|In many of my previous
articles I raised some warning flags about this latest trend toward coring
boat bottoms, pointing out the potential for some very serious problems.
Those warnings haven't been heeded as numerous builders jump on the cored
Well, folks, now the chickens are coming home to roost, only to find
that the fox has raided the hen house. You needn't take my word for it,
for I will show you actual pictures of the results.
Cored hull bottom failures are starting to show up in increasing
numbers, just as I had predicted. After all, the industry went though this
ridiculous situation in the 1960's, again in the early 1980's and now in
the decade of single digits. The industry love affair with cored
bottoms seems to run in twenty year cycles and here we are again.
The market is being filled with cored bottom boats.
Amazingly this re-experimentation with cored bottoms is not just
happening with smaller boats, but mainly with very large boats, exactly
where you'd least expect it, what with large boats involving such huge
sums of money, one would think that large boat builders would be a lot
more cautious and conservative. But, no, like a herd of lemmings, many are
making a mad dash toward the edge of the cliff.
Sea Ray is a company that has played around a lot with hamburger helper
for fiberglass. Over the years it seems they've done everything they could
think of to reduce the amount of costly glass and resin they use in their
hulls, including the pioneering the "putty boat," hulls that
incorporate spray-in polyester filler (stuff that looks exactly like
Bond-O autobody filler).
Sea Ray has used cores in their boats for many years, first for decks,
then for hull sides, and finally for bottoms. Sea Ray, like most other
builders, has had a lot of trouble with rotting cores. They've also had a
lot of trouble with their chopped strand mat laminated over plywood hull
stringers and other structurals rotting. So you have to wonder, if they
can't build plywood cored stringers and balsa cored decks that won't rot
away (and which aren't submerged in water), what on earth makes them think
that they can build boat bottoms the same way? And with balsa, no less?
Why It's Risky to Buy a Cored Bottom Boat
The risk of water entry into a cored bottom is obvious to most
experienced boat owners. It's like having a water-proof watch. Even my
Rolex Submariner will, over time, experience the seals aging and going bad
and water leaking into the watch. That has already happened twice. Of
course, a boat bottom is not built with the precision and care of a Rolex
watch, so how much more is it likely to leak? The answer is so much more
so that water ingress into the core is almost inevitable over time.
of hydraulic erosion. This test boring of a delaminated cored boat
bottom came out as a handful of mush. The brownish color is
pulverized balsa. A larger piece of balsa is visible at lower
Then there is the risk of improper handling, repairs, alterations, etc.
All it takes is for one improperly made screw hole to turn a boat bottom
into mush. Mush? Yep, mush. Take a look at the above photo that
illustrates the results of what happens when water gets into a core. These
core samples were taken from the bottom of a large Sea Ray which had
failed that was only three years old, one of several of this model that
had met with the same fate.
What you see there is a bottom laminate that was in the process of
completely disintegrating. The reason is that once water gets into a core,
a phenomenon called hydraulic erosion takes place. Due to the slamming and
pounding of the hull bottom on the sea surface, water contained within a
laminate or core will be compressed by the flexing laminate structure.
Thus, the bottom literally becomes a diaphragm pump.
Once ply separation occurs, the impacts of hull against water creates
hundreds of pounds of hydraulic water pressure within the laminate. The
pressure is so strong that it will erode the plastic and shred the glass
fibers. And that is exactly what you see in that photo. A hull laminate
that has been reduced to a slurry of plastic particles shreds of glass
fibers, much like the way the Colorado river carved out the Grand Canyon.
shot, the inner laminate is separated from the balsa
core. A large amount of dust fell out of this hole when
the plug was removed, some of which can be seen stuck
to the inner laminate. The dust is pulverized fiberglass
from pounding. No water was necessary to fuel the destructive
process of laminate breakdown.
People usually think that a balsa cored bottom would be far worse
because of the wood's ability to absorb water. So far, the evidence at
hand does not support that idea. Foam, because it is much softer, and not
at all fibrous, breaks down much faster under hydraulic pressure. In fact,
in all the test borings taken on this boat, the balsa itself was yet to
break down. The actual failure occurred because the cored bottom panels
were not properly terminated at the keel. In this instance, the major ply
separations occurred within the solid laminate (containing a LOT of CSM)
bottom made with a lot of CSM delaminates. The separated pllies
grind together and reduce the laminate to dust and shredded fibers
as seen here.
The Limitations of Surveys
Unfortunately, core problems are often undetectable during surveys
unless the problems are far advanced. That is particularly true when the
outer skins are particularly thick and neither sounding nor moisture
meters are likely to give an indication of trouble.
And in the case of our Sea Ray boat here, extremely little of the
internal hull is visually accessible, so not much of the internal hull can
even be inspected.
Thus, when buying a used boat with a cored hull, even a survey is not
going to prove a reasonable probability of a defect-free hull. Surveyors
ought to be shivering in their shoes anytime they approach a cored bottom,
so how much more trepidation should a buyer bring to the table?
So there you have it. The history of cored bottom performance is poor.
Surveys that don't involve destructive testing can prove soundness. Buying
a cored bottom boat, in my opinion, is little more than a roll of the
dice. The odds are not in your favor.
How About Cored Hull Sides?
No problem. Hull sides are not submerged and are far less likely to
become water saturated. The potential for hydraulic erosion is far
lessened even if it does. And because the sides are vertical, water will
collect at the bottom near the chine. Water saturation in sides is fairly
easy to detect: All you have to do is drill a small pilot hole on the
inside and see if water runs out.
Keep in mind, folks, that the industry tells us that cored bottoms are
lighter and stronger. Lighter, maybe. But here's your evidence of
The amazing thing about the boat building industry is that no one ever
seems to learn from the lessons of the past. Want a job in the marine
industry? Anyone can apply: no experience necessary.
Ray and Balsa Core Bottoms - Posted November 20, 2002
The Hamburger Helper of Boat Building,
Reviewed in the Light of History - Posted October 31, 1998