Chain Plates
This page describes the original chainplate design on the 10M and it's deficiencies. Pearson issued a recall for the repair of this problem in spring 1975.
Chainplate Gusset Recall - April 1975
This page also describes a refit procedure done by Graham Bryan (10M #187) on hull #60. Contact Graham about fabricating beams for your 10M. You will not get a better job from anyone. All the work on #60 was done by Graham or Gary Duke with Graham's guidance.
Original Design
In the original design the 10M had the primary chain plates were anchored to a knee about 6" aft of the main bulkhead. The knee was tabbed to the hull and there was an additional box section tabbed to the hull as stiffener. This configuration soon proved to be inadequate. The loading from the rigging pulled the knee inboard and dimpled the hull section to which it was attached.
There are a couple of problems with the knee/chainplate configuration. The hull section to which the chainplate knee is bonded is not stiff enough to support the loads without deforming and the bond itself is secondary. That means the chainplate knee was glassed in after the hull was complete and the bonding is secondary (mechanical) rather then primary (chemical). With primary bonding you get chemical cross linking of the resin layers to form a continuous structure. Secondary bonding is not nearly as strong as primary. And polyester resin is not very good in secondary bonds, especially compared to epoxy.
Pearson Recall
Pearson basically issued a recall authorizing owners to have re-enforcing added to the knee. The recall letter describes the hull deformation as cosmetic and not a structural problem. At least one 10M (#89) experienced a chainplate failure when the knee separated from the hull under load. The recall notice specified the lay-up for the reinforcing and it was to be done by local yards and billed to Pearson. This was back in 1975 and applied to hulls through at least #89. The original owner of #89 recalls being told by the dealer that his was the last boat recalled. I think Pearson did the reinforcing on #90 up to at least #101 before the boats left the factory. My guess is that all this was done with woven roving, cloth and polyester resin. I have seen the recall notice on a '74 10M in Milwaukee but I didn't get a copy (If anyone has one I would love a copy).
Design Change
At some point in the first half of the production run (after hull #101) Pearson made a change to the chain plate configuration. They removed the knee and replaced it with an aluminum I-beam unit to transfer the chain plate loads to the main bulkhead. The forward lowers still connect to a knee about 12" ahead of the main bulkhead. This change occurred sometime after hull 101 in 1975.
Bulkhead Differences
There is also a difference in the main bulkhead on the later chainplate beam configured boats. It is considerably thicker consisting of a 1/4" of glass, 3/4" plywood and another 3/8" of glass (photo). The chainplate knee boats don't seem to have the 3/8" glass layer since they were not designed to carry the chainplate loads.

Chainplate Knee Replacement - Chainplate Beam Re-Fit
On hull #60 the OEM knees were completely cut out and replaced with chainplate beams patterned after the ones Pearson used on later models but with some improvements. The first part of the re-fit was to remove the knees. You would think this should be a tough job. In fact it's made fairly easy by the weak polyester secondary bonding used to attach the knees. Open the edge of the tabbing with a chisel and pry it all away. After the knee has been removed the surface of the hull and the bulkhead need to be prepped for reinforcing with fiberglass because they are not as thick as the bulkheads in the later chainplate beam boats. This was done on both sides of the bulkhead.
On hull #60, biaxial glass tape 4 inches wide and both biaxial glass fabric and biaxial glass fabric with mat cut from rolls were used to tab both sides of main bulkhead to hull as shown in the photos. West System biaxial fabric has two layers of non-woven, flat, (straight fibers) at + and - 45 degrees orientation held together by light stitching. Non crimped fibers yield higher stiffness and strength than woven fabrics such as woven roving. (Lloyds will no longer certify hulls made with woven roving). Epoxy resin yields a much stronger secondary bond than the original polyester resin.
New Chainplate Beams
New aluminum I beam chainplates were fabricated by Graham Bryan. They were made from a thicker cross section than the Pearson beams and also have a cap welded on top to reduce torsional flexing (twisting) induced by upper shrouds.

OEM Chainplate Beams
The photo here shows the top of the OEM chainplate beams on #212. The corrosion is from water getting in between the SS chainplate and the aluminum beam. This is a minor problem compared to the same water getting into the main bulkhead on a more conventional configuration. I cleaned up the corrosion with a brass wire brush on my drill, filled the pitting with epoxy/filler and re-painted. I also spread some lanolin on the backside of the chainplate and over the bolts to help inhibit more corrosion caused by the dissimilar metals. Proper bedding of the chainplates will keep the water out. I removed about 3/8" of balsa around the slot in the deck and filled with epoxy. I cut a chamfer in the top to hold the sealing caulk and sealed with Boat-Life.
The photo here shows the top of the OEM chainplate beams on #212 after the repairs mentioned above. It also shows the polished chainplate. I polished them to reduce the chances for crevice corrosion and to make them look pretty. They do look pretty. (click to see the before photo)
The photo here shows port chainplate beam on #212 after repairs. You can see the epoxy work that was done at the hole through the deck.