The appendage package

We look at what goes on below the water on Cup boats while Tom Humphreys compares what we saw on Sunday

While yesterday's article showed the considerable variation between the hull shapes of the latest crop of Cup boats, this is also true of their underwater appendages, in particular the keel configuration.

The bulbs on Cup boats are humungous affairs. The all-up weight of a Cup boat is 24 tonnes, one tonne less than the Version 4 boats sailed in 2003. However design teams have been piling as much as 20.5 tonnes into the bulb, foil and the substantial internal structure necessary to keep the whole deal hanging from the boat. To give some indication a bulb of this size is typically what might appear beneath a 90-100ft long cruising yacht.

While some faster classes such as the Volvo Open 70, bulb weight is the biggest performance driver, the onus to remove weight from upstairs to put into the bulb is considerably less important with Cup boats. As Team Shosholoza Principle Designer Jason Ker has told us: "We could spend all of our design budget on structures and we’d only be a little bit quicker as a result." Saying this the bigger teams have been trying hard to hone weight from the structure. As a result the range in bulb weights tends to be around 200-300kg.

Between the bulbs the boats are using is a considerable range of bulb lengths from about 4-8 metres and these will also give some indication as to the moding of the boat - long slender keels indicate uprange moding, shorter fatter keels a moding for lighter conditions. Some keels are also 'squished' (oval in section) or even flat bottomed (D section) in order to keep the centre of gravity of the keel as low as possible within the 4.1m maximum draft allowed by class rule.

All of the bulbs are fitted with winglets and again these vary in size, orientation to the keel and position along as well as vertically on the keel. The winglets reduce induced drag and increase lift.

The rather substantial foils, necessary to hold a 19-20 tonne bulb in place, vary in terms of their cord length and also in how much they taper. However all are fitted with a trim tab, extending along the full length of the trailing edge of the foil, but again varying in their cord. These tabs are operated by a wheel inside each of the main steering wheels.

A new development allowed for this Cup is the ability to add twist to the trim tab, by cranking the bottom of the tab round by a further few degrees. Less tab but more twist for example could be used to reduce drag in waves or to increase the efficiency of the tab when in 'pointing mode' ie using the tab to the max to point as high as possible. After Sunday's unveiling we are none the wiser how many teams are able to twist their trim tabs - this would presumably be operated hydraulicaly from a separate lever-operated pump somewhere near the helming position.

When it comes to rudders their size seems to have stabilised or even have crept up a little compared to the 2003 generation boats. While a larger rudder represents increased drag and therefore a slower boat overall around the race course, with the racing expected to be ever closer this time around, teams may be placing more of a premium on manoeuvrability.

Again, while the hardware beneath the new boats was on display on Sunday, it is far from certain this will be what the teams will use come racing. For example bigger teams will have in the order of five to ten keels (that's a lot of lead) and looking at the long term forecast prior to a series such as Act 13 or the Round Robins, they will mode their boat accordingly.

Here Tom Humphreys of Humphreys Yacht Design gives his observations of the appendages on display on Sunday:


Alinghi SUI91



Approx length: 6m

There is an amazing waste of the lower 30-40cms of the fin thickness to reduce junction drag between the fin and bulb (the fin tapers inward towards the bulb - see photo on page 2). The bulb section is a squashed circle. This helps lower the VCG and increase the span of the fin, which is of major importance for lift generation. Circular section bulb shapes have lower wetted area than chined equivalents. This could be important in the characteristically lightish airs off Valencia. The bulb is fairly long though still - around a metre short of the longest on display. As seems to be the general trend, the winglets have been positioned around the mid-length of the bulb. The section shape of the winglets appears to be symmetrical unlike many of the other teams who have utilised asymmetrical section shapes. All teams have dealt with the planform shape of their winglets very differently, with SUI 91’s being of fairly long span, with a straight trailing edge and a reasonably high degree of curvature to the leading edge.

Alinghi SUI100



Approx length: 6m

In general the appendage package of SUI100 seems to be a refinement of 91’s, indicating the team to be probably pretty happy with where they sit within the design space. The rudders of 91 and 100 (see p6) are similar in terms of area and planform shape and are certainly near the small end compared to what other teams were showing. The bulbs appear to be of a similar length and volume though the fin of SUI100 seems to be swept forward a degree or so. The winglet section shape is again symmetrical and of similar planform to 91 though a little extra area seems to have been added.

BMW Oracle



Approx length: 4.75m



Approx length: 7m

USA 87 (top) and USA 98 (bottom) have vastly differing keel fins and bulbs with 87 having one of the shortest bulbs on display while 98 has one of the largest. While this could indicate a possible differing of opinions within the design group it is more likely, with the amount of research that has been undertaken by this syndicate, that it is to give them the ability to mode change with perhaps a series of iterations in between. The other noticeable difference seems to be with the fin areas. 98 appears to have less area in the fin, perhaps suggesting with the longer bulb that it has been designed with straight line speed in mind whereas 87’s short bulb and larger fin look to favour manoeuvrability and height out of the tack. There is also a dramatic difference in the placement of the winglets. 98 has them placed just aft of the fin trailing edge and intersecting with the bulb on its chine, whilst 87’s are positioned just forward of the tail. Both bulbs have very pronounced chines running aft. Whilst the fin and bulb of both boats differ markedly the rudders of both boats are very similar and on the moderate to small size in terms of area compared to the other teams.

