Re: [Paddlewise] Hull/Deck Seam Construction

Mark Arnold wrote: 
>
> 1. What is an "outside" seam exactly?
> 
Fiberglass kayak hulls and decks are made separately, and then the two
halves are joined in some fashion.  If a woven fiberglass cloth tape (looks
like a ribbon woven of fiberglass cloth and available in many widths)
impregnated with some sort of resin is used to glue the top and bottom
halves of the boat together, these gluing strips can be put either on the
inside or the outside (or both) of the boat.  

Appearance is not as much of an issue on the inside of the boat, so the
inside seams can be wider and stronger (more layers) than outside seams. 
In order to make the outside of the boat look the best, the seam (if there
is one) must be very narrow, and more carefully applied.  It stands to
reason that if all other things are equal, a boat built with both inside
and outside seams will be stronger than a boat built with only an inside
seam of the same thickness/construction.  It is possible, however, to not
use an outside seam, and still have comparable structural strength.  

> 2. Current Designs info says the "H" channel method produces a "seam that
has 
> a hollow center which accommodates the rudder cables inside".  How are
the 
> deck and hull glued to the "H" to leave a hollow center?   Is the hollow 
> center going to be a weak point particulaly up forward where there is no
need 
> for rudder cables.?
>
I've not seen (paid any attention to) this type of construction, but from
the description I'd guess that the weakest part of such a joint would be in
how the deck and the hull are glued to the H-channel.  Any time you have to
bond dissimilar materials together - especially in a long-lasting, tough,
and waterproof joint - the trick is to find an adhesive that sticks equally
well to both parts. And though such adhesive joints may be good when
'young', the bond may change with time and exposure.  The molded H channel
should be pretty tough, and shouldn't split in the middle. - although years
of UV exposure could change the toughness of the channel as well. 

 
> 3. What are the pro's and con's of each construction (or does it even
matter 
> to the paddler)?
> 
If the major consideration is ultimate boat strength, it would make sense
to opt for a heavy-duty layups joined by heavy-duty seams both inside and
out.  One can also argue for having a seam weaker than either the hull and
the deck construction (which _can_ be the case with both construction
methods).  In such a system, a catastrophic impact would break the seam
first, and leave the hull and deck pretty much structurally intact, thereby
making it easier to repair the boat.  With a real heavy-duty seam job, and
assuming the hull and deck don't break independantly of the seam, the boat
will be able to take a much bigger impact before breaking.  But if and when
it finally *does* break, it probably won't break along the seam, and will
be much harder to repair.  

It's ultimately up to the paddler to decide where and in what kind of
conditions they're going to paddle, and then specify the appropriate boat
layup and construction methods.  A Tsunami Ranger will need a much tougher
boat than a twice-a-year paddler.  Nobody wants to have a boat spring a
leak in the middle of a long tour or in real nasty conditions when a long
ways from shore.  Very light layups can also be serviceable for many, many
years, IF one is aware of the relative fragility of a boat so constructed
and 'babies' it accordingly.  All boats will eventually need some repair or
maintenance work. 


> 4. Are there other options for joining the hull and deck together besides
the 
> two I have mentioned?
>
Plastic boats truly are of 'unibody' construction, and there is no seam. 
Plastic is arguably the most maintenance-free boat material, but it suffers
in areas of weight, and (usually) relative hull speed.  In wood boats, the
hull and deck are held together with a combination of glue, nails or
screws, and sometimes a fiberglass skin coat.  Wood boats are
labor-intensive to build, but can of the lightest weights - and pretty
tough.

 
> 5. Can you tell by looking at the boat how the deck and hull are joined?
>
It's usually easiest to see how the hull and deck are joined on the inside
of the boat, as the gel coat and decorative strips/moldings on the outside
make it harder to see exactly how the hull and deck are joined.  The
manufacturer should tell you how the boat is built, but remember that each
manufacturer will point out the 'good' aspects of their own means of
boatbuilding, so take their comments with a grain of salt.  


Caveat:  I'm not a boat-builder, though I have played quite a bit with
fiberglass repairs over the years (and am currently repairing and
reinforcing the aged and cracked fiberglass vent cover for my attic fan). 
I too hope that the builders will jump in to add to the commentary.

Erik Sprenne
(at the southern end of Lake Michigan)
 

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