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Fiberglass
boats built with internal liners have been around for a long time.
Typically, a liner is a premolded internal component, the purpose
of which is to provide the basis for the interior layout. Over time,
this function has evolved and has slowly taken over the function
of providing internal hull structural support as well.
Hence the liner has evolved to the term "grid/liner"
where the function of the liner is included to take over the role
of conventional framing systems such as individually laid up glass-on-wood
stringers. Surveyors know that working with liners can cause problems,
not only with access for inspection of the internal hull - often
they make large parts of the hull inaccessible - but because of
the difficulties imposed by the design for bonding the liner to
the hull. If the surveyor can't reach the areas, neither can the
builder, and so the manner in which it is attached to the hull has
to be suspect unless proven otherwise.
The Wellcraft 34 Grand Sport is a good example
of a boat with a full liner forward that obscured most of the internal
hull framing. These boats also had major structural shortcomings
that resulted in serious structural failures. In most cases, examination
of what little could be seen of the internal hull would yield little
evidence of the problem. However, with these boats the bending and
twisting of the hull was so severe (they only had one bulkhead in
them) that the interiors were shifting and breaking up. If you knew
what you were looking for, it was relatively easy to detect the
problem. Just inspect the interior for unusual gaps between components,
broken joints, loose moldings, screws backing out and little piles
of wood dust where parts were rubbing together. If one isn't aware
of the problem, its quite easy to miss it because the effects may
be quite subtle until the whole structure begins breaking apart.
Yet for most boats with liners there was usually
some degree of access so that one could get at least a fair look
at the structurals. Now the growing use of grids is going to make
inspection of the internal hull more difficult than ever. The reason
is because grids are combining the function of liner and framing
system all rolled into one. The grids are reducing visual access
to the hull interior more than ever, often to as little as 10%.
This is a departure from typical liners that generally do not take
over the function of frames. Normally, the liner sits on top of
frames.
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This sailboat uses a premolded grid that completely
disbonded from the hull. The problem here was inadequate tabbing
which can be seen broken loose at left, center and right of
photo. Only the webs are glassed while the longitudinal section
(seen at top) is not bonded to the hull at all. |
If you thought lack of access gave you problems
with liner boats, you'll be even more thrilled with what you see
arriving in the near future. One-piece grids are being used to virtually
eliminate the traditional framing system, replacing it with a liner
system that is literally glued into the hull. Glued, you say? Well,
they call it bonding putty but an adhesive by any other name is
still a glue.
Those of you experienced with Hunter sail boats
will know what I mean. They were one of the first to use full interior
grids, albeit not necessarily a liner, and much of their product
line suffered massive bonding failures, including their large 60
footer.
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The framing system in this boat was reduced from
a 4 stringer system to 2 stringers because it was thought
that the liner would provide the needed extra strength. The
liner broke loose and the stringers started to fracture as
shown in this photo. The area of the break was completely
obscured by the liner and was not discovered before the bottom
cracked open. |
So what's wrong with grids, you ask? The reason
behind them is, of course, efficiency, making the boat easier and
faster to build. And that is always the signal for the surveyor
to open his eyes a little wider, for another interpretation could
be "cutting corners." Start with the fact that grid/liners
are basically glued into the hull. Now, I don't know about you but
I've never seen anything that was glued together being as strong
as a component that's all of one piece. That's despite all the loud
and fancy the advertising about adhesives. The only things that
glue together well are parts with identically uniform or mirror
profile surfaces. For example, gluing two pieces of wood together
that are perfectly flat makes for a very strong joint. But allow
the slightest surface irregularity or out of squareness and the
joint becomes very weak. Unfortunately, the interiors of laminated
hulls can hardly be called uniform. Our experience with bonding
putty in cored hulls tells us that there's not likely to be any
better level of success in this application than for foam cores.
See related article Hi Tech Materials.
Next, let's consider the functions of the structural
members that the grids are taking over. The function of the bulkhead
is primarily to prevent a hull from wracking or twisting in a torsional
mode. Additionally, it also serves as a transverse frame.
Stringers are longitudinal frames, probably better
represented by the word girder than frames since we traditionally
think of frames as transverse members. Stringers are uniquely critical
to power boats since powerboats travel at much higher speeds. The
forces that work on a power boat hull tend to want to break it in
half, similar to the way a bridge wants to collapse in the middle.
Thus, good, strong stringers are needed to keep the hull from bending
in this direction. The above photo shows what happens when this
principle is compromised.
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Bonding failures can sometimes be difficult to detect.
This broken liner bond was only discovered by sounding because
the break was obscured by a dirty bilge. In this photo, it
is cleaned for purposes of clarity. |
At a time when I taught a course on marine surveying,
I used a shoe box to demonstrate the forces that work on a power
boat hull. A small lead ingot was placed in the center of the box.
