On Thu, 20 Nov 2003 03:44:31 +1100, K Smith wrote:

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+

================================================== ==

Not at all true, wood has a very high strength to weight ratio. You
are confusing strength with stiffness. They are two entirely
different properties.

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



--

Chuck,
Okay then why the concern over rotted wood in the stringer?

Depends on the stringer. Did the manufacturer intend to have an FRP stringer
that just happens to to have a wood or foam core, or did the mfg put 3/32 inch
of glass to "encapsulate" a wooden stringer where the wood was actually bearing
the stress?

I would
imagine that the water got in through a poor job of sealing the wooden
stringer, so why not seal it up and not worry about it?

See above. I've attended surveys where a decayed stringer core has been
detected, and in some cases the stringer is condemned as a result and in others
it is not.

I know a fellow that had his boat (88 Sea Ray 300
Weekender) out of the water for three seasons while he dried out his
stringers and checked for moisture with a meter.

Hooooo, boy. Find out whatever it is that guy wants to buy and go into the
business of selling it to him.

You can't "dry out" decay.

Each case is different, but a stringer repair should be doable in a matter of
days, not years. I'm aware of situations where the stringer has been sliced
open, the old core excavated, new core material substituted, and the whole
works glassed back up. Another cure that surveyors have signed off on, (again,
depends on the stringer), involves building up the laminate to increase the
load bearing ability of the stringer perimeter.

I think he then bored some
holes in the stringer and filled with epoxy. Was he wasting his time?

Three years to do a cheap and dirty Git-Rot fix? Yeah, he was wasting his time.
:-)

His
complaint was that Sea Ray drilled limber holes through the stringers and
didn't seal the limber holes causing the water absorption. I'm just trying
to determine how wide and important of a problem is this.
Paul

Unsealed limber holes are fairly common in production boats. :-(

It is a bit griping how so many builders, (not just a few) turn out a product
that will maintain fair to good structural integrity for
12-15 years, and then price it at $250k or up- and the typical buyer needs a
20-year mortgage to pay for it.

WaIIy wrote:

On Wed, 19 Nov 2003 10:15:12 -0500, Wayne.B
wrote:

On Thu, 20 Nov 2003 03:44:31 +1100, K Smith wrote:

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+

================================================ ====

Not at all true, wood has a very high strength to weight ratio. You
are confusing strength with stiffness. They are two entirely
different properties.

Well, for the most part, she wrote a great post.


How would *you* know that, Wally?

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"Harry Krause" wrote in message
news:c3dhc2g=.82627d12784c03afb2097e72f24ffd36@106 9256316.cotse.net...
K Smith wrote:


Really? Gosh. There probably are 100 million wood frame houses in the
united states, some more than 100 years old. I suppose that because of
the weakness of wood and the difficulties of attaching it to anyting,
even itself, are really problematical, eh?

Don't twist those houses or they fall down. Don't suddenly lower the
pressure...the nails come out. Don't let the weather barrier get
compromised or they rot. Don't compare house construction to boat
construction. They don't even come close.

I suggest you take a look at some standard floor joist simple span tables
and see just what it takes to span 10 feet. Conventional wood is not very
resistant to flexing. A lot of other things are stronger.

An all composite, no wood boat is a lot better than one with wood in it.

Just because a lot of boats have been made with wood does not make it good.
It just means it is cheap and easy.

"Harry Krause" wrote in message
news:c3dhc2g=.82627d12784c03afb2097e72f24ffd36@106 9256316.cotse.net...
K Smith wrote:


Wood is not a very strong material at all.

Bull****. Wood is a very strong material when used appropriately.



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.


Yeah? Tell you what. Envision a wood stringer, say 2"x10"x16'. Tip it on
its edge. Now, try to bend it in the up or down plane with any load
similar to what you might find in a working hull. Now, build that 2x10
into an eggcrate sort of structure, with cross members of the same or
similar material. Now subject that structure to lateral loads. Doesn't
bend that way either.

This is real world construction here, Karen, not some crap you lifted
off a web site.



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.


The problem isn't wood in boats. It is the wood between your ears.





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)


Really? Gosh. There probably are 100 million wood frame houses in the
united states, some more than 100 years old. I suppose that because of
the weakness of wood and the difficulties of attaching it to anyting,
even itself, are really problematical, eh?



It might be a "normal" process when you glass over wood because resins
will not ever properly bond to wood

You ought to send your resume to Grady-White. I'm sure they'd be
interested in hiring you because in your opinion, the boats they build
are likely to fall apart any moment; their stringers are constructed of
XL plywood covered in fiberglass.

But what could Grady-White know about boat=building, compared to the
Australian bull**** artist, Karen Elizabeth Smith?




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.

An important principle to keep in mind when building small boats, eh?


More Karen lifts from engineering webpages, but no understanding of
materials or applications.








