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Knut Weberg

Why ballasted foils ?

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Just love the new keel-less AC boat. But why ballast the foils ? Would it not be possible to have both foils in the water, and - when overpowered - the windward foil gets a negative lift, pulling the hull down. As leeward side would have normal lift. You could probably get a lot more righting moment with such a system than you could achieve on the AC50. The more pressure on the rig, the faster you go, the more downward pull you have to windward. I understand the rolling-issue @ low speeds, but as soon as roll becomes an issue, the boat probably moves fast enough to make the foils effective

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There is this big tall thing that shoots up from the deck of the boat raises the center of gravity up above the boat, and without something to counter balance that, ass-over-teakettle it all goes.  And since the boat does not always move (you know, when the guys are getting on, or when the wind dies, or when they stop to take an impaled sunfish or whale off one of the scythes.... somehow not cleanly filleted by the blades.... and ETNZ claims the boats may be left in the water, as in, on a buoy?), they are not always going to rely upon movement to keep them pointing up.  Not to mention, these things are supposed to be relatable, meaning something like regular Joe sailor uses.... meaning not a water start like a giant sailboard.

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3 hours ago, Knut Weberg said:

Just love the new keel-less AC boat. But why ballast the foils ? Would it not be possible to have both foils in the water, and - when overpowered - the windward foil gets a negative lift, pulling the hull down. As leeward side would have normal lift. You could probably get a lot more righting moment with such a system than you could achieve on the AC50. The more pressure on the rig, the faster you go, the more downward pull you have to windward. I understand the rolling-issue @ low speeds, but as soon as roll becomes an issue, the boat probably moves fast enough to make the foils effective

The foils are so damn big that to optimize performance they need it to fly two-web-footed, without a third big web-foot at negative attack draggging alongside.

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4 hours ago, Knut Weberg said:

There is no reason at all to believe the (idiotic) claim from that article, that the boat has AVS of 90 degrees with full water ballast and no keel.

It is very reasonable to have a beamy monohull with enough RM to sail slowly without a ballast keel but yet having waterballast at small angle of heel. All it takes is to have metacenter higher than center of gravity, just like all powerboats and ships.

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I think the ballasted foils are strictly for self righting the boat in the event of a capsize. A big bonus if there's a big sea state and you have long races.

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The foils only provide righting moment / stability when moving. 

The cats had a huge beam to provide stability when they were getting up to speed. These monohulls only have the leverage once up foiling. 

All the time they are not foiling they will need something to keep that huge rig in the air. Ballasted foils are that thing. 

 

My question is, if the foils are only ballasted to provide minimal righting moment (for on birth etc) then could the AC75 be susceptible to knock downs in the pre-start? You'd have to be very careful sheeting on before you have sufficient flow over the foils. Similar to how in a dinghy if you sheet in too hard to early you slip sideways on the start, but instead of just slipping sideways they'd topple over? 

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1 hour ago, mozzy656 said:

The foils only provide righting moment / stability when moving. 

The cats had a huge beam to provide stability when they were getting up to speed. These monohulls only have the leverage once up foiling. 

All the time they are not foiling they will need something to keep that huge rig in the air. Ballasted foils are that thing. 

 

My question is, if the foils are only ballasted to provide minimal righting moment (for on birth etc) then could the AC75 be susceptible to knock downs in the pre-start? You'd have to be very careful sheeting on before you have sufficient flow over the foils. Similar to how in a dinghy if you sheet in too hard to early you slip sideways on the start, but instead of just slipping sideways they'd topple over? 

The foils provide some RM long before the mainhull is lifted into the air. Ballasted foils 1...1.5 tons each will not provide much RM at normal sailing angles, more likely barely enough to have AVS above 100 degrees, possible even less like 95 degrees. Most RM while sailing slowly is very much dependent on beam of waterline of the hull, just like in most ships, powerboats, dinghys and other unballasted monohulls. Metacentric height is very important to have any chance to gain enough speed to ever become foil borne.

They will definitely be very tippy and overpowered while sailing slowly, and prone to knock downs. Plenty of action required from the crews to adjust heeling moment produced by the sails to keep it going.

The rig is not huge at all according to Dan B, see: http://www.sail-world.com/USA/Americas-Cup---Dan-Bernasconi-explains-the-foiling-AC75---part-1/158994

Quote

With a minimal ballast package and a mast height of around 90ft the AC75 would be expected to have some stability issues when floating upright, or alongside a dock

'When the boat is floating upright, the righting moment is quite low, due to the small keel weight for that size of boat (2-3 tonnes of keel weight)”, Bernasconi says. “These will be very overpowered boats until they start foiling. 

A rig only 20% longer than hull is much shorter than in many cruising boats!!!

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3 hours ago, R15 said:

I think the ballasted foils are strictly for self righting the boat in the event of a capsize. A big bonus if there's a big sea state and you have long races.

The dimensioning case for foil weight is almost certainly based on requirement of self righting the boat, but it is not strictly for that. I'm not yet covinced 1 tons for each would be enough, but 1.5 tons on either side propably will be.

Foil weight also has significant effect on stability whenever boat is moving slowly and foil lift on leeward side is limited due to that. It is necessary to move center of gravity substantially below metacenter to allow for heeling moment and thus for the sails to accelerate the boat.

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37 minutes ago, NotSoFast said:

Ballasted foils 1...1.5 tons each will not provide much RM at normal sailing angles, more likely barely enough to have AVS above 100 degrees, possible even less like 95 degrees.

That didn't come out as I intended. 

"more likely barely enough to have AVS above 100 degrees, possible even less like 95 degrees" refers to the docking position of the foils, not sailing one!!!

