Effect of Hydrofoils and Canting Masts on Single Handed Sailing Catamarans by Crunderwood

Part one: a short description of Hydrofoils

Hydrofoils are essentially “water wings” – surfaces which produce lift submerged below the waterline resulting in the vessel being lifted from the water. Due to the relative density of water to air being 0.9982 kg/m^3 for fresh water, a hydrofoil can be up to 1000 times smaller than an aerofoil which produces the same amount of lift. Equally, a hydrofoil does not require the same velocity as an aerofoil to produce significant lift.
A great benefit of hydrofoils is the ability to “foil” upwind. The vast majority of high performance racing yachts have a wide beam with a relatively flat underside of the hull. This increases the potential for speed off the wind as the yacht is able to increase her effective waterline length. As the yacht starts to plane, her waterline length increases by 15%, which allows the yacht’s speed to increase. As the speed increases, the effective water line increases, thereby allowing the yacht to increase in speed exponentially (as long as the wind velocity provides the required power).
For a hydrofoil, however, the yacht lifts above the water at a speed much lower than her planing velocity. As she rises, her waterline length drops dramatically – for some yachts dropping by 98% – which according to speed-length equations should dramatically decrease in speed. This is not the case however as the skin friction also drops by equally large percentages – for an AC72, the drop in skin friction could be as high as 86%.
With enough power in the sails to produce a speed high enough to foil, a drop in skin friction on over 40% is enough to overcome hull speed, and once above hull speed, the unfavourable amplification of wave height due to constructive interference diminishes as speed increases.

Part two: Mast cant

Among the vast majority of racing yacht classes, canting the mast is forbidden according to the rules, however, there exists sufficient classes where mast cant is not specifically forbidden which allows for this to be considered.
Cant masts have great advantages over regular masts, for one the mast can be canted to windward, thereby increasing the apparent sail area. As the boat heels, normal masts become less efficient and induce drag and stress by trying to force the boat under the water due to the pressure created by the air flowing over the top of the sail. Cant masts keep the sail closer to vertical, increasing efficiency and reducing the down force on the hull. They can also increase efficiency by canting the sail to windward, this increases the apparent sail area, to more than the static sail area. When the mast is canted to windward it also decreases drag by lifting the hull instead of forcing it down. It works just like a windsurfer does, in fact windsurfers use cant masts and they have been around for a long time.

Part three: Single handed sailing Catamarans

The vessel being considered is an A Class Catamaran. “About the A-Class
A-Class catamarans are the fastest single handed racing boats in the world.
Most other racing classes are one-design whereas the A-Class is a development class and as a result it has become a pure high-tech boat.” As a development class, the A Class Catamaran allows for innovation within the class rules, meaning that hydrofoils and canting masts would be acceptable.

Part four: Application of concepts

Due to recent innovations in class construction, hydrofoils are regularly seen at championship events with several receiving high placings at the 2014 world championships. Speaking with several helmsmen of these boats, the set-up of the foils was relatively similar; the T-foil on the rudder was controlled using a twist rod system in the tiller, and the forward foils were raised and lowered as a standard daggerboard. Fore and aft rake of the forward foil was controlled by a system of blocks set up behind the foil on the topsides, allowing for the foil to be raked upwards (water pressure from forward motion results in raking downwards of the foil).
The canting mast system, however, is yet to be introduced to single handed sailing. This is due to the increase in thought process that even simple manoeuvres would require. For example, consider putting a foiling A Class through a tack:
1.       Drop leeward foil
2.       Rake foils to identical pitch
3.       As you steer into the wind, pull in the mainsail.
4.       Shift weight to centre of the boat whilst controlling the turn
5.       Steer slightly off the wind to heel boat slightly, release mainsail slightly
6.       Shift weight outboard to bring boat upright whilst raising old foil
7.       Pull hard in on mainsail, whilst bringing weight down horizontally.

This manoeuvre takes around three seconds for the highest ranked sailors. Now consider a canting mast being involved:
1.       Drop leeward foil
2.       Rake foils to identical pitch
3.       As you steer into the wind, pull in the mainsail.
4.       Shift weight to centre of the boat whilst controlling the turn, also release mast can to vertical
5.       Steer slightly off the wind to heel boat slightly, release mainsail slightly
6.       Shift weight outboard to bring boat upright whilst raising old foil, also cant mast to new windward
7.       Pull hard in on mainsail, whilst bringing weight down horizontally, finish canting mast to windward

Whilst only a few words long, the increase in effort results in greater energy expenditure, especially aboard an A Class where all work is manual (energy saved by use of mechanical devices is not worth the added weight).

Part five: Effect on Vessel

The largest perceived benefit of a canting mast and hydrofoils is the decrease in required speed to foil. This is due to lift provided by canting the rig to windward as the vessel remains relatively flat. This would increase the speed potential in light wind conditions, providing a great advantage over other foiling catamarans in these conditions. Similarly, the ability to cant the mast to leeward in heavy wind conditions provides the benefit of capsize prevention.
The largest perceived disadvantage of a canting mast and hydrofoils is energy expenditure during race conditions. I one round of a world championship race, competitors can expect to tack over 40 times. Increasing energy usage by a significant amount requires a way to restore energy reserves mid race, adding one more item for the sailor to worry about.

Conclusion:

Despite there being a large number of perceived benefits and disadvantages, no concrete conclusion can be drawn without further research. This means that no effect can be guaranteed until such a setup has been trialed at a world championship event.

Sources:

For further information concerning A Class Catamarans
For further information concerning Hydrofoils
For further information concerning Canting Masts – Not used for original thesis

The majority of information from the original thesis came from the libraries of the University of Strathclyde, Seattle University and MIT.
Information provided by sailors at 2014 Worlds provided in person.

I trialed a Canting Mast and Hydrofoils on an A Class in late 2015, however due to not being an advanced Catamaran sailor, I cannot comment further on the Effects

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