Spades no good for bluewater? - SailNet Community
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post #1 of 16 Old 02-21-2010 Thread Starter
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Spades no good for bluewater?

I've come to the conclusion from reading various threads spade rudders are not a good choice for bluewater sailing. They appear to be more susceptible to falling off or breaking. Why is that?
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post #2 of 16 Old 02-21-2010
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Hi Elkscout,

I guess I would ask, what accounts/information you read that led you to arrive at your conclusion?

From my limited experience ( I've never lost or damaged a rudder)

I think, spade rudders are more likely to suffer damage in a grounding.
But if you're in Blue water...grounding shouldn't be an issue.
You certainly wouldn't want a spade rudder that extends lower than the keel,
in a grounding.

The other argument might be that a great deal of stress is absorbed at the hull penetration having the potential to create leaks, or hull damage

A plus might be, that a spade will still offer some steerage, even if you've lost part of the bottom.

In waters where you are going to encounter lots of lobster pots, etc. having a skeg or partial skeg might protect the rudder better, a good skeg angle would help shed the line. But if you get the line caught between the skeg and the rudder..it could be difficult to extract.

I've sailed blue water with both, but I know there are folks here with far more blue water miles than I have that will have reached more definitive conclusions.

Tempest
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post #3 of 16 Old 02-21-2010
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Spade rudders are fine.

They are "out there", less protected.
If you are sailing blue-water, you will find yourself in some remote part of the world, with no back-up, should it get damaged.
However, sadly, this is no longer true...
Fed-ex can get you anything in a few days. For me, the spade is the better responding rudder and I would be happy to take it blue-water. After 10 years of sailing and 30 years offshore, I have never experienced a negative experience with a spade rudder. I thought I would by now, but it has never happened to me or anyone I personally know.
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post #4 of 16 Old 02-21-2010
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There is a school of thought that considers spade rudders unsuitable for true bluewater navigation boats... but I'd point out that most of the recent world records set in sailing were on boats that had spade rudders. The initial school of thought for spade rudders probably came about early in their development, when the materials and the stresses involved were not as advanced as they are today. IMHO, your generalization is a bit overly broad.

By eliminating boats that have spade rudders, you're going to cut the selection of boats you can choose from down considerably.


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I've come to the conclusion from reading various threads spade rudders are not a good choice for bluewater sailing. They appear to be more susceptible to falling off or breaking. Why is that?

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post #5 of 16 Old 02-21-2010
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A spade rudder is probably better than a some skeg hung rudders and certainly better than one attached to the aft end of the keel because it can easily be balanced. You can't do that with a fully supported barn door rudder. Also with a large number of skeg hung rudders, particularly the ones with smaller skegs the rudder and its post are strengthening the skeg, not the other way around. Like many items quality of construction is important.

Brian
Living aboard in Victoria Harbour

Last edited by mitiempo; 02-21-2010 at 12:28 PM. Reason: add
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post #6 of 16 Old 10-06-2010
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This was a good thread. Mi Tiempo's comments in particular. My Seafarer 24 is a small skeg barn-door arrangement and seems to be fairly high-maintenance but the boat is almost 40 years old so some fairly serious maintenenace issues can be expected. maintenance records show that metal work was done to the middle pintle gudgeon about 4 years back...the lower pintle/gudgeon is ( not surprisingly) next on the docket for renovation...
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post #7 of 16 Old 10-06-2010
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The spade rudder is more vulnerable to catastrophic fatigue failure for several reasons:

1. its essentially a 'cantilever', an unsupported 'pole-like' structure that by 'material strength' or 'stress analysis' due to its stress carrying 'geometry' is approximately 1/4th as strong as a 'barn door', etc rudder. Therefore a cantilevered spade rudder needs to be 4X 'beefier', etc. to do the same job as a rudder that supported also on the 'bottom' of the rudder. {Hold this in your mind for a minute: 1/4 as strong.}

2. the Shafting is commonly fabricated from common 300 series stainless steel. 300 series stainless steel has a 'problem' - fatigue failure. The design limit for fatigue service of common 300 series - called 'endurance limit' is ~30,000 psi, while the ultimate tensile strength UTS of the material is ~90,000 psi. {Hold this in you mind for a minute: add another 1/3 less strong = ~1/10th as strong ... because the shaft is allowed to 'flex' in the region of the cantilever connection to the hull}

So by simple 'static' analysis (using same materials and shafting diameter on both rudder types) a spade rudder shafting needs to be ~10X as strong as a 'barn door' or skeg hung rudder.

