Category: Lofting

As I was gaining experience lofting Justine’s lines on the plywood panels attached to the barn floor, I began to realize that unevenness in the floor surface was making it difficult to judge the fairness of curves that I needed to draw. I had shimmed the plywood lofting surface as I was laying it, nevertheless irregularities remained. These were a result of the barn being on a deteriorating foundation (I began lofting in 2003 and didn’t rebuild the foundation until 2006), the advanced age of the barn (c. 1900 we think), and the fact that the floor was comprised of a mix of boards of varying thickness.

Might I be able to do the lofting on my computer? As part of my work life I had coauthored two textbooks and become very proficient doing technical illustrations using a software program called Canvas. The program is loaded with features for architectural drawings, among other things, so making very accurate scale drawings of large objects would be possible. There were tools for drawing and making careful adjustments to curves, so producing fair lines through a set of points seemed will within reach.

Could I produce the full-size drawings that I would need?  I knew I wouldn’t likely need to print out the whole lofting. But I would need to print full-size half sections of the hull, and drawings to make templates for parts like the stem and transom. I would need to print things as large as 4′ x 6′. I was confident I would be able to find a way to get what I needed printed.

A full-size profile drawing of Justine’s profile would be about 21′ long. My laptop (on which I’d be doing the lofting) screen was about 13″ wide. The Canvas program allows for producing scale drawings over a huge range of scales very conveniently. You specify the scale you want. Let’s say you want a 12″ line on the screen to represent 24′ on the boat. Then you set the scale at 1:24. What you see on the screen are “rulers” at the edges of the viewing window marked to the true size on the boat. If the window was 8″ x 12″ on the screen, the rulers would be 16′ x 24′. Now let’s say you use your mouse or trackpad to draw a horizontal line on the screen by clicking and dragging. Small panes in Canvas’ menu bar will indicate the starting location, the ending location, and the length of the line in the full-size drawing. This is incredibly convenient. Furthermore, by “zooming” the screen view, you enlarge the view, but the original scale of the drawing does not change: zooming the view also “zooms” the rulers. When you need to, you can zoom the view to full-size, allowing very precise positioning of features in the drawing.

A variety of methods can be used to draw curves in Canvas. One drawing tool allows you to click on any number of successive points on the screen, and Canvas will fit a smooth curve that connects the points. This is a good way to begin to develop a fair curve. You plot the appropriate points from the table of offsets (there might be ten or more), then you enable the curve tool, and click near four of five of the points, including the first and last. After the last, Canvas will connect the points with a smooth (but rarely fair) curve. Then zooming to full size, you can carefully set the points on the curve to coincide with the corresponding points you plotted from the table of offsets. To further refine the curve, each of the points on the generated curve has a pair of “handles” that control the slope of the curve at that point. This gives you real control on the fairness of the curve. By iterating the locations of the points and the position of the handles, you can come quite close to matching the points from the table of offsets with a fair curve. (The table of offsets gives points to the nearest 1/8″, so it’s not necessary to have the curve exactly on each of those points.)

The Herreshoff Manufacturing Company built most of their boats upside down over a set of construction molds that determine the hull’s shape. To illustrate, here’s a picture of Justine’s construction molds in position on the barn floor.

Justine’s table of offsets specifies coordinates at 11 “stations” positioned 19 1/2″ inches apart along the hull’s length. These correspond to station numbers 2, 4, 6, …, 22. The lofting is done based on this set of points, and there is a construction mold at each station. Molds are also needed halfway between each of these stations, corresponding to station numbers 3, 5, 7, …, 23. (There is no need for a mold at station 1 because of the very small space at that position in the hull.) In all there are 22 molds, each spaced 9 3/4″ apart. Each station mold needs to be drawn full-size.

My computer lofting of Justine’s plan-view and profile-view lines is shown below. Using information provided with the plan set, I’ve added full-size lofting of the transom, as well as the profiles of the stem and keel plank, and ballast keel. “LWL” is the load waterline of the boat—this is the designed location of the waterline when the boat has all its equipment and crew aboard. The curved lines in the upper drawing represent the hull’s waterlines which include the LWL and lines in parallel planes at 6″ intervals above and below the LWL. The curved lines in the lower drawing are buttock lines that represent the hull’s contours in vertical planes along the hull’s mid plane and at 10″ intervals athwartships.

