Case Study Class 40, Part 1

In a recent article about the STP 65, SYDE introduced itself and the RPT to the SA readers.  We have been requested to add to the YD101 series.  In this article we will cover the overall process of the design and the tools employed by SYDE.  There are some similarities in this to the previous YD101 articles, such as the Basics. 

To demonstrate the process and tools, we are going to use the Class 40 as a case study.  The SYDE Class 40 is targeted at the growing northern European short-handed market sector.  The inspiration and the design brief for the SYDE Class 40 comes from local Dutch sailors.  Beyond Holland, we have also had some fruitful discussions with American sailors (contacted through SA forums!) about creating a nice all-around racing yacht in this largely downwind, and dominantly French, racing class. 

The kind of sailors in the target demographic are, to make light of it, in the throes of a mid-life crisis.  They are without a doubt experienced racers but not interested in 1. sitting forward of the primaries and 2. finding others who will sit forward of the primaries.  Mainly they are racing on the North Sea on distance regattas, but for many there are plans to race Round Britain and Ireland, the OSTAR or the Route du Rhum.  A few race courses of interest on the other side of the Atlantic are the Newport solo-twin, the Bermuda 1-2, and the single-handed Transpac.

As opposed to beating the rating, box-rule classes race boat for boat and have only a few hard limits such as maximum beam, minimum freeboard, and maximum draft, etc.  The goal is to draw a race winning boat on the water (which is generally but not always the  fastest boat.)  In the course of discussing the process we will cover some of the changes to Class 40 rule for 2007 as well.  

At SYDE, there are no drafting boards any more, we work 100% on computers and really 100% in 3D.  After collecting design goals and concepts, we start the real work: the necessary and boring weight estimate.  We, like most offices, use a spreadsheet for this kind of calculation.  In the early days, it takes on the form of a weight budget, rather than an accurate summary.  Fact is, this often means scaling off of known designs, both for weight and center of gravity.  Bigger items may be pinpointed already – mast and rigging, main engine/saildrive, keel and bulb, obviously.  This situation means your database is a major factor in success of later boats.  The rich get richer.

Fig 1 Fastship

After we have the shell of a weight estimate together, we start with the other side of the same equation – hull shape and hydrostatics.  For creating hull shapes, SYDE uses Fastship software from Proteus Engineering.  Everybody has their reasons for choosing their tools (and it doesn't hurt that it came with a beer bottle opener).  Modern hull fairing programs generally manipulate “control points” just off of the surface itself.  Fastship offers great tools for both course and fine control of these  control points, linking or freezing them, and updates the lines plan and hydrostatic calculations essentially in real time.  The end result can be used without conversion in most CAD and rendering programs downstream. 

The hull shape for the Class 40 has its roots in smaller planing sportboats and with a touch of displacement forms, which tend to have a more balanced section area curve.  As an aside, the section area curve is a graph of cross section area of the part underwater, plotted against position from bow to stern.  The SYDE Class 40 hull has a serious bias toward power reaching, but will not pitch forward dramatically when heeled.  We very deliberately are not “banging the corner”, avoiding the absolute beam limits of the class.  But, creating a better upwind boat is clearly working in the direction of more stability, and without added draft or displacement, that does mean making it a wide-ish boat.  The bow will be a little more pinched than most IMOCA yachts for performance in waves.  In evaluating candidate hull shapes at this stage, we look at the usual coefficients of form, free trimming heeled waterplanes, immersed volumes forward and aft of LCG when heeled, and of course the competition. 

Fig 2 Rhino with Class 40 novel deck edge study

The Class 40 does not have either 10 degree rule or the inversion test of the IMOCA 50/60's.  The latest edition of the Class 40 rules does have a “soft” maximum righting moment.  This comes in the form of a maximum force at the masthead for 90 degrees of heel.  It limits bulb weight, but also brings rig weight and VCG into the mix.  This leaves a bit of loophole open, because righting moment at 90 degrees has nothing to do with racing.  A hull will develop its maximum RM at a heel angle nearer to 50 degrees, and the stability at 90 degrees heel can be skewed, for example, by varying the hull-deck joint as shown below.   Essentially, the idea here is to simply remove volume from the area of the hull-deck joint.  This drives the center of buoyancy toward the keel, making the boat seem less stiff, only for the purpose of the test.   

For quick and dirty intuitive modelling tasks like these, SYDE uses Rhinoceros, from McNeel, and RhinoMarine, also from Proteus Engineering.  This design feature will  not be pursued by SYDE for the Class 40, as it will almost certainly be  banned.  Advising an owner to go ahead with such a novel deck edge would be risky. 

To sum up, we have looked into the first steps of the design for the SYDE Class 40: weights and hull design.  In the next part we will begin looking at the sailplan, the deck loft and eventually the initial performance prediction and structures.