34th America's Cup: multihulls!

Hi All, the C class cat experience is that the foils don't make a difference. When a cat is up on foils it is in a metastable condition. As the heeling moments increase the boat sinks as there is no immediate resistance to sinking. If the hull is in the water and the boat is heeled the hull provides immediate resistance. This provides a slightly greater righting moment and hence speed. See polar plot of Rocker(foiler) vs Apha (no foils). The wetted area resistance is very small. So many other factors have to be consodered to get the foiler to be faster. Peter S

 

Davidson Bow or Spoon Bow

Hi All,
The flat leading keel line is called a Davidson Bow. Look it up. monohull AC boats have used them for decades. Plus oracle and alingi used them in AC33. The idea is to get the water to go under the boat not around it. The bow knuckle is trimmed so its out of the water. If you look at the AC45s sailing they rarely put the knuckle in. As boats sail with leeway the leading edge of the boat never splits the water evenly. This creates a strong leading edge vortex on the lee side hence drag. The extension of this is like last years mini 650 winner, a scow. I think the sharp pointy bowed boat is going to slowly disappear. Peter

 

Smushing the waves versus slicing eh? I suppose the degree of flat water in the venues will make a difference. Scows can be quick in seas though it's more of a over than through approach isn't it?

 

The latest issue of Sail World has photos of them sailing (flying) in 20 knots of wind.
http://www.sail-world.com/australia/index.cfm?SEID=2&Nid=101785&SRCID=0&ntid=118&tickeruid=0&tickerCID=0

Too bad I couldn't include at least one photo, but they are copyrighted.

 

The ride can be harder but the results speak for themselves. 747 won a design award and it won the race. The Volvos also have a rounded bow now and are getting rounder. Once we get our heads around a sharp leading edge is not best it moves from fashion to function. We used to design "softer" bows but the sharp vertical bow became fashionable. Then we can figure out whats technically best. Wally has super scows on the drawing board and they where popular some time ago in various classes. Everything goes in cycles Peter

 

Both scows and pointy bowed boats exist among moths, both international and classic. My familiarity with race results (omitting the recent foiling moths) suggests the pointy bows have it in most conditions, but that the scows sometimes do well in windy conditions.
http://www.mothboat.com/Building/supermoth.html - scow classic moth
http://www.mothboat.com/Building/howidoit.html - mistral type narrow waterline classic moth
The photos below show:
left - mistral type classic moths
center - a Europe dinghy (can be made classic moth legal with a slight mod to roach of sail)
right - a scow type international moth (the hiking rack is legal in international moths but not in classic moths)

 

Hello Stephen, unfortunately your comment is based upon the popularity of the pointy bow vs the technical merit. You could say the same for soft sails vs wing sails. Currently the race results clearly say that they are successful and popular but critical investigation shows wing sails perform better then soft sails. Peter S

 

Plus I have observed the bow knuckle vortex whilst on trapeze on a 5m nacra cat. It was clearly starting at the submeged knuckle and streaming off as we were moving along. This vortex clearly shows me that the knuckle is producing drag. Peter

 

I'm a bit confused with this. Isn't the Davidson Bow more rating specific, designed to increase waterline length when a mono heels while staying within rating for measured waterline?

Isn't the knuckle we see on catamarans more a product of hull form, specifically the flatter high volume high lift profile? - rather than being there for any benefit on its own?

And finally, isn't the scow mini 650 designed to increase rightly moment and therefore speed reaching and running, the dominant angles in the transat? I thought the designer and skipper were clear that this was to the sacrifice of upwind performance - also clear from the race results when long upwind legs have been part of the course.

