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Exterior Deck and
Cabin Beams

by Paul Butler
illustrations by Marya Butler

Exterior beams, rails, and moldings have many uses on small boats. They reinforce decks and cabintops and still allow a smooth surface on the underside, which means a cleaner look below, more room, and less head banging than interior beams. Exterior beams also make the interior much easier to finish. On small hulls such as dinghies and dories, girder-type exterior beams serve as bottom protection and reinforcement, tie-down and lifting handles, and safety handholds (Fig 1).They can be installed on wood, glass, or metal hulls.

Figure 1

There are various types of exterior reinforcements, from small toerails laid across the deck to massive laminated girder-type beams that can provide very strong support, such as near a mast. Exterior reinforcements can also be in the form of traditional grabrails that provide support with light weight and are useful for tying off oars, dinghy fenders, and other gear. These deck and cabintop beams also provide good footing and are particularly useful at night to help in locating yourself on deck or cabintop.

Exterior beams may be located fore and aft, athwartships, or diagonally. Fore and aft beams are more traditional. They are often used for stowing oars and boathooks on deck, or routing rainwater for catchment. They can also route sail control lines to the cockpit, and rigged with lengths of shock cord and tie-downs, they are very handy to hold the various odd and assorted pieces of gear necessary on small boats.

Athwartships beams are more out-of-the-ordinary but probably provide better reinforcement to the deck and cabintop since they usually extend from one sheer clamp or carlin to the other. Though not as aerodynamic, when properly designed and built, they look appropriate on most modern hulls and can be very convenient for storage. They are also sometimes easier to build than other types.

It is often possible to design exterior reinforcements to accomplish multiple jobs. We once built a set of girder-type beams laid athwartships across the cabin-top. They had holes cut for stowing two large sculling oars and a long boathook, and also acted as a cradle for the hard dinghy carried upside down atop the cabin. The dinghy sheltered a ventilation hatch in the cabintop and also protected the varnished sculling oars from sunlight and weather.

Girder-type beams are much stiffer and can be less than half the weight of conventional solid beams. While their solid top and bottom provide great strength, the center is cut out at intervals to lighten the beam and to form tie-offs or hand-holds. When planning to build such a beam, consider the potential uses and make the cutouts large enough to accommodate a gloved hand, four Fingers of which should slip easily through the holes.

There are two different methods for building exterior beams, both of which have worked well for us — lamination, which involves epoxy gluing thin strips to build up the beam to required size, and sawn beams.

Laminated & Sawn

Decks and sweeping complex shapes make good use of laminated beams, which can be built from smaller strips and pieces to save money and can be made to conform to just about any shape (Fig. 2).
If you plan to build only one or two beams, it will probably be easier to glue them right in place on the boat than to build a laminating form or strongback.

Figure 2

If a number of beams are needed and the deck or cabintop maintains the same curvature, then it may be worthwhile to build all the beams on a laminating form. One like the bracket form shown can be easily changed for various shapes and can also be used to build beams right from the lofted lines of the boat (Fig. 3). This ensures accuracy and allows the beams to be built before the boat is completed and put in place later.

Figure 3

We lay a sheet of plastic over the boat or form to prevent the beams from sticking, then remove the beam for finishing when the epoxy has cured. PreFinishing "on the bench" is always faster and produces superior work. After all possible finishing is completed, we glue the beam in its proper location.

When laminating beams, make the laminations thin enough so that you don't need excessive force to hold them in place, but don't make them so thin and flimsy that too many glue lines are required. Most deck cambers will take 1/2-inch or 3/8-inch laminations, but radical shapes may require using laminations as small as 1/8-inch veneer, which will conform to a very tight radius.

After laminating the entire beam, remove it from the cabintop, deck, or laminating form and clean off the sides by planing and sanding. Laminating with strips slightly wider than needed in the finished beam allows us to clean off the glue drips and still end up with the proper width. The top edges should be rounded with a 1/2-inch or larger radius to make them safer on deck should you fall on them. Also, make any cutouts at this time using a sharp blade in a jig saw, and round the edges of the holes using a router bit or wood rasp.

Beams may also be tapered from top to bottom lo save weight and achieve a less bulky look. Tapered beams can look very clean and finished where parallel sided beams often look bulky and clumsy.

Plywood makes the best sawn beams, especially if there is an extreme or unusual shape to which the beam must conform, since optimum strength is provided by the multiple opposing laminations of veneer in the plywood, lf there is minimal shape in the beam, wide lumber will also work well, but plywood is preferable for maximum strength to weight. We build plywood beams by cutting a pattern to shape, scribing it right to the deck or cabintop location. Then we cut out and glue together as many thicknesses as necessary for width, finally finishing it, and attaching it to the deck.

Permanent & Removable

If you plan to attach the beams to the boat using bolts through the deck or cabintop, you can use those same bolts for clamping pressure while laminating and then for permanently attaching the beam in place. A fastening alternative for smaller beams is to simply epoxy glue them in place, after which all fastenings can be removed.We routinely glue wood to gelcoat and fiberglass laminate using WEST system brand epoxy. Both sanded and cleaned surfaces are wet out with catalyzed resin, then a thickened mixture of epoxy is applied to one of the mating surfaces. This provides a bond so strong that we routinely remove all fastenings.

If you want the beams to be removable, they can be bedded and bolted in place. Carriage bolts work well. Countersink the heads and seal the holes with wood plugs bedded in silicone. Removing the plugs for access to the bolt head or nut will probably destroy the wood plug, but they should be renewed each time anyway. To easily remove such a plug, drill a small pilot hole through its center, then insert a wood screw into the hole and tighten. The screwwill enter the plug and bottom out on the head of the bolt. As you continue to tighten it, it will lift the plug right out of the hole. For permanently attached beams, we also use epoxy fillets on the edges (Fig.4). This seals the edges of the beams from moisture, looks good, creates a stronger bond between the deck or cabintop, and increases the bearing area of the beam. These fillets look very good after they are painted, and the beams seem to grow right out of the boat.

Figure 4