Article - Issue 27, June 2006
The first Access Dinghy prototype on its maiden sail at Sandringham, seen sailing downwind under asymmetric spinnaker © Access Dinghy
Chris Mitchell is a man with vision – and a fleet of dinghies to prove it. He has helped hundreds of disabled people discover the freedom of sailing, through his Access Dinghy Foundation. Sally Wilkes found out more about his new prize-winning skiff that will power sailors all the way to the 2008 Paralympics in Beijing.
The SKUD18 (SKiff Universal Design) was selected on 7 November 2005, by the International Association of Disabled Sailing (IFDS), as the Paralympic vessel of choice in the two-person keelboat event. It then quickly snapped up its first accolade: the Australian Sports Innovation Award in February this year. The vessel is the latest and snappiest in the stable of Access Dinghy sailboats but, as Mitchell points out, it is not a disabled sailing boat per se. Rather, he is creating a universal craft that can be adapted for virtually anyone to sail, regardless of skill or physical prowess. “We take the most difficult and complicated cases and get them sailing, and the others just fall along in line.”
Skiffs for all
Admittedly the SKUD18 will take a little more taming than the Liberty; the Access Dinghy club boat design that the SKUD took inspiration from. At 5.8m long, this lead-assisted skiff (a type of dinghy) has an asymmetrical spinnaker and hi-tech rig making it the ‘turbo’ class specialised version designed for top-level performance and excitement – with a crisp response that is equally challenging for both able-bodied and disabled sailors. But while the skiff is high on thrills, spills must be contained – the Paralympic competition will be a strict one-design class to create equity for all competitors, with two sailors seated (and in most cases, strapped in) on the centreline. The addition of a spinnaker sail means the boat could get knocked down, so selfrighting and self-draining features are vital. The SKUD18 had to be designed for the worst-case tip scenario – a ventilated tetraplegic who can only move their tongue – making it imperative that all seats are clear of the water when the boat is on its side. Other design configurations allow at least one person to move around the boat, so edge seats also need to be kept out of the water during heeling (where the boat leans over). This combination of spinnaker and central seating is an innovation in this class.
Concept and design
The SKUD18 concept came to life two years ago and has involved collaboration with renowned Australian skiff designer Julian Bethwaite (who designed the Olympic class 49er skiff ), and naval architect Martin Billoch to bring it to this stage. From the beginning Mitchell knew he was going for a faster planing hull, rather than the more stable displacement hull of his earlier models. The final hull design is an 8mm PVC foam sandwich with bi-directional glass and polyester resin; a cheap and lightweight combination. The boat’s buoyancy chamber contains 500 litres of foam, forcing the skiff to remain upright with more than 100mm freeboard (the height of the boat that is above the water) when sailing with the buoyancy chamber totally flooded and the cockpit filled.
A wildly heeling dinghy needs a serious underwater counterweight, and this is provided by a bulb keel with 150kg of lead working as a righting device. Two oversized rudders also mean control is maintained during heeling, allowing the boat to heel to 45 degrees comfortably. Directional stability is also maintained by a large jib sail, which loads the bow up in powerful winds, keeping the boat on course and balancing the helm. Mitchell admits this feature was discovered by serendipity when trialling the Liberty, and was subsequently incorporated into the SKUD18. According to Mitchell, experiencing a wild heel is rather unnerving at first for a strapped-in sailor, but the helmsman has good visibility, since the stern rises in the air as the boat heels, and transparent Mylar sails give a good all-round view. The high boom removes the threat of being hit on the head – the scourge of small dinghy sailing when going about! Testing of the hull stability and performance enhancement was achieved through a mixture of experience, engineering, and trial and error, scaling down the ratios and lines of Bethwaite’s 79er skiff. According to Bethwaite, this type of boat design does not subscribe to neat equations. He fine-tuned the hull by towing the SKUD18 hull behind a speedboat and plotting drag curves, ensuring the design parameters such as keel weight, draft (keel depth in the water) and uncrewed mass displacement were maintained. The final design has low drag, meaning the boat heels fairly easily but gives a faster ride.
In technical terms, the boat has a sloop rig (this refers to the sail arrangement, including the mast, sails and ropes) with high roach, which, loosely, is the mainsail radius and an indicator of the boat’s speed capability. The rig’s auto de-powering function is a safety measure often used in boats of this class. By encouraging the top of the mast to flex, air is allowed to spill out of the sails at very high speeds in an attempt to avoid excessive heeling. The mainsail is fully battened (battens extend horizontally across the mainsail) for optimum performance and a self-tacking jib eases handling. The ‘boost function’,a tube-launched asymmetric spinnaker on a carbon fibre bowsprit, completes the sail tally, taking the boat above a speedy twelve knots. An asymmetric, rather than standard, spinnaker is particularly effective on fast planing skiffs like the SKUD18 because they are fast enough to generate an increase in apparent wind when reaching (sailing across the wind), so it is often faster to zig zag to the bottom mark rather than sailing directly downwind.
