Lofting Part 2
In lofting part 1 we talked about lofting, how it is used in boat building and the development of the rabbet. This week I am going to review two additional developments, mold reduction and transom rollout.
Molds are part of the temporary structure used to define the shape of a boat’s hull. The reductions are a drawing of the mold shape on the lofting floor. The molds are made from the body plan stations and must account for the appropriate structural systems (planking, framing and longitudinals, longitudinals are any structure that runs the length of the boat). In order to define the mold lines we need to understand how the lines were developed in the design. In the case of the sixteen, the lines are to the outside of planking (other designs could be different, for example a lapstrake boat might be to the inside). Another piece of information we need to understand is how the boat will be constructed in relation to planking, frames and longitudinals (PFL). In the case of the sixteen the frames are bent outside of the longitudinals. As a result the mold reductions in the sixteen need to account for the PFL thickness. Depending on the boat design other build methods could be used.
If you take a perpendicular slice of PFL at each station the length of the face of the slice will be more than the thickness of the material (think of cutting a delicious baguette at an angle). I am going to walk through the tricks we learned to measure that angle, but the underlying concept will be difficult for me to explain in this context. In general the measurement is a function of the distance between stations, right triangles and good old pythagorus. The angle (bevel) could be different at each point along each station. To determine the bevels we make a bevel stick from our master bevel board. The markings on the stick are at a distance from 0 on the master board equal to 2 times the station spacing (see pic). That bevel stick is placed in multiple places across three stations in the body plan to determine the bevels of the center station (for example a stick placed across 2, 3 and 4 would find the bevel at station 3. For stations 1 and 7 we draw in new stations (yes we can have as many stations as we want) to get those bevels.
Now that we have bevels along each station line we can move on to determining where the mold lines would be located. The distance from the station line to the mold reduction is PFL (planking + frames + longitudinals). The longitudinals have taper along the length of the boat so they can bend in place, as a result the thickness is different at each station (purposely not covering that math). To measure our baguette cut along all the station lines we make a clear mylar bevel board. We mark a reference line on the board at 0 degrees that represents the PFL thickness. We align the bevel line on the board at the appropriate bevel line on the lofting and mark the distance to our reference line (spoiler alert its going to be longer than what it was at 0 degrees, pic below). Once you make your marks, a line is drawn using finish nails and a batten. One other detail I am going to gloss over is making notches for the beam shelf (a longitudinal), the notches are represented in the pictures below. The size of the shelf is obtained from the plans and the lines are drawn using similar methodology.
Once we have our mold reduction lines we can make some molds. There are many different methods depending on builder preference and mold complexity. The method we used involved transferring the reduced line to a piece of mylar then to some stock. When transferring to stock you need to be mindful of the grain orientation of the futtocks (love this word). In addition, to the mold line we also transferred the bevels (the bevel on the edge of the mold helps to define the shape of the hull) from the lofting and a few reference lines. The rolling bevel on the futtocks is cut out on the band saw as a team. Where one person is driving and calling out bevels and the other is rotating the table to make the appropriate angles. This process makes one half of the mold which can be copied as a mirror image. This assembly is temporarily attached to the floor where they are connected with gussets and a cross spall of appropriate strength.
Roll out transom
Rolling out the transom, the flat (or not so flat) surface that forms the stern of a boat. Not all boats have them, but this one does. On the sixteen, the transom is raked and curved as such there is no view in the lofting up to this point that could be used to transfer its shape to a piece of stock. The development of rolling out the transom puts the shape in a plane where it can be transferred onto a solid blank of material. The roll out includes the shape, thickness and any bevels that might be required. We were given a detailed step by step instructions on how to generate these lines and translate them back and forth between various views to define the shape and thickness of the transom. I went through the process and I have a picture of the result below. I have more to learn on this process to be comfortable performing it without some guidance.
Building a skiff
We started building a boat! It is flat bottomed skiff, a 14’10” design from Atkin. This is a work boat that could be set up with oars or a small outboard motor. The flat bottom design provides stability that is desirable in a boat of this type. We are a team of 9 plus our instructor working together to build a boat. A different dynamic which is more fun for me, I always preferred my project classes.
The lofting for the skiff is simpler than the sixteen’s and was completed quickly. Once the lofting was done we could start building molds. The nice thing about working as a team of 9 was that while mold construction was underway I was on another team working to make the strongback. The strongback is a ladder type system made of sturdy lumber where the top is perfectly level. We used some edge jointed 2x12s, laser level and shims (i love shims) to make that happen. Other work that was happening simultaneously was building some of the first boat parts including the stem, transom, chine logs and apron. We were able to get all of the molds mounted on the strong back, a fun milestone. For the rest of the school this quarter we are almost 100% skiff mode. I am looking forward to seeing her far we get before holiday break.
https://www.woodenboat.com/boat-plans-kits/14-10-outboard-sprite
