METAL BULKHEADS
The bulkheads at the forward and aft ends of the engine room in both sailboats and powerboats should be constructed from the same metal as the decks but in some cases may be one measurement smaller in thickness. For instance where the decks are 3/16 inch (5 mm) you may use 5/32 inch (4 mm) steel for the bulkhead plate.
As was demonstrated in the Falklands War, aluminum can burn. For this reason, particular attention should be paid to insulating with fireproof material any aluminum bulkheads located where a fire may break out. The bulkheads that enclose the engine space, for example, will need special attention.
It’s common practice to make the bulkhead adjacent to station 0 from the same metal as the hull. This “crash” bulkhead is usually at the forward end of the waterline. Some classification societies and authorities, including boats built to U.S. Coast Guard survey and those built for sale and use in Europe, require this first bulkhead to be located 5 percent of the load waterline (LWL) aft of the forward end of the waterline. This is a sensible rule, but it sometimes takes up valuable space. Boats built to the Coast Guard survey will need the accommodation moved farther aft than would otherwise be necessary.
Some bulkheads may need stiffeners, depending on the size of the vessel, the metal used in the bulkheads, and the size of the particular bulkhead. These vertical L-angle or T-bar stiffeners are spaced about 12 to 18 inches (305 to 457 mm) apart and are installed with base of the L or T inward, thus making an excellent base for installing the cabin lining material. Some transverse stiffening also may be required. Check with the designer of your boat. The cavity formed by the L-angle can also be used to install the insulation.
Concerning those bulkheads that you’re installing before the deck and superstructure are in place: make sure that the height above the sheer or deck will allow you to cut the correct cabin top camber later. We always recommend that you don’t try to cut the shape for the cambered decks, cabin sides, and top camber at this stage; simply allow the top of the bulkhead to stand up square from the sheer. This advice applies to all metal and plywood bulkheads. Later, you can mark out the deck camber, the lay-in of the cabin sides, and the cambers for the cabin top and/or wheelhouse top. These cuts may be more difficult to make with the bulkhead in an upright position, but they’ll be much easier to mark out with all of the bulkheads in place, rather than one at a time before the bulkheads are erected. More experienced builders may prefer to mark and cut the bulkhead tops as they install each one.
If you’re building upright, and if many of the bulkheads are on a frame location, then it may be worth your while to include the basic bulkheads as part of the frame construction. Our advice is still to leave the tops square, as mentioned above. If you prefer, you may carefully work out the measurements and cabin-side angles of each bulkhead, and cut them to shape before installation.
PLYWOOD BULKHEADS
Intermediate and partial bulkheads are best built from plywood. You can use any suitable grade that has a marine glue-line. One way to test the durability of plywood is to boil it. A widely used 8-hour boiling test will give you a clear indication of its quality. Plywood provides stiffening and strength in many directions and will keep the weight of the interior down. Plywood bulkheads should be installed with the tops left square, so the areas above the deck can be marked and shaped at the same time as the metal bulkheads. If your plans do not state the thickness for the plywood bulkheads, keep in mind that the adjacent furniture and joinery will add stiffness and strength.
The transverse plywood bulkheads will need to be bolted in place, either to existing metal frames or to short sections of framing material commonly known as tags. The tags are 6 to 12 inches (150 to 300 mm) long and they’re spaced at the same intervals as their length. They’re welded to the hull to accept the bulkhead. The tags become necessary if a transverse bulkhead is located between frames, or adjacent to a frame but not at its exact location. It may be possible to alter the bulkhead’s location a small amount by bolting it to one side or other of the frame—but be careful not to create a space such as a berth that is too small or too long. You’d do better to install tags at a location between the frames. These tags provide more than adequate strength for bulkhead attachment. Don’t forget to pre-drill the tags at 4 to 6 inch (100 to 150 mm) centres to accept the bolts that will attach them to the bulkheads.
As you’re unlikely to be able to purchase plywood sheets large enough to make the bulkheads in one piece, you’ll need to join or laminate the sheets somehow. The thickness of the plywood bulkheads will vary, depending on the size of the vessel as well as the purpose and location of the bulkhead. Transverse plywood bulkheads are generally thicker than longitudinal ones. The designer of your boat may have specified the thickness required.
To form one complete bulkhead, you can use plywood of the specified total thickness and have this scarfed to the correct sheet size, or you can scarf or half-lap the sheets yourself. Our preferred method is to divide the thickness into two or more parts and then laminate two or more sheets face to face. For maximum strength, the joins can be widely staggered by alternating the joins in each layer. Plywood bulkheads, with the exception of those in the area of the mast in a sailboat, will not be exposed to great strains. The bulkhead adjacent to the mast can be strengthened by the addition of framing as required.
CORED BULKHEADS
If you are weight-conscious, you can consider one or more cored bulkheads. They can be used to divide the accommodation longitudinally, or to construct half-bulkheads such as those that form one end of a hanging locker or similar piece of joinery. The core material can be structural sheet foam, a light timber framing, or other suitable material that is both light and fire-resistant. The face plywood can be 3/16 inch (4 to 5 mm) and can be veneered with teak or a similar surface. The fiberglass bats used in house insulation are unsuitable for core material as they will soon shake down into a floppy mess when exposed to vibration and other marine operating conditions.
Bending and Installing Deck Beams
The material for the deck beams can be flat bar, L-angle, or T-bar. It makes sense to use L-angle or T-bar, as either of these, when installed with the flange down, will provide an attachment point for the interior lining material. The insulation for the deck and cabin top will fit neatly in between the angle or T-beams.
The beams can be bent using the hydraulic-jack and steel-frame method, or you may prefer to have them bent by a professional metal shop. Your plan will at least give you a camber figure—for example, 6 inches in 13 feet (150 mm in 3.96 m). If you don’t have a pattern, but you do have the numbers for the recommended camber, you’ll have to make a pattern using the formula shown. If you have patterns for the various cambers, you should make a master pattern out of plywood or timber. You will use the pattern to check the beams as they’re bent to the correct camber and also as a general pattern for cutting bulkhead tops.
In our designs, we recommend that you install the deck beams in one piece right across the hull. This method of installing the beams is much easier than trying to support short side deck beams while maintaining the correct sheer and curve of the deck/cabin-side intersection. Later, you’ll cut out the center of the beams and the section you remove will be re-bent for cabin-top beams.
You may need fore-and-aft Intercostal deck stringers, depending on the spacing of the beams and the size of your boat. The intercostals are best cut from flat bar and should be snaped in at the ends and welded in between the deck beams, as required. You could use a lighter angle for the Intercostal and have the flange level with the inside to provide a base for the lining material. The depth of the Intercostal should be the same as that of the deck beams as you may need to weld the underside of the deck plating to the Intercostal as well as to the deck beams themselves.
Once all the deck beams are in place right across the hull, then you can mark the position where the cabin sides intersect the deck, support the beams, and cut at the marked line on each beam. Next, install a carlin to accept the inner edge of the side deck plating. The carlin will be a vertical length of flat bar running around the inner edges of the cut inner ends of the deck beams.
You can, at this stage, make provision for hatches in the fore and aft decks or you can cut them out later and install any extra deck framing at that time. Once you’re satisfied with the framing for your deck, you can go ahead and plate the foredeck, side decks, and the aft deck area. Remember at this stage you are still tacking everything together; no long, continuous welding should take place until the whole structure is tacked together. See also the welding recommendations that come with your plans; usually only the outside seams are fully welded while other areas have stitch or chain welds as the final welding.