Note that in my lofting, I only show the even numbered stations, as these were the locations at which the points in the table of offsets were given. Before lofting Justine’s sections, I added lines representing the locations of all the odd-numbered stations as I would need to draw those sections in addition to the even-numbered ones.

My lofted sections of Justine’s hull are shown below.

There’s an additional wrinkle in drawing the shapes of the construction molds from the lofting of the sections. The table of offsets actually gives points on the outside of the hull’s surface. Between the outside of the hull and the construction molds there are 5/8″ planks as well as 7/8″ frames (steam bent “ribs”). Furthermore, the edges of the construction molds must be beveled to match the curvature of the hull at each point. So I used a two-step process to make full-size drawings of the construction molds: First, I produced lofted sections representing the outside of the hull, then I made an allowance for the reduction in mold size to account for the thickness of the planking, the dimensions of the frames, and the bevel of the molds. I drew a fair curve through the corrected points and then I was nearly done with the lofting. I just needed a way to print out my results!

I first explored options to do large-format printing directly from my laptop. My brother-in-law Win Fowler had a sail loft with a computer-c0ntrolled fabric cutter that could be fitted with a pen and draw on paper or mylar. He made some test prints on mylar sheet for me, but I was not satisfied with the accuracy of the printout.

I knew from experience that it would be very expensive to have large-format prints made commercially, so I developed a “work-around”: add a set of registration marks on a grid to my computer-generated mold shapes, and simply have Canvas print my drawing on a number of overlapping 8 1/2 x 11 sheets of paper. I found that if I was very careful in positioning the registration marks as I assembled the printed sheets, I could generate suitably accurate full-size drawings comprised of 8 1/2 x 11 sheets attached to each other with two-sided tape.

Coming next: building Justine’s construction molds.

To build a boat to a specific design you need a set of plans. (I bought mine from the WoodenBoat Store.) Plans vary significantly in level of detail. The plan set for my Flatfish consists of six sheets, the largest of which is about 3×4′: a table of offsets; a lines plan, a construction plan, two sail plans, and a plan with drawings of the spars and important pieces of hardware.

The table of offsets quantifies the location of a collection of x,y,z coordinates for points on the surface of the hull using a standardized format. These points are used by the boatbuilder to generate the designed three-dimensional shape of the hull.

The lines plan is a scale drawing of the hull shape as projected when viewed from the top, side and front/rear. The lines plan is produced by plotting points from the table of offsets and connecting them with fair (smooth, “eye-sweet”) curves.

The construction plan gives details about the various parts that make up the boat, such as shape, thickness, suggested materials (there are six different wood species in Justine), and some of the fasteners that are needed. The construction plan contains top, front/rear, and side views, with detailed information about how the various parts fit together. To me, this is the most interesting of the plans set because it gives some sense of complexity of the building project and it takes a lot of study to figure out some of the finer details.

The sail plan shows a side-view drawing of the boat with its sails raised and gives sail dimensions, area, center of effort, as well as details about the running and standing rigging. The Flatfish plan set includes two sail plans, one for the traditional gaff rig (quadrilateral mainsail), and one for the more modern Marconi rig (triangular mainsail).

The spars and hardware plan provides dimensioned drawings of the wooden mast and boom(s), locations of various hardware items on the spars, and three-view drawings of many of the bronze hardware items that were standard on the boats produced at the Herreshoff Manufacturing Company. For the Flatfish, the spars and hardware plan also provides dimensioned drawings and a perspective illustration for constructing a mold for the boat’s 1200+ lb. ballast keel.

It would be impossible to a good job building a boat of Justine’s size and complexity without first producing full-size drawings of the boat in top, side, and front/rear projections. The process of taking the data from the table of offsets and rendering full-size drawings is called lofting or laying down the boat’s lines. Careful lofting is of critical importance: mistakes in lofting, if not caught, result in a hull that deviates from the designer’s intended shape. Once the boat’s lines are lofted, numerous important parts of the boat can be drawn on the lofting, in full size. This generally includes the stem, transom, and keel plank/timber which comprise the “backbone” of most wooden boats.

