SEDATION 4  Boom Design

The AeroRig was not a commercial success for several reasons, mostly financial, and is no longer available.  Performance and ease-of-handling were marvelous but detractors cited the bulky AeroRig's boom as unsightly, if not downright ugly.  I must admit that my preliminary boom design was similarly awkward, albeit functional and easy to fabricate. Construction would involve hand lay-up of a very long (35') honeycomb-cored panel which comprised both sides and the bottom of the boom's "canoe hull" shape.  Gaps in the honeycomb would permit folding the flat panel into the desired shape, then the whole boom would be held together by a series of CNC-cut lateral braces that are laminated in-place.
Highly-stressed angled braces at the mast hole would provide "rocking" support but a full-length longitudinal stringer was needed along the centerline to prevent buckling under  extreme loads.  Strategic use of carbon fiber at several highly-stressed areas was inevitable.  One disadvantage of my canoe design was the cross braces' obstruction of a convenient space to stow the furled main sail.  The large side panels were not only ugly, but presented substantial surface area thus aerodynamic drag.  I adhere to the "form follows function" axiom but this boom just didn't seem "right".
Two years ago, an interesting design study was posted on the Net, by Joseph Oster, for a split-boom rig. Shortcomings and technical difficulties of his preliminary design, nicknamed the "Bucky Boom", formed the basis of my second approach.  [I've been a fan of Buckminster Fuller since 1980; when paring my home library down to "yacht size" Mr. Fuller's book Ideas and Integrities made the cut as essential reference material.]  I applied the concept of Tensiontegrity to the boom's design and developed this model.
The key, and most highly-stressed, components are the end-caps.  These must maintain the bowed carbon fiber spars in-place plus provide attachment points for the forestay and aft stay.  Two 35' longitudinal guy wires (possibly rods) provide the bowing forces and "connect" the upper- and lower-bearing assemblies as they rotate about the fixed mast.  Counterbores are provided, for the wires' and stays' adjustment nuts, as shown in this close up view.
The angular orientation of guy wire and stay  thru-holes is critical to optimizing component strength, so the whole mast assembly geometry required definition before I could proceed.  The ends of the bowed spars are inclined 8° from horizontal, thus the end-cap bodies with spars' pockets are angled 8° from the horizontal guy wire holes.  The forestay angle is 70° from horizontal and, therefore, the guy wires' plane.  These end caps will be CNC machined from billet 6061-T6 aluminum then electropolished to reduce stress points before hard anodizing.  Generous internal- and external-radii will be applied, but are not modeled in these design prototypes for analytical simplicity.
This transparent rendering, looking at the forward end-cap's upper side,  illustrates the relationship of the spar pockets, thru holes and counterbores.  The carbon spars are Ø3 OD for simplicity of fabrication.  Building 35' long tapered mandrels would be optimum but unjustifiably expensive.  Elastomeric discs, inserted into each spar pocket, will pad the components' interface and prevent excessive point-contact stresses.
The carbon spars are isolated from the main cage (shown in green) by molded-in-place elastomeric bushings during boom assembly.  The radial gaps between the spars and cage tubes can be seen in this view.  The lack of mechanical attachments (i.e. rings or set screws) eliminates stress points which could induce failure of the carbon spars, plus isolates boom vibrations from the mast bearings.  The mast is also elastomerically centered in the integral Mast Tube of SEDATION 4  which should result in quieter sailing.
The guy wires are split at the mast, to permit boom rotation, and attach to the lower bearing disc.  Recessed pockets on the disc's underside (shown at right) accommodate the guy wires' end terminations.  The disc rotates on a plastic ring bearing installed above.  A simple sleeve, which separates the upper and lower plastic bearings, provides vertical location of the boom assembly.
Viewed from the underside, the apparent simplicity of the "bucky boom" rig can be appreciated.  The lower bearing (shown in orange) and the upper bearing (metallic colored) transmit all forces to the mast.  The main cage's final configuration will be much more complex than the design model shown here.  A radial clew track will occupy the upper forward surface on the main cage for the self-tacking jib.  This should automagically provide the optimum 5° slot angle between the foresail and mainsail.  An attachment point for the mainsail's luff track, at the cage's rear, is shown but will probably be optimized later.
Space between the bowed spars provides an ideal mainsail storage environment.  Simple "tennis net" material can hold the sail while providing ventilation for quick drying, unlike the previous "canoe hull" design.  Integrating a "lazy jack" furling system is trivial for this configuration.
The boom is positioned vertically with the guy wires 76" above side deck level to avoid noggin' bashing :)  Finite Element Analysis of the carbon spars' loading is next.  Results of the FEA will determine the spars' optimum laminate schedule and an iterative tweaking of all components' geometries will follow.  The masthead rigging design will be quite a chore but the elegantly simple configuration will be worth the effort.  In the process of documenting this project, I think further simplification is possible by eliminating one of the guy wires.  That approach will be pursued post-FEA.