Swing Keel 'Fluttering'

Thought this subject might be interesting to some on this forum that might not have heard of the latest development with the growing popularity of swing-keeled vessels.


SCUTTLEBUTT EUROPE #488 - 28 MAY 2004

Produced by Seahorse magazine and boats.com Europe, Scuttlebutt Europe is
a digest of sailing news and opinions, regatta results, new boat and gear
information and letters from sailors. Contributions welcome, send to
editor@scuttlebutteurope.com


THE TRANSAT: BONDUELLE ANNOUNCES WITHDRAWAL FROM RACE
Plymouth, England: Following the withdrawal from The Transat yesterday of
Roland Jourdain's brand new Open 60 Sill, so this afternoon her sistership
Bonduelle has also been forced to pull out.

While the withdrawal of Sill was disappointing for Jourdain, the decision
for Bonduelle skipper Jean le Cam has been more traumatic still. Sill was
forced to pull out of The Transat after experiencing severe cavitation
problems with her keel, that the fitting of a newly cast keel bulb last
week failed to solve. In contrast Bonduelle has retired despite to date
having experienced no keel issues.

"We never had a problem with Bonduelle, but exactly the same boat did have
a problem, so we don't want to take the risk," commented designer Marc
Lombard. "It is something that is very nasty and which never happens on
sailing boats. It is something you find on aircraft bombers and is called
'fluttering'. But we are not even sure it is exactly this problem because
it is very difficult to analyse."

With uncertainty over their boat's reliability, Jean le Cam in conjunction
with his sponsor Bonduelle have announced their retirement from the
Transat in order to allow them time to find a solution without endangering
skipper and boat and to focus on their primary objective – November's
Vendée Globe.

Their decision comes despite Bonduelle winning her first ever race, the
1000 Milles de Calais, just three weeks ago. The team point out that this
race was held in light conditions and Bonduelle has yet to sail in the
heavier conditions in which Sill experienced the severe vibration. Lombard
says on Sill the vibration in the keel occurred when the boat was sailing
in excess of 20 knots.

Aside from the technical issues competing in The Transat was thought to be
essential if le Cam was to comply with the qualification requirements for
the Vendée Globe. Denis Horeau, the new Race Director of the Vendée Globe
says that a new Notice of Race for the non-stop single-handed round the
world race will be published this coming Tuesday and will include details
of the qualification procedure for his race. There is a possibility this
will allow Sill and Bonduelle to qualify by sailing their own 'race' as
was allowed for Javier Sanso and Bernard Stamm prior to the Vendée Globe
four years ago.

Fifteen IMOCA Open 60 boats will now take to the start line of The Transat
on Monday, 31st May starting at 1400hrs amongst a fleet of 37 boats in
total.

http://www.thetransat.com

 

Canting Keels

There's a thirty year(more or less) legacy of canting keels most of which have functioned well. It will be real interesting to see what Marc Lombard finally determines as the cause here.
As I understand it these two keel fins are 100% carbon where many in the past have been metal...
Fascinatingly complex problem...
(Can everybody in the world agree to call these keels once and for all canting not swing keels? Please! Back in the 70's swing keels were all the rage remember ,on trailer sailers-and they weren't canting keels.(geez-sorry-pet peeve))

 

Doug - Dang! Your pet peeve almost exactly matches mine, in reverse
I hate the term "swing keel" when it means "ballasted centreboard", but don't mind it much when applied to "canting keels".
The answer, obviously, is to just outlaw the term completely, since we both hate it

Steve

 

"swing keel" outlawed

Yes! We've changed the world! Ah, do ya think "they'll" listen?
Perhaps it's interesting that on many of my rc models I'm using all carbon extremely thin(6-7%t/c) fins to support a canting bulb and that many fixed keel models use simarly thin all carbon fins with no fluttering even at high model speeds.
It may be that the torsional stiffness is low and the cg of the bulb so close to that axis that it can move-just guessing.I had that happen once with a fin that twisted too easily and the boat shook like it was attached to a vibrator! Again, from model experience I know that a carbon fin laid up using too little double diagonal woven reinforcement will twist extremely easily while being very stiff in bending; it's really weird like it has some kind of mechanical hinge!
Looking forward to what they find out....

 

Heh heh! Yup! Torsional stiffness/rigidity is often ignored in isotropic materials, since it is usually fine if there is sufficient bending strength. Carbon doesn't have that advantage, and while longitudinal and hoop strengths are stressed, torsional strength is often left alone.
Steve

 

Ask any dinghy sailor and hear about "fluttering". It is an everyday occurence if there is, with luck, enough wind to plane. It happens earlier in boats with thin metal boards.

 

Fluttering Centre boards and keel are not the only thing that flutters.
Having been involved in the marina industry you sometimes see marina piles that also flutter in high tide range areas. Note in some costal area in Australia 400mm Dia x 12mm Wall Steel pile's have been known to snap because of this.
The cause is the pile is the right Dia (aspect ratio) at the flow rate to vibrate solution increase Dia (aspect ratio) problem solved. In our case 400mm dia to 425mm.

Cheers Specmar.Aus

 

From the Transat site:
"IN DETAIL:

With five days to go before they set sail upwind across the North Atlantic, the very last thing teams wish to discover are severe structural problems with their boats. However, this is exactly what has been experienced by three of the teams due to take Monday's start of The Transat.

