[Public-List] Bulkheads

Thu Feb 4 10:44:27 PST 2016

Here's a caution  regarding using thickened epoxy filled holes to improve
bearing area stress distribution for the chain plate joints.
   - the chain plate connection to the boat is an important structural
connection.  Here's a little tutorial on strength of materials: There's a
property of materials called the modulus of elasticity, for steel its
30,000,000, aluminum: 10,000,000, plywood 1,000,000 and unreinforced epoxy:
15,000 psi.  Using epoxy as a structural component actually makes the
connection less strong.  Making plugs out of G10 which is a glass
reinforced epoxy would be a different story.  I don't have any
data detailing what the modulus of elasticity is for G10 but I expect that
its at least as good as plywood.  Using steel, aluminum would improve the
bearing capacity.  G10 would be as good as plywood and maybe better.  But I
wonder if all of this is needed.  Here's a couple thoughts:
    - the only time we've seen chain plate failures is when the wood has
been damaged by water.  Insuring structurally sound wood is the key.  If
you got water damage and your doing a serious restoration then the
bulkheads can be replaced which Towney talked about.  If you just want to
go sailing knowing the chain plates are solid then local replacement of the
damaged area with appropriate backing plates tying everything together is
reasonable.
    - we found years ago that the 1/4-20 fully threaded fasteners were
marginal and have recommended for years to replace them with 5/16 bolts
which has be described previously in this thread.
    - backing plates that capture all three bolts is easy to implement  and
vastly increases bearing area to distribute bolt tension loading which
becomes significant as the wood softens...backing plates better accommodate
wood deterioration than washers.
    - the knees in the v-birth have to be structurally sound, a reasonable
improvement was also discussed in this thread.  On my Alberg the fiberglass
tabbing was separating from the hull so I redid them while also doubling
the length of the knees and glassing them in.  Lengthening the
knees spreads the load out over a larger area reducing stress.

With everything structurally sound go sailing BUT pay careful attention
to the keeping the water from leaking in around the chain plates.  They
require constant attention.   I don't know of a fix that can be done with
out requiring periodic maintenance.  Does anyone know a way?

Jim Mennucci

On Tue, Feb 2, 2016 at 10:15 PM, Hernán Scarnichia <
public-list at lists.alberg30.org> wrote:

> Michael,
> I thought about adding bushings. There is an article on wooden boat #206
> from 2009 that describes using G10 tubes and plates to reinforce bulkheads
> this way. https://goo.gl/photos/vCozrEUgKh3GdGNH8
>
> On my boat the knees for the forward lower shrouds were too rotten and the
> tabbing was de-laminating from the hull so I rebuilt them using 3/4" G10
> and made them longer (below the shelf) and better tabbed to the hull. No
> bushings needed here as G10 is hard enough.
> I will probably replace the plywood on the aft lower knees with G10 too.
> For the upper shrouds the bulkhead is not rotten but the bolt heads have
> pulled a little bit because of the small washers used. I'm planning on
> using a 6"x12"x1/2" G10 backing plate on each side of the bulkhead and
> adding some extra bolts to better distribute the load on the wood and
> through bolt it to the hull tabbing too. Increasing the bolt size to 1/2
> should take care of the slightly deformed holes on the bulkhead.
>
> What size of chain plate will you use? What do most Albergers use when
> replacing chain plates?
>
> From my calculations my current chain plates (3/16x1-1/4) will start
> deforming at 4200 lbs. I've read it is recommended to have chain plates 30%
> - 50% stronger than the wire so that would be 3/8 x 1-1/2 with 4 1/2" bolts
> each.
>
>
> On Tue, Feb 2, 2016 at 5:44 PM, Michael Connolly via Public-List <
> public-list at lists.alberg30.org> wrote:
>
> > Thanks to all who responded to my request.  There is one question still
> > unanswered. Has anyone used bushings around the chain plate bolts to
> > increase the bearing surface area of the bolts as they pass through the
> > plywood bulkhead? Thoughts about making an even larger hole in the
> plywood.
> > Michael #133
> > .
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