel has for & aft lower shrouds, these chainplates may be through bolted to other minor bulkheads, or the cabin sides or other reinforced attachment points. There are many other types of attachment methods, but we are going to limit ourselves to those methods whereby the chainplates or the shroud attachment points penetrate the side deck. This is an area that can leak, and when it leaks can allow water to saturate the owwden structure used to secure the shrouds and support the mast.

The problems arise when, because of the aging of the sealant materials an/or the working (movement) of the chainplates due to tension and shock loading stresses, the seal between the deck, chainplates and sealant loses its watertight integrity, allowing water to penetrate the seal. Water penetrating this seal can migrate to the coring materials of the deck and cause rotting of the core material [endgrain Contourcore balsa for the Express 27]. It can also expose the bulkhead to water, which will penetrate its plywood laminates. When moisture and temperature conditions are right, rotting at this location will occur and its effects can be disastrous and lead to dismasting of the vessel!

Generally, attachment to the bulkhead is accompanied by through-bolting the chainplates to the bulkhead. In most instances, these bolts pass through the chainplates, through the bulkhead and through a backing plate which is used to spread the compressive loads created by the tightening of the chainplate bolts.

The most common type of chainplate penetratio9n of the deck is simply a slot in the deck, that the chainplate passes through. A "beauty plate" of stainless steel with a similarly sized slot for the chainplate to pass through is provided to dress up this area. The beauty plate also acts as a mechanical part of the deck penetration seal. The beauty plate may also be found to be screwed to the deck's surface.

Renewing this seal is easier done than explained. With the rig up or down, remove the screws securing the beauty plate and pry the beauty plate off of the deck. Scrape and remove as much of the old sealant as possible. Apply new sealant generously to the chainplate slot in the deck and the screw holes for the beauty plate, replace and refasten the beauty plate. When done, clean up the excess sealant and stand and tension the rig. The new sealant must cure with the rig under tension (tensioning forces may cause movement of the components being sealed, which might cause a loss of sealing integrity after the sealant has cured.

What sealant to use? We do not want a permanent seal here. So, this rules out #M 5200 or Polyurethane adhesive/sealant. We are left with Silicone or Polysulfide. Of these two, Polysulfide is the better choice for this application. Its properties work well between fiberglass and metallic surfaces. Besides being the best choice for sealing the particular materials involved, an added benefit is the fact that Polysulfide takes a while to cure, allowing time to stand and tension the rig.

How often should this job be performed? Probably every year if the rig is left up over the winter. The reason for this is the mast and standing rigging are still subject to changing tensions due to temperature variations and shock loading due to wind gusts. It is also more likely that the vessel will not be as well covered with the rig up. this exposes the chainplate penetrations of the deck to freezing and thawing and exposure to the sun's detrimental effects. If the rig is struck for winter, it will probably suffice to renew the sealant every two to three years.

A significant percentage of the sailing vessels I've surveyed show that this is an area which is not getting enough attention. Repairs to the bulkhead (because of rotting) entail replacement of the bulkhead, or at least the insertion of a graving piece to replace that which has rotted. This can be an expensive proposition and one that is probably best left to the boatyard to repair.

Detection of this problem may be as simple as a visual inspection of the area of the bulkhead, where the chainplates are through-bolted. Any discoloration of the bulkhead surface in this area may be indicative of a moisture problem. Generally some staining will show up but there can be a problem that doesn't show up on the surface because of the wood having been varnished or oiled. In this case, other means of detection must be employed, such as the use of a moisture meter.

Deck core rotting is very difficult to detect and may (or may not) be detectable with a moisture meter. If your decks are cored with end grain balsa wood, the rotting if having gotten a start will probably be very localized. Weter does not travel crossgrain in wood very easily. If plywood coring has been used, you can expect that damage by water incursion and rotting of the core may be extensive, the reason being that the end grain is exposed to the water in this instance. Unless you are proficient at fiberglass repair, it would be best to have a boat yard effect repairs to the deck when you have a core rotting problem.

With the simple preventive-maintenance required to thwart this problem and it taking a minimum amount of time, knowhow and tools to perform, do yourself a favor and take care of this problem before serious and costly damage occurs!

Reproduced with permission of Northern Breezes, Inc, Minneapolis MN.

NOTE: In your editor's experience the greatest vulnerability of the Express 27 to damage is from leaks into the core of the deck around the chainplates and through-bolted deck fittings. Ideally they all should be removed and drilled oversized, filled with epoxy and redrilled for the bolts, caulked and refastened. An owner recently reported significant damage to the core of his deck on one side of the mast step, not an especially easy repair job. He said that there had apparently been leaks around through-bolted fittings nearby. An article last year on chainplate leaks recommended fastening a line to the chainplate and running it aft to a winch and tensioning it enough to pull it aft in the slot, removing old caulk in front of the chain plate and re-caulking and then run the line forward and back to a winch and pull the chainplate forward so that the aft side can be cleaned and re-caulked. I haven't tried it yet but intend to when the snow melts.