IOMICA Forum

Modern yachts above 10 metres have wide transoms so that an aft cabin can be fitted. Sailing reports in sailing magazines indicate that boats with narrow transoms sail better than ones with wide transoms. I think that this is because the underwater shape does not change that much when the boat is heeled whereas wide transoms present an asymmectrical shape underneath the water when the hull is heeled.

Why do IOMs have wide transoms - after all they don't have little people on board?

Hull design is always a compromise. It is dangerous to blindly follow the shapes of full-sized boats as there are other factors at work (eg the weight of the crew on the rail of a wide boat can be very fast).

A wide transom with a flat run aft promotes planing. However this usually causes an increase in whetted surface. Also a wide afterbody changes its immersed shape when heeled and that can lead to broaching. If you sail in generally moderate winds, this is a good choice as it will plane sooner and longer that other shapes.

The boat that won the 2006 IOM event in the UK (Hoylake.West Kirby?) was a narrow, hard chine design. From what I read, the conditions were pretty extreme (wind and waves) and I surmise that the narrow hull penetrated the waves better.

For very light winds a design with mimimum whetted surface (ie. towards a circular cross section) should be more effective.

Just my thoughts