For most sailing yachts, losing a rudder is probably the most catastrophic structural failure other than losing the keel.Rudder failure happens with distressing regularity. This paper examines the hypothesis that the underlying reason is designfailure. There are many qualitative decisions to be taken in the design calculation process. Example calculations arepresented which show that the maximum rudder force generated in steady state conditions is easily underestimated. For atypical spade rudder of a typical modern production sailing yacht, the normal rudder force should be calculated using aboat speed of at least 125% hull speed, and a force coefficient of at least 1.3. Care must be taken in selecting an appropriatevalue for the allowable stress of the material used for the stock.
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