In their story “Working-Class Wing” (Professional BoatBuilder magazine No. 140, page 60), Greg and Patrick Johnston describe their development of the variable geometry wing sail, or VGWS, as a practical alternative to the articulated rigid-wing sails that are gaining notoriety in the run-up to the 2013 America’s Cup. The Johnstons’ wing has the advantage of being composed of two fabric skins over flexible composite battens that respond to changes in outhaul tension and controlled mast articulation to impart varying degrees of curvature to the foil-shaped wing sail. The result is an extremely adjustable foil that is durable, affordable, reefable, and furlable—all traits that, so far, elude rigid wings.
Details of mast construction and staying, and specialized boom design for a VGWS rig were too extensive to include in the print edition of the magazine. They are available here, and more information can be found at the Johnstons’ website, www.advancedwingsystems.com. —Ed.
VGWS/Mini Transat Rig Details
The design details below have been sucessfully used on our rigs over many years and have proved to be simple, robust, and reliable. These are only guides, and I expect that the design and size of critical components will be engineered as required.
The mast section is suitable for a Mini Transat extruded-alloy mast tapered above the hounds. Sail tracks are set at an angle to enable taper and minimize weight. Mast is extruded with the sail track intergral. The upper tapered section of the mast is joined with an angled section. It’s simple and cost effective, with no welding required, as the tracks are bonded and riveted into position.
Untapered Section size
- Fore and aft: 5.7″ (144mm)
- Side to side: 7.1″ (180mm)
- Wall thickness: 0.07″ (1.8mm)
Linear Taper above hounds over 3.3′ (1m) to section size
- Fore and aft: 2.8″ (70mm)
- Side to side: 3.9″ (98mm)
- Wall thickness: 0.07″ (1.8mm)
- Estimated weight: 1.6 lb/ft (2.39 kg/m)
Hounds fitting has tang for cap shrouds above the forestay tang in order to shift the compressive loads back toward the neutral axis of the mast section. A sheave for the jib is incorporated, and the support wire for the spreaders is attached just below the jib sheave. We recommend fitting a bonded, tapered doubler inside the mast section at the hounds fitting and extending down one mast diameter below the fitting.
- Stainless steel 316 TIG-welded
- Tang thickness: 0.16″ (4mm)
- Curved base plate thickness: 0.08″ (2mm)
- Sheave webs thickness: 0.08″ (2mm)
- Spreader support tang thickness: 0.08″ x 0.08″ (2mm x 2mm)
- Jib sheave size: 60×10
- Fasteners: 1/4″ and 3/16″ Monel rivets
- Alloy doubler backing-plate thickness: 0.07″ (1.8mm)
- Tangs drilled for toggle pins
Spreader Hinge Detail
Can pivot about the horizontal axis and is fixed about the vertical axix.
Spreader attachment is with two pins to the circular stainless steel band. This sits on a plastic bearing surface with flanges at top and bottom to capture the stainless band. The band runs inside the sail tracks via reinforced slots in the mast. The spreaders are then supported by an upper wire that runs up to the hounds fitting. This arrangement allows the spreader to move with rig loading while keeping the band in a constant orientation to the mast.
Standing rigging is 1×19 stainless steel wire rope for the forestay, cap shrouds attached to the hounds fitting via toggles, and lower shrouds that are very lightweight as they are used only to locate the spreaders and control mast bend. The upper wire supporting the spreaders is the same size as the lowers. For a Mini Transat, we expect the sizes to be:
- Forestay: 1×19 1/4″ diameter
- Lowers: 1×19 1/8″ diameter
- Upper spreaders support: 1/8″ diameter (or larger if a forward baby stay is used).
All running rigging exits near the front of the mast at approximately 3.9′ (1.2m) above the base and is staggered so each is separated vertically by 2x the slot length. Halyards can then be turned through blocks and led aft to jammers. Full mast rotation will result in a length change of approx. 5/32″ (4mm) and will not restrict sailing operation. The main halyard should be a 2:1 system with twin small-diameter tails led through exit blocks at the top of the sail tracks; this keeps the loads down. A block is attached to these inside the mast, a larger diameter rope fed through it and dead-ended inside just above the exit slot near the mast base. This rope then exits the slot and is turned at the deck. The main-halyard exit needs to be the lowest one on the mast to ensure that the tails can be pulled down enough to attach to the sails. Jib halyard is as per normal, exiting via a slot near the mast front. Spinnaker halyard is as per jib. Any other halyards or lazyjacks, etc. can be arranged externally on the rear face of the mast. Boom outhaul is a multi-purchase with the tail exiting the boom near the g/neck and led to a turning block close to the mast base. Typically a 4:1 system will allow the tail length to change with mast rotation while having a small effect on the outhaul.
We presently use this system for our spinnaker and outhaul with no issues or problems with mast rotation.
The boom has a deep section hinge allowing horizontal articulation a small distance behind the gooseneck to facilitate mast rotation with respect to the boom clew end. In this rig the gooseneck is a hinge near the mast centerline that accommodates the vertical articulation, or up-and-down movement in the boom, while the horizontal articulation of the forward boom section is determined entirely by the rotation of the mast.
Mast Base Pivot
The pivot we use is a stainless steel ball that sits in an alloy base plug; it can be captured via a keeper plate. The pivot ball is attached to a hinged base plate to allow the mast to be stepped, raised, and lowered. I would increase the size of the fitting and the keeper to ensure that the mast cannot become detached from the boat in a dismasting. We have been using this method for many years, and it works well.
Rig Set Up With Single Swept-back Spreaders
Sail top chord is set at 0.4 chord at the boom. The upper mast panel is bent aft to get the hounds fitting as close as possible to alignment with the vertical rotational center of the section. This then stands the top of the mast up to windward with rotation; also, when rotation is dropped, the section then lies off just a little. This works fine.
Sail Plan to Suit RG 650
The arrangement is a single swept-back spreader with spreader at 55% of P (length of the sail’s luff) above the base pivot pin, with a forward baby stay attached to the spreader, if required. (This may require increasing the size of the upper-spreader support wire.) We currently use a spreader swept back at 25°, although 21° will be fine. Running backstays, etc. can be attached near the sail tracks on the forward surface of the mast.