Twin Keels came up in the Old Ideas thread and I found the discussion interesting, particularly as I've owned a modern high-aspect ratio twin keel boat (RM 1050) and have sailed others. I find them a great solution to a problem: sailing in tidal areas, like Brittany in France and shallow draft, though the high aspect keels aren't necessarily that shallow. I thought I would post some of the comparative polars Marc Lombard's office has done between single keel and twin keel versions of the same boat (RM 1060). Remember that the single keel has 30cm more draft than the twin keel, so that is skewing the results. My sense is that a single keel of the same draft would be still possibly a little a faster upwind in light air but that it would be very close. In any event, I think the question is largely moot since most people who choose twin keels are choosing them over centerboard/internal ballast boats (dériveurs intégraux) and twin keels are in my opinion much better performing up wind than centerboard boats. Anyway I thought these numbers might be interesting to some. These are percentages measuring the gain of the single keel boat v. the twin keel one upwind and downwind across the wind range from 4 to 25knts, increasing by 2knts in each column.
VMG près 7% 6% 6% 5% 5% 4% 4% 4% 4% VMG portant 3% 2% 2% 1% 1% 1% 0% 1% 1%