Then I picked up the box and it buckled because it was too weak
to carry the weight. Next, the lid was put on and taped in place.
Taping the box lid in place represents the function of the deck;
it is a horizontal bulkhead. When lifted, the box still buckled,
although not quite so badly as when the lid is not taped in place
because the secure lid adds strength. Just not enough.
Then I put two 1/4" square strips of wood
on the bottom of the already buckled box and set the ingot on top
of the strips. Without putting the lid on, I picked up the box.
By now you get the picture that, even though the strips of wood
weren't attached to the bottom of the box, they served to distribute
the load of the ingot over the length of the bottom. When lifted,
the buckled box did not buckle at all.
Now lets introduce grids into bottom of the hull
and see what we've got. Not surprisingly, two different boats I've
seen have grids that do not make any attempt at providing continuous
stringers. One had stringers in the liner running half the length
of the hull, while the other had a section four feet long where
the stringers disappeared in the middle. Imagine taking a boat and
cutting out four feet from all four stringers in the midships section!
On this boat, the stringers were eliminated so that they could put
in a very large fuel tank. The same thing was happening to this
boat as our cardboard box with the ingot.
Additionally, many of these grid systems are found
to have the fault of creating dog legs in in longitudinal framing
systems. I you don't understand the problem with this, just imagine
a bridge span with a dog leg in it. A dog leg changes direction
of the load suddenly, creating a local high stress point which may
result in stress cracking and failure.
The problem here is that the design of the grind/liner
may involve so many trade-offs that the designer ends up compromising
the essential structural elements. In order to get components to
fit, structural elements are sacrificed. In and of itself, this
is a problem. But now let's add to the mix the fact that the whole
grid system is glued into the hull. Combined with a compromised
structural system, how well is it going to hold up?
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When full inner liners are used, internal plywood
partitions are usually secured to the liner with screws. If
the liner is loose and working, it is very likely to result
in misalignment of internal components. In this case, the
large gap and shifting of a partition was a dead giveaway
of a structural problem. Further evidence was in the form
of screws backing out of parts fastened together. |
I'll let you be the judge. In the meantime, these
new designs are going to require some new surveying techniques since
what you previously were surveying visually will no longer be visible.
Fortunately, there are some techniques that will partially overcome
these limitations.
Because the use of grid/liners will essentially
act as an inner and outer skin, somewhat similar to a cored hull,
finding disbonded areas shouldn't be particularly difficult. If,
because of the liner, builders cut down on hull laminate thickness,
as I believe they will, then detecting disbonding will be even easier.
If you thought hammering on the bottom of a hull was a primitive
survey technique, wait until you hear what I'm about to suggest.
If a liner bonded to a hull comes unglued, in all
probability a gap will open up. Sounding with a hammer is not going
to detect this gap because the bonding putty is considerably less
dense than glass laminate. The hammer may return a sudden change
in sound, but then again, its just as likely not to. The answer
lies in using something quite a bit heavier than a hammer, such
as a heavy chunk of wood like a piece of 2" x 4". Now
give the hull a moderate hit with the wood.
If there's a gap between hull and liner, the two
parts should hit together and return a sort of clacking sound -
the two pieces hitting together. The sound will come across as sort
of a double clack, first the wood hitting the hull, and then the
hull and liner contacting. This technique is highly likely to find
major areas of disbonding where the interior is not accessible.
Naturally, going over the whole bottom with a big
hunk of wood will require the strength of a 24 year old linebacker.
So to reduce the effort, we can go over all the internal areas that
are accessible with the hammer. This is because the parts of the
liner that are glued to the hull are going to be quite thin. The
hammer should return the usual disbonded sound. Once you've done
all of the interior that you can, then you target only the more
limited areas externally for the big hit.
For a 30' boat , this may entail about 30 minutes
more work, mainly because of the additional interior sounding that
you would not otherwise be doing. Its well worth the extra effort
because if you ignore this, as I've explained above, the risks of
getting tagged for not discovering a major problem are unacceptably
high.
To summarize:
- View gridded systems and large liners with great suspicion.
- Take every opportunity to sound out the internal bonding points
wherever accessible
- Examine the grid system for structural design faults such
as non-continuous stringers and dog legs. When such indicators
are present, be extra cautious.
- Examine the fit of the interior components for signs of working,
misalignment, loose screws or little piles of dust caused by
abrasion of working parts.
- Use these new but crude techniques to further prove out the
grid bonding.
- Remember that the grid is the framing system: if its broken
loose from the hull, the effects are no different than top hat
stringers that are broken loose.
- Remember that the best liability insurance you can get you
can provide for free yourself by doing as good a job as possible.
- Take pictures: document your file with the good, the bad and
the ugly. There's nothing like photographic evidence to prove
your point.
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