So, can a spade rudder be used for 'blue water' and survive all that dynamic stress load cycling? .... yup it can! It's just that the the shafting - the component that usually breaks' - has to be 10X stronger, larger, better and more critically designed, using 'better' and more fatigue compatible materials, larger bearing, etc. etc. etc. ..... or it will fall off by fatigue fracture of the shafting - about 1 million load cycles above that 30,000 psi limit.

All that stated, a spade rudder is a MUCH better hydrodynamic device - less drag, less prone to 'stall', usually 'under' the boat and therefore less vulnerable to 'ventilation-cavitation' etc., requires less power to steer, etc. etc. etc. but its HAS to be ultimately more expensive and much better built because the job that its 'shafting' has to do in 'cantilever stress and enhanced fatigue vulnerability' is much much more than a pintle hung rudder.
A guestimate would be TEN TIMES stronger ... just because its a 'cantilever' made of quite fatigue inferior materials.

Note - I didnt even mention 'crevice corrosion' which is enhanced and further propagated by fatigue/embrittlement - two nasty and simultaneuos failure modes.

For my boat for the open ocean I simply will not have a spade rudder because its just too damn vulnerable due to geometry, high stress in the WRONG area/component and vulnerability to fatigue fracture .... because its a 'cantilever' and its shafting is STAINLESS STEEL. To build a 'good' one takes a LOT of extra expense to do it 'right'.

Last edited by RichH; 10-06-2010 at 11:07 PM.
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post #8 of 16 Old 10-07-2010
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Not quite that simple

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The spade rudder is more vulnerable to catastrophic fatigue failure for several reasons:

1. its essentially a 'cantilever', an unsupported 'pole-like' structure that by 'material strength' or 'stress analysis' due to its stress carrying 'geometry' is approximately 1/4th as strong as a 'barn door', etc rudder. Therefore a cantilevered spade rudder needs to be 4X 'beefier', etc. to do the same job as a rudder that supported also on the 'bottom' of the rudder. {Hold this in your mind for a minute: 1/4 as strong.}



.... because its a 'cantilever' and its shafting is STAINLESS STEEL. To build a 'good' one takes a LOT of extra expense to do it 'right'.
For your point #1, this assumes that the skeg is strong enough and rigid enough to provide complete support to the bottom of the rudder. I was astonished years ago when I had my rudder off how flimsy the skeg was. The boat was S&S designed from a reputable builder. Clearly, as someone mentioned, the rudder was supporting the skeg here.

For the latter point, increasingly spade rudders are being made with composite rudder posts so the engineering concerns change.

I think it is not a simple question of spade rudder -bad; attached rudder - good. Rather it is a question of a properly designed and built rudder of whatever design - good; poorly designed/built - bad. My current boat has a skeg rudder; before this I had several boats with spades. I had confidence in all of them and each had advantages and disadvantages but it was the total package that mattered.

Back home on Lake Ontario after something over 36,000 nm circumnavigator. Not surprisingly there is a lot of stuff I want to get done on Ainia both cosmetically and functionally. Getting an early start so it will be ready to go for next summer (Lake Superior?).
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post #9 of 16 Old 10-07-2010
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Talking

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Originally Posted by elkscout View Post
I've come to the conclusion from reading various threads spade rudders are not a good choice for bluewater sailing. They appear to be more susceptible to falling off or breaking. Why is that?
If you were not a well-regarded previous poster here, one might assume this is a troll. (To be followed up with a question about why flax sails are not superior to dacron and the virtues of tarred hemp.)

When it comes to sailing boats across oceans, all sorts of hull/keel/rudder designs work well... IF designed and constructed for self-sufficiency off shore.

Next time I talk to the owner of Rain Drop, mooring two walks over from me, I'll kid him about his poor boat choice!
And then there's another Cascade 36, on another row at our YC, with a Tahiti round-trip behind him....

Come to think of it, are you sure you're not just trying to stir the pot on a quiet fall day?


L

Last edited by olson34; 10-07-2010 at 10:21 AM.
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post #10 of 16 Old 10-08-2010
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To be followed up with a question about why flax sails are not superior to dacron and the virtues of tarred hemp.
It is good that you pointed this out.
I am astonished at how many sailors are trusting their
personal safety to Dacron sails. Over the years I have
seen many Dacron sails that have torn, and quite a few
that have suffered total catastrophic failures.
Nylon sails are much worse, as they can cause any boat
to become completely out of control, and the failure mode
of the fabric is always only moments away at any time.
In my entire sailing career of more than four decades,
I have not seen a single flaxen sail, that required any
repair of any kind.

Islander 30 II 'COOL'
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