Alinghi keel

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jwilson

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Reading newspaper report this morning that Alinghi may have found a way to cant their keel legally, but no-one knows how yet....

I think I've sussed it. It will be a floppy keel. Canting keels are specifically illegal in AC rules, but keel trim tabs are allowed. If the keel stub was made floppy/springy side to side, with a trim tab near the base, hydrodynamic forces under sail to windward would swing the bulb to windward without any specific canting mechanism. Even a foot or less of movement on a bulb 15 feet down would be a guaranteed racewinner against non-moving bulb keels

Quite proud of myself. I wonder if I'll be proved right. The beauty of this in AC terms is that the keel will look absolutely normal when the boat is out of the water.

I'm not a racer, I just like to sail fast......
 
If I understand correctly (and I very probably don't), then the trim tab would be lifting the keel to windward. This is the same as the rig is trying to do, so surely it would be bad for performance. My understanding is that with a canting keel the canting force needs to be at the fulcrum so that the keel bulb stuck to windward asks in the same way as the lard sitting on the rail.
However, I have always had some interest in the idea of a trim tab on the keel so that it is 'lifted' to windward in medium wind conditions.
 
You might be onto something, but not the way you describe it working. A springy keel with that massive bulb on the end would try to remain vertical when the boat heels giving the same effect as canting the keel to leeward. However, that would probably be decreed a canting keel.
 
Sorry I think that's a non-starter. If the trim tab is going to lift the keel bulb then the weight of the bulb is supported by the hydrodynamic forces and is no longer applied to the hull. Result:- large reduction in righting moment! /forums/images/graemlins/frown.gif
 
I'm profoundly ignorant about the America's Cup so this may be a silly question. For a keel to be called canting does it have to be angled so that it is out of vertical with the hull?

If this is so, could the top of the keel simply be pushed across the bottom of the boat so that it remained vertical but the weight was all to windward? /forums/images/graemlins/confused.gif

Just a thought
 
Ye gods, what is racing sailing coming to ? Moving 15 tons of keel and bulb across the hull ? What happens in a tacking duel ?

With all this info technology, it could be transferred to a digital processor and we could all play it in a giant simulator, with people throwing buckets of cold water over us to keep in touch with real life.

Back to basics, with straw in the crews quarters, and proper yellow oilskins and Sou-westers.
 
Looking at the photo and having read Matthew's comments, it seems that there might be some flairing at the top of the fin to increase the lateral stability and reduce the sag. 20 tonnes certainly is a lot of weight to pitch at the end of a 4 metre fin.
 
It is incredible that they throw that bulb around on the end of a narrow 4 metre fin and nothing breaks.

Alinghi had a spare bulb in their compound in Auckland, and when you actually stand next to it you realise what a big chunk of metal it is. Written on the side of the bulb:
"Weight - 20 tonnes. Property of Alinghi sailing team. Do not remove." /forums/images/graemlins/grin.gif
 
The trim tab on a keel goes down to leeward, giving additional 'weather helm' effect to counteract leeway - horizontal lift to weather. This also tries to pull the keel/tab assembly to weather - ie against gravity. On a normal stiff keel there is no further movement of the bulb because of this, though it probably does cause a small amount of extra heeling. This is performance-sapping, but insignificant in comparison to the positive factor of the lift to weather effect. Look at how badly any yacht with lee helm goes to windward.

If however the keel stub was springy, this lift would lift the bulb outwards, increasing the righting moment, exactly as a canting keel does. In true canting-keelers, they move the keel bulb so far out to weather that the keel stub virtually ceases to act as a leeway-stopper, hence that function is taken over by unballsted additional daggerboards.

The dynamic forces could certainly outweigh gravity trying to bend the keel the wrong way - if dynamic forces couldn't outweigh gravity, 747s wouldn't fly, planing hulls wouldn't plane, and hydrofoils wouldn't 'foil'.

Of course Alinghi might have found a way to use rig loads to bend the keel the same way, or maybe both factors - rig load transference - and hydrodynamics - are used.....

Or it could all be a myth designed to psych out the opponents currently racing....
 
[ QUOTE ]
If however the keel stub was springy, this lift would lift the bulb outwards, increasing the righting moment, exactly as a canting keel does.

[/ QUOTE ]

No no no... If the trim tab applies a force which 'cants' the keel to weather then it's the force on the trim tab which is acting against gravity pulling the keel down - there will be no extra righting moment conferred to the hull. In fact there will be less righting moment since the force on the trim tab will also confer a heeling moment to the hull.

Mike
 
"Another sailing website" thinks it may work like this.

bendykeel.jpg


I think that's more likely. And very clever.
 
Now what if those rods were connected to the shrouds on their respective side? The increased tension on the windward shroud would do the same job, possibly with the aid of some multiplying device such as a pulley system.

Cameron
 
As you say, very clever. If true it would encourage the development of intentionally bendy fins. I'm not sure it would be legal, though.

One of the Public Interpretations apparently reads:

'No, any induced deflection or alteration of the position of a fixed appendage would contravene the definition of "fixed".'

I suspect that this very clever system (if it exists) would fall foul of this interpretation and the rule giving rise to it. If I were on a boat that had this, I would definitely want to get a clear ruling on it before it was too late.

Edit - I also can't imagine the stresses involved, and how much extra structure would need to be built up in order to accomodate them. Maybe not that much, but maybe a lot. I also would not volunteer to be the guy that releases that mechanism just before the tack - that must go with a bang!
 
I think it all depends on the exact wording of the rule. As I guess you could argue that you didn't induce the bend, you just fixed it.
And I agree, those rods must be massively strong.

Not totally sure about the idea of linking it to the shrouds though, can't imagine that the crew would want to give up the very fine adjustment in the shrouds.
 
You are correct that the forces exerted on the keel from lift will be disadvantageous, but this will be more than overcome by the movement to windward of the overall CG of the hull/rig/keel combination. It doesn't matter much where the weight to windward is (eg 20 crew on windward rail or keel canted to windward) as long as it has moved the CG of the hull up to windward.
 
Sorry, I still disagree. Maybe we're at cross purposes. Take the bendiness to the extreme where the keel is effectively just hinged on the bottom of the hull. Now add a trim tab or foil which makes the keel 'fly' to windward of the centre-line of the hull. You've moved the CoG of the keel, but it's weight is being supported by the water flowing over the foil (tab) and is not contributing righting moment to the hull.

Mike
 
Agreed. One can also think of it another way - imagine a second boat were sailing along beside Alinghi, with ropes attached to the bulb. Ignoring the drag of the ropes in the water, it pulls the bulb towards the surface (achieving the same effect as a trim tab pushing the fin/bulb towards the surface) and reduces - not increases - the righting moment.

Or think of it another way. The trim tab in your scenario (edit - sorry, not in your scenario but in the other scenario) is lifting the bulb (counteracting the force of gravity) - effectively reducing the weight of the bulb, as measured by the righting moment it applies to the stub of the keel fin. Carried to extremes, the "lift" will bring the bulb all the way to the surface - and push the spreaders into the water. OOPS.
 
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