Minimum propeller diameter is a calculation based upon the fact that below a certain diameter, it does not matter how much power you can supply to a propeller or how fast you spin it, it simply will not be able to develop enough thrust to keep the vessel under way in all sea conditions.

Users, particularly users of sailing vessels, should note that this minimum diameter is often far larger than they would expect, often exceeding fitted propeller size. This does not mean the data is wrong, it does mean that your vessel cannot be guaranteed to maintain way against any sea conditions, so when it blows up moderately rough be prepared to run away to a safe harbour or anchorage.

Formula

• D = 4.07 x ( square root ( beam WL feet x Hull draft (exc. keel) in feet ) )

Our example vessel with 8.5 foot beam and 2.3 foot draft requires an 18 inch prop to drive the hull safely in all sea conditions, since we have chosen 16 inch as maximum permissible we can say that while the vessel is safe in most weather, should a storm warning be issued we should head for port.