It’s usual to assemble the frames over patterns that have been lofted by the builder or supplied with the plans. As mentioned earlier, you may prefer to use a plywood or steel area for this purpose. Make sure the area is level and that it will provide a firm base on which to assemble the frames.
A boat has two basic types of framing: transverse framing, generally referred to as the frames, and longitudinal framing, usually known as the stringers, which also includes chine bars, deck stringers, and the like. Here we are discussing the frames.
In metal boats, transverse-framing material may be flat bar, L-angle or T-bar. Your plans will most likely stipulate which is appropriate. For many years, flat-bar frames have been favoured in steel boats. Many designers have given this advice. The reason usually quoted is that L-angle is hard to protect from corrosion, and that the angled portion adds unnecessary weight. More recently, however, we have considered angle in a more favourable light.
Against the above objections, an argument can be made for angle. The flange will provide an excellent place to attach the lining material. Corrosion problems can be overcome by using prime-coated materials and sprayed-in foam insulation, which is now common practice in metal boat hulls. Regarding the extra weight of angle, I believe that this is not a problem in larger and heavier displacement boats. All that has been said about angle can also be applied to T-bar frames. Aluminum boats will have transverse frames made of angle, T-bar, or a proprietary extrusion that has some type of bulb or flange.
Some builders may prefer to have the deck beams included as part of the original frame construction. If you prefer this arrangement, you’ll find it’s best used when you’re building upright. Also, including the beams at this stage may interfere with the installation of the larger and heavier items (such as the engine, etc.) in the hull at a later stage of the construction. If you are building the hull inverted you will find that the deck beams interfere with access under the boat. My experience is that the deck beams are best installed after the hull is fully plated and already turned upright and the heavier items are already installed in the hull. It is easier to check for a fair sheerline before installing the beams. In some cases—for instance, if your boat has a bulwark—this last objection may not apply, To summarize, if you’re building upright, then you may consider installing the deck beams as part of the original frame, but if you’re building inverted, don’t install the beams until after the hull is upright and preferably with the engine, etc., already placed inside the hull.
After you have established how many frames you’ll need, and which material you’ll be using—L-angle, flat bar or T-bar, it’s now time to start cutting the correct lengths of material to form each frame. An angle grinder fitted with a suitable wheel can be used for cutting the frame material to the correct length and angle. Some builders prefer to use their oxyacetylene equipment for making these cuts, and no doubt you have your own preference. Cuts made with the angle grinder are more accurate and will be preferred by many builders. A neat trick is to make up strips of cardboard as templates for the angle joins on the frames. Use cardboard that is the same width as your frame material. Lay two cardboard strips directly over the joint on your patterns, ensuring that there is sufficient overlap to allow you to cut through both layers of cardboard using a straightedge that bisects the angle. You have now created a pattern that forms the angle required for both parts of the framing material. Transfer these angles to your lengths of framing material and now you can neatly cut each angle to provide the basis for a perfect join. You may prefer to use a carpenter’s bevel-gauge or a plastic protractor to obtain the correct angles. You can always clean up your angles by using the grinder, but it’s preferable to make the correct cuts in the first place.
Next, tack-weld the frames together. After checking against the patterns, make the final welds. It’s worth noting that a very small incorrect angle at the chine can become a large error at the sheer or keel. The frames may be made up in two halves, or one half on top of another, and then opened up like a clamshell to form the frame. You must carefully check the fully assembled and welded frames against the patterns and one half against the other. Accuracy is vital at this stage. It’s not a good idea to tack the various parts of the frame to the steel master plate or loft plate. Frames assembled in this manner can have built-in tension that will cause them to change shape when released from the loft-plate floor.
A good way to avoid distortion is to follow the same sequence for assembling each frame. For instance, place a tack-weld at the center of each angle joint and let it cool for a few seconds before tacking either ends of the angle. Several frame sections can be done in sequence, ensuring that minimum time is lost through waiting for welds to cool, before proceeding to the next step in the assembly process. The object is to keep the job moving forward, without setting up stresses in the frames, and avoiding unnecessary delays in the work schedule.
Once you’ve tacked the frame together, you should be able to move it about and check the accuracy against the master patterns that have been scribed on the metal or plywood loft floor. When one side of the frame is tacked together, you should turn it over and tack the other side. Again, check the accuracy against the master patterns.
You’ll be installing some form of headstock across the frame. This headstock may be used to support the frame on the strongback or bedlogs (see below). Make sure you install other bracing between the headstock and the sides and bottom of the frame, otherwise it will be too flexible and impossible to set in position on the strongback.
Mark all of the important reference points on all frames. Include such points as the load or datum waterline (LWL or DWL), the sheerline, the deck line (if this is below the sheer), and any other points indicated on your full-size patterns. Finally, please follow the designer’s specifications for making your frames; never overlap the ends of the frame bar where they join, in the misguided belief that you’re making the boat stronger. Overlapped metal can harbour moisture and promote corrosion. It also adds unnecessary weight and looks unsightly, as well as giving your boat an amateurish appearance. On the same theme, don’t add extra reinforcing plates or permanent gussets at the frame joints; these items were necessary for frames in wooden boats but add extra unnecessary weight in a metal hull.
There are several ways to make the various cuts in the frame to accept the stringers, deck shelf, and sheer stringer. One method is to divide each area between the chines into equal spaces and, using a square, mark in a notch for each stringer. These notches may then be cut while the frame is still on the loft floor. If you prefer this method, it may be better to cut the notches before tacking the frame together; cutting the notches will probably distort the frame part, so this is best corrected before you assemble the frame.
We recommend standing up the frames and then marking in all of the stringer locations on the frames, using a batten to simulate the fair curve of each stringer. The next step is to take the frames down and cut the slots. Finally, check each frame for accuracy before reinstalling it in its correct location. This method is time-consuming but it does ensure that you get a fair set of stringer notches and, in turn, a fair set of stringers. This method also makes sure that the final frame is still the shape intended by the designer, and in due course it will contribute to building an attractive and fair hull.
