Spades no good for bluewater?

[cormeum]

Originally Posted by RichH

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'.

Yeah that pretty much sums it up. Note also that as the shaft flexes the bearing holding the shaft in place has to be engineered to account for the additional stress. It's a tradeoff beween mechanical robustness and hydrodynamic efficiency.

[JohnRPollard]

Originally Posted by olson34

If you were not a well-regarded previous poster here, one might assume this is a troll.....Come to think of it, are you sure you're not just trying to stir the pot on a quiet fall day?


L

Hmmm. He posted that question back in February.

Nothing wrong with the question at all. There are different views on the suitability of spade rudders for extended off-shore sailing, particularly in remote areas with little support.

Certainly spades can be built robust. But, speaking anecdotally, we do hear plenty of reports of boats abandoned due to lost/damaged rudders, the vast majority of which end up being spades. Yes, spades are by now the more common rudder type, so statistically that would hold true. But it's still unlikely that the majority of boats crossing oceans have spade rudders, which means spades are disproportionately represented in this failure category. Suggesting that as typically designed/built on production boats, they are not as robust as configurations that provide rudder protection.

There's a lot of debris floating in the ocean. Not all waters are carefully charted. Not every sailor is at 100% proficiency all the time.

How would the typical spade fare here?

http://www.sailnet.com/forums/pacifi...-back-usa.html

[Boatsmith]

Yes spade rudders break their shafts and fall off more than skeg or keel hung rudders. just the fact. Pac cup racers have to have an emergency rudder. Spade rudders are far more efficient than others and balancing reduces the steering load a lot. I prefer a fast sailing boat and these are usually spade rudders. Its like driving a car. A'72 LTD wagon is probably going to break less than a Corvette.

[puddinlegs]

Go over to sailing anarchy's cruiser's forum. Ask Bob Perry directly, or do a search for some of his posts on the subject. He's a fan of spade rudders these days, and has/can explain the difficulty of building a skeg hung rudder. Build quality has much to do with either type as anything. And about Pacific Cup/Transpac, etc... requiring an emergency rudder, it's a requirement for ALL boats.

[mitiempo]

It's all in how it is designed and built. Spade rudders have been around for a long time. Below is Arion designed in 1950 by Sidney Herreshoff and the first large sailboat built in fiberglass. Launched in 1951 she is still sailing.

[Boatsmith]

Yes emeregncy rudders are required for all boats in the Pac races. But far and away the vast majority of entrants are spade rudders and rudder failure has in the past been enough of an issue to require the emergency rudders.