Skimming Disk Amas Plans and Templates Download

Skimming Disc Amas
Designed by: Mark S. Palmquist
Dimensions: 23.875" x 36" x 9"H [60.6 cm x 91.4cm x 22.8cm H]

Skimming disc amas are designed to provide hydroplaning lift. They are easier to mount than hydroplaning wings or hydrofoils.

Advantages:
• They may be connected to a hull with a single crossbar, therefore, they don't get in the way of paddles.
• They are lighter than traditional displacement amas.
• They are positioned above the waterline so they create no drag at low speeds
• At 9 knots of boat speed they provide approximately 180 LB of lift [82 Kg]
• The lift increases with the square of the boat velocity
• At 12.7 knots of boat speed the discs produce 360 lb [162Kg] of lift
• The discs have approximately 27 lb [12Kg] of buoyancy
• They are perfect for sailing kayaks, sailing canoes and narrow, light sailboats up to 4' wide. They work best with hulls which have minimal rocker and are light weight.
• The skimming discs do 1 of 2 things: They either provide more stability with your current sail area, or, allow you to carry more sail area and go faster with less heeling and less reefing of sails.

Skimming Disc Amas - How do they work?

There are 3 kinds of lift:
• buoyancy lift
• dynamic lift, underwater
• dynamic lift, skimming

Buoyancy lift relies on creating a hole in the water. To create the hole a typical boat hull will push water sideways and downwards/upwards . Lift is equal to the weight of water displaced.

The growth in buoyancy of a hull grows in a linear fashion. With every additional inch/cm of submersion the lift and drag of an ama or float increases.

Underwater dynamic lift works by bending water downward off the trailing edge of a hydrofoil, essentially an underwater wing.

Skimming lift works by deflecting water downward off a relatively flat, inclined plane. The leading edge must be higher than the trailing edge to produce a deflection. This creates a hole in the water behind the skimmer that grows with speed. The faster the speed of the skimmer, the larger the hole. Just like displacement lift, skimming lift is equal to the volume of the hole, but with much less wetted surface area.

The heeling force of a sail grows exponentially. With a 41% increase in wind speed the heeling force doubles.

Both kinds of dynamic lift also grow exponentially. Dynamic lift can limit or eliminate heeling.

Is a skimming disc better than an underwater wing? All current sailboat speed records in the past decade have been with sailboats incorporating skimming hulls (windsurfers, kiteboards, Sailrocket 2)

Underwater foils are subject to underwater turbulence and skimming hulls are subject to surface waves. The skimming disc akas (crossbeams) are designed to flex allowing the disc to step over most waves while maintaining positive lift at all times.