[Alberg30] Genoa Travler
Larry Morris
larmor at myexcel.com
Mon Nov 24 10:05:51 PST 2003
I vote for the ain't broke don't fix it solution. By the way how hard
do you hit your dock anyway? I think if you hit a piling hard enough to
dislodge the genoa track or egg shape the bolt holes, you will have
bigger problems to deal with.
Larry Morris
Solstice #501
-----Original Message-----
From: public-list-bounces at alberg30.org
[mailto:public-list-bounces at alberg30.org] On Behalf Of Roger L.
Kingsland
Sent: Sunday, November 23, 2003 3:42 PM
To: dk.campbell at sympatico.ca; Alberg 30 public list
Subject: Re: [Alberg30] Genoa Travler
Don;
To answer your question ("Why would you want to try to weaken the
attachment point for wind and genoa forces on an AL 30?"), first the
short
answer followed by the long answer (for the insomniac Albergers) that I
composed some time ago to sort out the issues for myself. Any feedback
would be appreciated.
Short Answer:
I am sure the "if it ain't broke, don't fix it" crowd will point out the
thousands of Albergs and other boats of the era having a similar toe
rail
detail that haven't sunk yet. But, this detail simply does not stand up
to
the logic test and, in my humble, newbie opinion, it is definitely
broke.
The problem is the fastening system requiring great strength and
rigidity
(the structural connection between hull and deck) is the wrong type
(shear
vs. friction) and is constantly subjected to forces that weaken it
(bumps
into the dock and genoa sheet loads).
To me, the solution is to create a good, bonded friction connection
between
hull and deck and to separate connections for the loads on the toe rail
and
genoa track. Thus the question about moving the traveler inboard.
Also,
the wind load on a 400 square foot sail at 30 knots is under 2,000
pounds
(assuming the sail is still intact in such wind), half of which goes to
the
sheet. If the sheet enters the block at 45 degrees, the upward force on
the
track is less than 800 pounds (500 pounds at 30 degrees). It is
relatively
easy to distribute with the proper backer plate.
Long Answer:
I am in the process of replacing my toe rail on #148 and I have the
impression the rivets were meant primarily to hold the deck and hull
together during chemical bonding. The rivets, which I drilled out and
filled with Marine Tex were too small and too deteriorated to have any
appreciable tensile or shear strength and were located near the outside
edge
of the joint overlap where they would not be effective in creating a
friction connection (but would do a good job of holding the hull and
deck
together
while the joint cured). I replaced them with 3/16" SS rivets 4" OC
about
1-1/2" inboard (my hand still hurts).
The thru bolted toe rail fasteners (3/16" SS bolts, 6" OC) seem to be
the
primary hull/deck mechanical fastener. The problem with these is,
because
they also hold the toe rail, they can not be tightened sufficiently (the
teak doesn't have sufficient compressive strength) to create a friction
connection (like that created by a clamp or Chinese handcuffs) at the
intersection of hull and deck. If the chemical bond fails, the only
remaining bond is the aluminum rivets (which are marginal) and the shear
connection created where the SS bolts that pass thru the hull/deck
material.
A shear connection is only as strong as the surface area that "pushes"
against the sides of the connector; in this case, the sides of the holes
in
the hull and deck (each less than 1/2" thick) forced against the bolt
shaft.
Also, unless there are no gaps between the fastener and the
material being fastened, the shear connection comes into play only after
the
chemical bond fails. So instead of the mechanical fastener adding to
the
strength of the chemical bond, its strength is only realized after the
chemical bond
fails. I suspect the chemical bond is stronger so, if a force
sufficient
to break that bond is applied, a second application of the same force
would
also cause failure of the mechanical fasteners. Glug, glug.
Ensuring bolt holes are filled with epoxy (often a messy, hit-or-miss
process) or using fully threaded screws would allow both connections to
work
in
concert but, the limited strength of a mechanical fastener in shear
would
still be a problem. A screw with a smooth shaft at the upper of the two
layers being fastened will create a friction connection but its strength
will be limited the thread grip on the relatively thin lower material.
The dual function of the fasteners creates another problem. That is the
fasteners that hold the hull and deck together also hold the toe rail
(AKA,
bumper) onto the deck. Just a minor bump against a piling can
cause movement of the bolts which will enlarge the hole through which
the
bolt
passes (I removed bolts forward that had a lazy "S" shape). In
addition,
any eccentric load on the genoa traveler attached to the top of the rail
will "work' the thru deck bolts. Close hauled, where loads are higher
and
more longitudinal, the depth of the toe rail will create shear
resistance.
When loads are transverse, as in when the sail is out (especially when
held
out by a whisker pole), the thin toe rail does little to prevent bending
in
the bolts which are essentially cantilevered from the deck. I suspect
this
is why we read postings such as this one from George Dinwiddie, "Most of
my
leaks in the lockers under the salon windows have come from leaking
genoa
track bolts." The result is water tight integrity becomes entirely
dependant on the bedding compound (which can also be effected by
movement in
the toe rail) and the sheer connection no longer works in concert with
the
chemical bond (per above).
Roger Kingsland
Chief Financial Officer (AKA, check writer)
Mahina Manu, A30 #148
N40° 29.288'
W79° 54.228'
Author's Disclaimer; This email was produced exclusively by the sender
and,
in the interest of expediency, without the benefit of editing by others.
The sender, thank goodness, is a much better architect/sailor than
speller/editor and, frankly, constantly laments an obvious flaw in
"spell
check," it does not know what the author is thinking. Please accept the
sender's sincere apologies for any "typos" that may appear in this
document.
If present, they are certainly unintended and hopefully do not cloud the
message, or spawn any unnecessary lawsuits.
----- Original Message -----
From: "Don Campbell" <dk.campbell at sympatico.ca>
To: "Alberg 30 public list" <public-list at alberg30.org>
Sent: Sunday, November 23, 2003 11:44 AM
Subject: Re: [Alberg30] Genoa Travler
> The WTC fell when it went out of column and the support was too weak
to
hold the weight due to gravititational forces of the mass. Why would you
want to try to weaken the attachment
> point for wind and genoa forces on an AL 30?
>
> "Roger L. Kingsland" wrote:
>
> > I have considered moving my genoa traveler inboard about 2" so I
could
attach it directly to the deck instead of the toe rail. Would this be
permitted under the class racing rules?
> >
> > Roger L. Kingsland, AIA
> > Managing Partner
> > Kingsland Scott Bauer Associates (KSBA)
> > N40° 27' 49" W79° 57' 59"
> > 3441 Butler Street
> > Pittsburgh, PA 15201
> > www.ksba.com
> >
> > 1(412) 252-1500 X101 - Office & Voice Mail
> > 1(412) 779-5101 - Mobile (no voice mail)
> > 1(412) 252-1510 - Fax
> >
> > +---------------------------------------------------------------+
> > | This Old Boat by Don Casey |
> > | http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
> > +---------------------------------------------------------------+
> >
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