Case Study Class 40, part 2

This is the continuation of a Case Study by Schickler Yacht Design & Engineering for YD101.  In the first part, we began looking at the process and tools utilized by this yacht design company in connection with their Class 40 yacht. 

A preliminary hull shape has been chosen to go forward with the other aspects of the preliminary design.  This shape will be revisited after a more complete study of the resistance.  At the time of writing, an intensive optimization project is ongoing to fine tune the Class 40 hull shape, particularly the stern, as well as the location of water ballast, sailplan and appendage positioning, using precisely the same parametric tools as some America's Cup yachts.  Namely, the suite from Friendship Systems has been generously made available in the framework of an academic research project which incorporates the Class 40.  The suite consists of a hull shaping tool, a force balance (VPP), a potential flow code, and an optimizer.  These tools will be combined with know-how and weather data gathered for a racecourse in the UK, to arrive at a race winning design.

At about the same time as the preliminary hull design, the sailplan is begun.  The sailplan of the Class 40, set on a carbon fibre mast with running and topmast backstays, is dominated by an enormous square-top mainsail, and features a roller furling blade jib.  To get a handle on just how big a main we could put on the boat, we called a journeyman sailmaker in Valencia.  While cup boats are going out with about 4m of headboard length, he felt that offshore boats can handle almost 2m these days.  Total upwind sail area is 115 m2, per class rules.  The class limits each yacht to 8 sails in racing, including the storm jib and trysail.  This leaves six sails, two of which are not limited in material.  The general plan is to have a laminated (3DL, D4, ...) main (with 3 reefs) and furling genoa, a staysail, a code 0 and two spinnakers; one asymmetrical XXL and one symmetric.  An aside, wisely, the loophole in earlier versions of the Class 40 rule, allowing rotating masts, has been slammed shut.  Banning halyard locks, on the other hand, seems to be out of touch with racing trans-ocean, and may be protecting the fleet or the builders.

The tool of choice for developing such frequently updated 2D drawings and 3D models is parametric CAD.  SYDE uses the Pro/Engineer, while others prefer TopSolid, Solidworks, Solidedge, etc.  The decision to buy Pro/E came from our experience in the America's Cup arena.  Challenging components, like masts, keels, bulbs and rudders were shaped by systematic span-wise variations, linked to either formulas or graphs, to reach optimal and fair shapes.  It takes some time but the results are amazing.  Put another way, shapes like those used on the keels of our SY-X 24 can be controlled with great accuracy.  As a bonus, the package comes integrated with a powerful Finite Element (FE) program, allowing rapid automated sensitivity studies of design parameters on component stresses.

The sailplan of the Class 40 is just a tiny fraction of what Pro/E can do, but it does this task quite neatly.  Move the mast aft by 2 inches and rake less by 0.5 degrees, and the areas, centers, relations, and drawings are all updated to match.  Handy for doing a lot of iterations and creating new boats nearly the same size.

Fig 3 Pro/E with sailplan

For the creation of the deck loft in 3D, Pro/E starts to come into it's own.  A set of steps, each executed in order, define the development process with associated parameters, dimensions, and angles.  Once set up in a robust way, many variations can be modelled and tested.  Still, like the sailplan, most of the updating is done manually, by tweaking the dimensions one at a time.

For our Class 40 example, the cockpit is designed for good sail handling by a small crew (1 or 2), but offers enough space that 4 to 6 could operate the yacht without interference.  One unique element of the cockpit layout is the prominently placed utility winch, slightly to starboard near the side the companionway.  Operation of this winch, which has all halyards and reef lines led to it, takes place behind the shelter of the “cuddy”.   It was originally intended to drive the “bitch” with a coffee grinder (or an electric drive when not racing), but the latest iteration of the rule ban the grinder outright.  The companionway hatch itself can be sealed watertight when slid aft and hinged down onto the angled hatch frame.  The A-sails are set on a sprit at the bow, though this must be retractable according to the latest iteration of the Class 40 rules.

Fig 4 Deck loft in Pro/E