Hi Richard,
We have a 455GL and cruise between 1500 to 2500Nm a year. Main is fully battened slab reefing and genoa is 135% Elvstrom. As a rule of thumb sailing upwind I normally sail at half or (just over half) of the apparent wind speed. That is until the apparent wind reaches 16kns. Using your example, I would expect 4.5 to 5kns on a flat sea with no tide. We do have a folding prop which helps. In mast furling may give a slight reduction in performance. If your sails are 10 years old they have probably seen their best. I replaced an 11 year old main a while ago and it made a big difference in how the boat performed. Obviously a clean bottom helps a lot. Be aware that Polar Diagrams are calculated on perfect conditions.
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