Skeg or Spade?

[model 10]

Originally Posted by hodgmo

This makes sense... but on a sailboat does this imply that, while a balanced spade is best for maneuverability (dog fighting maps to match racing), a skeg rudder is preferred for long passages? This interpretation seems contrary to several thoughtful comments in this thread that seem to prefer the spade for all sailing applications.

I don't know about sailboats but planes need the directional stability of a vertical fin.

[TeddyDiver]

Originally Posted by hodgmo

This makes sense... but on a sailboat does this imply that, while a balanced spade is best for maneuverability (dog fighting maps to match racing), a skeg rudder is preferred for long passages? This interpretation seems contrary to several thoughtful comments in this thread that seem to prefer the spade for all sailing applications.

In principle yes, but there's a major difference between spades and skeg hungs in sailboats. The aspect ratio which in most spades is much higher making them more effient. High aspect skeg rudder is more demanding to engineer, and more costly. High aspect rudders bend and having a "hinge" along the span has issues.. In aeroplanes we don't see high aspect stabilizers due the lack of leeway which is allmost constant in sailboats. There's also broaching tendency to deal with when the maneuverabilty of a spade rudder is much better.

BR Teddy

[K_V_B]

[QUOTE=Sandero;2140851
I am still curious about the speed improvement in a 20 mile sail with a spade over a skeg with similar underbodies and sail plans.[/QUOTE]


The speed improvement may not be significant. However, a more efficient rudder means the autopilot doesn't need to work as hard. On a cruiser that matters...



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[Stumble]

Originally Posted by Sandero

Of course with stronger high materials a spade's strength these days, as you point out is no longer weaker than a skeg. Frankly I am not familiar with what designers and builder are doing today. The skeg may have been abandoned.

I am still curious about the speed improvement in a 20 mile sail with a spade over a skeg with similar underbodies and sail plans.

It depends on the speed of the boats. Drag goes up with the square of the velocity, so a fast boat would see a larger gain than a small one. On a windward/leeward race course the difference is likely far greater, first because a spade sails faster, but it also comes out of tacks at a higher speed.

To directly quantify this you would need some significant time running fluid dynamics that I think is beyond anyone here.

[El Pinguino]

If we can include aircraft lets include merchant and navy ships as well... 99.9999% of which have semi balanced or 'spade' rudders...

Its all to do with how you build it... I have seen good spades and bad... ditto skegs.

The last day job had twin Becker(sp) rudders ( hyper efficient 'spades'... essentially with a trim tab sort of a thingo on the trailing edge ) that could be 'barn doored' ..ie 30* port on port rudder, 30* stbd on the stbd..at 20 knots if you really wanted to stop the ship in a hurry. Only tried it once... back of the boat didn't fall off.. rudders didn't fall off either.

At the end of the day a well engineered 'spade' is better than the best skeg hung ruddder... imnsho

[Jim Cate]

Interesting point, Pingo. I'd never heard of that kinda brakes before... how much did it
take off the stopping distance?

And how is life in Chile? We are missing your voice on the Comedy Net!

Jim

[El Pinguino]

Originally Posted by Jim Cate

... how much did it
take off the stopping distance?

And how is life in Chile? We are missing your voice on the Comedy Net!

Jim

Hola Jim,
rather a lot compared with either just putting the props into neutral pitch or going astern and making lots of bubbles. They reckon the quickest way to stop a ship is to go slow ahead with the prop still bighting to take the initial speed off and then go astern. Doesn't upset the plumbers quite as much either.

Sunny but cold just now .... rain next week.

Did a delivery on a Benny Oceanis 57 up to Vina del Mar area week before last... motored whole way into Nlys and rain much of the time. The boat was 'fit for intended purpose' but motoring into a sub 1 metre head sea and swell isn't the intended purpose......
Not much else exciting going on.

[valhalla360]

Originally Posted by hodgmo

This makes sense... but on a sailboat does this imply that, while a balanced spade is best for maneuverability (dog fighting maps to match racing), a skeg rudder is preferred for long passages? This interpretation seems contrary to several thoughtful comments in this thread that seem to prefer the spade for all sailing applications.

During normal straight ahead flying, the planes rudder is pure drag and is kept straight so it creates no lift. The ability to streamline the skeg into the tail is more beneficial than the loss in lift efficiency as most planes fly straight ahead the vast majority of the time.

With a sailboat, the rudder is always being used to create lift (when sailing) as you need that lift to counter leeway. But you have to be able to do it equally well on either tack, so you can't just form the skeg to maximize lift and efficiency on the port tack or it will be bad on starboard tack.

