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

[Roger Pihlaja]

Lou,

OK, this is starting to make some sense now.  Portland cement has an unusual
property that may be what's causing your problem with swelling or bulging in
the centerboard trunk.

Without going into the chemistry, when Portland cement cures, water
molecules are actually incorporated into the solid crystalline
microstructure of the cement.  These water molecules occupy space and to
make room for it, the crystal structure must expand slightly.  Normally,
this water comes from the water that was originally used to make the cement
slurry.  Macroscopically, what we perceive as the cement "drying" is in
reality, the free water in the slurry being incorporated into the crystal
microstructure.  There is crystal growth & the individual grains in the
cement slurry become locked together in a hard rock-like matrix.  Because
cement curing is actually a series of chemical reactions, this is why
hydraulic cement will cure under water.

The crystal growth has another effect.  In situations where the cement is
poured into a mold which restrains the crystal growth, the cement curing
reactions strengthen the solid material by placing the entire mass under
static compression.  Note that unreinforced cement has almost no tensile
(pulling apart) strength; but, quite a bit of compressive strength.  This is
why concrete is reinforced with rebar in situations where tensile strength
is required.  Usually, there is some unreacted Portland cement left in the
solid concrete even after the cement is said to be fully cured.  Any water
that finds its way into the cement, say thru damage or some sort of
microcracks, will react with this residual Portland cement.  The cement will
expand and tend to seal off microcracks and even "heal" small amounts of
damage.  Normally, this is a "good thing".

GBI did not build the early Rhodes 22's in-house.  Instead, the construction
was out-sourced to a number of contract boatyards.  The quality of
construction at some of these boatyards was not of a very high standard,
which is why Stan eventually started building his own boats in-house.  This
is also why GBI will not recycle & warranty Rhodes 22's from this era.   My
fearless hypothesis regarding the problem with your centerboard trunk is
that the FRP laminate was too thin &/or of poor quality.  Water has leaked
into the cement thru this questionable FRP laminate.  Over time, this water
has reacted with the residual Portlant cement & expanded to give you the
bulge we observe today.

My initial reaction is that you are going to have to grind away the bad FRP
laminate in the centerboard well + some cement.  Then, you are going to have
to rebuild the walls of the centerboard well with new FRP laminate.  Ugh!

Let me think about this problem a little more?

Roger Pihlaja
S/V Dynamic Equilbrium


----- Original Message -----
From: <steadilsr at verizon.net>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Sunday, December 05, 2004 9:11 PM
Subject: [Rhodes22-list] Re: HELP!! CB trunk has delaminated!


> Roger, Todd , Peter T and all Rhodies:
>  Need advice on pics being posted.
>
>  After pulling out the stuck CB last week I decided to use my dremel tool
> attached to a flex shaft & 120grit sand minidrum to shave about 1/16 " of
the
> fibreglass away which is bulging out into the upper trunk.
> During the wk I carefully measured the CB thickness using two  straight
edges
> for references I meas the distance btw the edges along the length of the
> board.
>  i then cut strips of plywood conforming to those depths as test sticks.
I
> used those depth sticks today to measure how much glass i was trimming
away
> with the dremel.
>  Its diff to photograph but my trunk  as it has 2 apparent sections of
depth.
> The upper trunk where the lip is sealed with the neoprene and the lower
> section which opens to the sea.  My lower section is not bulging and there
is
> plenty of room, but the upper is too tight.
>  After sanding the head of the board down I was able to drop the depth
stick
> in and moved on to the middle of the board.  The section to the rear of my
> pivot was tighter and soon I foung my wall had cracked and the glass
crumbled
> away revealing the dry cement.  I know how to patch the hole with epoxy,
but
> now I cannot sand the walls with out risking similar problems.
>  I am posting my pics for reference and will have more next week.
>  Any advice welcomed.
>
>  note: Even if a surveyor had told me not to buy this boat due to a stuck
cb,
>  i think i would have gone agains his advice due to the condition of the
hull
> and other parts.  I am still convinced my boat will sail this summer!
> Lou
>
>


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