On Tue, 24 Jan 2006 01:11:52 -0500, DSK wrote:


"Frank Boettcher" wrote...
As a sample of one, my experience has been that if a manufacturer puts
fasteners into the core without going all the way through, you are
much more likely to get core rot.

Pretty much guaranteed IMHO if that fitting has any stress
at all on it.

... I repaired quite a bit of it and
found it all related to that practice. I had a hatch cowl, head
ventilator, several escutcheon plates, and some others which were all
caulk and screw into the core. All eventually resulted in core rot.
When I repaired it I converted all of those to through bolt.

Ummm, I hate to tell you this, but that's actually worse for
cored lay-ups.

I disagree. see below

Balsa core is not very strong in compression. Actually,
neither is plywood, but it's stronger than balsa.

All wood is stronger in compression of the end grain than compression
of the side surface. There is less chance to develop a permanent
depression on the end grain than the side surface. And end grain
balsa is stronger in compression than side grain plywood, plus lighter
which is why it is used. Load up your cabin trunk or deck with
plywood and you have a top heavy craft requiring much more ballast to
get the centroid right. No good.

When you
tighten the bolts, you compress the core. When the fitting
is loaded, the core compresses further (think stantion base
plate) and then when the load is removed, it doesn't expand
again (or at least, not 100%). Now the bolts are very
slightly loose. Repeat. Now water is guaranteed to be let in.

If you put fasteners (which would have to be wood screws or some type
of universal thread screw) into the core once you have achieved the
appropriate torque that is pretty much it. If it moves and leaks you
don't know until the damage is done. And periodic retightening will
only strip out or weaken the bond. When you through bolt with machine
screws and nuts you can 1.) see the leak if it develops because it
will come all the way through, 2.) periodically retorque the fasteners
to tighten up the seal or remove and rebed with the ability to get a
good seal because of the use of machine screws and nuts.

Frank I am very glad to hear you have not had any rot in
your core since making your repair. But that isn't the
textbook method and it's not any more work to do it that way.

The repairs I made solved the problem in the areas they were in. I
sold the boat several years later, checked back periodically with the
new owner and they were still providing trouble free service.

The answer to this problem is to not have any core in the
area of fittings.

Good plan but most builders do not do this.

The original builder, if putting in core in the first place,
should cut the core to a template around all fittings, and
taper the core down all edges so that the two skins come
together and can be reinforced easily in the area of highly
loaded fittings. Ideally the edgees should have a double
radius, S-curve.

Look at a Morris, Oyster, or Baltic, and you will see this.
Look at an Ericson and you will see core with a flat taper
or bevel in the decks. Some eras of C&C did this as well,
but they also put in plywood or milled PVC blanks in place
too for some models in some eras. So did some other
builders. But many builders just toss the core in the mold
and slap cloth over it.

Almost everyone does it for high structural penetrations. very few do
it for the low structural connections that usually end up causing the
problems. My problems were not caused by stantions, standing rigging
chainplate penetrations, pulpits or any other high load items. Mine
were caused by low load items. Many times, after market items like
covers and cowls are attached with snaps or fasteners that are just
screwed into the core and most manufacturers don't make allowances for
these items.


Maxprop wrote:
Damned if I can recall what manufacturer did this, but I recall seeing a
boat on which all thru-bolts were first drilled oversize, then filled with
solid resin--epoxy, I presume--and then redrilled to the proper, smaller
diameter.

The problem with this method of repair is that you're
cutting away the strength memeber... the skin. But the epoxy
filler is stronger in compression than the core, so that's good.

One method to dig out the core & leave the skin is to take
an Allen wrench and put it in your drill, and work it around
the edges of the existing hole. Put tape under, fill with
epoxy (I use hi density filler mixed in), then drill out the
original sized bolt hole.


.... No way any moisture could get into the core with that system.

It still can if the fitting is not bedded properly. That's
why the answer is to have solid glass in the area of fittings.

.... I
have Airex foam cores in my hull and deck, but I still do the same thing if
installing something. It makes for a lot of work, but it's a safe system.
I also overdrill holes into the core through just one laminate (snaps, for
example) and fill them with epoxy, too. Probably overkill in my case, since
Airex won't absorb moisture at all.


No, but freeze-thaw cycles will still cause progressive
delamination if any water gets into it. And Airex is also
weak in compression, thru-bolts will crush it and cause leaks.

Cored laminations are a higher level of technology than
solid glass. They're lighter, stiffer, more elastic, can be
engineered to have all kinds of desirable properties. People
who say "I hate cored fiberglass" are saying "I want my boat
to be heavier, weaker, slower, and less stable."

But cores aren't foolproof, they're not even user-friendly.
The real killer of cored structure is lack of maintenance.
How long has it been since all deck fittiings were rebedded?
Going on 2 1/2 years for me, and I'm thinking about doing it
again. But then I was raised in the old school where you do
this *every* year.

This turned out to be really long, sorry about that. But
it's an important issue. This should have come under the
"projects" thread earlier.

Fresh Breezes- Doug King

Balsa core is not very strong in compression. Actually,
neither is plywood, but it's stronger than balsa.


Frank Boettcher wrote:
All wood is stronger in compression of the end grain than compression
of the side surface.

That's true.

However, *balsa* is not stronger that way than even cheapo
pine plywood (unless of course there are gaps in the ply).



.... And end grain
balsa is stronger in compression than side grain plywood

I don't think so, but I bet there are figures out there
somewhere.

... plus lighter
which is why it is used.

Plus the bonding properties and lack of wicking.

... Load up your cabin trunk or deck with
plywood and you have a top heavy craft requiring much more ballast to
get the centroid right. No good.


Agreed, but a lot of boats are built that way. And a lot of
people assume 'heavy = strong.'


The repairs I made solved the problem in the areas they were in. I
sold the boat several years later, checked back periodically with the
new owner and they were still providing trouble free service.


Good work, then... you must have done an excellent job
bedding them. I'm never that confident, plus I really hate
deck leaks.

DSK

"DSK" wrote in message
news
Balsa core is not very strong in compression. Actually, neither is
plywood, but it's stronger than balsa.


Frank Boettcher wrote:
All wood is stronger in compression of the end grain than compression
of the side surface.

That's true.

However, *balsa* is not stronger that way than even cheapo pine plywood
(unless of course there are gaps in the ply).



.... And end grain
balsa is stronger in compression than side grain plywood

I don't think so, but I bet there are figures out there somewhere.

Check out the Wooden Boat magazine archives. There is so much esoteric data
on the properties of wood as to be tedious. I'm sure this issue has been
addressed, and probably redundantly.

Max