Balanced spade rudders a liability?

[Capt_Flrp]

And I thought an expert was anyone with a briefcase more than 50 miles (nautical or otherwise) from the main office.

[seasick]

"Could the additional hydrodynamic forces from heeling further aggravate the wear and tear of this set up?"

Heeling tends to relieve the hydrodynamic forces on the rudder of a monohull. It is the continuous slewing for lack of a better word, where the stern yaws to the side that tends to really load up a rudder.

Picture if you will the dynamics of how a water skier turns. He leans over and the ski carves across the wake. This same dynamic occurs when a vessel surfs down a wave and heels over. One side of the hull is buried much deeper than the other and the vessel carves to windward, broaches to a degree, and as the stern slews around, the forces loading up on the rudder(s) are tremendous.

I know this is a touchy subject for many so I'll leave it to say, spade rudders are great just make sure you go with an auxiliary rudder windvane like the Hydrovane so you can get back to port when they finally fail.

[Brob2]

Originally Posted by ObiWanSand

"Multis stay mostly at the dock", has very little to do with fact.

that was just a playful poke at the dualies. Sort of like a Vålerenga Fotball/ Lillestrøm SK thing.

[Sailor Doug]

Spade rudders steer astern way way better than attached. A plus for risk assessment if you count docking accidents.


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[leftbrainstuff]

Originally Posted by Stumble

There is absolutely no reason a spade rudder can't be as strong as a attached rudder. The only question is what are the relative strengths of the design. Hung rudders may be stronger, and it may be easier to design them to a given strength, but cantilevered beams (i.e. Spade rudders) are well understood design problems other than doing the math it isn't difficult.

Any cantilevered design, rudder or otherwise, will have much greater bending moment induced loadings on the rudder shaft and bearing surfaces.

This will translate to much higher shear and tensile loadings at the location of maximum bending moment when compare to a skeg hung equivalent.

For a rudder this is primarily where it meets the hull.

We engineers abhor cantilevered designs. A double shear design is far superior in terms of durability and transfer of loads.

A spade rudder has three primary benefits over a skeg hung rudder:

1) lighter
2) more efficient hydrodynamically
3) cheaper to manufacture

For a race boat 1 and 2 are important. For a yacht builder who prioritizes 3 I would expect to suffer lower durability.

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[Dockhead]

No one has mentioned the most important thing, which is hydrodynamics. The rudder, like the keel, is a wing, and its ability to generate lift is crucial to getting upwind.

Skegs, whether full or partial, really hurt the hydrodynamic properties of the rudder. Imagine an airplane wing, with a kink in it -- ick.

My boat has a partial skeg, and the rudder is very large, which are compromises intended to make the rudder very powerful but with a better lever arm, and with some protection, compared to a spade rudder. But the downside is that it is less efficient, and the great size of my rudder costs drag.


I think I prefer Dashew's approach, which is a pure spade rudder, but massively overbuilt structurally, and with a sacrificial tip which will break off in case of impact, prior to bending the rudder shaft. Downside here is just cost, but you get superior performance, and sailing a boat with a really effective rudder is such a joy.


With catamarans, I wouldn't waste a nanosecond of thought on it. Cat rudders are shorter and so less likely to catch on anything, and you've got a backup in case something does. Just enjoy and don't worry about it.

[SeeMonstrEd]

if you are really worried about rudder failure, buy the boat you like and invest in a properly sized drogue. This will provide you with a backup steering method as well as a sea anchor. I have had several issues with my rudder and purchased a drogue before my last ocean passage. I found one the right size used from Bacon Sales for a reasonable price.

[Dockhead]

Originally Posted by leftbrainstuff

Any cantilevered design, rudder or otherwise, will have much greater bending moment induced loadings on the rudder shaft and bearing surfaces.

This will translate to much higher shear and tensile loadings at the location of maximum bending moment when compare to a skeg hung equivalent.

For a rudder this is primarily where it meets the hull.

We engineers abhor cantilevered designs. A double shear design is far superior in terms of durability and transfer of loads. . . .

Far superior if you assume like sizing of the structural members and bearings. "You engineers" also know that any cantilevered design can be made just as strong as some given other design, by properly sizing the structural elements.

So that's what you do.

Done right, however, it is expensive.

SetSail » Blog Archive » What is the Best Rudder Configuration – Spade, or Skeg Mounted?

[Dockhead]

Here is what Bob Perry says about it:

"A partially balanced spade with a carbon stock and blade can be extremely strong. This would be my choice on a boat over 40'.

