sailboat beating to windward

Holding it all together: The Hull/Deck joint

It was a blustery December day in 1999 and we were bashing our 1962 built sailboat to windward across the Mona Passage on our way to Puerto Rico from Samana, Dominican Republic. Waves smacked the sides of the hull and broke across the low freeboard of our bow as we motor-sailed as close to the wind as possible with the little 2 cylinder raw-water cooled Volvo banging away below. We were used to going to windward by this time, it felt like the only point of sail we knew, but the confused cross seas made this passage a little rougher than most. I got a bit of shock when I stepped down into the cabin and noticed water on the floor and flowing from the bow towards the aft bilge. There’s never a good time to bring the ocean inside your boat, and when you’re out of sight of land it’s a particularly bad time to be hunting for leaks. When I reached the v-berth where we normally sleep and felt the completely soaked mattress I had a pretty good idea where the water was coming from, along with the realization that there was nothing I could do about it now. It was the hull to deck joint letting water in, and while it wasn’t going to sink us it was enough to make an unpleasant ride a little bit more unpleasant.

Now that I’m a marine surveyor I have a special fascination with hull to deck joints. It’s an incredibly important part of boat building that’s usually completely ignored by most boat buyers, yet can tell a story about the builder in a way that few other areas of the boat can. There are three main types of hull/deck joint and several different ways to secure each of the three. There is the external or outward flange, the internal flange, and the overlap or “shoebox” type of joint.

External Flange hull to deck joint
External flange hull to deck joint (drawings courtesy of GoodOldBoat.com)
Inward or internal flange hull to deck joint
Inward or internal flange hull to deck joint
Shoebox, or overlap type of hull to deck joint
Shoebox, or overlap type of hull to deck joint

Normally an elastomeric sealant or methacrylate adhesive is used between the two pieces of fiberglass to bed and secure the connection. Then either sheet metal screws or through bolts are used to tighten and hold the joint together. For what should be obvious reasons through bolting with backing plates or washers is always the better choice but that usually requires two workers, one on the outside and one on the inside to secure all the bolts and nuts while the sheet metal screw is quick and cheap. The problem lies in that fiberglass does not hold sheet metal screws well and they have a tendency to work themselves loose. Some people will take pliers and bend the tips of the sheet metal screws to make it more difficult for the screws to work themselves loose.

Shoebox, or overlap type hull to deck joint with sheet metal screws
Shoebox, or overlap type hull to deck joint with sheet metal screws

The external flange hull/deck joint is another ‘ease of production’ decision that builders make to cut costs but as always there is a price to pay for that cost savings. With an external flange the builder can just leave a little excess fiberglass on both the hull and the deck pieces, one person can through bolt the joint from the outside, cut them flush after they are connected, slap some rubrail or toe rail on it and call it a day. But then the external flange joint takes the brunt of any docking mishaps as that external piece is always the first to hit. See the photo below for a typical worn external flange hull/deck joint.

Crack along external hull to deck joint from impact damage
Crack along external hull to deck joint from impact damage

The internal flange is a common method, and often one of the most secure. To through bolt the joint it takes one worker inside and one outside working in unison, not always an easy feat. The inward flange of the hull provides a ‘shelf’ for the deck to lay on and be secured to.

Inward flange hull to deck joint, notice the vertical through bolts
Inward flange hull to deck joint, notice the vertical through bolts

The best way to secure a hull/deck joint

The one characteristic that separates the ‘just ok’ hull to deck joint from a well done, secure hull to deck joint is adding a layer of fiberglass to the inside of the connection. This can be done to any of the 3 types, but most commonly seen on the overlap or ‘shoebox’ type joint. Sometimes this glass is added before the rubrail and screws are driven through the connection and sometimes after. This additional layer of fiberglass both strengthens the joint and adds another barrier to water getting through. It’s an added effort and cost to the builder, and is done before the bulkheads and tanks are installed, but is commonly seen on high-end, quality boats.

Glassed over hull to deck joint
Layer of fiberglass added over the hull/deck joint to strengthen and increase water tightness. Notice the screw/bolt heads sticking through the glass in a row above the arrows. Don’t mind the mildew, this is a typical Florida boat anchor locker.

How do I know what type of hull/deck joint is on my boat?

Some types, like the external flange, are apparent from the outside of the boat. For others the best place to sight the hull/deck joint is usually in the anchor locker and in cockpit/lazarette lockers and looking upward at the joint. Here you’ll see if it’s screwed or bolted, fiberglassed in or not. If you see vertical fasteners than you are most likely looking at an inward flange, and if you see horizontal fasteners from outside you are probably looking at an overlap, or ‘Shoebox’ type of hull to deck joint. Many of today’s boats are built with large grid liners installed inside the hull that hide/prevent access to the hull/deck joint.

Next time you’re on your boat, or maybe you’re shopping for one, pay a little more attention to what is holding the deck on to the hull and how that connection is made. Are you looking at a top-tier boat builder who is trying to make the best connection possible, or a mid-level builder who is trying to increase production and lower cost?

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