The Bowsprit and New Sail Plan
A large diameter sheave through the center of the sprit gives a 2:1purchase for extending the spar. The line goes through a clutch on the launch tube and thence to a cockpit winch.
A router/trimmer in the tool holder being used to turn the foam core.
All of the unidirectional carbon fiber for the highly loaded bars for the sprit had to be stretched out to be both perfectly straight and equally loaded. Blocks of wood preglued to both ends of each carbon layer were fastened solidly at one end and then tensioned with a spring balance. They were then fixed in place before the whole gooey mass was clamped together.
After the unidirectional carbon core had cured and the blanks sawn to shape pre pregged carbon ribbon was laid on transversely with this basic jig. Then it was vacuum bagged in place.
Making the launch tube in a PVC tube mould
Vacuum bagging the walls of the launch tube with a PVC tube both inside and out made a clean laminate
Each half had a moulded rebate to joint the two sections together.
Making the stainless fittings to mount the tube. These had to be capable of transferring the 5 tonnes of compression loading into the deck and stem.
The extended bowsprit.
Why we Needed a Bowsprit
The original sail plan gave us a single headsail on a furler, a #2 sized genoa. It was not a good arrangement. It was too large upwind in anything over 10 knots, too small under 8 upwind, or 15 reaching. The problem was exacerbated by the poor shape of the sail. The material was cheap, heat-set dacron, and from day one, the sail was spilling it’s fillers, stretching hideously, and developing a huge saggy leech, tamable only by ferociously hooking it with the leech line. So the sail was either too big or too small in almost every situation we sailed in.
New sails were definitely essential and we had to decide on the material and size of the sail. It was quickly apparent that one size would not fit all. We wanted a boat that would sail well upwind as well as down, and in a wide wind-range to boot. Clearly, we needed more than one headsail. Furthermore, removing the # 1 had to be possible and painless since windage and weight of the extra sail forward would definitely hurt our performance if we had to go upwind in heavy conditions for any length of time.
A bowsprit with a continuous-line luff-furler was our only option. Now, we would be able to drop the sail furled up in almost any weather. However, we also had to be able to carry and launch our anchor from its original position. Furthermore, since we didn’t want to permanently increase the length of the boat with a fixed spar the bowsprit had to be retractable.(Marina’s measure everything. A fixed bowsprit would add at least 5% to our daily charge.)
We chose to build a retractable bowsprit and launch-tube which was able to take the high compression loads imposed by the bobstay. The bobstay, in turn, had to resist the 2.5 tons of luff tension required for the #1 when going upwind.
Construction Details
The foundation of the bowsprit is a light foam core. This was made by putting a small router in the tool holder of the lathe and turning a long block of foam that I had laminated over a metal tube. The tube was removed after core was shaped, its only purpose was to enable the foam to be held in the lathe and keep the foam straight whilst it was shaped.
To take the almost 5 tons of compression load a carbon/epoxy bar was laminated inside a U shaped aluminium mould. In a plant making parts for the aerospace industry these would have been produced in an autoclave with at least 5 atmospheres pressure. Lacking a pressurized autoclave I used multiple screw clamps pressing on a solid bar inside the U-shaped mould. This was a low-tech but cheap, simple, and very effective method. Carefully weighing the amount of carbon and later the weight of the finished product gives the fiber-to-resin ratio, in this case 62/38. Additionally, since there was no vacuum bag to complicate things, I could apply tension to all the carbon fibers making sure they are absolutely straight inside the laminate. This is especially important when making components that are to be in compression. Once cured this bar was split in two on the table saw. Unlike glass fiber laminates carbon is easily machined with tungsten carbide tools. These two bars were then set into rebates machined into the foam core. At the outer end of the sprit they fit against another short bar which forms the backstop for the bobstay and luff furler attachment and at the inner end another one that holds the sheave for the extension/retraction mechanism.
To complete the bowsprit all that was required was for the outer skin to be laid up. This part of the sprit had to take the side-loading , essentially negligible compared to the compression load, but still an important consideration. The bowsprit behaves like a cantilevered beam in this plane with the fulcrum being the point at which it contacts the launch tube. In practical terms this means more transverse fibre needs to run around the bowsprit here, to ensure the core doesn’t crush/collapse and it is also the area where the most layers of lengthwise unidirectionals are laid. Vacuum bagging at atmospheric pressure, though not ideal, was adequate for this part of the layup.
The launch tube was built using a mixture of carbon and “S” glass fibers which produce a good structure for compression loads. A large PVC drainage pipe was used as the mould. The tube was laid up in two sections, by glueing a strip of PVC along one edge of the split pipe a rebate was formed in the mould. This meant that the two sections could then be glued together and the finished tube had a double-wall thickness where the side load of the bowsprit was taken. To further strengthen this area, a strip of foam was added and carbon and glass fiber laminated over it, making small beams on either side of the tube.
The bobstay chainplate in the hull was built by first laminating a small frame into the hull. The lowest practical position for this was at the bottom of the chain locker, once cured in position, kevlar line, saturated in epoxy was threaded through holes drilled in the frame and through another bored through the bow and around a low friction ring. Multiple wraps equally tensioned were used to give an attachment easily able to take a 20 ton load, more than 6 times the expected bobstay load.
The bobstay is a dyneema line with a shockcord inside it. When the sprit is retracted it keeps the stay taunt and clear from the path of the anchor when dropping or retrieving.
