|SJ23 Tech Tip H07, (Issued 2002-08-22, Bob Schimmel)|
SJ23 Hull Construction and Assembly.
|Some members have asked where to place a bilge pump in a SJ23 and others
have concerns about water freezing between the hull and the hull liner during winter
storage. In a discussion with Gene Adams (Port Gardener Sailboats),
I learned some very interesting information. Remember that Gene was
the "last 20 feet of the assembly line". He corrected any flaws in
|HOW IS THE KEEL ATTACHED TO THE
HULL - While the keel (ballast) on certain San Juan models is bolted
on the outside of the hull, the ballast of the shoal draft keel of a SJ23 is encapsulated inside the
keel stub. The
ballast consists of lead shot (pellets) mixed with resin and poured into the
keel stub. The bonding process ensures that the ballast stays in
place when a boat broaches. With this pouring technique it is
difficult to control the exact position or the density of the ballast like
with a bolt on keel. On the other hand there are no keel bolts to
corrode or leak and it
creates a water tight seal around the ballast, provided you don't run
into a rock. Compromises again!
To compensate for a slight difference in the position of the ballast between hulls, Clark equipped the SJ23 with an adjustable center board. In practice though, the position of the center board has more to do with balancing the center of lateral resistance with respect to the center of lateral force.
Sometimes the ballast sits so low inside the keel stub that it forms an air gap between the top of the ballast and the hull liner. This is great for sailing performance but this gap should be filled to prevent water entrapment, especially if you live in a northern climate. One symptom of an air gap is a stress crack around the center board lift cable attachment on the cabin sole. To test for an air gap, gently tap on the floor around the keel area. If you hear a hollow sound, you have an air gap. The fact that it sounds hollow is the indication that it is dry down there, which is good. Water inside this void is definitely not a good thing in a freezing climate. Tests should be made to determine the size and depth of the void. On Panache I drilled a 3/8" hole through the floor, just behind the pedestal, and filled the void with cold cure epoxy using a tiny funnel. The void required 1/2 litre of resin to fill. It goes without saying that if the void is filled with water, it must be pumped out and dried before pouring in the resin.
HOW IS THE HULL LINER ATTACHED TO THE HULL - After the keel is cured (and cooled) in the manufacturing process, a thick layer of mush (lightly catalyzed putty called Thicksol) is sprayed over the inside of the hull in the area where the liner will sit. Then the liner is lowered in place and a vacuum pump attached over the liner so air pressure compresses the liner uniformly to the hull. Any excess mush is squeezed out along the edge of the liner. The fact that it squeezed out is verification that the bonding process has worked. The vacuum is maintained for 24 hours till the whole works solidifies to a cured state. If done correctly, all voids are eliminated, creating an extremely strong hull to liner bond. If you look inside the cockpit lockers, near the front, you might see some of the cured mush that oozed out. Oozed out mush is quite weak on its own and can be easily broken off. Mush that adheres between two inside surfaces creates a very strong sandwich. (Gene thinks the mush may go by the trade name of Fix-All. It consists of resin and filler mixed to the consistency of mayonnaise).
WATER BETWEEN THE HULL & HULL LINER -
It's a fact of life that eventually water will get inside the hull from
leaks, condensation, spills or replacing the knot meter impellor while the boat is
floating. A popular location for the impellor is just forward of
the keel, under the head. While the impellor performs well here, a
lot of water can easily flow under the liner from where it is difficult
HOW IS THE DECK ATTACHED TO THE HULL - The deck is sealed to the hull flange with butyl rubber and mechanically fastened with machine screws through the toe rail. The toe rail adds a tremendous amount of reinforcement to this joint. The hull flange is actually an extension of the top of the hull, turned inwards to create a lip that the deck rests on. It is quite substantial, having a minimum thickness of 1/8". A bead of butyl rubber is laid on top of the flange and seals the joint when the deck is lowered and aligned to the hull. A few pop rivets are judiciously set in place to lock the alignment of the deck to the hull for attaching the toe rail. Finally the toe rail is bolted on, starting at the stern, going forward.
