Kevin Dibley is one of our favorite designers – we just like that he seems to get it and we are always happy to feature his work. Here’s his latest…

As soon as you see the word IRC and connect it with anything between 38’ and 43’, you will be excused from thinking heavy production type lead mines. It is this type form that seems to do well under this rule and not until you get into the 48’ plus lengths that lighter displacement hull forms can start performing to the rating. The cost comparison of a TP 52 and a 42-footer both in build and campaign costs is far more than triple so we have a group of yachties out there that want their yacht to light up and fly but are either forced to go heavy and slow to have any chance under IRC, or go to a different rule/handicap system all together.

There is talk from the RORC that they are going to review this anomaly, but in the meantime we have to work with what we have.

Recently we were commissioned to do some Preliminary work on a new IRC 42-footer. We needed to find out where to draw the line in the sand without having to go heavy displacement, but not so light that we ended up with a great design that could never race to her rating. It had to perform well on the wind, as well as light up on reaching conditions which most of our Coastal races are down here in Australia and New Zealand.

The first item on the job list was to review what was currently racing out there in this size range. We looked at designs that have had decent success in this size range. Designers such as Christian Stimson, Mills, Corby, Farr, Ker, as well as some of the production boats such as Beneteau.

We then started looking at sail plans until we picked one that we thought was about right for the type of racing she will be doing, as well as for general comparison.

Now the real work began with hull modeling. Using our Maxsurf modeling software, we tried out different scenarios on weight, volume distribution, prismatic’s, measurement trim, and even looked outside the square with soft-chines and other configurations. We ended up going through 16 different models.

We then put the designs through our Wins-VPP software to work out where the strengths and weaknesses were in each. It is important at this stage to keep the same sail plan on each as we were only checking hull shapes at this stage. We are able to race all the models in a section of the software called ‘Regatta’ and from this we can see which yachts were coming out ahead of the others in various courses such as Windward/Leeward, Olympic, Offshore, and Circular Random. Four stood out and from those, we started looking at appendage and sail plan changes and how the VPP’s changed accordingly. Our goal from the beginning was for a fast reaching yacht that could hold her own on the wind with similar racing yachts of her size. So as we analyzed the data, we kept these stronger angles in mind.

We were now able to pick the top two yachts and from this we started looking at the IRC Rating. IRC is a rating rule that is administered by the RORC Rating Office in England. It is used worldwide. Though the rule is published and guidelines can be followed to ensure you keep within the spirit of the rule, the actual calculations of IRC ratings are kept secret and contain subjective elements that change annually. The only way a designer can ’crack’ it as such, is to study the trends of previous and current yachts and to then guess where the rule may be heading the following year.

The spirit of the rule requires that designers and owners not seek means of artificially reducing the rating of a boat. In other words, increasing performance without a corresponding increase in rating. In general, the RORC have done a great job in this, but of course it is the designers job to make the yacht as fast as possible within the rating.

RORC allow designers to get a fixed number of trial certificates per boat per year so we can see how various changes affect the rating. We went through that exercise to see how the two yachts rated against each other and from there we were able to see how our VPP’s were able to deal with the rating given.

It sounds like a lot of work to reach a goal, but that is our job. And though we come up with our own results and opinions on where the client should go, another design firm may come up with something completely different based on their own interpretations. This is what makes yacht designing so interesting. Every yacht has their day and every designer has their particular way and style. 


LOA                                         12.80 m                  (42’ – 0”)
LWL                                        11.70 m                  (38’ – 4”)
BEAM                                      3.89 m                   (12’ – 9”)
DRAFT                                     2.90 m                   (9’ – 6”)
DISPL [MEASURED]              5,875 kg’s               (12,950 lb’s)
IM                                           17.133 m                (56’ – 2”)
J                                             5.00 m                   (16’ – 4”)
LP                                           5.25 m                   (17’ – 3”)
ISP                                         19.24 m                  (63’ – 1”)
STL                                         6.71 m                   (22’ – 0”)
P                                            17.30 m                 (56’ – 9”)
E                                              6.15 m                 (20’ – 2”)