FIBERGLASS BOATBUILDING: Hull-Deck Joints

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Sailboat deck going on hull

My last missive in this continuing series on plastic boat construction dealt with internal structures within a hull and how they help support and stiffen a boat. This time we’ll look at how the two biggest pieces of a plastic boat, the hull and deck, are married to each other.

Almost all builders these days first install a boat’s interior and then close up the hull by placing the deck, another very large fiberglass part, on top of it. Large pieces of equipment, such as engines, electrical generators, and water and fuel tanks, are also installed while the deck is off. Sometimes owners later discover they cannot remove such equipment from inside the boat, as there is no deck aperture large enough to accommodate it. A few builders–Catalina Yachts is a good example–install all mechanical equipment after decks are installed to make sure this never happens.

The bond between a deck and hull is critical. Ideally, it should include all interior bulkheads and partitions. The best practice is to tab these separately to both the deck and hull so that the finished boat is a solid unitary structure. Often, however, these important vertical structures are merely tacked to the deck (or worse, to a deck liner) with adhesive putty. In all cases, the exterior perimeter of the hull and deck must be directly joined to each other. This primary hull-deck joint is often the area most likely to leak when you are actually using a boat. Normally you only learn about such leaks sailing closehauled into a fresh breeze with the boat well heeled.

There are many different ways to engineer a hull-deck joint, particularly if you consider the myriad methods for installing caprails, toe rails, and rubrails on top of them. Their basic architecture, however, falls into three categories. The most common method on sailboats is an inward horizontal hull flange mated to an outward-facing flange or margin on the deck. Another option is the so-called shoebox joint, where a downward-facing vertical deck flange fits like a shoebox lid over a vertical lip on the hull. In the third variant both the deck and hull have outward-facing horizontal flanges that are joined together.

Hull-deck joints
Basic hull-deck joint variations

However a hull-deck joint is formed, what’s most important is how the two surfaces are joined together. Most high-quality joints these days are bedded with a tenacious adhesive sealant (3M’s 5200 compound is often used, for example) and are through-bolted at regular short intervals (1/4-inch bolts on 6- or 8-inch centers are highly recommended). An acceptable alternative is to glass over a bedded joint with fiberglass tape so that the hull and deck effectively become one part. The most bulletproof method is do all three: bed the joint, through-bolt it, then glass it over.

Preparing a deck joint on sailboat
Prepping an inward hull flange prior to installing a deck. The surface is roughed up so the adhesive sealant will adhere better

Unfortunately, many hull-deck joints are of rather poor quality. The most common practice, especially on mass-produced boats, is to bed the joint with an adhesive sealant and fasten it with self-tapping screws rather than bolts. Screws can be installed much more quickly than bolts, but they make poor fasteners when threaded into fiberglass laminate. An even worse practice–and you will find this on some boats–is to fasten the joint with lightweight aluminum pop rivets.

Otherwise the difference in terms of quality between the basic types of joints is open to debate. Some prefer the double outward-facing flange because the joint is outside of the boat. It is much easier to install through-bolts on a joint like this; it’s also easy to assure a good fit and trim away excess material. Others dislike these joints because they are more likely to be damaged in collisions. The flip side of this argument is that repairs can also be made more easily. The other types of joints, by comparison, are better protected but are also harder to install and repair, as the working surfaces are hard to access. Probably the most problematic type is the shoebox joint, as the tolerances between the hull and deck are quite fine and a close fit is hard to achieve.

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7 Responses
  1. David Thompson

    Hi wonder if you can help I have a Laser dinghy and just discovered that the deck has come apart from the hull both port and starboard midship – about 75 cms in length either side . Have you any advice on self fixing or other contacts for advice
    Thanks
    David Thompson

  2. Charlie

    @David below: I’m not familiar with the deck joints on Lasers, which are apt to be different from the joints on larger boats described in this post here. I’m guessing, however, you can easily glass over the joint on a Laser to repair the damage you describe. It would be best to glass over the entire joint, rather than just the affected area. And first get all the moisture you can out of the interior of the hull. Check online for groups of Laser owners who likely have good info on these boats. cheers, charlie

  3. mike

    I wonder if you can pass along the answers to the above two questions to me? Also, do you know, specifically, what the hull-to-deck bonding material was that was used on older phantoms and lasers? If it worked for 25 years, I’d sure like to know what it was. If you do not or can’t know, do you recommend 3m 5200?

  4. Charlie

    @Mike: I’m not sure what two questions you are referring to. Can you clarify please? And as I mentioned to David, I have no knowledge of the deck joints on Lasers, or Phantoms for that matter. For rebedding a joint 5200 is a safe bet. It’s a very tenacious adhesive sealant. Again, however, it might be a good idea to track down some Laser (or Phantom) experts to get their views.

  5. bannana man

    Leave a comment

    @Mike: I’m not sure what two questions you are referring to. Can you clarify please? And as I mentioned to David, I have no knowledge of the deck joints on Lasers, or Phantoms for that matter. For rebedding a joint 5200 is a safe bet. It’s a very tenacious adhesive sealant. Again, however, it might be a good idea to track down some Laser (or Phantom) experts to get their views.

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