Karl Denninger wrote:
In article ,
Rich Stern wrote:

Many manufacturers are touting "no wood/no rot" construction, and some are
using a composite grid system (fiberglass over some type of foam) to replace
traditional hull stringers and forms. Several bay boats I am interested in use
this type of construction. Of course, from a marketing perspective, it sounds
great. But I'd be interested in some real world opinions. Is such a boat less
prone to flex? Does it result in a more solid boat? Are there other problems
to be aware of? Any pre-purchase inspections that can be accomplished,
assuming it's even possible to see the below-deck structure?

Comments appreciated.


Sure - it sucks.

Here's the problem - no access at all to it,

But the lack of access applies no matter the stringer materials, so
there's no particular disadvantage.

and the problem with wood is
NOT that its wood.

Wood is not a very strong material at all. Tougher hardwoods have very
moderate strength in compression, but other than that wood is a very
weak material with low resistance to flexing & even worse performance in
tension. i.e. bend a piece of wood to simulate a load in the middle; one
side is in compression & the other is in tension. The wood will bend
easily & a great deal, it will usually then break as the tension side
fails. So I'd like to submit the problem with wood is that it's wood.

Most anything designed in wood is strength for strength much much
heavier than almost any other material, steel, glass, even ferro. This
is because the material is inherently weak & this weakness is aggravated
by the difficulties of attaching it to anything, even itself (mechanical
fastenings, nuts & bolts are about the only real fix)

Here's how damage REALLY happens to a cored structu

1. Some small amounts of water get in there due to improper sealing of the
core.

Closed cell foam, which structural core foams use, don't absorb
moisture. There are specific standards & test procedures to verify this,
so this can only happen if the wrong foam for the application is used.

2. Wave action and boat motion cause the two panels to compress against the
core. This is a NORMAL process. But with that small amount of water in
there, it will NOT compress. It thus acts like a hydraulic ram, deforming
the core.

It might be a "normal" process when you glass over wood because resins
will not ever properly bond to wood, but foams are different, the foam
material provides an excellent mechanical key to bond with the resin,
increasing the notional bonding area many times over AND the foam is a
plastic just as the resin in the covering is, giving a chemical bond as
well, so again proper choice of materials means there is no so-called
"pumping".

3. The panels "relax"; there is now a vacuum in the space. Since it is not
cmopletely sealed, it draws in more water.

I'm only reviewing this so it's up to whatever you want to make of it,
but gees louise; it's a vacuum?? & then water can get in?? This is a
good description of how water gets into timber stringers but .......

4. Complete, just like a jackhammer, until the core integrity is destroyed.

5. If the core is wood, it will EVENTUALLY rot, but the damage to the fiber
and bonding - the PRIMARY damage - had nothing to do with that. Note that
synthetic cores will delaminate MORE READILY than wood, as they have NOWHERE
NEAR the strength of wood in terms of resistance to compression damage.

Wood will rot as suggested but it rots because it's wood & wet wood.
The foam will never rot even if it gets wet.

How do you avoid this?

Simple.

1. Don't do that. Specifically, NO CORES IN HULL BOTTOMS. Ever.

The racing boats still use full foam construction for ultimate
lightness vs strength, however use of cored scantlings below the water
have not been in favour for many years as suggested. However in this
discussion I thought we were NOT talking the entire skin just a prefab
foam cored stringer/frame system??


2. Cores in decks and hullsides (above the waterline) are acceptable,
PROVIDED they are properly encapsulated. This means that there are
NO PENETRATIONS without the edges of the core being sealed with
epoxy. No way for water to get in, no problems. Note that this
means that hardware must be THROUGH BOLTED; screwing it down into a
cored structure is NOT ACCEPTABLE.

No bill

3. IF these rules are followed, then wood is a SUPERIOR coring
material, particularly, for decks, end-grain balsa. It has inherent
rot resistance and is a LOT stronger than PVC or Divynicell cores,
and its very light. Plywood makes the best transom cores; nothing
else comes close in terms of structural strength.

Balsa wood is about the same as other timbers for strength, the
devotees pretend it's stronger , but for equal weights of material to
carry an equal load, balsa isn't all that special & being timber it's
basically a weak material just waiting around a while till it rots.

Stringers, ideally, should not have a core in them at all. The best
stringer systems are hollow fiberglass "top hat" designs. Those can NEVER
rot and, properly engineered, are hellishly strong. They're also rare as
hell; only a few production builders have ever used them.

Not so rare all benatuas are built on a hollow all glass boxed grid
system. The issue is how any stringer or frame system is attached to the
skin, again glass or foam will always bond better than any timber.


Stringers should not derive their strength from the core;

I'd like to also disagree with this if I may. Hollow sections are never
as strong as three dimensional webbed or bulkheaded sections. i.e. say
in steel a rolled hollow section (RHS) of a given weight is never as
strong as as a universal column (RSJ) of the same weight. This is
because the top & bottom flanges can better do their respective
compression & tension jobs when held apart & kept parallel by the
central web. Hollow sections buckle like well... a hollow section:-) In
foam construction the foam becomes the "web" it holds the two skins
(flanges) apart AND (not hollow) timber beam (or stringer) operates the
same way, in any cross section when under load (if you could slice it
like bread);
(i) one side the wood's cells would be in compression
(ii) the outer surface would have the most compression acting upon them,
(iii) as you looked further down the slice you'd see the compressive
force getting less & less till around the centre there would be no load
whatsoever (as if there were no load on the beam at all) then,
(iv) as you proceeded further down the wood cells would start to see a
tension force trying to pull them apart &
(v) this force will increase till at it's maximum at the other outer edge.

Hollow uncored beams can work but only if they're wall thickness is
excessive OR they're fitted with bulkheads or frames at appropriate
intervals, to keep the outer load carrying parts of the beam from
buckling or moving relative to each other. (the reason bamboo is so
strong relative to it's weight?? it has bulkheads/ring frames & is also
better than ordinary wood in tension)

should be short and wide rather than tall and narrow. If you have to
core them, Marine XL plywood is a good choice,


Any sort of wood makes a bad "core" because it can never properly bond
to & therefore position & transfer load across to the surrounding load
carrying sections; all it can do is act as a spacer & a heavy wet
rotting one at that:-)

but its not necessary for
virtually all boats as a properly engineered stringer doesn't need a core
for strength - you can use CARDBOARD - just to hold things in place while
the resin cures!

If you are using completely hollow stringers or structural members
thats fine, however again; you'll need to design them thick walled (&
heavy) enough to resist deformation (with glass mostly it's compression
force buckling) & if you do that then it will weigh more than the same
strength using a closed cell structural foam core or if you choose
bulkheading &/or frames.


K



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