SJ23 Tech Tip H07 - Hull Construction and Assembly

SJ23 Tech Tip H07, (Updated 2024-03-09) Bob Schimmel, Gene Adams, Glen Moore.	Index
SJ23 Hull Construction and Factory Assembly. INDEX - Keel attachment, hull liner, water under floor, deck, deck construct, weight, hull assembly at factory.
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) and Glen Moore (Clark Boat Company), I learned some very interesting information. You should know that Gene was the "last 20' of the assembly line". He corrected any flaws in a hull that was tagged with a defect. Glen worked inside at various jobs.
HOW IS THE KEEL ATTACHED TO THE HULL - While the keel (ballast) on certain San Juan models is bolted to the outside of the hull, the ballast of the shoal draft SJ23 is encapsulated inside the keel stub that is part of the hull. The ballast of the first production hulls consists of lead shot (pellets) mixed with resin poured into the keel stub. Bonding the ballast to the hull ensures it stays in place when a boat broaches. In later hulls (don't know the serial number) lead castings were placed in the keel stub and similarly held in place with thickened resin. With either technique (especially lead shot) it is difficult to control the exact position or the density of the ballast as is possible with a bolt on keel. On the other hand there are no keel bolts to corrode or leak and the stub creates a water tight seal around the ballast, provided you don't crack the fibreglass by running into a rock or thump the hull down on something hard. Compromises again! To compensate for a slight imbalance in the fore/aft position of the ballast, Clark equipped the SJ23 with a fore/aft 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. The factory placed the board in the center hole. On some SJ23s the ballast sits so low inside the keel stub that it leaves an air gap above the ballast, between it and the hull liner. This is great for sailing performance but this gap could be filled to prevent water entrapment. Water inside this void is definitely not a good thing in a freezing 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 above 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. A test could 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 poked a stick down till it bottomed out. I filled the void with cold cure epoxy poured through a tiny funnel. The void required 1/2 litre of resin to fill. It should go without saying that if the void is filled with water, it must be pumped out and dried before pouring in the resin. Another thought on this void. BALLAST - In the original hulls the ballast consisted of lead shot mixed with thickened resin poured into the keel stub adjacent to the center board trunk. In subsequent hulls the ballast consisted of lead castings placed in the keel stub with thickened resin poured in to prevent movement. 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 Q-cell) is sprayed inside the hull, covering the entire area where the liner will rest. 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 liner is bonded completely, ensuring maximum strength. 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 easily break off. Mush that adheres between two rigid layers creates a very strong sandwich. (Gene Adams says the mush consists of resin and filler mixed to the consistency of mayonnaise). REMOVE WATER BELOW THE HULL LINER - It's a fact of life that eventually water will get inside the hull from leaks, condensation, spills or pulling the knot meter impellor while the boat is floating. A popular location for the impellor is below the head, just forward of the keel. While the impellor performs best here, a LOT of water will flow inside the hull under the liner from where it is difficult to remove. This is the reason why I no longer remove the impeller with Panache floating. On Panache I enlarged the hole through the floor below the head so my arm can just reach aft to the edge of the mush by the compression post. This allows me to soak up most of the water with a sponge. Regardless of how the water enters the hull, all of it will flow to the edge of the mush that bonds the liner to the hull. While the closed cell foam mush can't absorb water, the edge of the mush is NOT sealed against the ingress of water. If the mush has separated from the hull or liner from pounding the hull too hard into steep waves, the water will fill the gap, where it will be "impossible" to remove. Its the understatement of the year that recommends removing this water prior to freeze up! So.... "If you are concerned about water freezing between the mush and the hull, a possible solution is to drill a 3/4" diameter hole through the floor liner, just forward of the table pedestal. This is the lowest point on the curved floor and is directly above the ballast. Drill very carefully, about 3" deep, stopping at the top of the ballast. DO NOT drill into the cured resin that seals the top of the ballast." Gene Adams. Assuming the space above the ballast is filled with foam, the 3/4" hole will become a tiny "bilge," that water can flow into. Use a small tube connected to a suction pump to remove any water. Eventually you will remove all the water, leaving the area under the liner dry and OK to freeze. Once the "bilge" is dry, plug the hole with a thermos bottle plug to prevent spilled water from flowing in. A removable thermos bottle plug allows you to conveniently monitor the "bilge" for new water. 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 tightened to speed nuts below the hull flange. 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 in itself, having a minimum thickness of 1/8". A bead of butyl rubber is laid on top of the flange and seals the joint with the deck resting on the hull flange. During assembly a few pop rivets are judiciously set in place to lock the alignment of the deck to the hull so the toe rail can be bolted on with accurate placement. Bolting starts at the stern and ends at the bow. I wish I could report that the toe rail is bent to conform to the hull, alleviating stress, but that is not the case. I discovered this when I tried to replace several toe rail screws and the end of the rail started to pop out. So I stopped and pulled it back into place using long bar clamps and lots of lumber. This is a lot of work on your own. NOTE: The toe rail and hull to deck joint and seal is shown at right. However, the picture shows my improved and stronger bolting technique of aluminum washers and nylock nuts, not using the factory original speed nuts. See Tech Tip B26. DECK CONSTRUCTION - The deck is a sandwich construction using 1/4" plywood in the core to save weight. The majority of the cabin top is balsa core except under the mast step. Solid fibreglass is used to reinforce the deck 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. HULL WEIGHT - "The design weight of a San Juan 23 is just under ~2700 pounds. The cabin and the interior pan were made with a chopper gun that sprays resin and fibreglass. 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 one of the reasons why the people who built it did not stay in business long. As a dealer I got to install the second axle on the trailer. The ~2700 pound design weight is right at the upper edge of what a single axle trailer can support. If your boat happens to be slightly heavier, you need two axles." Gene Adams.
