Saturday, December 31, 2011
We are building a spar factory for this job and the more rote each task becomes the better we like it. The painstaking and really fun part is setting the routines up and seeing them work. In this factory the production run is relatively short so there is still some handwork and still the possibility of messing things up through human error. We have not yet brought on the automaton robot zombie boat builders or the next generation of software to replace them so we do have to be careful through each phase of this job
All of the spars we are building in this run will be of eight-stave birds mouth construction. We begin by cutting the staves to their widest dimension on the table saw then joining them with a slash scarf joint to make up the required length for each spar. The joints are roughed on a band saw then finished in a router jig and glued together on a bench
The staves are then clamped into a jig on the backboard of our spar bench. The taper is cut into them with a circular saw adapted to slide along that jig and smoothed with a router set up in a similar way. In this way each stave comes out exactly the same as the other seven that make up the spar. The position of the batten and its wedge clamps is changed for each spar. That position is determined by a little exercise in graphic geometry where we start with the diameter of each column and proceed to determine how wide each facet of an octagon must be to encompass that diameter. Remember how many times we protested the uselessness of geometry in high school? I loved geometry.
So far the staves are scarphed together, birds mouthed and tapered just as in any hollow wood spar of this type. Silent Maid 's spars will be lined with carbon fiber cloth and rod vacuum bagged to the inside face of each stave. In this way a very stiff spar of a smaller diameter, wall thickness and weight can be built. In this case the carbon is in the form of a woven layer, a 1/4" rod set in a groove down the middle of the stave, and a layer of unidirectional cloth. In effect the wood is now a matrix for the carbon fiber which is doing the real work. All of this depends on a good bond between the carbon and wood as well as keeping the amount of epoxy to a minimum, hence the use use of a vacuum bag.
After the carbon the first layer that is not part of the finished spar is the release fabric. This soaks up excess epoxy and can be peeled off the finished composite easily. Then a layer of plastic to keep the glue out of the breather fabric. The breather allows to vacuum pumps to pull evenly on the entire bag. There are two pumps one for each half of the bag. The air is drawn out through nylon tubing with the ends sealed and small hole drilled in it. The tubes are taped to a batten and care is taken to keep the tubes in contact with the breather fabric over the length of the spar.
The first step is to set up a group of staves so they are perfectly straight inside the vacuum bag. We joined plywood panels with strips of wood hot glued to their top face then hot glued spacers to that base to hold the staves in place. The simple jigs pictured to the right were used to align the staves. After the alignment was complete the staves were lifted and a sheet of thin plastic placed over the ply and spacers kept the staves from being glued to the set up. All faces of the stave that are not to be covered with carbon are sealed with packing tape to make the epoxy cleanup as easy as possible.
The Workshop has a large compressor so two Venturi type pumps are used to draw a vacuum. They draw this 40' long bag to 9 lbs of pressure per square inch and the compressor is up to the task of maintaining this pressure overnight. A close inspection for leaks is the last step in the process. These usually occur where the hoses enter the bag or where there are folds in the bag. Since everything is sucked into the bag these are easily plugged with mastic
There is something very cool about using the atmosphere for a clamp.
The finished staves are bundled and held in place with hose clamps so the fits can be checked and plugs for the ends fashioned. More geometry and handwork. Once this is done they can be glued. That is a bit of a gooey juggling act as the bundle of staves tends to fall apart until they are all together and at least one hose clamp is on. One of the basic tenets of boatbuilding is, Never ever cut a piece of wood to its exact length until you absolutely have to. I am sure Noah chiseled this into a tablet and it will be found someday. Because our staves are long we can tack them together with dry wall screws in the very ends. The drywall screw holes will be removed when it is essential that the spar be cut to length. Once the glue is set the spar can be 16 sided with a power plane and roughly rounded with a spar plane to prepare it for the lathe.