[Public-List] Bulkheads

Sat Feb 6 17:14:01 PST 2016

Chip,
on $414 the knees looked just like your picture, not even touching the
liner.
I considered glassing them to the deck but decided against. I did  not want
to cut a big chunk of the liner off and figured if the originals were not
attached to the deck it might be better to only tab the new ones to the
hull.

On Sat, Feb 6, 2016 at 3:26 PM, Chip Dance via Public-List <
public-list at lists.alberg30.org> wrote:

> I think someone, earlier in this thread, mentioned the knees supporting the
> deck (or something along those lines). I'm curious about this. on #456 it
> appears as though there was a liner around the knees that was cut out by a
> PO, but from what I can tell, the knees don't appear to ever have been in
> contact with the deck, or even the liner. Can anyone shed some light on
> this for me?
> Since I will be rebuilding the knees, is there any concern with tabbing the
> knees to the underside of the deck? Is flexibility between the deck & the
> hull a necessity?
>
> The link below shows a photo of what I'm talking about.
>
> https://www.dropbox.com/s/erhr89fwfju6ikm/SAM_0896.JPG?dl=0
>
> Thoughts?
>
> Chip
> #456
>
>
> On Sat, Feb 6, 2016 at 12:23 PM, via Public-List <
> public-list at lists.alberg30.org> wrote:
>
> > Don
> >
> > You're right about the epoxy modulus...I asked google what the modulus
> was
> > and it gave me the value in pascals and I missed a digit when converting
> to
> > psi...but nonetheless the strength of unreinforced epoxy is still too low
> > to be used structurally.
> >
> > You bought up a very good point about the aft stay knee.  I'm a little
> > surprised but I don't think there's any collected knowledge that The
> > association can fall back on.  There's been no failures that I'm aware
> of,
> > hence no knowledge.  There's enough fiberglass covering the wooden knee
> > that its conceivable to me that the connection in some cases is working
> > without the wood playing a role, my hunch.  There's a lot of boats out
> > there that don't maintain the deck seal and no reported problems.  Can
> > anyone add any experience to this?
> >
> > The mast pumping that occurs when the boat falls off a wave and the load
> > release that occurs when tacking does subject the lower chain plates to
> > cyclic loading.  If the lower chain plates arn't in line with the shrouds
> > there will be moment loading which can work to enlarge bolt holes over
> > time.  Seems like association members when refurbishing their chain plate
> > seals should check for looseness and take action accordingly.  Wet wood
> in
> > this situation would accelerate the wear.
> >
> > My two cents
> > Jim Mennucci
> >
> > Sent from my iPhone
> >
> > > On Feb 4, 2016, at 3:45 PM, Don Campbell <dk.campbell at xplornet.ca>
> > wrote:
> > >
> > >  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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