 

On a mono the Davidson Bow with fair sides (no hollow) is designed to retain fair/convex waterlines as the boat heels. Its more then just trying to increase waterline length, overhangs are the best way to acheive this. But it also decreases the angle of incidence with the water and reduces spray drag (mono or multi). If you try to design bows with NURBS or Solid modelers you always get hollows and difficult geometry at the knuckle. Its a very subtle thing and is best done by manual lofting. Peter

 

Have you got any pictures of examples to illustrate that point? Looking at the mono AC bows I'm still struggling to see the same concept transferred to the 45s and 72s.

http://www.cupinfo.com/en/americas-cup-2013-boats-ac72-ac45.php

http://www.cupinfo.com/en/americas-cup-2013-ac72-catamarans.php

 

Modern catamaran trim

Hi - heres a good example of a modern cat hull in good trim. The bow knuckle is out of the water. The water goes under rather then around the knuckle. If you try to trim a hull that is not designed to do this the stern will be too low in the water. The reverse bow is a function of a pear shaped fwd section. The hull sectional areas are designed to provide a hull that drives very flat. A conventional hull (getting bigger from keel to gunnel) requires it to trim down to gain bouyancy. This means the stern rises and then the hull hobby horses" this is why a flat is used at the stern to damp the hobby horsing. This "new" type of hull has a Cp=>0.7 which means it has lots of bouyancy at its ends. It also uses the T rudder to damp its motion vs a stern flat. Obviously this all is irrelevant when foiling. This is why if you look at the videos of NZ and USA taking off they go bow down initially. The drive from the rig is being resisted by the fwd hull bouyancy. When they lift off, the balance changes and its only supported on half of the length so the bow droops. They will need to learn how to fly and trim so the boat doesn't dive in the transition. I suspect that Oracle may have broken the daggerboard in this sort of maneuver . ie if they were doing a reasonable speed and tried to fly, but then it dived/swerved the foil side load could exceed the max load it is designed for in "normal" service. I'm am a yacht structures engineer and this is the same problem for swing keels in fast monos. If you design using a SF=2 or 3 say and consider the keel as horizontal as in conventional design this is underdesigned if you calculate the possible hydrodynamic load if the boat swerves in a seaway at high speed for instance. Hope this helps. I'm also interested in the hullshape of NZ vs USA17. NZ has very flat sides whereas USA17 sides cirve inward then recover to maximum width. It also seems that the insode hull shae is different to the outside hull shape in USA17. NZ is nearly symetric. NZ has very flat bottoms whereas USA17 has elliptical bottoms. Survival of the fittest, will soon take over! Peter

 

As a yacht structures engineer, i would have thought you would make more mention of the structural difference between USA and NZ AC72`s... to me it seems, that ETNZ is miles ahead of USA with their boat.

The USA oracle boat, firstly looks like it trims very badly but even more scary is watching the entire structure twisting horribly when loaded up and flying a hull. Have you seen this? Compare this to the ETNZ boat, which has been shown flying along at much higher speed and loadings, yet the entire structure maintains much more stiffness with no marked twisting noticeable.

With the lifting foils allowed on these machines, I think the hull shape differences are going to amount to bugger all... if you watch the videos of ETNZ, they have very little of the hull in the water most of the time (even when 1 hull is still immersed, the lee foil is still carrying most of the boats weight)

 

Hey Doug - How you doin??
On the point of the the boards. Team NZ used different foils for day 1,2 and 3. They had Toyota on them. They were curvier than their flying version. Either day 4 or definitely day 5 they went with quite different boards and rudders (no Toyota). Plus I saw a photo of Oracles AC45 and it had different daggerboards on each side. So they could do a run one way then do it the other way and get immediate feedback on the differences very neat. Don't need two boat testing. I suppose a cat is two boats!! I'm sure we will see many variations of boards and rudders before the testing is complete. Seems your not a prophet anymore. Peter S

 

Agree'd on hull shape, they will lift the lee hull as much as possible (via foils) yet retain longitudinal stability. Yes Oracle was very twisty but you have to remember the boat is just out of the box and they haven't settled/bedded the structures in yet. Probably not on full pretension yet. The AC45s used to twist horribly when new. They bed in the connections and wet set them in after they settle/stretch a bit. Plus I'm not sure what the huge rear beam is about yet. Everyone is saying its for aero lift. I havn't done any numbers to figure out how much lift it could provide, but its improbable to do this as it needs to be trimmed somehow. Plus its at the back in all that dirty air. I suspect it has some purpose yet to be disclosed. Its simplest use is a walkway. If you watched AC33 they crawled from side to side on narrow beams. This is impractical in the coming short races. Especially for the helm. Jim can run back and forth over that easily, steer anywhere as needed. So as "simple is best" I think its a runway. Peter