Building a polar for the boat you sail

Many offshore crews are routing on numbers computed for a fictitious yacht. Offshore coach Stuart Greenfield shares where those numbers come from, why they go wrong, and how to replace them with your own.

I acquired a 1970s Ron Holland half tonner, called Silver Shamrock, with a Half Ton Cup win to her name and a set of assumptions built into her that no longer applied. She had been designed and campaigned under IOR, and IOR shaped everything about her: the sail plan with overlapping genoas, the way she was designed for the rule then. I then rebuilt her for doubled-handed racing, went to non-overlapping headsails and added a code zero and asymmetric spinnakers.

Whatever performance model had ever existed for that boat died the day I did that. The hull was the same. Nothing else was. A polar built around a big overlapping genoa tells you nothing useful about a boat now carrying a blade and a code zero because the crossovers have moved, the reaching angles have moved, and the entire shape of the curve between sixty and one hundred and sixty degrees has changed. I was not looking at an inaccurate polar. I was looking at a polar for a different boat. – Full report