JJohn Swarbrick is known to most of us of late as the designer of the new JS 9000, but he has been involved as principal and associate designer for a number of high-profile and interesting projects; designing Tokio II for Chris Dickson's Whitbread campaign in 1993/94, and doing design work for a number of AC campaigns. With the recent talk about various new hull appendages, most of it surrounding the new TNZ yacht, John was kind enough to take some time to help explain what is and isn't going on. Enjoy. -Ed

I have had past involvement with three AC syndicates via limited design consultation on Hull geometry only. In each case I had advanced the possibility of appendage geometry that would increase the effective sailing length and enhance the wave making resistance properties. My favoured proposals were never investigated; I presumed my ideas were too complex to encompass the method by which these syndicates were evaluating hull candidates, all three syndicates evaluated (via tank testing and CFD) hull configurations separate of appendages (keel, bulbs, rudders etc). Final VPP calculation was by way of all hull modeling being awarded the same appendage geometry, i.e. to compare apples with apples.

The following history on IACC Appendages dates back to my verbal interchange with Chief Measurer in early 1989:

Under IACC rule version 1.1 9 May 1989 it was possible to design a yacht with greater than 20% of its displaced volume contained within the appendages. I was working on a design that had in excess of half its volume contained in a torpedo like structure. The water plane area was possibly to be only a few square metres in measurement trim.

I wished to know if my design proposal was rule legal and after advice from my mentor Alan Payne decided to contact the IACC Chief Measurer. In a telephonic conversation with the Chief Measurer I described geometry of a hull/appendage configuration as above.

The Chief Measurer¹s response after a long silence ³this is not what the rule intended², he asked if I would describe in writing the geometry I had conveyed to him so that he may simultaneously post this information globally to all AC syndicates...I declined his invitation.

The Chief Measurer stated he would consider my question of IACC rule address. After a week or so had passed I again contacted the Chief Measurer and was informed... ³ this is a moral problem and Rule issue that did not require answering², in that I did not warrant an answer as my question (application) was not through a registered AC syndicate. Miraculously 4~6 months later the first rule measurement calculation amendment just happened to significantly address my above mentioned design.

IACC RULES IN SUPPORT OF APPENDAGE APPLICATION AND DESCRIPTION OF GEOMETRY.

RULE 10. LENGTH

10. No part of the hull or appendage(s) throughout their range of rotation, at or below the measurement waterline plane shall extend forward of MWL or aft ALM.

Rule 13.3 The immersed volume of the fairbody (canoe body) shall not be less than 80% of the immersed volume of the complete yacht in measurement condition.

Rule 14. BEAM

14.1 Maximum overall beam, without penalty, shall be 5.50 m. The beam of the yacht shall be measured between verticals at each side of the yacht in measurement condition.

Rule 16. HOLLOWS

16.1 Hollows in the surface of the hull below a point 100mm below the sheer line are prohibited except: (a) in association with an appendage and its skeg or fairing: (i) from the forward end of MWL to a point at .25 LBG, may b extend hollows may extend transversely 125mm each side of the centreline; and (ii) aft of 0.25 LBG to the aft end of LBG, hollows may extend transversely 250mm each side of the centreline:

(b) the yacht may be any shape below the bottom of the fairbody (canoe body), provided the shape complies with rule 10 and 14.3.

Typically the immersed volume of the keel strut (s), ballast torpedo, rudder (s), canard, wings, winglets displayed on all IACC yachts to date have been in the range of 7%~8% of immersed volume.

The aft body geometry drawing as shown is solid throughout its form, and is approximately the remaining 12%~13% available under the rule. Pursuant to rule 16.1 (a) (ii) the appendage is attached longitudinally within the 250mm buttock line commencing around station 6, terminating at ALM. Part of the appendage is above the measurement trim waterline, therefore water plane area will have to be included in designers declared water plane calculations pursuant to Rule 47.

It is possible that the buoyancy can be varied independently while still holding the same external surface geometry i.e. by reducing the form to a shell of minimal thickness, attachment may be by way of struts within the region allowed under rules 16, 16.1 (a), (b). This type of configuration has been what has been recently referred to as the second skin on TNZ.

In order that the appendage does not touch the canoe body, the amount of clearance will be have to be met by mechanical engineering that best maximizes hydrodynamic flow, similarly the appendage geometry may have to be designed to concede to hydrodynamic forces, so as to not infringe rules. It is also possible that the gains made by moving the stern wave further aft allows for a structure that can geometrically tolerate the passage of turbulent water between appendage surfaces and hull and still be superior

Consideration of appendage due to its close proximity to the canoe body will be more akin to total hull hydrostatic calculations and hydrodynamic outcome. The mechanical engineering and final geometric detailing may best be undertaken by actual sailing. Station 10 (aft) as shown is the static waterline on all IACC yachts to date, the appendage as I have shown (shaded area) increases the waterline length by approximately 10%, the yacht will be faster.

If the appendage were designed in such a way that seawater was ostensibly contained and therefore have an effect on inertial and gravity properties (as separate from an appendage that has been designed to produce or assist in hydrodynamic properties through form shape) over and above the yachts measurement displacement argument may be raised that this contained seawater is in effect ballast .....intentional or not.

SUMMARY

America¹s Cup history is a history of challenged rules. TNZ may well be testing the current accepted designs methodology that has resulted in boats of pronounced similarity, the same way Ben Lexcen did with Australia 2.

However, the immersed volume rule has many and varied applications and, in my opinion, significant potential remains to substantially enhance boat speed and performance.

I remain acutely aware of other seemingly unexplored areas of rule Ointerpretation¹ that remain possible, I can still hear Alan Payne¹s words... ³You will probably require an army of lawyers in the process²

TNZ appear to have escaped the casualty of ³collective experts² that so often sideline innovative design. TNZ are to be applauded for innovation although in my opinion they will be challenged by their final opponent.

John Swarbrick