To put the challenge of designing for this demanding group of sailors into perspective, Mitchell classes someone with cerebral palsy as: “among the hardest to get sailing competitively… because they have spasms in their hands and limbs, and can have difficulty getting their hands and fingers to follow instructions from the brain. They may also have communication problems on crewed boats as speech can be impaired.” Mitchell’s team has a vast tool kit of innovative electronic servo-assist controls and winches at its disposal, to help disabled sailors make use of their strongest or most useful body part to control the boat. Generally, the most physically disabled sailor would take the helm at the rear of the boat and use a manual or servo-assisted joystick to drive the rudders, while the second person – someone with arm strength – would control the sails. It would be possible to have a severely disabled crew controlling the whole boat using rope handline functions, or for just one person to use a four-way joystick for steering and pulling in the sheets. As both sailors sit on the centreline in the Paralympic setup, traditional boundaries between helm and crew are blurred and both sailors could operate any of the controls. Servo-assist controls and winches can control the helm (with a variable speed option) and the rig. Manual controls include joystick or push/pull lines, a cleat console for easy sail control, and a single speed sheet-type winch for easy handling of other rig functions and the keel. The servo controller range includes a variety of electronic joysticks and secondary control switches. Sip and puff controllers could be incorporated, but in practice they are not widely used. The servo winches on all Access Dinghy sailboats are powered by worm drive permanent magnet DC motors, typically used for windscreen wipers but marinised to give a high IP53 corrosion resistance rating. The winches use a glass bead coated friction drum and can pull tension in both directions, so a single winch can trim both port and starboard sheets. Likewise,a single winch can raise and lower the spinnaker by hauling on the halyard (rope) attached to the head of the sail.
Mitchell has spent years working with disabled would-be sailors to come up with creative solutions for boat control. For example,a joystick can be operated with any body part, such as the hand, chin or tongue. The operating load would simply be reduced to accommodate the tongue. “Another person we got sailing could only move their tongue against their cheek, so we fixed a strip with pressure switches across the face to give three points of activation at each station. They could control the lot with that,” explains Mitchell. Another configuration uses a magnet attached to the cheek to operate reed switches. Access Dinghy are able to incorporate their own electronic systems into the SKUD18, including a 32 channel wireless transmitter that can operate 16 winches. This means that, for extremely disabled or novice sailors,a person with a controller can remain on shore and effectively override the boat’s controls should a problem occur – rather like a remote-controlled model yacht. However,Mitchell aims to keep the electronic side as simple as possible, in order to keep costs down and avoid malfunctioning. Those with sensory as well as physical disabilities can also sail the SKUD18, though they would be more limited by the weather conditions. Blind sailors can orientate themselves using a talking compass that beeps according to the optimum angle to the wind, and can communicate with a shore-based guide via a radio link. An established system of insignias and verbal communication between competing boats also exists in blind dinghy racing.
Functionality and feedback
The boat’s functionality lies as much in what it cannot do as what it can, creating equity between sailors of different physical capabilities. For example, an able-bodied dinghy sailor normally uses a manual rudder to alter the course of the boat, and makes constant (and often subconscious) microadjustments to the tiller to account for changes in direction, swell and wind sensed through the tiller. The SKUD18 does not allow this. “You can’t incorporate sensitivity of this kind into a joystick or it would disadvantage someone like a tetraplegic who may have no feeling in their hands, but has a small degree of movement,” says Mitchell, who argues that it should be against class rules to allow steering that is too light.
Full servo future
Mitchell is passionate about widening the acceptance of full servo sailing. “There’s nothing disabled about using a joystick,” is an oft-heard mantra. This may sound obvious given the joystick’s ubiquity among computer gamers, fighter jet pilots and motor boat pilots, but it is quite a leap of faith for experienced small boat sailors to hand over their manual controls to servo controllers in what is such a fast-reaction sport. Mitchell would love to see disabled and able-bodied people competing in clubs across the world using his boats. “That is our goal: for people sailing full servo to out-sail the able bodied who are sailing manually, and get disabled people joining in everyday events rather than having special classes.” Future technical developments include the possibility of a power-assisted swing seat that allows a disabled person to shift their weight during heeling. Mitchell is also working to extend accessible sailing to poorer countries around the world.
Access Dinghy Sailing Systems was supported in its development of the SKUD18 with an innovation grant from the Australian Government, through its business unit AusIndustry. For further information on the SKUD18, and Access Dinghy’s other vessels, see www.accessdinghy.org and http://www.bethwaite.com
Biography – Sally Wilkes
Sally Wilkes is a freelance science journalist based in Brisbane, Australia. Email: email@example.com