When I built my 14’5″ Biscayne Bay Sailing Skiff, I prepared a surface for lofting by attaching two 4×8′ plywood sheets together end-to-end to form a 4×16′ panel. I painted it with a coat of flat white paint, and did the lofting on the floor of a study in our house. Justine required a larger surface for the lofting and she was to be built in our barn in Georgetown, Maine. So I began by fastening painted plywood sheets to the upstairs floor of our barn, forming a surface 6×20′ on which to begin the lofting. I started this lofting in 2004.

So now that you know something about what a lofting is, how do you actually make one? I began by doing a lot of reading. Most boatbuilding books include a chapter or two on the topic, and I read several before I began lofting my Biscayne Bay Sailing Skiff. [A favorite book of mine that has a good treatment of lofting is Bud Macintosh’s How to Build a Wooden Boat.] Then I dove right in and learned by doing.

Lofting begins by drawing a grid of perpendicular lines spaced at convenient intervals consistent with the locations of points in the table of offsets. I started drawing the profile of the hull, as viewed from the side. By taking appropriate values from the table of offsets, I plotted a series of points that mark the full-size profile on the drawing. In lofting the boat’s profile, you plot out the front of the stem, the bottom of the keel plank, the transom, and the top of the uppermost plank that forms the side of the hull (known as the boat’s sheer line).

Once you have a set of points that represent a smoothly curved part of the hull, you need to connect the points to form a very fair curve. This is accomplished with the aid of a long relatively thin piece of wood, called a batten, fixed to the lofting at several places along the curve so as to intersect the plotted points. It requires some fussing with the locations at which you fix the battens to the lofting, so that the batten takes a very fair shape while hitting (or nearly so) all the plotted points. It helps to have someone experienced (I had Scot) to give you a hand when you are starting out using battens. You need a collection of battens with different cross sections so you can draw lines of varying curvatures. The battens need to have an even grain, with no knots, so they naturally form smooth curves when bent.

Once I’d drawn the hull’s profile, I began to plot more points from the table of offsets to create an additional set of lines that represent sections of the hull at 10″ intervals out from the hull’s mid plane (fore and aft + vertical), again as viewed from the side. This set of lines (called buttock lines) produces something akin to a topographical map representing the hull’s shape as viewed from the side.

Next, the “plan view” of the hull shape, that is, the shape looking down from above, is plotted out. Because the hull is symmetric (the port side is a mirror image of the starboard side), it’s only necessary to loft one half of the hull shape. The profile lofting and the plan view lofting are drawn with a common axis, specified by the table of offsets, and both are superposed on the lofting. The only difference between the “lines plan” provided in the set of plans and the curves described so far in the lofting is that that the lofting is full-size. In theory, if you had an enlarger that could simply scale up and reproduce the lines plan, you would not have to loft those. But I am sure that with current technology you’d get a much more accurate result by doing the lofting yourself. The table of offsets for the Flatfish includes plan-view coordinates at the boat’s waterline, and additional sets of coordinates at 6″ intervals both below and above the waterline. These lines, called water lines, are also drawn on the lofting and the set of waterlines can be visualized as a topographic map depicting the hull shape.

Finally, a third set of lines is drawn on the lofting giving a set of “sections” that represent the cross section of at fixed intervals (in the case of the Flatfish, 19 1/2″) from stem to stern. It’s as if the hull is sliced like a loaf of bread, and the shape of each slice is drawn. If you ever studied mechanical drawing, you know that the shapes of these sections can be derived from measurements that you make on the profile- and plan-view drawings. Lofting books explain how to do this, and by doing it this way the section lines will be consistent with the faired lines in the profile and plan-view lofting. This set of curves is that comprise the sections is critically important for constructing the Flatfish, because these curves determine the shape of forms onto which the boat’s frames (“ribs”) are steam-bent and held in shape while the boat is constructed.

For mathematically-knowledgable readers I can make the relative orientations of the profile-, plan-view, and sections a little more explicit. Imagine an x axis running from the boat’s mid plane out to the port (left) side, the y axis as pointing upward, and the z axis running forward toward the boat’s stem. Then the profile curves are in the y-z plane, the plan-view curves are in the x-z plane, and the section curves are in the x-y plane.

So a complete lofting gives three sets of curves related to side, top, and fore/aft views of the hull shape. Each set can be thought of as contour-lines on a map, and with some practice your mind can visualize the shape of the hull directly from the lofting.

The flatfish lines plan looks like this: