SLITHERSKIFF - A Very Fast Single Person Pedal Powered Racing Watercraft

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The 4.5-metre Slitherskiff was designed in 2009 for river sprints and marathons.

Built for exceptional speed on fairly calm water, it is a pedal-powered racer with highly advanced fluid dynamical design features integrated into a lightweight surface-skimming hull, fabricated in polystyrene foam reinforced by polythene skin and steel tubing endoskeleton.

The endoskeleton provides the rigid mounting framework for seat, canopy, controls, pedal crank, sprockets, chains, Scotch Yokes, oscillator arms, hydrofoil rudders and twin propulsion tailfins.

Most fluid dynamical features are underneath. Stepped like a hydroplane in the first quarter of its length, it skims the surface above a certain speed. Air passages opening atop the hull near the bow curve down through its interior to vent under the bow through slots with overlapping sharkskin flaps; and at the hydroplane step's trailing surface. When underway, Bernoulli Effect interposes air layers between water and both undersides, reducing drag with 800 times lower fluid density and 50 times less dynamic viscosity.

Bow section air layer is trapped by ribs and main section layer by PVC skirts on each side underneath. Air can only exit at the trailing edge between the two propulsion tailfins.

From hydroplane step to trailing edge, the underside has a concave curve profile resembling a lifting aerofoil's dorsal surface. This modifies flow velocity differential, generating extra lifting force in the hull – a concave surface underneath works just like a convex surface does above.

Natural sharkskin covers all underparts. Sharkskin's epidermal denticles have little fins that reduce vorticity and spaces between denticle surfaces capture recirculating pockets of laminar flow in a reattached Reynolds flow condition – providing the lowest CD. It's why sharks can briefly attain 120 Kmh.

Topsides, a pedal crank mounts above the hydroplane step on the centre line. Up to four sprockets with a Shimano-type gearchange engage the chain. Sprocket pairs mount at intermediate positions along the centre line. At least two sprocket chains comprise the powertrain to the Scotch Yokes – possibly three. This powertrain passes under the seat to a final drive cluster behind it of up to eight sprockets with Shimano-type gearchange. Both sides of this cluster, Scotch Yokes are mounted on the same axle. Yoke oscillating plates drive thrusting arms, which connect to their corresponding tailfins.

The tailfins, oppositely synchronised, thereby waggle like fish tails without waggling the hull. Very efficient – no cavitation. They're captive to tiller-like assemblies mounted on the endoskeleton. Tailfins each comprise seven thin steel sheet laminations 30 cm deep x 30, 60, 100 and 150 cm long for graduated flexibility; a covering layer of rubber/latex/silicone; and sharkskin outer jacket.

Seat is a lightweight padded plastic bucket type on an endoskeleton-mounted slide rack, with position adjustment notches.

Mounting gear selectors, two steering levers control two coupled lifting hydrofoil rudders by rope and pulleys, mounted on tillers forward of the tailfins.

Canopy comprises hiking tent materials including nylon, flynet, velcro and flexible fibreglass rods.

Estimated prototype cost: ≈$3,000;
Potential earnings: corporate sponsorship, prizemoney, royalties;
Design applications: shipping;
Benefits: fuel efficiency.


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  • Name:
    Ciarrai Macgiolla Comghail
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