HULL ASSEMBLY at FACTORY - "Regarding the assembly of the SJ23 hull. There are five major sections; the hull, the centerboard trunk, the ballast, the interior pan, and the deck that together make the shiny finished fibreglass boat you see. HULL - The first step in building an SJ23 was to spray gel coat into the female hull mold. If a stripe was required then a striping dose was applied to the mold as a separate gel coat color. The hull was laminated over that, consisting of layers of woven roving, the thickness of which was selected to handle the hull stress. The hull was hand-laid, using woven roving with chop-strand mat (fibreglass mat) between the layers of roving. Chop-strand mat has a bad rap because there are many who do not understand that a bit of chop-strand mat really helps to interdigitate the layers of roving to make a strong laminate. You can drive a wedge between layers of pure roving laminate to separate them; but not so when chop is bonded between the layers of roving. Chop makes excellent "stickum". However, you need a very skilled chopper gun operator. The chop was monitored by weight at each lamination. The assembly crew was very attentive this. Don Clark was responsible for quality control, material cost and saving labour. CENTERBOARD TRUNK - The centerboard trunk was made in the "small parts" department, on its own mold, and bonded into the bottom of the keel stub by the assembly crew. The centerboard trunk also included the hollow table pedestal that accommodates the centerboard lift cable. Feel the inside of a centerboard trunk and you will notice it has a smooth, finished surface. This means it is impermeable to water which is sort of important since the water rises inside to a about a foot above the cabin sole. BALLAST - Once the trunk bond was cured the lead ballast was added in the keel sump. The first production boats received a mix of lead shot & resin that was poured in and later topped off with resin to seal it. Later production boats received two precast lead pigs that were placed in the keel stub on either side of the centerboard trunk. The pigs were secured by pouring a resin/thixotropic mix (Q-cell) to bond them in place. INTERIOR PAN - The keel was capped with the installation of the interior pan (hull liner) which is about 18' long. It includes the V-berth, the main cabin and the quarter berth, with bumps and burbles in between that meet up with the bulkheads and flanges that bond to the hull. The pan is very important to create longitudinal stiffness in the hull. - One of the neatest things about the assembly of an SJ23 is the vacuum process that bonds the pan to the inside of the hull. The inside of the hull was smoothed to get rid of bumps, then cleaned and finally a bed of chopped strand mat (fibreglass mat) shot on all the hull surfaces that mate with the pan. A vacuum hose was connected to the floor of the pan (by the pedestal). The pan was craned to the hull, aligned over the pedestal and lowered inside the hull. When the pan was down, the vacuum was applied, spreading the micro balloon putty to the edges of the pan, sealing it to the hull. As the vacuum built up, suddenly there was no pathway for air to flow and "thwok!" the pan was sucked down tight to the hull, bonded in place. (You can see the oozed out putty inside the settee lockers). The vacuum was eased when the resin gelled. - After that the unfinished area around the base of the pedestal was smoothed over and "painted" with interior gel coat to match the final finish. By the base being raised, there was an anterior chamber for the tip of the vacuum hose to be secured during the vacuum procedure. To watch it happen was rather crude: the vacuum hose was a 1" or 1 1/4" black poly pipe. Once the area around the hose tip was sealed with a gob of butyl rubber all that air was sucked from between the interior pan and the hull. The workers then made a batch of micro-balloon putty ( Q-cell filler) and formed a dam around the pan perimeter (the base of the settees and the edge of any pan surface that contact the hull). This blocked air entry between the hull and pan, and the famous "thwok" followed shortly after. I was easily entertained in those days, and I always thought the process was pretty cool. The pressure that is achieved by sucking is much more effective and uniform than piling on a bunch of lead shot ballast weights. - The sandwich construction technique creates the dual benefit of a finished floor with added stiffness. All the pieces here serve a dual purpose while minimizing weight. Thank you Don. DECK - And finally the deck was installed on the hull, sealed with butyl rubber and thru-bolted to the toe rail for strength. 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. Anyway... this description should be corroborated with the construction manual." Glen. BULKHEADS - I presume the bulkheads and teak trim was installed in a completed hull. Bob. ------------------------------------ SJ23 CONSTRUCTION MANUAL ---------------------------------------- To Bob: "An SJ23 construction manual would be very helpful if I could find one. I have some change drawings, but no complete manual. There must have been something in the shop, but I have never seen one. I talked to the people in the shop but they never gave me one. A great deal of knowledge about how to build these boats was in their heads. I am starting to put together a manual to pull as much information out of my head and get it on paper. When I have a finished manual I would be happy to share it with you. You have done a great job of documenting details of the boats and you have answers I don't have." Gene Adams. PS: I never received this manual from Gene prior to his retirement. I show you this dialogue to demonstrate the processes during the 1970s and how difficult it is to get accurate answers when documentation was a low priority.
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