Park Avenue Boom
Mould frames set up
First laminates in position
Milling the gooseneck bars
Gooseneck bars glued in and foam wings almost complete. Top chord lying on the bench has had carbon laminated to it. This was made of two laminations of Douglas Fir, the second is inside the boom. By the time this boom was made all the original cast components of the gooseneck had failed and been replaced with parts machined from solid billet and bushed with bronze.
The method used to accurately align all the sheave axle bushes, in this photo a solid block of timber drilled in the mill and clamped in place is the guide for leech sheaves. For the gooseneck ends the holes milled in the composite bars were used.
Shiny with resin about to be sanded off for paint. The extra glass for impact resistance against the cap shrouds shows as an obvious bulge here but was faired out before painting.
Mainsheet, boombrake and vang tang. Sail bag in this photo was about to be retired.
Boom with new lazy bag installed.
Why we Built a PARK AVENUE BOOM
After we had made the decision to improve the sailing performance of the boat with fairly expensive new sails we did not want them to be damaged by the original single-line reefing system. While the existing system had the virtue of being the cheapest possible option for Beneteau, it had serious shortcomings for us.
Foremost of these shortcomings was the double purchase on the luff. It did damage to the mainsail in two ways. Firstly, by having all the frictional loads of pulling the leech in going through the luff first, the sail was seriously stretched. Secondly, having a line on both sides of the sail crimped and abraded the cloth. The boom had to be modified to add additional sheaves to change the system. Upon removing it we then discovered that the boom had a serious permanent bend. With hindsight, we saw how lucky we had been not to have had it break during the Atlantic leg between the Azores and Portugal. It appears that the builder ignored the spar makers specifications and supplied the boat with a boom having only half the moment of inertia required. This apparently is a commonly adopted plan to chisel a few dollars from their costs. A new boom was required, initially this was done with a used aluminium spar, and the old tubular lazy bag frame. It served us reasonably well but by the time we reached San Francisco was showing signs of it’s age and since we had a great facility at our disposal (a very much appreciated payback for Peter volunteering to build the American prototype of the SV 14, an Alex Simonis designed, ballasted performance dinghy for disabled sailors, SV 14 on facebook,) Spaulding Boat Works, building a new boom was feasible.
This one could be specifically designed to take the sheave arrangement for our single line reefing and have inbuilt “wings” to hold a new sail bag.
Construction Details
First the gooseneck was laid up. This was done by making two flat bars of carbon fiber, “S”Glass and epoxy. These would later be fitted with bronze bushes and laminated into the boom. The offcuts were tested to destruction in a hydraulic press. Each one started to fail at about 20 tons so no concerns there!
The basic structure of the boom would be a central box made of PVC foam and carbon fiber but with its top, the piece in high compression when sheeted hard going upwind, having a Douglas Fir core. As the boat has mid boom sheeting the boom also has to resist considerable bending forces. Both the sides of the central box and the wings that hold the lazy bag resist these loads.
A series of mould frames were cut and assembled on the spar bench to take the foam core. Another mould was made to bend the heated foam over. It took only seconds to heat the foam sheets under the buildings infra red space heaters!
As the photos show the foam was sheathed in carbon fiber which was vacuum bagged in place for the curved wing pieces or laid flat on the vacuum table for the sides of the box section before they were assembled into the mould frames. The flat components were then glued and laminated into place.
Foam stringers and edge pieces were then fitted to the lower wing section, the stringer spacing determined by how much the foam could span without shearing when 14psi was applied by the vacuum bag. A destruction test with lead weights on a small cross section sample answered this question.
The gooseneck bars were first machined to install the bronze bushes for the sheave axles and mast hinge and then glued and laminated into place. In approximately the correct position holes were bored through the boom box sides and sail bag wings and with a mandrel made to suit the axle bushes concentric holes were cut for the outside bushes using a hole saw. (see photos for a clearer explanation!)
A “T” section laminate was moulded and installed into the bottom chord of the boom to take the vang and the area between it and the gooseneck given additional fiber reinforcement to take the high compression load the vang imparts.
The top chord of the boom had unidirectional carbon fibre laminated to both sides and it, too, was glued in place. Once the positions for all the control lines were established the cut outs were made to reduce weight where extra strength was not required. These holes in the top also make it very easy to confirm that the multiple lines for reefing the sail are not twisted. Any crossed lines would cause a huge amount of friction. To seal the edges of these cutouts, strips of carbon laminate were 1st vacuum bagged on the table and then bent and glued in place.
The foam edges of the wings were then rounded to shape and all the remaining carbon fiber laminated and vacuum bagged in place. Where the boom could hit the cap shrouds a substantial reinforcement of glass cloth was added to protect the otherwise fairly light construction of the “wings”. Enough resin was then painted over the finished surfaces so everything could be sanded for paint without the danger of removing any of the thin carbon skin.
The main sheet and boom brake attachment strops go through glass/carbon composite tubes. Free floating low friction rings limit any twisting load on the gooseneck as the sheet is eased since the tubes are positioned to align the pressure in approximately the same plane as the gooseneck pin.
Plastic bolt rope track was attached with both screws and 3M Hi Bond tape to attach the sail bag. The lazy jack lines terminate on the boom and go through pockets sewn to the new bag (lower photo only) so even with the lines in place to hold the sail when reefing the bag can remain folded inside the boom when sailing.