[Rhodes22-list] My Centerboard delamination

[Roger Pihlaja]

Lou,

No gelcoat on your centerboard you say?  Quack?

I took another look at your photos.  Man, if that's not gelcoat; then, it's
some of the thickest bottom paint I've ever seen!  It's hard to believe that
any boat builder would build an underwater part like a centerboard & just
paint over the raw FRP laminate.  However it got that way, assuming you are
correct & that delaminating layer is really old bottom paint; then, I would
strip all the bottom paint off your centerboard down to the raw FRP
laminate.  Fix any cracks or delaminations in the FRP laminate.  Then, I
would put down an unthickened epoxy tie layer, 20 mils of epoxy with West
422 Barrier Coat additive, and a couple coats of bottom paint.

By the way, if you really don't have any gelcoat on your centerboard; then,
you should consider doing some dimensional comparisons with your rudder
blade.  With no gelcoat, the FRP laminate in the centerboard may be swollen
due to water absorbsion &/or freeze/thaw cycles.  This could be another
reason for the interference fit in your centerboard well.

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: Thursday, December 09, 2004 2:27 PM
Subject: Re: [Rhodes22-list] My Centerboard delamination


> >  Roger,
>
>   I may be missing something here but my CB does not have any gelcoat
> on it at all.
>   It was obvious to me that if it was there originally, it came off or
> was removed and the board was sanded down to the glass mat and
> painted with bottom paint.  I was referring to sanding this cracked
> and heavily layered section down to the glass and then painting it
> with bottom paint.
>   Indeed , an new board would most likely have been gelcoated but I am
> not planning on doing that to this board at this time.
>   Lou
>
>
>
>
>
>
>
>
>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_le
a
> >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.
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>
>