[Public-List] Bulkheads
Don Campbell via Public-List
public-list at lists.alberg30.org
Thu Feb 4 12:45:56 PST 2016
There are a couple of other things that have not been discussed in this
thread. One is the movement of chainplates under load and with a weakened
deck this tends to be greater than designed or thought of. There is
possible sideways movement, fore and aft, of the chainplate from the point
of attachment of the wire to the top chainplate bolt. If there is any
luffing of sails, backwinding, or shock loading from tacking, it can be more
than one realizes. Any movement will cause added wear, and the
possibilities of working away at increasing deck wear and bolt hole size
within bulkheads. I have used epoxy plates on top of the deck to reduce
movement of chainplates and reduce the water inflow, while allowing sealants
to be added under thread pressure. George has a couple of photos in the
"maintenance, disorganized, chainplates" section of the website. These
plates considerably decrease the leverage from the pin holding the wire to
the top bolt that is possible with flex sealants and weak decks. This
system also ends most periodic maintenance of rotten or waterlogged deck
core, (provided the core is sound to begin with) and extends the interval
between replacing sealants in the space between chainplate and deck.
As for Modulus of Elasticities of epoxies, Gougeon's "Gflex" (registered
trade mark) is 150,000 p.s.i. unreinforced. This is a very easy epoxy to use
and the testing of it comes very close to the specs for their 105 resin base
for most of the other tests. Gflex is superior in adhering to several
surfaces, including oily woods, when compared with or to other epoxies.
The bonding of either the bulkhead or the knee is to the hull. This was
originally done with polyester resins and the core of the knee was made of
plywood. This was, in effect, a type of truss, similar to the deck
composition. With water infiltration, the plywood (bulkhead or knee) has
rotted in many, many hulls without the owner knowing it. It is not until the
plywood rot gives way under load, and the tabbing then gives, that we
realize the knee fails. Thinking the rig is structurally sound is far
different from knowing it is sound. It ought not be something that one finds
when you are out on a race course, or out in 25 knot winds, because even
quick thinking to get on the opposite tack, head to wind or the sail down ,
may be too slow in practice. Test drilling of small holes or moisture meters
may give some idea of troubles. In many cases the deck core will be weak as
well and so it offers no strength to resist the chainplate pulling through
the deck. I do not think that backing plates will make much difference if
the core of the truss fails. The backstay chainplate offers a different
problem. There is a moment of force there because of the design. The top
bolt is aft of all the the others because the chainplate is angled aft at
the top. Thus if the top bolt fails from a bad knee, then the leverage is
increased on the lower bolts and shear is more likely. As the chainplate
revolves around the bottom bolt as the others shear, and it holds because
there is no longer a moment to shear it, once straight with the stay, there
is a major overload in a straight pull to cause the shear then, and there is
no deck that can hold that so the backstay flies free. One needs to be sure
that structurally sound is based on fact not fancy.
The differences between knee and bulkhead structurally are fairly
simple. The bulkhead holds a stay as well as supports the deck (which in
the case of the AL30 supports the mast base) and reduces torque on the
hull. Any change to a bulkhead will change these three purposes. It is
important to consider any fix by adding only a small amount of material to a
bulkhead as to how it relates to all three. A knee only holds the stay.
The one thing you do not want is to have a rig come down. It is very
expensive to replace and probably more dollars than the boat is insured for,
so the boat becomes a write off with the insurance company. Getting it back
from them is one thing, having to pay for the rig replacement after that and
getting insurance again can be frought with interest, (and lots of principle
when it comes to money).
Don
-----Original Message-----
From: Jim Mennucci via Public-List
Sent: Thursday, February 04, 2016 1:44 PM
To: Hernán Scarnichia ; Alberg 30 Public List -- open to all
Subject: Re: [Public-List] Bulkheads
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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