Apologies to those who might feel this subject is starting to dominate a proa forum, but it keeps cropping up.  Better to get it done and dusted me thinks.  The remained of this post is a quote from Tom Speer's Basillicus presentation.

http://www.basiliscus.com/CSYSpaper.pdf

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 "Speed is not the only reason for hydrofoils. They improve seaworthiness, too. "Amazingly, the hydrofoils help to cure most of the bad habits associated with multihulls, such as: pounding, quick motion, snap-rolling in beam seas, difficulty in selfsteering, the shaking of wind out of the sails, lee-bow burying, and pitchpoling or capsizing in freak waves." (Keiper, 1996). These attributes are due to raising the Fig. 1, Williwaw sailing at Pacific Multihull Association speed trials, 1975 hulls out of the water and away from wave disturbances, by the increased dampening provided by the hydrofoils, and through the increased stability afforded by widely spaced foil units. The latter arises from the fact that hydrodynamic forces increase with the square of the speed, while forces from buoyancy are independent of speed. Thus the boat acquires additional stability when it needs it most as it is driven hard or surfing down waves.
Hydrofoils also allow tailoring of the motions in waves through their heave stiffness. For fully submerged foils this is done through the design of the feedback control system and for surface piercing foils by the choice of chord and dihedral, which dictate the rate at which the wetted area of the foils change with
height.

"Against these advantages must be balanced some severe drawbacks for hydrofoils. Hydrofoils add weight and complexity. It is important that they be retractable when hullborne to reduce the considerable added drag due to their wetted area. When retracted, the hydrofoils add windage and may occupy deck space.
However, the most severe drawback is their propensity for collision. Even small debris or seaweed can cause damage or significantly degrade performance. And whereas the chances are a collision with a modest size object and a hull will be a glancing blow, the hydrofoil leading edge meets everything head on. This is a severe liability for a coastal cruiser where the chances of encountering debris is high. Finally, grounding of a foilborne hydrofoil can be a catastrophic event.

"In major collisions, such as with a semi-submerged shipping container, the craft has a good chance of escaping severe damage because the hydrofoils can be made to be sacrificial, breaking away and allowing the craft to decelerate more slowly while the hull passes over top of the object. The hydrofoils may be destroyed, but the boat has a better chance of avoiding catastrophic damage.
Therefore, the hydrofoil cruiser may be a niche vehicle, best suited to offshore passage making, where the chances of minor collisions are reduced, the avoidance of catastrophic damage is most needed, and where the hydrofoil's speed and seaworthiness can be a greater asset.
The need to fly exerts a powerful influence on the rest of the design, particularly with respect to the need to keep weight to a minimum, have a powerful efficient rig, to have a structure which is stiff and capable of handling the large loads at concentrated points from the hydrofoil attachments, and a hull shape which is
compatible with stable transition to foil borne operation on takeoff. These factors argue for a boat that is
designed from the outset to be a hydrofoil, rather than adding hydrofoils to an existing boat."

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