Rudders that can spin freely through 360 degrees are not going to provide significant lateral resistance when left to spin free. Also small rudders in the end of the hull can reasonably be designed to take the hydrodynamic loads imposed when skidding down a wave - for the Shuttleworth cats a safety factor of 1.5 on a perpendicular blade at 25 knots; similar to the redesign on the 18m proa, which are based on SF of 3 on perpendicular blade at 20 knots. Such rudders will take the loads of beaching the boat and hull will be in less than waist deep water once firmly grounded.
Flat bottom on the lee hull reduces draft while maximising the span efficiency of the rudders.
It is a matter of identifying the risks as best as possible then designing to mitigate them and ultimately operating within those constraints. Understanding and testing the limits are a challenge particularly on a developmental design.
Rick
On 20/01/2014, at 11:08 PM, Rob Denney <harryproa@gmail.com> wrote:
Retractable rudders allow for minimum draft and provide nothing to act as a lever to flip the boat. A 15m x 8m raft drawing 150mm/6" is a very stable platform. However, flipping multihulls under bare poles is pretty unusual. The other advantage is that you can run the boat up the beach and step off in shallow water past the breakers rather than in them.
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