One of the odd characteristics I've noticed in foilers is that they seem to stay level side-to-side, even at high speed. Don't they heel? How is that possible?

--
Brett
2012 Goodall C2 with 2014 Hulls (warranty)
1992 Hobie 18 w/ SX Wings (Sold)
Tucson, AZ
--

Depends who is sailing them.

--
Greenville SC

Offering sails and other go fast parts for A-class catamarans
--

I was wondering if the lift could be increased on the leeward side to increase the righting moment. The S9 seems to have this capability.

--
Brett
2012 Goodall C2 with 2014 Hulls (warranty)
1992 Hobie 18 w/ SX Wings (Sold)
Tucson, AZ
--

Yes, they heel, but that's slow because it means you have loaded the boards up laterally. Lateral load equals drag.

I don't think the S9 is setup for differential lift to generate righting moment. It would be to hard to adjust the roller from tack to talk. I don't think Charlie has ever checked the flap angle, but since the gross AoA is positive, I don't think you could get enough flap angle to provide down force. It will stabilize itself without a sailor on board though...

It’s been explained to me that yes you want to fly them flat and that The leeward foil will actually provide windward force, effectively causing the boat to point higher.
The key to any boats speed is smooth input, foilers included.
Consider our non foilers, what’s fastest for us? Sail it as flat as possible while the windward hull is just out of the water. Why? To eliminate the drag of one hull while still maximizing presented sail area. If you’re foiling then hull drag isn’t a concern so you just sail it as flat as possible.

--
Joshua

Texas Gulf Coast
'82 Prindle 16 (Badfish)
'02 Hobie Wave (Unnamed Project)
‘87 Hobie 18 (Sold)
‘89 Hobie 17 (ill-advised project boat, Sold)
--

ropewalker

I was wondering if the lift could be increased on the leeward side to increase the righting moment. The S9 seems to have this capability.

I suppose in theory you could but I have heard (and seen a few times) that a boat set up to run with the windward foil up becomes quite unstable with both boards down.
I know you can induce negative lift that will suck the boat down to the water.
I am specifically referring to the Flying Phantom.

--
Joshua

Texas Gulf Coast
'82 Prindle 16 (Badfish)
'02 Hobie Wave (Unnamed Project)
‘87 Hobie 18 (Sold)
‘89 Hobie 17 (ill-advised project boat, Sold)
--

ropewalker

I was wondering if the lift could be increased on the leeward side to increase the righting moment. The S9 seems to have this capability.

In a stationary situation, the total vertical force on the leeward hull is 0, that’s why the boat doesn’t fly or sink, except for the range of motion that allows the foil. So when that hull is stable vertically, no effect on the heeling moment.

What is different, I guess, is on the dynamic part, on a puff for instance. Both hulls will tend to lift and now the leeward hull can actually lift. At that period of time the axis of rotation is no longer the leeward hull but the center of gravity of the boat, and therefore there is a different effect on heeling. But when the puff is over and conditions are stable again there’s no permanent effect. I think that may explain why “it takes longer”, as said above.
Of course things are more dynamic than stationary, so the final effect is probably different all the time in practice.
That’s my academic analysis (or guess). Wish I had that practice... not happening in the near future unfortunately..



Edited by Andinista on May 10, 2019 - 07:49 AM.

We foilers _feel_ just like anyone else! How dare you...

Ohhh, you mean _heel_ --

When you are foiling, you are working constantly to trim -- usually the main -- to keep it flat. The main is carried depowered, if needed, to keep the boat flat or with minimal heel. "Incorrect" sail trim, spilling some wind, is faster than perfect trim on a foiler. Very odd, but that's what it is.

When downwind, the target is flat or minimal leeward heel. Upwind, gentle windward heel is very advantageous.

The S9 has extra powerful ride height control flaps, so they contribute a lot to keep the boat flat. It's an outlier -- this effect is so strong that in some conditions it'll keep foiling, flat, after the sailor has fallen overboard.

Why do we want to keep it flat? Because the foil horizontals ("wings") have a particular angle (slight dihedral like airplanes), and you want to maximize the lift, particularly at takeoff time. And the dihedral contributes stability, just like on an airplane.

When heading upwind, the best configuration is slight windward heel. This looks and feels "wrong" from a cat/dinghy perspective, but windsurfers are the right model to think of. If you can get the sail to be "on top of you", now the sail is lifting you. Reduces load on the foils, so less drag == faster. Then, the lift from the wings is now pushing you to windward, so you track better. Moths/Wazps/UFOs do this pretty aggressively, on cats you do it just a little bit due to the width of the platform.

On downwind legs, it's tempting to use windward heel, but it's too unstable; it's harder to stop the roll to windward (like on a dinghy) and you end up with the boat capsizing on top of you. Talking with various foiling cat sailors, people find that carrying the kite makes the boat more stable, even in strong wind, when foiling downwind. So kite and a bit of leeward heel. It's also damn fast.

Foiling cats do use differential rake (more AoA on leeward foil, less on windward foil). You can set this on S9 and Whisper, however it's damn fiddly to switch it when you tack. So I'd do it sometimes on my Whisper when the conditions are marginal but it's not my favorite. The Nacra 17, Flying Phantom and other race oriented foilers have the same foil controls but you can trim from the wire, so the crew will adjust them on every tack/gybe.

One more thing - when foiling, the boat is super sensitive to rudder, twitchy. And the rudder has wings. If a rudder lifts out of the water, its reentry is _uneven_, as one of the wingtips enters ahead of the other. So you get a jolt on the rudder, which jolts the rudder that is in the water. Because the boat is so twitchy, it can jolt the whole boat. My arms are always in tension when steering as I am trying to control for that kind of thing.

Martin;
Thanks for that great description.
It sounds like the technology is developing rapidly.
One of these isn't in my budget yet, but I like to keep up with new developments.

--
Brett
2012 Goodall C2 with 2014 Hulls (warranty)
1992 Hobie 18 w/ SX Wings (Sold)
Tucson, AZ
--