AC40

[17mika]

Question remains as to why the loadings were higher. The lateral loadings I can see as bad luck/exceptional case, but why are the longitudinal stresses 70% higher. The boat wasn't going faster. So it seems likely the deceleration was at least partially worse due to the hull shape. Which seems like a missed factor in the design

I think it is mainly related with geometry of ac40 vs ac75.

given the relative higher ratio between ride height and lenght, the nosedive of the 40 are likely tonoccur at a higher angle to waterline, so if speeds are similar, more deceleration is not surprising. it just happens i guess because if a 40 fioter ventilate the foil, the bow can point down the mine more before it hits the water than what happens with the 75

still I am surprised no big similar capsizes happened with the small boats in previous cycle though. maybe last cycle 40s were slower so capzizes were less brutal... now I guess the 40 is not that far in speed from the 75

 

[Mozzy Sails]

The 40's have the jib tack on the bow so they don't have to stick the bow in very far before the jib scooping with water also starts to slow them as well.

I think them corkscrewing on the way down and picking up the side loads will just be how the sails are loaded and the trajectory at the moment the rudder let go. Unfortunate... but you only have to spend a few moments scrolling through pitchpole central to know they're are a million ways to go down the mine.

 

[Stingray~]

The 40's have the jib tack on the bow so they don't have to stick the bow in very far before the jib scooping with water also starts to slow them as well.

I think them corkscrewing on the way down and picking up the side loads will just be how the sails are loaded and the trajectory at the moment the rudder let go. Unfortunate... but you only have to spend a few moments scrolling through pitchpole central to know they're are a million ways to go down the mine.

I think it was during their nose dive when Alinghi's jib clew burst, it may have loaded up big-time too. That breakage may even have served as a bit of a safety-valve.

 

[enigmatically2]

I think there are several things that we can point to why the design would be worse (including bow shape). But that should have been allowed for. So just saying the forces were higher than before isn't an excuse

 

[Stingray~]

I think there are several things that we can point to why the design would be worse (including bow shape). But that should have been allowed for. So just saying the forces were higher than before isn't an excuse

I wonder why the rudders are not longer/deeper.

 

[Sailbydate]

I wonder why the rudders are not longer/deeper.

Drag

 

[Stingray~]

Drag

Sure, but in an OD Class that can do 40 knots then how much difference would an extra foot of rudder make? Why make that safety sacrifice?

 

[enigmatically2]

If the rudder had been longer in this case, would it have solved the problem? The boat was already pitching beyond what the rudder could prevent (presumably), soa longer rudder might have delayed the rudder losing grip, but not stopped it. That might have made the crash more gradual, or even worse. I'm not sure

 

[Stingray~]

If the rudder had been longer in this case, would it have solved the problem? The boat was already pitching beyond what the rudder could prevent (presumably), soa longer rudder might have delayed the rudder losing grip, but not stopped it. That might have made the crash more gradual, or even worse. I'm not sure

Hard to tell. Am disappointed about how the nice animation they provided does not include the rudder.

 

[Sailbydate]

Sure, but in an OD Class that can do 40 knots then how much difference would an extra foot of rudder make? Why make that safety sacrifice?

I get your point. On the other hand, why not add a keel, with a lump of lead on it?

Chasing speed is the name of this game.

 

[The_Alchemist]

I think there are several things that we can point to why the design would be worse (including bow shape). But that should have been allowed for. So just saying the forces were higher than before isn't an excuse

Exactly! It looks like they only used forces that were generated in the past on their AC75 as a design targets. Very surprised they are now just talking about modeling the loads of impact.

 

[buckdouger]

The one thing that lurks in my mind is whether the analysis and fix has been a bit too hasty in that it addresses this newly discovered load case but more time wasn't dedicated to ensuring there aren't load cases that weren't immediately obvious.

 

[david r]

longer masts are faster on foil boards, but that is probably another false equivalent

I get your point. On the other hand, why not add a keel, with a lump of lead on it?

Chasing speed is the name of this game.

.

 

[JALhazmat]

If you are foiling you ride as high as you can to reduce drag

making the rudder a foot longer and keeping the front foil arms the same length isn’t going to do much for the overall balance, then if you make the foil arms and the rudder longer all you do is just ride higher, so for Barcelona, that might actually be a good idea Because of the Shit sea state.

 

[shanghaisailor]

So much nashing of teeth and beating of chests. This was a prototype that did an unexpected thing that wasn't allowed for. Organisations with much bigger R & D budgets get it wrong all the time. For example Tesla has just recalled over 80,000 vehicles delivered in China (some produced here and some imported from USA) and even worse Ford Motor Company has recalled over 620,000 vehicles because of an engine fire risk that has resulted in around 20 fires - and they weren't even pushing an envelope.

It shouldn't have happened, I know, but not the first time an engineering design has failed when pushed beyond normal limits or, apparently, well within normal limits

 

[dogwatch]

Not really a prototype, given that it has been supplied to paying customers. And a mandatory purchase, to boot.

 

[The_Alchemist]

So much nashing of teeth and beating of chests. This was a prototype that did an unexpected thing that wasn't allowed for. Organisations with much bigger R & D budgets get it wrong all the time. For example Tesla has just recalled over 80,000 vehicles delivered in China (some produced here and some imported from USA) and even worse Ford Motor Company has recalled over 620,000 vehicles because of an engine fire risk that has resulted in around 20 fires - and they weren't even pushing an envelope.

It shouldn't have happened, I know, but not the first time an engineering design has failed when pushed beyond normal limits or, apparently, well within normal limits

It hasn’t been pushed as a prototype. Every team was required to buy one “as is” and then use them for races crewed by sailors who do not have experience in this type of boat. The boat has auto pilot features that allows inexperienced sailors to crew them. They are even offered for sale to the public!

Also, the recalls are not applicable to this situation. The have 20 incidents in 600,000, that is 0.00003%. The AC40 has broken its bow on 100% of the boats.

 

[The_Alchemist]

The one thing that lurks in my mind is whether the analysis and fix has been a bit too hasty in that it addresses this newly discovered load case but more time wasn't dedicated to ensuring there aren't load cases that weren't immediately obvious.

I agree. I am no structural engineer, but just putting in their fix doesn’t look like it will be enough to withstand those forces.

 

[The_Alchemist]

Question remains as to why the loadings were higher. The lateral loadings I can see as bad luck/exceptional case, but why are the longitudinal stresses 70% higher. The boat wasn't going faster. So it seems likely the deceleration was at least partially worse due to the hull shape. Which seems like a missed factor in the design

Yes, the bow was designed to catch more air and water to quicken liftoff and allow the bow skip over waves. A concept they ignored when modeling crashes.

 

[The_Alchemist]

The bow shape wouldn't mater if they could keep the rudder foil from breaking loose. Maybe having 2 rudders like on the foiling cats is actually a good thing in spite of adding a bit of drag. Plus they gain RM using rudder differential on the windward rudder on the cats.
This whole idea of making a trimaran without the amas might not be such a great idea...

But they sail with very little heel, so two rudders have no purpose.