NOTE: The assembled toe rail and hull to deck seal is shown at right. However, the picture is from Tech Tip B26 and shows my improved and stronger bolting technique, not the factory original using the speed nut. Go to Tech Tip B26 to see the Clark's bolting technique.
DECK CONSTRUCTION - The deck is a sandwich construction using edge grained balsa in the core to save weight. The majority of the cabin top is balsa core except under the mast step. Plywood is used to reinforce the decks where a stanchion or winch would be mounted. You will find plywood under the aft part of the deck as well. Plywood was the material of choice at the time to create compression strength and save weight.
OVERALL HULL WEIGHT -
weight of a San Juan 23 is just under 3000 pounds. The cabin and the
interior pan were made with a chopper gun that sprays resin and glass. It is difficult to maintain a uniform thickness with a chopper gun
unless you have the touch of Rembrandt. If you have such a touch you do
not need to build boats! The result is that boats varied widely in
weight. The extreme was the last boat I worked on, which weighed 4000 pounds,
empty. This was 1000 pounds over weight, which is why the
people who built it did not stay in business long. As a dealer I got to
pay for the second axle on the trailer. The 3000 pound design weight is
right at the upper edge of what a single axle trailer can support. If your
boat happens to be on the heavy side, you need two axles." Gene
HULL ASSEMBLY -
Regarding the assembly of the
pieces below the sheer line.... There are three sections to a SJ23 hull. The hull, the centerboard trunk, and the
pan, which together make the shiny finished fibreglass boat that you see.
The first step in building the boat is to spray gel coat into the hull
If a stripe is required then a
striping dose is applied as a separate gel coat color. Then the hull
is laminated, which consists of layers of woven roving that are engineered to handle
the hull stresses. Call Don Clark on that. The hull is
hand-laid, using woven roving with chop-strand mat between the layers of
roving. Chop-strand mat has a bad rap because those who know little do
not understand that a bit of chop-strand mat really helps to interdigitate
layers of roving to make a stronger laminate. You can drive a wedge
between the layers of a pure roving laminate to separate them; not so when chop
is bonded between the layers. Chop makes very good stickum. However, you need a skilled chopper gun operator and
chop is monitored by weight at each lamination. We were very
attentive to this monitoring.
The centerboard trunk part was made in the "small parts" department, on its own mold, and bonded into the hull part by the assembly crew. The centerboard trunk part also includes the table leg, which is hollow to accommodate the centerboard cable. If you feel the surface up inside your centerboard trunk, you will note that it is a smooth, finished surface, which means cured and water impermeable.
The interior pan is a very large piece about 18' long, that includes the v-berth and all the way aft to the quarter berth, with bumps and burbles to meet up with bulkheads and flanges to bond with the hull. The pan is very important in creating longitudinal stiffness in the hull. Surfaces such as the settee vertical sides (back rest) add a lot of structural strength, as well as making a place for the kids to kick.
One of the neatest things about how the boat is assembled is the vacuum process that bonds the pan to the hull. The sandwich gives the dual benefit of providing a finished floor surface as well as added stiffness. All the pieces here serve dual purposes, while minimizing weight. Thank you Don. The inside of the hull is cleaned to get rid of bumps and stuff and a bed of chopped strand mat is shot on the surface that mates with the pan. The pan is craned into the hull and a vacuum hose is connected to the floor of the pan through a hole. The vacuum is turned on and a micro balloon putty is spread around the edges of the pan inside the settee lockers to seal the edges. Suddenly there are no pathways for air to flow to the vacuum hoses and "thwunck!" the pan is sucked down to the hull and is bonded. The vacuum is eased when the resin gels.
Anyway... this description should be corroborated by the
construction manuals. Gleno.