Originally Posted by robert sailor
As I recall
Dashew uses double the normal ABS/Loyd's standards for spade rudders but he also has much greater requirements for the design of the bearing areas.
We have had this discussion before and there is some suggestions that failure rate of production spade rudders is approaching 1% overall but if you consider that the % of production boats that are sailing offshore is very low then the failure rate of spades offshore could be much higher.
Whatever you want to believe losing your rudder in a fin keel/spade rudder boat offshore is not something you want to have happen so the idea of having a back up system is a good one and if you are racing
in any major race
(I don't include the ARC as it doesn't qualify as a race) a back up rudder is part of the requirements.
If you don't believe that spade rudders suffer a fairly high failure rate then ask yourself why all major ocean racers require a back up rudder system. While all designs of rudders can and do fail spade designs seem to top the poles.
As to losing appendages, we're in agreement on that. And I'm about as familiar with it as I'd like to be, given that a very close friend was on a boat which lost
her rudder. And subsequently wound up self destructing in the surf, on a Mexican beach as a result.
- The owner was too cheap
to have the boatbuilders doing the modifications, use all of the proper materials. Tried to save a few hundred $ on a half million dollar boat. But then that had always been his nature... still is.
On the Dashew's & ABS safety
factors, I'm thinking that the 2x minimum ABS number is for keel
floors structures. But my book's not here, so I can't check.
And while more spade rudders fail than say the type which are on pilot cutters, there are I think, a LOT more spade rudder equipped boats out there than any other type.
As to them failing, there's one or two things I didn't mention.
- While a lot of their stocks are undersized IMHO, more often than not, it's the connective bits, cores, & skins which are more problem prone.
- The failure problem is HUGELY exacerbated by cruising & racer/cruiser designers following the trend of racing boats in their never ending quest to get weight out of the ends of the boats. And this is VERY often done at the expense of sufficient strength in rudders.
And yeah, it results in offshore racers needing to have a safety
net like this Kurt Hughes Multihull Design - Catamarans and Trimarans for Cruising and Charter - J46 Emergency Steering
Although, I, personally, am a big fan of the boats running 2, kick up rudders, & them carrying spare blades onboard as well. It's a good nod to having learned something from the multihull
crowd (racing type, not cruising).
I can also say that I'm a fan of the monolithic, composite rudders. Wherein the blade & shaft are all one piece. They're milled out of a big pre-laminated block, which has more, & stronger fibers where the shaft on a standard rudder would be. And when they're done machining it, said fibers & location are the shaft.
Given the materials which they're made of, & how heavily they're autoclaved. Assuming that a designer
puts in anything resembling sane safety factors, I'd think that they're tough to beat in terms of performance & long term reliability
As always, the bottom line in terms of what's put onto boats, is those that write the checks. So the designers can draw up things as tough as they like, but if they want to keep their jobs, they'll do what needs doing to keep both the bean counters, & the public, happy.
EDIT: On this 1% thing. I know that if it were cars losing critical systems at even 1/10th of that rate, there'd be a lot of automakers out of business. And some REAL Fat lawyers.