In the Open 60 class for the last two weeks there has been major concern about the canting keels of the brand new Marc Lombard designs Sill and Bonduelle. Since their launch within the last month the keels on both boats have developed an alarming vibration when being pushed hard. During the recent 1000 Mille de Calais race concern over their keel's integrity was enough to prompt Roland Jourdain on board Sill to retire from the race. Ironically her sistership, Jean le Cam's Bonduelle, went on to win the race.

Since then both Sill and Bonduelle have been taken out of the water to have their keels fully checked. It appears the vibrations are occurring due to their keel foil twisting when the boat is powered up. After discussions with designer Marc Lombard and the structural engineer Herve Devaux they have attempted to solve this on Sill by casting a new keel bulb and fitting it to the foil, but in a position further aft to alleviate the twist. However this modification has not worked and the team are now back to square one. As a result Sill has been forced to withdraw from The Transat. "

I can't believe they moved the bulb back! That's the worst thing they could have done to solve a flutter problem! The center of mass must be at or ahead of the elastic axis. They should have moved the bulb forward.

Classic flutter occurs when the bending frequency coincides with the torsional frequency. When there's a change in angle of attack on the surface, the lift makes the surface bend - and the motion is greatest at the tip, of course. The mass of the bulb will tend to make the bulb lag behind the motion of the tip, twisting the keel. If the center of mass is behind the elastic axis, the inertia of the bulb will tend to twist the keel in the same direction as the bending, which is destabilizing. If the center of mass is ahead of the elastic axis, the inertia of the bulb will tend to twist the keel in a direction that opposes the change in bending, stabilizing the situation.

So the two fixes are to move the mass forward or to stiffen the surface (usually in torsion). This is one reason you always see engine pylons on jet transports designed to hang the engine out in front of the wing. The engine acts like a balance weight to stabilize the wing against flutter.

I'm not surprised their new bulb wasn't effective in preventing the vibration.

 

Illustration

[/IMG]Just to illustrate Tom’s words (I hope ) I have made a little drawing of the keel seen from the buttom.
The circle represents the mass or the center of gravity of the bulb, and the ellipse the keel.
In the first situation we have a steady state condition, I’ve shown this with zero angle of attack but it could be any angle as long as it is steady.
Mass behind the twisting axis situation:
1: suddenly a change in angle of attack occurs the keel responses by generating a lift force an starts to move the keel. The bulb resists the acceleration by applying a force in the opposite direction of the lift force to the keel and thereby creating a torsion moment.
2: The moment twists the keel increasing the angle of attack, increasing the lift force, increasing the bulb force, increasing the torsion moment….. increasing….. If the change in angle (from + to -…. + to -) of attack coincides with the eigenfrequency of the keel the structure will start to bend at twist violently until the velocities will be large enough to dissipate the energy to the water in some kind of energy equilibrium.
Mass in front of twisting axis situation:
1: suddenly a change in angle of attack occurs the keel responses by generating a lift force an starts to move the keel. The bulb resists the acceleration by applying a force in the opposite direction of the lift force to the keel and thereby creating a torsion moment.
2: The moment twists the keel (this time in the opposite direction) DEcreasing the angle of attack, DEcreasing the lift force, DEcreasing the bulb force, Decreasing the torsion moment…. and the system becomes more stable.

The above explanation is of course very simplified, but I hope it illustrates the idea.
What is eigenfrequencies ?? eigenfrequencies are certain frequencies in which structures looses damping. Every structure has a lot of them (theoretically infinite) but usually only the once that occur at the lowest frequencies have interest. A good example is if you drive a car with a unbalanced front wheel, at a certain speed the car starts to shake, but if drive faster or slower the shaking stops. At the shaking speed the unbalance excites the suspension structure at one of its eigenfrequencies. If you increase the mass (bending ) or inertia ( twisting) you can lower the eigenfrequency of at structure, and same will happen if you decrease stiffness. At eigenfrequencies you will always have trouble (unless you can introduce some damping) and in this case it seems you have double trouble when the torsion and bending frequencies coincides because they assist each other so nicely.
By moving the bulb back I guess they have lowered the torsion frequency a bit but increased the torsion moment, and it sounds plausible that it takes them to square one. :idea: Instead they should just put up more sail and increase the boat speed until it stops vibrating, it costs no money and will increase the chance of winning the race .

 

Notes from Scuttlebutt

* From Preston Wake: There is also one other big boat with a canting keel
and twin foil to be launched this year it is Magnitude 80, the Andrews 80.
Although the lead foil is a dagger board similar to a laser.


* From Dave Beck: While I cannot speak for Mike Golding, my guess is that
his KISS solution to his ballast strut problems will be to install a more
robust hydraulic drive motor. The performance advantage of canting ballast
in these light boats is too great to ignore, and the acceptance of powered
ballast struts is a done deal. Every innovative technology goes through a
period where the best engineering solutions are sorted out in the hard,
real world. These solutions then become commonplace and accepted as normal. Anyone who has sailed a performance oriented canting ballast boat knows they are too fast and too much fun to ignore. This is the way to do it.

 

It seems to me that a catamaran is a better solution to the problem of getting a lot of weight out to windward of the rig.

 

capsize

That or a tri or a Bradfield type foiler that develops unlimited righting moment. But then you flirt with disaster every second you're off shore.
At least the canting ballast boat is self righting....

 

There is a practical experiment going on now:
http://www.thetransat.com/en/default.aspx

We can look after the fact to see which technology is more reliable.