Emirates Team New Zealand



aP



NZL 84 (top) and 92 (bottom) also have quite different ballast packages. 84’s bulb is fairly long with very D-shaped sections and a hard chine while 92 has a shorter bulb and more cylindrical in section for reduced wetted area. As with Oracle this points towards a provision being made by the team to mode change with the longer, heavily chined, low VCG bulb being suited for breezier conditions, while the shorter, fatter, lower wetted area bulb for the lighter stuff. Winglets have been positioned similarly on both boats with their leading edges lining up with the fin’s trailing edge.

China Team



Approx length: 5m

The most interesting feature of Team China’s appendages is that the winglets appear to be twisted to some degree, with an asymmetric section shape.

There also seems to be a swelling on the bulb at the fin junction. It's difficult to imagine that this is helping hydrodynamically in any way…possibly a repair job?

Skinny rudder


Desafio Espanol



ESP 88 - approx length: 4.75m
ESP 97 - approx length: 4.2m

The keel fin seems to be swept forward on both boats. The bulb slightly longer on ESP 88 with smaller nose radius and maximum thickness not carried quite so far aft. The ESP 97 bulb appears to have more volume/weight. Winglet placement is similar though planform shape varies. 88 has a very elliptical trailing edge and straight leading edge while 97’s winglets are more trapezoidal.

Their rudders are very similar with small fore and aft fairing at root. There will always be some degree of separation in the flow at the junction between rudder and hull (also fin and hull/bulb) and these sorts of fairings can help to minimise this.

Victory Challenge



Approx length: 4.5m

A very short D-sectioned bulb. The bulb shape is quite aggressive in profile with maximum thickness carried a long way aft with a dramatic reduction in vertical thickness on top of the bulb towards the tail. The winglets intersect with bulb on the chine to reduce junction drag. The winglets have a very elliptical leading edge. The trim tab has much larger chord at the root than the tip possibly in an attempt to increase the loading on the fin at the tip, reducing the fin's impact on wave drag when the tab is employed.

Rudder fairly skinny


+39 Challenge - ITA 85



Approx length: 6.1m

+39 fits into the long bulb category. The bulb has quite an aggressive shape with a small nose radius and maximum thickness carried a long way aft. The bottom of the bulb runs into a progressively more pronounced chine further aft, with the winglets leading edge positioned just forward of the fin trailing edge, just above the chine. The section shape of winglets is asymmetric. The fin profile is fairly tapered towards the tip and has an unusual concave leading edge.


Areva Challenge - FRA 93



Approx length: 5.6m

This moderately long bulb (possibly changed from a longer bulb the night before unveiling) has a progressively more pronounced chine as you move aft. The winglets appear to have symmetrical section shape and intersect with bulb on chine. Their planform has an elliptical trailing edge with a straight leading edge lining up the trailing edge of fin. The fin has reasonably large area with quite large tab (approx 30% of fin chord).

Luna Rosa



ITA 86 approx length: 4.75m
ITA 94 approx length: 5.5m

The bulbs on ITA 86 and 94 are quite different. ITA 94 has a longer, skinnier bulb whereas ITA 84’s is shorter by about a metre and fatter as a consequence. It is interesting to note how far forward the bulb centre of gravity is on ITA 86, being well forward of the fin. The fin may have been positioned aft in an attempt to push the centre of lateral resistance aft to aid balance and manoeuvrability as attaching the fin aft of the bulb CG is unlikely to be of any structural benefit. It is also very difficult to have any control over the placement of the bulb on these boats without upsetting longitudinal trim, since the ballast weight accounts for more than 80% of the total displacement. The winglets on both ITA 86 and 94 are pretty high area with straight trailing edges and an elliptical leading edge. Winglet positioning is similar on both. 86 winglet section appears to be symmetrical whilst 94 appears to be slightly asymmetric.

Rudders similar in planform and of moderate area compared to the rest of the fleet.


Mascalzone Latino





ITA 90 - approx length: 6m
ITA 99 - approx length: 4.5m

ITA 90 (top) and 99 (bottom) again have quite different bulb designs indicating a desire to be able to mode change as conditions and circumstances change through the course of the Cup. Both bulbs are quite similar in section shape though ITA 90’s is longer. The fin is also larger area on ITA 90 with a big trim tab suggesting this package has been designed for uprange conditions. Winglets on both bulbs are asymmetric in section with similar planforms and are positioned slightly further aft of the fin trailing edge on ITA 99. Rudder area is slightly greater on ITA 90 with a longer tip chord.


United Internet Team Germany - GER 89



Approx length: 4.5m

Fairly short, circular section bulb and not as squashed as most other designs, suggesting a desire to minimise wetted area. The winglets are slightly asymmetric and positioned a foot or so aft of the fin trailing edge. Rudder very skinny.

Team Shosholoza - RSA 83



Team Shosholoza has one of the longest bulbs on display at around seven metres with a soft chine running aft. The winglets are asymmetric in section with the leading edge just forward of the fin trailing edge and intersecting with the bulb at chine. The rudder is undoubtedly one of the largest of all the teams with very little taper and a long tip chord.

On the following pages:
page 2 - bulb front profiles
page 3 - bulb side profiles
page 4 - bulb rear profiles
page 5 - bulb winglets
page 6 - rudders