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-   -   Spade rudders vs blue water passage making (http://www.sailnet.com/forums/general-discussion-sailing-related/101042-spade-rudders-vs-blue-water-passage-making.html)

deniseO30 07-03-2013 12:53 PM

Spade rudders vs blue water passage making
 
THERE! FINALLY, A BLUE WATER DISCUSSION THREAD FROM ME

"Rudder Lost at Sea" "--------" comes to mind...

Given the rarity we hear about lost rudders (and keels) it makes me wonder if the older boat designs were just more rugged then newer boats.

Or? Are owners of older boats just very very lucky?

Can a spade rudder be made stronger then the boat it's mounted in? I say yes, and it's not necessarily a good thing.

Rudder tube to deck or cockpit vs top bearing and stuffing box below water line?

Rudder to skeg or keel; the weak point seems to be the bottom where it is held in or on the keel/skeg I've seen allot of looseness in many boats in that area, which seems to be a very neglected piece of hardware below the water line.

Would I be somewhat correct in guessing; rudders with support at the bottom are actually under built and weaker then a spade rudder in the same size range?

Just wondering what the Jury thinks :)

krisscross 07-03-2013 01:20 PM

Re: Spade rudders vs blue water passage making
 
I have seen a lot of boats with seriously messed up spade rudders due to a hard grounding. It would not be my choice for serious cruising in areas we could call 'less-developed'.

TQA 07-03-2013 02:03 PM

Re: Spade rudders vs blue water passage making
 
I would only go blue water with a spade rudder if I also had a Hyrovane fitted. This gives you a back up rudder as well as the windvane controlled self steering.

capecodda 07-03-2013 03:30 PM

Re: Spade rudders vs blue water passage making
 
A few years back a buddy of mine lost a spade rudder on a LH54 about half way between Norfolk and Tortola. I had helped deliver the boat to Norfolk from up here, and luckily for me could not continue to Tortola.

They lost steering in the middle of the night. Next day they sent the youngest crew member over the side to find the rudder gone. They made it to within 10 miles of Tortola towing wraps for crude steering. 500 odd miles of zigzagging, with a quartering wind which was lucky. It was written up in one of the sailing mags, cannot remember which one.

The culprit in this case was crevice corrosion on the stainless rudder shaft at the bearing. Not an easy spot to inspect. Even if it was hung on a bottom hinge too not sure it would have made a difference.

krisscross 07-03-2013 03:51 PM

Re: Spade rudders vs blue water passage making
 
Quote:

Originally Posted by capecodda (Post 1053960)
The culprit in this case was crevice corrosion on the stainless rudder shaft at the bearing. Not an easy spot to inspect. Even if it was hung on a bottom hinge too not sure it would have made a difference.

Perhaps not, but the forces exerted on a rudder are really substantial. Having a bottom hinge transfers the load down and splits it between all hinges. There are not many keel hung rudders that suffer from a broken shaft. Frankly, never heard of one.

RichH 07-03-2013 04:13 PM

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.

capecodda 07-03-2013 04:17 PM

Re: Spade rudders vs blue water passage making
 
Quote:

Originally Posted by krisscross (Post 1053973)
Perhaps not, but the forces exerted on a rudder are really substantial. Having a bottom hinge transfers the load down and splits it between all hinges. There are not many keel hung rudders that suffer from a broken shaft. Frankly, never heard of one.

Yea, I agree with that. Forces are big on a spade, and in fact my current boat has a skeg hung rudder...although given the limited offshore we do, the primary advantage is catching a few less lobstah traps:)

BTW/any idea how to inspect for crevice corrosion on a shaft hidden in a bearing without dropping the rudder?

RichH 07-03-2013 04:23 PM

Re: Spade rudders vs blue water passage making
 
Quote:

Originally Posted by krisscross (Post 1053973)
Perhaps not, but the forces exerted on a rudder are really substantial. Having a bottom hinge transfers the load down and splits it between all hinges. There are not many keel hung rudders that suffer from a broken shaft. Frankly, never heard of one.

A pintle hung rudder (shafting) by its 'structural beam configuration' is FOUR times stronger than a cantilevered spade rudder .... size for size, that is ....; and yes indeed, they too sometimes fall off, usually as a result of NO inspection/maintenance of the attachments.

RichH 07-03-2013 04:27 PM

Re: Spade rudders vs blue water passage making
 
Quote:

Originally Posted by capecodda (Post 1053989)

BTW/any idea how to inspect for crevice corrosion on a shaft hidden in a bearing without dropping the rudder?

NOPE, you really have to drop it and inspect it firstly with a pocket microscope, then 'dye penetrant' (magnaflux, etc.) ... then the 'clock' to see if you exceeded approx. one million (estimate) load cycles above the 'endurance limit'. With stainless steel you only get about 1 million load cycles that exceed the 'endurance limit' ... after that value, the metal is usually considered 'tired' (embrittled). This is the same reason that old airplanes are scrapped after a certain number of 'take offs and landings' .... their metal gets 'tired' (weak).

capecodda 07-03-2013 04:40 PM

Re: Spade rudders vs blue water passage making
 
RichH...that's interesting. I don't think I've ever met anyone who dropped their rudder and did the inspection you describe. Sounds like a good idea before taking a long offshore trip on a less than new boat.

My recollection isn't perfect, but I think this rudder shaft was around 4" in diameter solid stainless. LH did lots of work in Taiwan, so I wonder about the quality of stainless.

I know Fontaine Yacht design has also done some of these shafts in carbon fiber. In talking with them, it was not only weight but also less bend, which reportedly made the bearing work better.


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