[Rhodes22-list] Re: CB trunk has delaminated!

Tue Dec 7 18:43:59 EST 2004

Lou,

I'm seeing several different types of wear & damage on your centerboard.

The damage in:

http://www.rhodes22.org/pipermail/rhodes22-list/attch/200412/07/23_wide_shot
_port_side_b_1.jpg
http://www.rhodes22.org/pipermail/rhodes22-list/attch/200412/07/CB_lower_AFT
_edge_worn_down.jpg

looks like pure long term abrasive wear to me.  It appears your centerboard
is a very tight fit in your centerboard well in the area around the pivot
point and around the aft lower edge.   Before you reassemble your
centerboard, consider putting about 20 mils (0.020 inch) of West System
epoxy with the 422 Barrier Coat Additive over the entire centerboard.  The
422 Barrier Coat Additive is powdered aluminum & will provide a much harder,
wear resistant surface.  But, you will need to open up the clearances
between the centerboard and the centerboard well, or abrasive wear like this
will reoccur.  The centerboard operation will also tend to always be
"gritchy" when the clearances are this tight.

The damage in:

http://www.rhodes22.org/pipermail/rhodes22-list/attch/200412/07/CB_flaking_n
ear_pivot__st_1.jpg

looks like a brittle compression failure of the gel coat to me.  This damage
was probably done over a relatively short period of time.  I would guess, as
the centerboard was starting to jam, you &/or the previous owner tried to
force the centerboard into & out of the centerboard well.  Does the location
of this damage correspond to a location in the centerboard well where there
is a lot of bulging?  You will have to remove the failed gel coat back from
the failure site to where the bond with the FRP laminate is sound.  The gel
coat will have to be faired out.  Then, you will build the gel coat back up
to the original level with thickened epoxy.  Don't forget to use an
unthickened epoxy "tie layer" as I've previously described.  One of the nice
MOC properties of epoxy is that it is a much more ductile material & has a
higher bulk modulus vs. the polyester gel coat on the centerboard now.  The
epoxy repair will be MUCH less likely to suffer the same sort of brittle
compression failure.  In fact, I'll make a fearless prediction & tell you
that, as long as you properly prep the surface & use a "tie layer", the
centerboard gel coat will fail somewhere else before the epoxy repair fails.

I see at least 3 kinds of damage in:

http://www.rhodes22.org/pipermail/rhodes22-list/attch/200412/07/CB_lower_lea
ding_edge.jpg

There is long term abrasive wear on the flanks of the centerboard.  The
elliptically shaped dent on the leading edge looks like a ductile failure,
"impact crater".  I also see what looks like a delamination crack emanating
from the elliptical dent. (i.e. the elliptical dent was probably the crack
initiation site)  Interestingly, although the elliptical dent is a ductile
failure, the crack is a type of brittle failure, so you have a mixed mode
failure at this site.  I would guess the elliptical dent & delamination
crack involved something on your trailer, perhaps the frame on one of the
centerboard rollers.  The irregular shaped gel coat failure near the
elliptical dent looks like another brittle compression failure.  The
compression failure may have been another result of the extreme compression
loading involving the tight centerboard/centerboard well fit.  It looks like
you may also have a delamination crack on the leading edge at the very
bottom of the centerboard.  The abrasive wear pattern on the flanks of the
centerboard suggests that this delamination crack occurred because the sides
of the centerboard were getting squeezed.  This compression loading on the
flanks of the centerboard would have resulted in a high tensile stress
(pulling apart) on the centerline of the leading edge radius.  This tensile
stress was relieved when the leading edge delaminated & cracked.  I would
guess this crack is not very deep, probably not much below the gel coat.
Previous posts have talked about how to repair delamination cracks with slow
curing epoxy.

The wear pattern in:

http://www.rhodes22.org/pipermail/rhodes22-list/attch/200412/07/CB_top_stb_e
dge_fwd_sheav_1.jpg

looks like iron oxide residue left behind by stainless steel in an abrasive
wear situation.  You will need to clean off that iron oxide before recoating
the centerboard with West System 422 Barrier Coat Additive.  I would also
give the blocks and mounting hardware a close inspection.  They may need
replacement based upon the amount of iron oxide left behind on the
centerboard.  You might consider putting a big washer of Teflon or nylon in
between the centerboard and the mounting hardware on both sides of the
centerboard when you reassemble.  The plastic washer will be a sacrificial
wear surface.  Plastic washers like that can be purchased at a good hardware
store.

There, that's probably more than you ever wanted to know about your
centerboard!  Aren't you sorry you asked?

Roger Pihlaja
S/V Dynamic Equilibrium

----- Original Message -----
From: "Lou Rosenberg" <lsr3 at MAIL.nyu.edu>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Tuesday, December 07, 2004 1:29 PM
Subject: [Rhodes22-list] Re: CB trunk has delaminated!

> This is the second posting of these pics the 1st was thru the main
> server site and somehow the list didnt recognise my email
> address..very strange!
>
>   These pics were taken in my basement last night showing the leading
> edge of the CB, the top looking down, the aft bottom edge and the mid
> leading edge.
>   Lou

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