[Rhodes22-list] My Centerboard delamination

[Lou Rosenberg]

>  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_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.