The Mech team has been working steadily over the past couple of months on construction of our Microtransat boat’s hull. The hull is being built using the “cold molded” construction method. This method involves first building a skeletal mould that defines the shape of the hull. Thin layers of softwood strips are then bent over the mould and laminated together to form a shell that will become the hull core. Finally, carbon fibre is laminated to either side of the core to make a very stiff, light and strong hull structure.
We’ve made some good progress on the hull mould: read on to see what we’ve been up to.
The first step in building a quality boat was to organize our workspace. The team did a great job of rearranging our work area to accommodate construction of the 5+ metre Microtransat hull – a much larger project than we’ve tackled before. The diagram below shows our workshop layout.
The lockers at left have been mostly filled with tools, many of which were donated to our team by sponsor Summit Tools. We finally have a collection of tools that gives us the ability to work quickly and efficiently.
The work tables shown at the right side of the diagram were our first construction project. They were made in a single session from 2 x 4 and ¾” plywood donated by Windsor Plywood. See the tables in the picture below. So far they’ve been providing good service and standing up well. One of our members discovered, however, that it’s really not too hard to saw through the tabletop, screws and all. 😉
Next we turned our attention to building the “strong back” – essentially a long box beam that forms the base of our boat mould. In one session, we built the basic structure in two sections, again with material donated by Windsor Plywood. In the next session, we joined the sections to create one long beam and added diagonal bracing. The webs of the beam are of ½” plywood 16” deep with flanges of 2 x 3 at top and bottom. This construction makes for a beam that is extremely stiff. The process of bending many stringers and veneers over the mould later on creates a strong aggregate reaction force that tends to bend the ends of the strong back upward. Robust construction is necessary to counter this bending force, keeping the mould free from distortion.
The next step in preparing the strong back was the addition of the transverse members shown in the picture below. These members were fastened perpendicular to the strong back centreline with precise separation to accurately locate the frames that would be fastened to them later.
To provide even greater resistance to the forces pulling the ends of the strong back up, heavy weights were added to each end. These also help to keep the whole mould from shifting after it is aligned.
The final step was levelling the strong back. With a laser level to create a level reference datum, we precisely measured the distance from this datum to the top of the strong back rails. The structure was then carefully levelled using height adjustment bolts built into the bottom of each leg. Having the top of the structure very level gives us a good reference to work with when levelling the frames later on.
The team designed the basic shape of the hull last summer, but until early this year the final hull lines had not been developed. With the aid of Philip Barron, Naval Architect at UBC SailBot sponsor Robert Allan Ltd, we created a set of hull lines by first building a 3D model of the hull using Rhinoceros 3D (with Orca plugin) and AutoCAD. These hull lines are sets of 2D lines representing the 3D shape of the hull in a manner similar to topography lines on a map. The hull lines are taken as a series of slices horizontally (waterlines), vertically (buttock lines) and transversely to the hull axis (stations). See the image below of the resulting lines.
Right now we are particularly interested in the stations, which define the shapes of the frames we need to build and attach to our strong back. Later these frames will play the main role in defining the shape of the hull mould as longitudinal stringers are bent around them.
The frames are the most important part of the mould for our hull. On their own, they resemble nothing so much as a stack of over-sized bread slices. When they are positioned on the strong back, however, the shape of the coming boat hull finally becomes apparent.
Despite their importance in shaping the mould, the frames will not be part of the final hull structure, so they are constructed from relatively cheap, but stable medium density fibreboard (MDF). The shapes of the frames were first printed full size on sheets of Mylar. Mylar is more durable and less prone to expanding and contracting with changing humidity than paper, making it more suitable for this job. With the Mylar taped to the MDF, the shape of each frame was transferred to the material with multiple pinpricks.
An important feature added at this point was a set of cross hairs on each frame. When the frames are arranged in space as they should be to form the hull shape, the cross-hairs all fall on a straight, level line parallel to the axis of the hull. These cross hairs were the key to precisely aligning the frames later.
With the frame shapes carefully marked on the MDF, we were ready to cut them out. Straight lines were cut using a circular saw and a jigsaw was used to cut the curved sections. The final shaping was done with a surform – a sort of cheese-grater type tool that is ideal for shaping MDF.
Mounting the Frames
Getting the frames perfectly aligned to match their theoretical position in the model is the key to achieving a fair hull with the intended shape. The transverse members on the strong back ensure that the face of each frame ends up on the correct transverse plane. The challenge then is to make sure that each frames is aligned in the correct position within its plane.
To align the frames, we first drilled a large hole centred on the cross hairs of each frame. The cross hairs were then carried through the hole by gluing thread across. Next we set up a laser oriented parallel to the strong back and boat axis, as seen on the illustration below. Starting from one end we positioned each frame on its transverse member such that the cross hairs were centred on the laser beam. Then, using the levelled strong back top as a reference to ensure frames were level, we fastened each frame on.
At that point we had all the frames fastened in their correct position and orientation. However, we needed to brace them to make sure they stay that way once we start bending stringers around them. We cut sections of MDF to form a box between each frame. First checking that each frame was level from top to bottom, we fastened these box sections in, locking the whole structure in place. The result is what you see in the picture below. Notice the alignment holes still visible in each frame.
To complete our mould, the final step is adding longitudinal stringers. The stringers are long, square ¾” x ¾” strips. We cut these from clear poplar donated by Windsor Plywood using a table-saw borrowed from the Royal Vancouver Yacht Club. The clear (knot-free) and straight-grained nature of the wood in the stringers is important in ensuring they will bend smoothly over the frames.
Once the stringers are in place, we’ll be ready to start laying on the cedar veneer strips to create the core of our actual hull. Stay tuned for more pictures soon!