"Under 40' where helm pressures are less you can do an unbalanced rudder on a skeg. The rudder will not work as well as a spade but you may at least feel safer. Skegs are hard to build in grp molded boats and not always strong. I have been doing a few partial skeg hung rudders where I think you get the best of both worlds, i.e. a lower bearing for strength and some balance area to offset helm presure. Remember, the stock for a spade ruder has to be engineered for both bending and twisting moments while a skeg hung rudder is engineered only for twisting moments. So, a spade rudder will have a much bigger stock than a rudder of roughly the same size with a skeg. It has always bothered me to have a critical bearing right at the bottom of the skeg where you would be most likely to bang it."

Cruising with a spade rudder - Cruising Anarchy - Sailing Anarchy Forums


He talks only about helm pressure, which I don't think is the whole picture of the hydrodynamic question.

But he adds a number of interesting points, especially, the difficulty of making skegs strong on GRP boats. A full skeg rudder might look strong, but on a plastic boat it might not actually be strong.

[seasick]

I think what Mr. Perry was referring to is, if you aren't using a split mold it is difficult to design in an appendage like a skeg or partial skeg and still get the boat to release from the mold. To actually design and build a skeg with sufficient lateral strength is not such a challenge.

[Dockhead]

Originally Posted by seasick

I think what Mr. Perry was referring to is, if you aren't using a split mold it is difficult to design in an appendage like a skeg or partial skeg and still get the boat to release from the mold. To actually design and build a skeg with sufficient lateral strength is not such a challenge.

Maybe. You probably know more about it than I do. But I think the point remains -- don't assume that any skeg is necessarily going to be all that strong.


My own boat, as I've said, has a partial skeg, and like the rest of the boat, the rudder, shaft, bearings, etc. are all massively overbuilt. Dashew hates partial skeg rudders, and one of several reasons is because he says lines will get caught in the joint.

Well, it just so happens, that the one time I've been caught in a pot line, that's exactly where the line got stuck, and it was quite a hassle getting it free.

My next boat will have a Dashew-style spade rudder.

[Stumble]

Originally Posted by leftbrainstuff

Any cantilevered design, rudder or otherwise, will have much greater bending moment induced loadings on the rudder shaft and bearing surfaces.

This will translate to much higher shear and tensile loadings at the location of maximum bending moment when compare to a skeg hung equivalent.

For a rudder this is primarily where it meets the hull.

We engineers abhor cantilevered designs. A double shear design is far superior in terms of durability and transfer of loads.

A spade rudder has three primary benefits over a skeg hung rudder:

1) lighter
2) more efficient hydrodynamically
3) cheaper to manufacture

For a race boat 1 and 2 are important. For a yacht builder who prioritizes 3 I would expect to suffer lower durability.

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Perhaps so, but so what? If you know the loads will be higher then you simply design it stronger. Saying a spade isn't as strong as a skeg has no bearing in reality, you need to know what spade and what skeg, because both can work fine if designed correctly.

The downsides to a skeg however are the added drag, decrease in efficency, extra weight, extra wetted surface, etc. none of which can be designed away because to do so results in a spade rudder anyway.


Sure it's possible to design a skeg with enough lateral strength, but it's just as easy to design a spade with enough lateral strength. Th skeg is just superfluous.

[Kerry1]

"Sure it's possible to design a skeg with enough lateral strength, but it's just as easy to design a spade with enough lateral strength. Th skeg is just superfluous."
__________________

The problem with the above statement and others like it, is that there is a vast difference between what is possible and what actually occurs. I have a spade rudder and can appreciate in certain conditions it is a weakness. Production boats are designed with a certain level of safety margin vs cost. I think for anyone who is contemplating using boats with spade rudders for extended travel and so exponentially increasing the risk of encountering extreme conditions will, (or should) be thinking in terms of a jury rig in case of rudder loss/steering failure and plan accordingly. I notice (in recognition of their inherent design weakness) that cars still carry spare wheels.

[Jim Cate]

My own boat, as I've said, has a partial skeg, and like the rest of the boat, the rudder, shaft, bearings, etc. are all massively overbuilt. Dashew hates partial skeg rudders, and one of several reasons is because he says lines will get caught in the joint.

Well, it just so happens, that the one time I've been caught in a pot line, that's exactly where the line got stuck, and it was quite a hassle getting it free.

DH, that one is pretty easy to correct: at the lower bearing on our partial skeg there is a short length of ~10 mm rod welded to the housing. This extends vertically downward past the gap between the skeg a nd the balance area on the rudder blade. This simple device has prevented lines from becoming entrapped ever since launch. A similar device could be retrofitted easily... could be attached with screws if welding was not feasible.

Jim

[Dockhead]

Originally Posted by Jim Cate

DH, that one is pretty easy to correct: at the lower bearing on our partial skeg there is a short length of ~10 mm rod welded to the housing. This extends vertically downward past the gap between the skeg a nd the balance area on the rudder blade. This simple device has prevented lines from becoming entrapped ever since launch. A similar device could be retrofitted easily... could be attached with screws if welding was not feasible.

Jim

Thanks -- a good tip.