Re:[Paddlewise] Hull/Deck Seam Construction

>>Date: Wed, 15 Dec 1999 11:52:03 EST
From: MJAkayaker_at_aol.com
Subject: [Paddlewise] Hull/Deck Seam Construction

I was reading some kayak manufacturer's literature recently (trying to
prepare for buying my next boat) and would like to get some more information
concerning the methods used to join the hull and deck of FIBERGLASS kayaks.
Necky talks about an "outside seam".  Current Designs describes using a
vinyl
"H" channel with hull and deck glued to the channel rather than to each
other
(at least I think that is what they meant).  Current Designs also says that
"outside seams" are available.

1. What is an "outside" seam exactly?<<

A bonded seam on the outside of the hull and deck. Most likely fiberglass
tape bonded across the seam line with fiberglass resin or gelcoat (sometimes
even a layer of resin in the tape and a layer of gelcoat over that).

Aside:
Somebody asked about Boreal Designs outside seams--it looks to me to be a
tape (probably nylon or polyester cloth rather than fiberglass) that has an
existing pattern in it and clear resin to let the pattern show through.

The process to mating the deck to hull using an outside seam is:
Tape the deck and hull together every 4 or 5 inches and run a strip of
masking tape over the joint on the outside. Seam the halves together on the
inside with fiberglass tapes (sanding or another way to roughen the surface
to get a good bond is necessary on vacuum-bagged boats but not on hand-laid
boats). Next peel off all the tape on the outside and place another strip of
masking tape along the seam line where you want the tape, place two more
rows of masking tape one on each side of the one that bridges the seam. Peel
up the middle one and carefully sand the gelcoat between the other two tape
strips to get a good rough bonding surface without tearing up the strips of
masking tape on each side (or retape if you do). Fill in and smooth any
major gaps or holes between the hull and deck with a fiberglass putty or
paste. Wet out (with resin--or if this is going to be the last step, with
pigmented resin or gelcoat) and lay in the fiberglass tape between the
masking tape strips and peel up the masking tape before the resin kicks off.
Let that side cure and turn the kayak over and do the other side. Often two
more strips of masking tape are taped on each side again and gelcoat (which
includes a smoothing and/or surface sealing agent) is put over the first
layer to leave a smooth hard shiny finish. Some are even sanded and buffed
at this point (or sanded between layers).

>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 particularly up forward where there is no
need
for rudder cables.?<

The H extrusion (usually of vinyl) most often has the outside leg longer
than the inside leg and often has a hole in the crossbar large enough for a
rudder cable to easily run through. The H is placed around the hull and the
deck is fitted into the top of the H. Some manufactures place a glue or
sealant into the H at this time to prevent the water leaks that are common
with many kayaks built with extrusion if this is not done. The glue (if
used) is not intended to be structural but is used as a sealant. (One
exception to this I know of is Eddyline's polycarbonite kayaks--such as the
Merlin--where this glue is what holds the hull and deck together and into
the extrusion.)
Once the parts are mated together in the extrusion several masking tape
strips bridge the seam and hold it together. Next the inside is seamed with
fiberglass tapes much as was done above with outside seams. Once the resin
cures the masking tape is removed and the seam is completed.
The strength in this H molding (extrusion) system comes from the fiberglass
tape rather than the vinyl. As with any system the better the materials and
the more material thickness used the stronger and more ridged the seam (but
the more materials used the more it will weigh too).

>3. What are the pro's and con's of each construction (or does it even
matter
to the paddler)?<

Caveat: I'm a manufacturer and we use outside seams (although early on we
made a few boats with vinyl extrusion seams).
Outside seams are much more labor intensive and therefore more expensive
(how much more depends on the amount of extra labor used and the cost of
labor where they are built). Current Designs $150 (US $ I believe) extra
reflects the extra labor costs. $200 is probably more typical for a US
manufacturers retail price difference.
As some have already mentioned vinyl seams have a reputation for being
leaky. This usually occurs near the ends of the boat where the inside seam
has been applied at some distance from the applier. The applier is using
(most likely) a brush on the end of a stick and can't really see how good a
job he is doing. If enough resin doesn't get where it saturates the cloth
and the gap where the cloth bridges to go over the inside leg of the H
(therefore doesn't leave any pin holes or bubble holes this can cause a
leak. In use the water that falls on the deck that flows down to and is held
in the channel formed by the upper outside arm of the H can get inside (if
not well sealed) This can happen either from gravity or from suction due to
air pressure changes inside the compartments (noted in the bulging in or out
hatch lid discussion). As someone mentioned the best test for this is to
stand the kayak on end and fill the end that is down with about 4 feet of
water (or up to the hatch opening). If water comes out the seams you found
your leak (but unfortunately these seam leaks can be very difficult to fix).
If this is not the source of the leak close the hatch and let the water
inside sit over the hatch area (sometimes it is the hatch seal sometimes it
is how it is mounted to the deck that leaks). Other common leak points are
where holes such as for rudder cables have been put through the deck and
sometimes it is the rudder pin hole or end toggle holes. Note: Seams are
usually leaking above where you first see water coming out.
Other differences are minor, pound for pound a well done outside seam is
probably stronger than a well done extrusion seam but this is not a big deal
because kayaks rarely break in this area. They are usually stronger at the
seams because the seams most likely are the thicker and stiffer areas of the
kayak because there is the extra material in the seams adding thickness to
this area. Serious damage to both systems is rare but would usually be
easier to fix (and a professional repair would most likely be less expensive
and look nicer) in the outside seam kayak. This is kind of splitting hairs
because few of us (we will make Doug Lloyd and Tsunami Rangers and surf and
rock garden kayakers an exception here) are going to have to deal with this
kind of damage.
(Note: someone suggested that it would be better to have a kayak break in
the seams because it would be easier to fix. This is wrong, it is much
harder to fix a break across or along a seam than breaks in other areas of
the hull or deck because of the greater complexity in the seam area. Also a
weakened seam as suggested is more likely to produce a catastrophic failure
with the hull and the deck coming partly or even completely apart. Not the
kind of thing you want to have happen out to sea or after hard contact with
a rock along a cliffy shoreline. It will be a lot harder to duct tape up
over a seam for an emergency repair as well)

>4. Are there other options for joining the hull and deck together besides
the
two I have mentioned?<

Yes but they are not much used in modern fiberglass kayaks except
sit-on-tops.


>5. Can you tell by looking at the boat how the deck and hull are joined?<

Yes if you know what to look for or feel for. For example, you can usually
get a fingernail under a vinyl extrusion to confirm it is an extrusion.You
will often see a wider dark (or seam color) line inside under the fiberglass
tapes (which is the short legs of the H) if clear resin and no cover coating
is used inside. Unsheathed rudder cables emerging from the  seam in back and
the seam area inside the cockpit area point to an extrusion seam as well.
Fiberglass outside seams will not likely have as crisp an edge if you look
closely.

Matt Broze
http://www.marinerkayaks.com



>This may be asking for more than can be described here on paddlewise, but I
would appreciate any information you  may be able to give.

Mark J. Arnold
MJAkayaker_at_aol.com<

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