Lofting Part 1
Lofting is drawing the lines of a boat full size and using those lines to develop molds and patterns. The process helps ensure a boat is accurate in its layout and pleasing in appearance. Drafting, discussed in week 5, is drawing the lines of a boat at scale. The beginning steps of lofting and drafting are identical, however drafting covers only 30% of the necessary steps to loft a boat. A drafting, similar to the one we completed of the “sixteen”, contains all the views to communicate the 3d shape of a boat (to the outside of planking) in a 2d format. At the end of the day it is an exercise that helps prevent us from making a crappy boat! The process of lofting can be applied to other craft that have fair lines, such as airplanes.
There are many methods of lofting, plenty of text to read and videos to watch. Lofting is a great way to teach boatbuilding because its a set of steps that can be laid out in a methodical manner. We aren’t learning to build boats by eye. The boatbuilding method we are learning is called the Prothero method (it may also be considered the “west coast” as well but don’t quote me). It was developed by Bob Prothero, one of the founders of the Northwest School of Woodenboat Building in 1981 (check out Wooden Boat #249 for an article with more detail on the Prothero method). One of the key elements of this method is that it relies on an accurate lofting. We spend our time up front, making sure everything works out before cutting one timber. Pencil lead and 1/8” plywood is much cheaper than boat building materials.
At the time of this writing my teammate and I have not completed the lofting sequence. So I am dividing up lofting into 2 parts (hopefully it will only be 2 parts). A cliffhanger!
To start we cleared the floors in the shop and stapled down 1/8” plywood and painted it white. We are working in teams of two and our shop has 9 loftings going on simultaneously. Each team develops their lofting within a 6 ft x 20 ft space. Once the paint is dry we lay out our grid and start placing lines of the boat in the same sequence as the drafting exercise. There are a few take aways for me that make this process unique. The selection of batten is important for developing fair lines, in drafting we had one plastic spline that we used for all but the tightest curves. In lofting you pick out battens (long pieces of wood at various cross sections) to draw your curves. The larger the cross section the more fair a line but it may not be able to make all the curves. Smaller cross sections can make the curves but may not generate a sufficiently fair line. I was also introduced to the bundle batten, a number of thin wood strips, riveted together at one end. The strips slide against each other allowing it to make tight curves. This batten was a necessity when laying out the stations in the body plan. The general rule is to use as stiff a batten as possible to make the curve. I also gained a better understanding of the usefulness of the unique numbering system used in the table of offsets (table that identifies the points for many of the lines). Each point is 3 numbers, 3-1-2 for example, which represents 3 feet, 1 inch and 2 eights of an inch. Weird right? Well when you are in a team working to plot points its easier to call out 3 numbers than some combination of empirical measurements. I am not going to bring up the metric system … but I should. The battens are all held in place when drawing the lines with finish nails. You can’t just place the nail on the point because the batten will be in the wrong place and your line will be incorrect. The nail has to be offset slightly in the correct direction, another skill to learn.
Once the lines are drawn and proven out we are 30% complete. Next we move into the developments. These are the steps that generate the lines for patterns and molds. The first development is the rabbet. I talked about the rabbet in week 3 when I made a stem for an imaginary boat. The rabbet is a groove that runs along the length of the backbone and accepts the planking. The plan for the sixteen only provides the location of the rabbet line. From the exterior of the boat the rabbet is where the planking and the backbone meet. The planking is recessed into the backbone (check out carving the rabbet in week 3) and we need to understand angle and depth of the groove that accepts this planking. These locations are represented by the back rabbet and bearding lines. To further complicate the development this boat is designed with a split keel which means the backbone is split into 2 pieces, the keel and the apron, at the back rabbet. This method is easier for construction (the rabbet only needs to be carved at the stem) but requires more lines in the lofting. Depending on the angle at which the planking meets the boat we need to use various geometric gymnastics to determine these additional lines. There are a few different methods but the most challenging is when the planking meets the backbone at an oblique angle. In this boat that is when the back bone turns into the stem and transom. To develop the lines we make multiple projected views where know dimensions can be used to find the necessary points. These points are translated back to the profile view and then the body plan (spoiler alert we use these lines to make the molds). Like I said gymnastics.
The next developments are mold reduction and transom rollout and will be discussed in more detail in part 2.
Lofting is a mix of repetitive tasks, head scratching geometry and trouble shooting past errors. I am starting to learn the relative importance of all the lines and points, some are more important than others. It really is a chicken and the egg situation, if I had built a boat with traditional methods already this lofting would make more sense. However, if I was building a boat having not lofted it I wouldn’t know the boat as well and I could make costly or time consuming mistakes. I am trying to focus on process and not just pushing to get it done. The better the lofting the better the boat. But I want to build a boat!!
Half Model
We built a half model of the sixteen. Half hull models are used to show the lines of a boat in a three dimensional format. Half models, along with being cool to look at, plan a ship’s design ensuring it meets the designers vision. The process starts with identifying the waterlines in the profile view, to determine material thickness. In our case they are the 3/4” in height which works well with dimensional lumber. The half breadths of each water line are transferred to the stock using sail pins (a method we have used before). Once the water lines are on the stock they are cut using the bandsaw. The stations are used as reference lines to help align the lifts when gluing. When the glue dries you cut the sheer and start working the hull to shape using progressively smaller tools (number 4 plane, block plane, then spoke shave). I didn’t finish mine (because we cleared the floor for lofting) but its real close and you can get the idea. If a designer was working on something new they might start with an existing design and modify it to obtain a different outcome.
