Increase your boat’s speed by up to 17%
Adding a pair of skimming disc outriggers to your canoe, kayak, or small monohull can greatly increase speed potential.
Skimming Disc Outriggers
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 outriggers
• 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.
Where to buy?
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visual comparison
The chart and images below show the equivalent sized float needed to meet the same total lift generated by the skimming disc. Note, with increasing boat speed, the dynamic lift of the skimming disc grows while the lift of the floats remain the same.
The faster the boat travels through the water, the larger the hole,
The larger the hole, the greater the lift.
The 90% of the energy required to create the hole in the water behind the disc is imparted into the disc’s upward lift and only 10% results in drag.
The skimming disc’s wetted area decreases with increasing speed while a typical outrigger’s wetted area will increase with increasing hull speed. It is the decreasing drag that gives the skimming disc the advantage.
the science…
Comparing the force leverage arm of the sail and a leeward skimming disc ama (or hydroplaning wing)
Torque= distance x force x angle of attack
Assuming the angle of attack of the sail and the disc are similar,
Lift Force= surface area x fluid density x velocity^2 x lift coefficient
Assuming the center of pressure of the sail is 10 feet up and the disc is 5 feet out,
The distance arm of the sail is 2x the disc
Assuming a sail area of 100 sf and a disc area of 5.7 sf
The surface area of the sail is 17.5x the disc
Assuming the sail is a double sided airfoil of zero thickness and the disc is a single sided flat plate
The lift coefficient of the sail is 2.5x the disc
Assuming the the boat and wind are traveling the same speed on a beam reach, the apparent wind speed over the sail is 1.41x greater than apparent water speed under the disc,
The sail has a 1.41^2= 2x fluid velocity advantage
Assuming the sail is bending air and the disc is deflecting water and the density of water is 850x more than air,
The disc has a 850x advantage over the sail
adding it all up…
The sail's advantages 2 x 17.5 x 2.5 x 2 = 175
The disc's advantage = 850
The disc lift has a 850/175 = 4.8x lift force magnitude advantage over the sail's heeling force
What this means…
The sail's lift is dynamic.
While an outrigger has more buoyancy lift it has no dynamic lift.
Linear buoyancy falls behind a sail's dynamic lift causing more of the outrigger to get wet with increasing speed
The disc's lift is dynamic. It grows at the same rate as the sail's lift.
However, because the disc has a built in leverage arm advantage, and planing surfaces become more efficient with increasing speed,
the wetted area of the disc (or wing) will actually decrease with increasing hull speed.
The only dynamic advantage that the sail has over the disc is apparent fluid flow.
Apparent fluid flow is greatest when tacking into the wind and least when going down wind.
Coming out of a tack, when the boat is traveling slower, the wind has a dynamic advantage for a short while until the boat accelerates.
The key is to tack quickly, or, let the sail out, fall off, and then as the boat accelerates, pull the sheets back in.
This is what windsurfers must do to keep from being pulled on top of the sail.
As long as wind speed is not 3x greater than the boat speed, the boat won't capsize.
In most cases, your boat will be traveling faster than the wind.
A wingmaran should be capable of traveling at least 2x the speed of the wind.
Inwhich case, the leverage arm of the disc or wing is actually > 4.8,
so the faster you go the less likely you are to capsize, and the more weight is shifted to the disc or wing but without pushing the lifting surface down