Morgan 382 Chainplates

WARNING: Technical sailing jargon ahead.  This post dives head first into the tangs and bolts of sailboat rigging.  An attempt on my behalf to define all the nautical terms below would render this post as palatable as a tall glass of salt water.  This one is for the sailors out there….

Pilgrim’s standing rigging and mast were replaced by the previous owner in 2002.  This was done while the vessel was based on Lake Erie and since the re-rig she has remained on the lake.  Subject to six month sailing seasons on a fresh water lake the rig is in excellent condition.  The new mast is an Isomat NG80, double spreader rig few feet taller than the original.  Pilgrim’s original cap shroud deck fitting was replaced with a new fitting capable of accommodate the upper stay. 

The components of our cap shroud chainplate.  Top - Deck fitting for cap and upper stay.  Left: Below decks bracket.  Right: Backing Plate.

Over the past couple weeks we have re-moved, inspected, and re-bedded all six of Pilgrim’s midship chainplates.  Fortunately we discovered very little corrosion and no stress cracks or hole elongation.  Other than the two cap shroud deck fittings, we believe all the chainpate hardware was original equipment.  I highlight “was” since we replaced all the thru-deck fasteners in the system.

Thru-deck fasteners?  Yep.

Port cap shroud chainplate installed.  Note rusty nut on thru-deck fastener. 

Cap shroud chain plate with thru-deck and thru-bulkhead fasteners in place.

 We were surprised to discover that the rig loads are transferred thru the deck to the chainplates below via four 3/8” stainless steel machine screws.    Initially I was shocked and worried by the unfamiliar set up.  Is this system strong enough?  Why are the thru-deck fasteners 3/8” and the thru-bulkhead ½”?   

Time for some internet homework…Stress Terms defined via Wikipedia…

A:  Compression = Squeezing the material =  The stress state caused by an applied load that acts to reduce the length of the material along the axis of the applied load. 

B:  Tensile = Pulling the material = The stress state caused by an applied load that tends to elongate the material along the axis of the applied load.

C:  Shear = Faces of the material sliding relative to one another =  The stress state caused by the combined energy of a pair of opposing forces acting along parallel lines of action through the material (e.g.  cutting paper with scissors)

In terms of relative strength of stainless steel fasteners they are able to tolerate the greatest about of stress in compression, followed by tensile, and finally shear.  Thus a M382 thru-deck fastener’s, under tensile stress, are 3/8” while the thru-bulkhead fasteners, under shear stress, are ½”.

Ok so back to the boat.  All six of the M382 midship chainplates are of similar design.

Chainplate assembly for starboard, forward lower stay.

Relying on the four 3/8” stainless steel machine screws to transfer the rig loads.  None of the screws displayed signs of fatigue or elongation, but they all possessed a small degree of surface rust.  We also discovered surface rust covering a couple of the interior nuts.  Since we expect these fasteners are original to the vessel – 35 years old – we replaced all the thru deck fasteners with new 3/8” machine screws with nylon lock nuts.

Starboard cap shroud chainplate re-installed with double nuts on the thru-deck fasteners.

The new fasteners for the cap shroud chain plate were a bit too long.  Rather than cut off the excess we chose to add a second, lock nut, to the assembly. 

Personally I would prefer the vertical, bulkhead, plane of these chainplates to be longer.  I believe a longer vertical section would distribute the loads over a greater area of the bulk head.  But there is not history of issues or failure with the original design.  On our Morgan these primary bulkheads are nearly 2” thick.  They consist of outer layers of ¼” plywood façade surrounding 2 layers of ¾” plywood. 

While the chainplates were absent we also took care of some structural (tabbing) and cosmetic (painting) projects in the salon area.  See our “Paint In Lieu Of Headliner” and “PaintIn Lieu of Headliner, The Portside Story” post for more details.

Port aft lower chainplate (fore ground) and cap shroud chainplate (background) re-installed.

I will discuss corrosion discovered on the backing plates and re-bedding the chainplates in upcoming posts.

Check out our Chainplates Photo Album for additional images and notes.