Sail Deck Cutback

There has been some recent discussion of cutting back the sail at the deck. It was popular in the C class in the 1970s, especially when aerodynamic engineers were involved. Patient Lady I was the most extreme, but Scimitar had a similar sail platform. I think the issues were sealing the sail to the deck, getting the area higher into stronger wind, and creating a space for the crew to cross when tacking.

Why isn't this seen more, especially with wings and single sail rigs?

Lohring Miller

 

Aren't the reasons that you mentioned pretty compelling ones? As a windsurfer I'm aware of the "closing the gap" issue but in reality a lot of the time 'closing the gap" doesn't seem to be as effective as theory claims it should be. While windsurfer sailmakers made a big deal about 'sealing the endplate' (or whatever they called it) at one time, in reality there is often a gap between the sail foot and the board* and the size of that gap doesn't seem to create a massive change in real life performance.

I seem to recall that Canoes (where you normally run around the leach so getting under the foot isn't an issue) tried and dumped low-footed sails. It would be interesting to get some windspeed readings down that low; maybe there just isn't much happening?

As always, I'll be fascinated to see what Tom and other experts say!


* the planform required to balance the board in light winds and tack the board means that there is often a gap.

 

Half the trouble, I think, is that everything around deck level is a mess of turbulence anyway. But at the recent C Class Worlds Groupama was sporting a trampoline sweeping wing foot, and it was given credit for some of their very visible upwind performance advantages.
But as for cutting the sail back and having a shorter foot and more chord further up, I think the main trouble is that your twist control goes to hell.

 

Don't forget Sail rocket:

 

Of course the newest version gets the best of both worlds with the ground effect airfoil portion adding to the main wing's aspect ratio. It's still cut away toward the base and heeling moment isn't an issue. However, lift distribution for lowest induced drag is.

Lohring Miller

 

It depends on how much lift carry-over you can achieve between the wing/sail and deck/hull. If you could get 100% of the lift to go down to the water, then you'd want a planform like a half-ellipse to get the least drag for the height of the rig. But that's not possible because the wind drops off at the surface, so the lift is going to decrease there even if the rig goes to the water.

The technical answer is the rig produces lift by deflecting the wind to windward, and you want the deflected velocity in the wake to be uniform along the span for minimum drag and taper linearly from foot to head to get minimum drag for a given heeling moment. You want the combination of planform, twist and camber to get this result.

The near-triangular shape of a conventional sail rig deflects the air more at the foot, and this results in a self-created header that reduces the lift at the foot compared to what you'd expect if the lift was proportional to the planform shape. So the lift per unit area is less at the foot than elsewhere in the rig. If you have an area limited class like the C class, this is bad news. You'll want to cut away the area at the foot, while still creating the same lift there, and put that area where it can do more good.

You can start with the kind of wake you'd like to achieve, calculate the spanwise lift distribution that would produce that wake, and then design the planform shape, camber and twist that will give you the lift distribution. Ideally, you'd have all parts of the sail loaded equally so no section was near stall when others were only lightly loaded, and the planform will have the same shape as the spanwise lift distribution. If you do this with the simplest theory (the same theory that says an elliptical wing has the least drag) and a uniform wind, you get the following shapes as you vary the size of the gap:


That theory exaggerates the effect of small gaps, but you can see the pattern. The smaller the gap, the more bottom loaded you can make the rig. Bottom loading allows you to generate more lift for the same heeling moment. You can use that edge in heeling moment to make the rig taller and reduce the drag. The tradeoff looks like this:


If you compare the latest C-class designs with earlier boats, you'll see the rigs are a lot taller. They are finding that the reduction in drag due to the larger span is worth the loss of lift that comes from being heeling moment limited with a higher center of effort.

So the short answer to your question as to why more boats don't cut away their sail at the foot is it depends on what kind of lift carry-through you expect to get at the foot, how bottom-loaded you want the rig to be, and whether or not you need to have the clew at maximum chord in order to control the twist using leech tension.

There are other boats that have used extensions of the wing below the deck-level to increase the span of the wing instead of cutting the planform away at the foot. One is Macquarie Innovation,


and another is 17:


But to make this work, you have to have the wing completely sealed to the deck so there's no gap.

 

Groupama did a clever thing, by shortening the foot chord from the front element. This way they can keep the foot depth lower downwind, when hinging the 2 elements for more power. Down low the wind is weak and app wind comes more from in front, hence you don't want lots of camber there. Apparently neither of the AC72s came to think of this, as judging by eye their camber was way too large above the deck, while trimmed for downwind.

Groupama's trampoline would be pressed to a hollow on the windward side and bulge upwards on the leeward side, so there was effectively a lift carry over to the trampoline.

 

The AC72 class rule severely limited what could be done with the planform shape. It had to fit within the shaded boundaries of the attached figure. There was an additional restriction that forbade the Groupama planform:
"10.11 The leading edge of the wing shall be straight (within a tolerance of 0.003 m) below grid C3."

The angle of attack of the foot was even less for the AC72 than the C-class because of the jib.

Aotearoa addressed this problem with main element twist, which they implemented in the lower half of the wing.

 

I wondered what the purpose of the below deck faring was on OTUSA was. I assumed it streamlined the below deck rigging. Does this actually increase the rig's aspect ratio? I would assume that interference from the hulls would make this faring ineffective unless the apparent wind angle was very small.

Lohring Miller

 

The apparent wind angle is small. Typically 20 - 25 deg.

It doesn't increase the aspect ratio - it increases the effective span.

The beams and fairings acted as winglets, too.

 

I looked up for fun during the cup and got something like 14 degrees at 10 m height. At 1 m about 11 degrees. And it did not change much for the downwind legs...

I remember you telling that the aft fairing was there to guide the foot vortex under it, to extend the effective span too. Very clever.

 

Sorry about my amateur's terminology. I was using aspect ratio (AR= Span^2/Area) and apparent span interchangeably.

Lohring Miller

 

Hi Everybody,

There is a risk I repeat what Mikko has already explained:
When looking at true wing gradient and apparent wind gradient foR an A-Cat windward in 10knts TWS. The bottom part of the wing/sail has very little opportunity for a large AoA.

If you want to design a wing which can be put at full use in these conditions, the alternative is :
Either your wing chord increases going down, with maximum chord on the trampoline (if as large as the trampoline, not very convenient for the crew to change side).
Either your leading-edge (or the part of the section in front of the mast) , must be reduced going down, it is a way to increase AoA, and get some twist with a flat structure.

A sail design software of an A-cat mate, using VLM achieves the same result with a standart mast/sail rig: It seems that if the sail chord decreases going down the tramp, there is significant lost in power, which offsets most of the end-plate gain.

Cheers Everybody

EK

 

In other words, the sharp reduction in leading-edge of Groupama C should be observed considering the shape of the leech at similar heights.

Putting a stick on the leading edhe and the trailing edge of the wing, set with a little camber, would materialized the wing chord.

While no twist in the structure, moving the stick down will exhibit a twist of the sail-chord.

Sorry for my poor English, hope it remains understandable

Cheers

EK

 

I expect much depends on the rules you are sailing under. If mast height is restricted as well as sail area, the marconi seems to be preferred. If mast height is not restricted but sail area is, a tall elliptical sail like those shown on the C Cats seems to give most bang for the buck.