Carbon-Fiber Spinnaker Pole
Block plane clamped to a simple jig both shapes the individual strakes for the pole taper and bevels the edges to the correct angle for jointing.
Gluing up the half sections. The moulds were shaped to hold each strake securely in position.
One half rounded to its finished shape
Glueing the two halves together
Tyre valve fitted into a temporary end cap to enable equal pressure internally as the vacuum on the outside was pulled down. Making the initial tube over length allowed the vacuum bag to be sealed onto the tube rather than having to go over the support brackets.
Vacuum being applied, only a few more inches of mercury to go. Surplus resin is already seeping into the absorbent scrim through the perforated plastic laid over the peel ply.
Turning the Delrin spacers for the end fittings. An old file ground to shape makes a good tool to turn the rounded shape.
A lanyard made of a dyneema sleeve with a shock cord core is clipped to the life lines when lowering and raising the pole up or down the mast. This stops the pole swinging wildly around. Once in position it is clipped back on the pole
Leather protection prevents the baby and shrouds from damaging the lacquer finish.
Why we needed to change our spinnaker pole
After blowing up our last symmetrical kite on the trip down to San Francisco from Alaska (it had served us well, over 10,000 miles in the tropical sun) we decided to change to asymmetricals that could be either tacked to the bow sprit or flown from a pole if sailing flatter angles. (And, since we had turned into floundering geriatrics, add spinnaker socks. The side benefit here is that, as the Mini Transat sailors do, we can now “reef” the kite in squally conditions instead of laboriously dropping, wooling, repacking and hoisting with the passage of every squall) Because the boat does not plane or even surf very often, sailing near dead down wind is quicker than gybing through hotter angles so a pole is essential. Since we needed to be able to swap between the sprit and the pole, the pole end had to be directly above the end of the bowsprit. Our old pole was a typical “J” length and also was of marginal strength for heavy reaching loads.
Design
As in any project the first thing to decide was what the purpose is and what compromises are acceptable. In our case with the pole, the one thing that we could not compromise on was strength and reliability. We decided that a pole of only slightly less weight than that of the appropriate aluminum section would be acceptable. This meant we could build a composite structure of carbon fiber with a thin Douglas Fir core. This enabled us to easily taper the ends without building a complex mould.
Construction
The first step was to machine the fir to size. The pole was to have a maximum diameter of 125mm and be a little under 6 meters long. The Douglas Fir needed to be scarphed to length and then sawn into vertical grain flitches 3mm thick ( this is about the same weight as aluminium 0.5mm thick and has about the same compression strength). These would then be reinforced with unidirectional carbon fibers vacuum bagged in place on a perfectly flat table. A series mould frames were cut from plywood, the changing diameters determined by bending a full length batten to the desired tube cross section. The pole was to be made in two halves of 6 sections each. A pattern was then made of the finished shape and fastened to the work bench. Each strake was then laid on the pattern and planed to shape and beveled with a small jig clamped to a block plane. Only the tapered ends had to be done this way as the the bevel on the central parallel section had been done when ripping them to width on the table saw.
The strakes were then laid into the mould and the 2 half sections glued together. Once cured and left to fully harden for a few days one of the sections was planed and sanded to its finished profile. The two sections were then glued together. Exactly shaped clamping blocks made this a simple operation.
The carbon to the outside was to be both along the length of, and around the circumference of, the pole. The first to provide compression strength and the second to hold it “in column”. All would need to be vacuum bagged in place. This raised the possibility of crushing the tube, so first the ends of the tube had to be sealed and the whole pole made airtight by epoxy coating it. Once this was done and it was lightly sanded all over it was ready for the next step.
A tire valve was fitted into one of the end caps. This would enable a positive pressure to be added as the vacuum bag pressed down on the outside of the tube. One atmosphere of vacuum is about 14 psi so over the whole length of the pole about 3,400 pounds is pressing on the tube. Any variation in the wall thickness or minor imperfection in the glued joints could be enough to allow the tube to loose its cylindrical shape and fail catastrophically, if no opposing force was applied. Once the first of the outer laminations have cured this is no longer a concern for subsequent laminations.
Since we were going to use the beak and bell from our old pole which was only 75mm in diameter we used Acetal Delrin to turn a pair of spacer blocks to fit the 90mm end diameter of the new pole.
It was then sprayed with 10 coats of UV blocking clear polyurethane. We also sewed a leather sleeve to protect the pole from the baby stay whilst raising or lowering it on the mast.