AC 75

capta

capta

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Jun 4, 2009
5,091
Pearson 530 Admiralty Bay, Bequia SVG
I just saw a pretty well done video on the AC 75 LaRoche-Posay, and I must say, it may be the ugliest sailboat I've ever seen. It was also impossible to look away, lol.
I'm a fan of foils, and were I in my 30s, I'd probably be putting in a great deal of effort into figuring out how to include foils in a comfortable cruising boat. Properly designed foils could really calm the motion in heavy weather, even if one is only moving between 5 and 10 knots. Of course, sailing flatter water in reasonable winds, one could really zip between the Windward and Leeward Islands, at breathtaking speeds, should one wish.
Zoom, zoom, zoom.
 
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dLj

dLj

.
Mar 23, 2017
5,158
Belliure 41 Back in the Chesapeake
I have to agree - foil boats are really ugly - but man would I love to sail on one! A good foiling cruising boat would be a game changer! Not that I'd ever be able to afford one...

dj
 
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shemandr

shemandr

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Jan 1, 2006
8,072
Slickcraft 26 Sailfish
Not only are they ugly but they lack grace in their motion. They remind me of a Pelican landing. Constantly on the edge of a crash.
 
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C

colemj

.
Jul 13, 2004
1,126
Dolphin Catamaran Dolphin 460 Mystic, CT
These large foiling monohulls came about because the owner of the defender of the Americas Cup hated Larry Ellison and wanted to get away from his foiling catamarans out of spite.

Foiling catamarans make immanent sense, are relatively easy to design with simpler engineering, fewer critical mechanics, and look more natural (see pic).

There are even several production cruising catamaran designs that now use partial foiling with C-foils to decrease displacement and increase lift. Probably trimarans also, but I'm not as tuned on those.

I agree with many that those foiling monohulls with the bug arms are butt-ugly, an engineering nightmare, and terribly awkward. However, there are designs being made that partially lift the boat, like described above for catamarans. The IMOCA 60 Dali foils are an example. They are still much more complicated than achieving the same thing on a catamaran.

IMO, full foiling will never be a cruising thing. Just too dangerous hitting a wave wrong and crashing back down at foiling speed. Not to mention the maintenance of the foiling systems - particularly on monohulls.

However, using foils to reduce displacement short of full foiling are already here in cruising catamarans, and I expect this to be around the corner in performance-oriented cruising monohulls.

Mark

Screenshot 2026-08-22 at 9.37.32 AM.jpg
 
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capta

capta

.
Jun 4, 2009
5,091
Pearson 530 Admiralty Bay, Bequia SVG
The foils we used on the multihulls in the '60s had a huge front foil that folded up over to bow when not in use, and a couple at the stern. They were heavy, cumbersome things, in a time when advanced airfoils were just beginning to be investigated.
But the one thing I must say, when the foils were down and the vessel sailing lowly, the foils definitely dampened the motion. However, as practical equipment on a cruising boat, they would increase draft and make docking a more exacting operation, probably not a good trade off..
 
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shemandr

shemandr

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Jan 1, 2006
8,072
Slickcraft 26 Sailfish
When I see wings on an airplane that resemble the foils in Mark’s pics I’m getting off. I wonder what Arvel Gentry would think?
 
C

colemj

.
Jul 13, 2004
1,126
Dolphin Catamaran Dolphin 460 Mystic, CT
capta said:
The foils we used on the multihulls in the '60s had a huge front foil that folded up over to bow when not in use, and a couple at the stern. They were heavy, cumbersome things, in a time when advanced airfoils were just beginning to be investigated.
But the one thing I must say, when the foils were down and the vessel sailing lowly, the foils definitely dampened the motion. However, as practical equipment on a cruising boat, they would increase draft and make docking a more exacting operation, probably not a good trade off..
Click to expand
Foils on multihulls today are really just reshaped daggerboards, and raise/lower the same as they have for eons. The difference is the materials technology that make these foils possible without breaking, along with boat designs that produce faster boats that make the foils more efficient.

That Gunboat G4 in the pic above is a bit of an extreme example, but even those foils are pretty a much normal daggerboard-type and tuck up under the hull when not deployed. The C-foils on cruising catamarans that partially lift the boat are just regular daggerboards with advanced materials and shapes.

Mark
 
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colemj

.
Jul 13, 2004
1,126
Dolphin Catamaran Dolphin 460 Mystic, CT
shemandr said:
When I see wings on an airplane that resemble the foils in Mark’s pics I’m getting off. I wonder what Arvel Gentry would think?
Click to expand
Likewise, when I see keels on sailboats that look like airplane wings, I'm getting off.

Water and air are completely different mediums when it comes to foil sizes and shapes. Consider that just one of those 13' AC75 foils fully lifts the 17,000lb boat. To put that in perspective, a Learjet 60 weighs 2,000lbs less and needs a 43' foil to lift it. And the Learjet has speed that allows a wing that short.

Also consider that solid sails on sailboats DO look like airplane wings. Complete with flaps, spoilers, and ailerons.

I thought Arvel Gentry's expertise was in sails, and not foils?

Mark
 
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shemandr

shemandr

.
Jan 1, 2006
8,072
Slickcraft 26 Sailfish
No argument there.
I’m just amazed that those relatively small foils can lift a 17,000 boat! Yeah I get air and water are different but I don’t know anything about fluid dynamics. I can sorta understand that the more dense fluid, water, could create more lift than air. I appreciate the comparison to the Learjet as it furthers my amazement. I don’t even know how you factor in the speed of a jet vs. a sailboat -even a foiling one. But the foilers are out there so empirically it must work.
I think Arvel’s expertise was in foils - I believe he was an aeronautical engineer by training.
 
thinwater

thinwater

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Mar 26, 2011
4,007
Corsair F-24 MK I Deale, MD
Form follows function. If it works, it is beautiful.

Not for cruising or casual sailing, of course. Single purpose.
 
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jssailem

jssailem

SBO Weather and Forecasting Forum Jim & John
Oct 22, 2014
24,696
CAL 35 Cruiser #21 moored EVERETT WA
shemandr said:
I think Arvel’s expertise was in foils - I believe he was an aeronautical engineer by training.
Click to expand
Yes he was an Aeronautical Engineer. Worked for Boeing in So Cal. Convinced Boeing that they shoul allow him to use their wind tunnel and computers to work on the sails for the America’s Cup boats.

Here is one of his practical articles from 1973.
 

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jssailem

jssailem

SBO Weather and Forecasting Forum Jim & John
Oct 22, 2014
24,696
CAL 35 Cruiser #21 moored EVERETT WA
If the studies that Gentry made in the 1970’s interest you, here is a link to some of his work. Articles
I looked for the original site today and was greater to spam. The original site appears to be down.

North Sails current designs were heavily influenced by Gentry’s work.
 
dLj

dLj

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Mar 23, 2017
5,158
Belliure 41 Back in the Chesapeake
jssailem said:
If the studies that Gentry made in the 1970’s interest you, here is a link to some of his work. Articles
I looked for the original site today and was greater to spam. The original site appears to be down.

North Sails current designs were heavily influenced by Gentry’s work.
Click to expand
I believe his daughter is, or was, working on a web site to house all of his work. I've no idea where that stands.

dj
 
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