In sailing configuration of foils while mostly in archimedean mode it might even be as low as only 50 degrees, but hopefully more, depending mostly on hull beam and freeboard, which are not known at this time.

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5 minutes ago, rgeek said:

May be it's because that's what they weight when you make them out of steel?

if only steel would be allowed, some money could possibly be saved

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OK. Seems there is no way around ballasted foils. But I still don't understand why the windward foil i a lever only. Of course, if you are less than overpowered, this concept makes sense, but doesn't it make sense to stick windward foil in the water with a negative lift when overpowered ? This way, you can, in effect, adjust the width of your imaginary cat

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8 minutes ago, Knut Weberg said:

OK. Seems there is no way around ballasted foils. But I still don't understand why the windward foil i a lever only. Of course, if you are less than overpowered, this concept makes sense, but doesn't it make sense to stick windward foil in the water with a negative lift when overpowered ? This way, you can, in effect, adjust the width of your imaginary cat

Speed in a foiling boat is as much about reducing drag as it is about righting moment .. ETNZ have clearly decided against having a second foil in the water. 

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47 minutes ago, Knut Weberg said:

OK. Seems there is no way around ballasted foils. But I still don't understand why the windward foil i a lever only. Of course, if you are less than overpowered, this concept makes sense, but doesn't it make sense to stick windward foil in the water with a negative lift when overpowered ? This way, you can, in effect, adjust the width of your imaginary cat

If you have a lot of material with infinite strength, make it available for all the AC teams and they probably will allow negative lift and make their plans accordingly. Without such material having a boat with practically speaking almost unlimited load is very difficult to design and engineer to hold up, and it would be extremely heavy to even try doing that. It's not a trimaran with a shroud connecting mast and windward float taking the load effectively. Heavy mast also due to enormous loads and narrow staying base means more weight for the canting keels, which they call T-foils, as long as they want it to be self righting.

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well, someone needs to do the maths and i guess ETNZ have done exactly that.... a ballasted foil lifted to windward creates RM... the leeward foil creates lift.... but how much mass in each?

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9 minutes ago, Tropical Madness said:

well, someone needs to do the maths and i guess ETNZ have done exactly that.... a ballasted foil lifted to windward creates RM... the leeward foil creates lift.... but how much mass in each?

Think  1-1,5 tonnes per foil was estimated Yes, and I assume that the windward foil won't be heavier ;)

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16 hours ago, Kenny Dumas said:

1.5 tons on a 15 ft lever arm aint too bad.  And it's nothing like the power required to move most canters.

Where did you get lever arm value of the 15 ft?

Anyway that righting moment 1.5 tons * leverage is only small part of total RM when boat is moving and leeward foil is providing upwards lift, whether foiling yet or not.

The main purpose of ballasted foils is ability to self righting, and initial RM from zero boat speed. Not for powering once moving fast.

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6 minutes ago, NotSoFast said:

Where did you get lever arm value of the 15 ft?

Anyway that righting moment 1.5 tons * leverage is only small part of total RM when boat is moving and leeward foil is providing upwards lift, whether foiling yet or not.

The main purpose of ballasted foils is ability to self righting, and initial RM from zero boat speed. Not for powering once moving fast.

I think DB said that the draught would be about 5m or 16 feet when foils in lowest position. Not sure if he gave any other numbers

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22 minutes ago, jonas a said:

I think DB said that the draught would be about 5m or 16 feet when foils in lowest position. Not sure if he gave any other numbers

That's my understanding as well, that's why I ask if there was somewhere other dimensions as well.

If the foil cant axis is 1 m above water, it means max distance from cant axis to the foil tip is 5 m + 1 m = 6 meters. (20 ft) If foil wing would be 3 m wide, max leverage from center of wing to axis = 6 m - 1.5 m = 4.5 m. (15 ft) But if no other number is given there is a large error limit. And foil arm has some weight as well, moving Center of Gravity of foil closer to the axis. But the leverage does not end at the axis. It's all the way to center of pressure of leeward wing of the foil.

More like 15 m than 15 ft when foiling for the foil weight. And then significantly more RM from hull & rig & crew & ... weight.

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The only moment arm that matters is the distance from the hinge to the center of mass.  If you include the boat and other foil, you need to include the ocean and earth.  15 ft is a good estimate.

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2 hours ago, Kenny Dumas said:

The only moment arm that matters is the distance from the hinge to the center of mass.  If you include the boat and other foil, you need to include the ocean and earth.  15 ft is a good estimate.

True but the location of the "hinge" varies according to the boats foiling status .. full foiling, full floating or something in between.

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Come on guys - basic physics - both wing/tees are the same so the righting arm is the combined cg of those plus the boat and combined CG of the foils isn't going to be that far off the c/line of the boat anyway,  So RM is going to be displacement times c/line to centre of effort of the combined tee and arm lift vector.

In effect seems pretty daft not to have had a fixed keel with a bulb of only 1-2 tons maybe for capsize resistance and get the sideforce for light airs a lot more efficiently than from the arms/tees, which will be draggy in that config.   Will also be crap for maneuverability in the light non-foiling airs.

Then wouldn't have needed stored power to wave the damn things around.   Might have taken a few knots off the top speed but that doesn't matter two hoots for match racing.  And they've already taken a few knots off with a semi rigid sail plan.

Look at this thing and doesn't make much sense really, trying too hard to invent something new ( fail, as ButchDS has already done that ), keep the sailmakers happy ( why? ) make it look as though the crew is actually doing something apart from winding winches ( fail as would bet you'll see selftacking headsails when/if they are used ), match racing ( fail as won't be allowed to get too close and if non foiling will be diabolically bad ).

Rant over - may all be way different when the rules themselves come out!

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