--

Mole wrote:
"Harry Krause" wrote in message
news:c3dhc2g=.82627d12784c03afb2097e72f24ffd36@106 9256316.cotse.net...
K Smith wrote:


Really? Gosh. There probably are 100 million wood frame houses in the
united states, some more than 100 years old. I suppose that because of
the weakness of wood and the difficulties of attaching it to anyting,
even itself, are really problematical, eh?

Don't twist those houses or they fall down. Don't suddenly lower the
pressure...the nails come out. Don't let the weather barrier get
compromised or they rot. Don't compare house construction to boat
construction. They don't even come close.



I'm not comparing house construction to boat construction. The point,
and perhaps I was too subtle, is that wood frame construction is strong
enough for houses, and wood is strong enough for boat stringers,
assuming whoever does the design does it properly and the design is
correctly implemented and the wood protected.

Karen Elizabeth Smith "gets off" on these little tangents of hers, but
they mostly are absurd or border on it. Wood is a fine boatbuilding
material, and has been for thousands of years. It certainly is "strong
enough" to be used as a boatbuilding material. Does wood have
shortcomings? Of course it does, but so does every other boatbuilding
material.

Ms. Smith is quick to castigate for what she perceives to be "problems"
in the designs and manufacture of boats generally, and, more typically,
boat engines, a position that really is not defensible, if you take a
look at the barely floating derelict of a boat she calls her own and the
rusting piece of crap "diesel outboard" engine with which she
underpowers it.



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The problem is that fiberglass is not water proof. Water migrates thru the
fiberglass and soaks the stringer. You can not seal a wood stringer with
fiberglass. Epoxy is more likely to keep the stringer dry but it is a lot
more expensive so nobody uses it. Even with epoxy any compromise is going
to let the water thru.

Most are laying enough fiberglass on the stringers to provide the majority
of the strength even after the wood gets soft. Some have figured out that
they don't really need the wood for strength and switched to foam. The foam
is just there to as a form for the glass.

I'll bet if you drill a hole around those motor mounts on your Tolly you'll
find quite a bit of fiberglass.

"WaIIy" wrote in message
...
On Wed, 19 Nov 2003 08:40:18 -0500, "Paul Schilter"
paulschilter@comcast,dot,net wrote:

Chuck,
Okay then why the concern over rotted wood in the stringer? I would
imagine that the water got in through a poor job of sealing the wooden
stringer, so why not seal it up and not worry about it? But does an
older
boat like an 89 Sea Ray depend on the wood for its strength or the
fiberglass coating? I know a fellow that had his boat (88 Sea Ray 300
Weekender) out of the water for three seasons while he dried out his
stringers and checked for moisture with a meter. I think he then bored
some
holes in the stringer and filled with epoxy. Was he wasting his time?
His
complaint was that Sea Ray drilled limber holes through the stringers and
didn't seal the limber holes causing the water absorption. I'm just
trying
to determine how wide and important of a problem is this.
Paul

I know the mid 80's Wellcrafts - 34 ft or so- were famous for rotten
stringers.

It would be interesting to know how much some og the boatmakers rely on
the wood in a stringer for strength and just pretty it up with epoxy.

I know in my little Tollycraft, it would take a wrecking ball to move
the stringers my engines are mounted on.

Lawrence James wrote:

I suggest you take a look at some standard floor joist simple span tables
and see just what it takes to span 10 feet. Conventional wood is not very
resistant to flexing. A lot of other things are stronger.

Well, of course, but that's not the point, is it? The point is that
properly designed, implemented and installed, wood floor joists are
strong enough. And that's all they have to be. As to the flexing issues,
my house has built-up "truss" joists of wood, and my floors don't flex
to the point you'd notice it. Further, large areas of the main and
second floors of my house are covered in ceramic and marble tile,
installed the usual way, and we have no cracks in the mortar or tile. If
there were substantial flexing of the wood subfloor (held up by wood
trusses), we'd have some mortar cracks.




An all composite, no wood boat is a lot better than one with wood in it.

Again, I suggest you take that up with Grady-White and other
manufacturers of small pleasure boats who continue to use wood in the
construction of their boats. GW can build boats any way it chooses,
without worrying too much about price points, since it already is at the
top of the price chart. It chooses wood.


Just because a lot of boats have been made with wood does not make it good.
It just means it is cheap and easy.

It's easier to build a small boat's structure of composites or foam. The
stuff can come out of a mold and be glued into the boat. No special
skills required.






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Lawrence James wrote:

The problem is that fiberglass is not water proof. Water migrates thru the
fiberglass and soaks the stringer. You can not seal a wood stringer with
fiberglass. Epoxy is more likely to keep the stringer dry but it is a lot
more expensive so nobody uses it. Even with epoxy any compromise is going
to let the water thru.


What's the impact of water on XL plywood?



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