Luc and Raps have expressed interest in foiling.  I’m well aware that Rob’s focus is upon simplicity and that foils add complexity.  So I’ll commence by discussing the general merits and demerits of foils before discussing specifics.

Added weight and change of design loads goes without saying.  But the benefits of efficient foils have been demonstrated from Moths to C-Class, AC72s and the likes of Sail Rocket: dramatic increases in potential speed.  If the design isn’t efficient then it’s possible to add complexity without additional speed – see discussion of Rocker the C-Class cat.

Another concern Rob raises is risk of breakage.  For this reason, HP foils are beam rather than hull mounted, thereby reducing the risk of hull damage.  One point to consider concerning foiling is that when up on foils the risk of hull damage is further decreased.  If the foils are not mounted through the hull and can kick up in the event of an impact then arguably the hull may have improved prospects for surviving impact with a semi-submerged object.  I’d rather sacrifice a foil than hull integrity.

The next concern of Rob’s (from our past discussions) relates to what happens when a foil is fouled (by plastic bags or weeds) or broken by solid objects.  Could this precipitate a (pitch pole) capsize?  The empirical evidence is thin on the ground.  Suffice to say, increased speed = increased risk.  Can a catastrophic capsize be ruled out?  No, but the risks are not limited to foiling.  The quest for speed through ever taller or larger rigs entails risk.  Perhaps an argument for foils and wing rigs is that increased efficiency (higher lift to overall drag) allows for a (ever so slightly) smaller rig. This question can’t be answered more conclusively in the absence of a discussion of the specifics of foiling.

So what is the empirical evidence?  Hydroptere and AC72’s have crashed catastrophically.  C-Fly broke the shaft on a canard rudder (I was never a fan of the use of aluminium in this location) yet landed gracefully despite a speed of 27 knots at the time.  A sample of one doesn’t amount to statistical support for the concept being fail safe in this situation.  The other craft may well have been travelling a lot faster.

That said, the C-foil concept is far more appealing offshore.  Luc observed:

It is hard not to be impressed by the inherent stability of the UptiP foils in recent racing catamarans (GC32, Nacra20 FCS, Flying Phantom, ... ). I wonder in what configuration these could be made useful in a HP?

If my understanding for a catamaran is up to date, the main foil is designed to carry 80% of the weight and the rudder foil about 20%. The vertical part of the main foil prevents leeway, the horizontal part provides lift, the twist and hook stabilize pitch, height and yaw. The rudder foil controls yaw and trims pitch. The main windward foil retracts, the rudder foils do not. 

The problem is that foiling is a dynamic situation, brought about by wave conditions and changes in apparent wind due to gust and changes of direction.  Often the rudder foil is required to provide down force.  After all, the rudder has to counteract the rotational pitch poling force that arises due to the rig centre of effort being above that of the drag centre of effort (6 degrees of freedom and all that).  If the rudder cavitates and breaks the surface – catastrophic foiling failure.  If the bow buries with a surface piercing bow foil, the foil area increases and so does the lift.  Whereas both systems can provide automatic pitch control (t-foils with flaps and wands) the canard foil has a smooth rather than catastrophic transition to bow down displacement.

Captian rapscallion observed:

Why not go all the way and make the boat a foiler??
Use the foil configuration found at the bow of C-fly on the leeward hull.

I’m interested in this concept but it will be no mean feat to get it right (and very easy to get it wrong).  C-fly are keeping their cards close to their chests. Patents and seeking commercial partners etc.  That said, their patent is presumably limited to the super-ventilated canard foil they’ve developed – but not the super-ventilated foil configuration.  See the picture of HMCS Bras D’or, upon which this super-ventilated canard foil technology was created.

Where C-fly, I believe, have further developed this technology can be seen in the image below:

I’m not going to explain the differences between cavitating foils, base ventilated foils (Sail Rocket), and super-ventilated foils.  Suffice to say, replicating the foil seen below will undoubtedly face property rights hurdles and a steep learning curve.  C-fly have been developing this for 15 years, much time spent tank testing.  This is literally rocket science (check the guys bios).

A final observation: foiling provides a smoother ride which may prove to be less fatiguing, perhaps increasing the capacity of the crew to exercise their seamanship skills.

Much of the preceding is of a conjectural nature.  There are no definitive answers to the question of whether to foil performance craft or not.  In the final analysis, foiling may end up winning most races, but won’t create the degree of accessibility that simplicity does.  Then again, look at the life (participation, viewing and commercial interest) that has been stirred into the Moth class, C-Class and AC class.

David