Join Date: Jul 2000
Thanked 233 Times in 220 Posts
Rep Power: 18
Re: Spade rudders vs blue water passage making
A spade rudder by its configuration is a rotating 'cantilever' ... the least strong of all structural 'beam' elements. That said it is easy for such to be built strong enough to sustain a cantilever to equate to the service of any other rudder configuration. The main problem with cantilevered spade rudders is the metal choice of the rudder shafting - stainless steel; so, such is additionally vulnerable to cyclical stress (fatigue) and at the same time macroscopic crevice corrosion in any fatigue cracks that do (will) develop - two modes of failure all at the same time.
Usually, a valid 'blue water' design will have adequate scantlings or design safety factors to insure that the critical stresses of critical components are at a minimum and so that the stainless steel elements will be selected to be well under from whats known as 'the fatigue endurance limit' of the material (usually at less than 30% of Ultimate tensile strength or a safety factor of 3). Many 'blue water' designers use a Safety Factor of 4 or more on such 'critical' components, "just to be sure".
A coastal design will usually calcualte out to a safety factor of around 2; an inshore design at 1.5.
Rx - nothing to be afraid of with a spade rudder on a purpose built blue water design IF the designer 'really knew what he or she was doing' with respect to design for fatigue endurance. The problem with spade rudders lost at sea is they are invariably lost on 'coastal' designs that werent designed for such service or were designed around 'cost control' measures.
So, when buying your new Blue Water boat with the contract condition of FS of 4 or greater for that Spade Rudder shafting .... you WANT a sample 'coupon' of the metal for submission to physical tensile testing including 'mill certifications' and 'chemical/physical analysis certifications' to validate that the metal is what its supposed to be and not some 'crap' ordered from an 'economy' source of materials - gonna cost ya, but its worth it - IMO.
Last edited by RichH; 07-04-2013 at 12:16 AM.