Another advantage of spade rudders is all the gear that controls the rudder can safely be downsized with the torque needed to turn the rudder reduced by something on the order of a factor of 10.
- A skeg hung rudder is hinged at the front edge and the quadrant must be able to apply enough force to turn and hold the rudder broadside to the flow of water (or at least to the maximum turning angle).
- With a semi-balanced spade, the water hitting the area in front of the shaft, is trying to turn the rudder in the opposite direction of the force applied behind the shaft and the overall torque needed is drastically reduced. This makes it far more practical to attach a temporary rudder stock to steer the boat if the wheel system fails.
- You don't do a fully-balanced rudder as there would be no helm feel regardless of rudder angle. Also in the event of a failure, the rudder could stay at an angle overriding any jury-rigged rudders steering capability.

[Snowpetrel]

Its probably been mentioned, but control in astern is much better with a spade rudder. As is your turning circle, so overall the boat is way easier to manuever in tight quarters.


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[Ericson38]

I had spades on my two Ericson 38s and that was a fast boat design, but in heavy beam reach seas at the Channel Islands those boat's rudders would stall once the steering correction became too large. This would slow the boat down. A stall reduces boat speed to about 1/2 on the E38.

High aspect has high lift, and narrow stall tolerance. On a broad reach it was worse, and I would depower the main (reef and flatten) just so the head sail would pull the boat, rather than pull and main sail push.

I think the partial skeg (not full skeg with rudder completely behind) with some rudder at the bottom being ahead of the rudder post arrangement is better in that there are three bearings, neutral steering, and a bit more span on the whole foil (spade section and leading skeg foil) that results in a much more stall tolerant appendage when the going gets rough and large seas will knock you off course when they occasionally reinforce each other. A stall (even partial) results in a slow or fast round up, and a lot of chaos as the sails flog.

For the strength to be better than a spade though, the skeg has to really be built to take loads, not go for a ride and look good. The Bill Dixon Taswell (and maybe Oyster and Moody) rudders have skegs that run about 3/4 the way down the rudder span.

The u-2 spy plane, at max alt, with very high aspect ratio wings, flies at the point of stall of the main wings to reach max alt and hold it. The local windflow AOA 'window' is so sensitive to get to this balancing act, that the skill can't really be taught in a simulator.

[Ericson38]

Originally Posted by K_V_B

The speed improvement may not be significant. However, a more efficient rudder means the autopilot doesn't need to work as hard. On a cruiser that matters...



Sent from my iPad using Cruisers Sailing Forum

I would say that on a real cruise of say 50 nm leg in heavy weather on a beam or broad reach, or a run, a couple of stalls each hour running with a spade will result in a slower crossing than with a balanced design with partial skeg.

On a close reach to beat, a spade should be better.

[Dockhead]

Originally Posted by Ericson38

I would say that on a real cruise of say 50 nm leg in heavy weather on a beam or broad reach, or a run, a couple of stalls each hour running with a spade will result in a slower crossing than with a balanced design with partial skeg.

On a close reach to beat, a spade should be better.

Why do you think stalling is a function of spade vs other types? A spade rudder will stall more only if the aspect ratio is exaggerated. Better hydrodynamics mean more power in general, and so unless that power is given away too much in favor of less drag, like very thin racing rudders, a rudder with better hydrodynamics will always be better.

Edit: Oops, just saw your previous post, where you explain this yourself. A cruising boat doesn't want the appendages to be exaggerated (like on the boats Polux likes and posts so many of). A cruising boat, especially one intended for heavy weather and high latitudes, wants an extremely powerful and stall-resistant rudder, and will give up a bit of speed to have it. But there is no reason why you can't make that rudder a spade -- it should just be large and with moderate aspect ratio.

One thing I like about my present boat (Bill Dixon design) is the rudder, which is taller than I am and seemingly as large as a garage door. The skeg does not go 3/4 down the rudder -- more like 1/4, and I think it would be better without any skeg at all, for the reasons we've been discussing. It is immensely powerful, and I have never, even in crazy circumstances, stalled it. Yet it is balanced enough to require quite little force. It will not be as fast as a thin racing rudder, but I don't think the cost is all that high.

[Stumble]

Originally Posted by Ericson38

I had spades on my two Ericson 38s and that was a fast boat design, but in heavy beam reach seas at the Channel Islands those boat's rudders would stall once the steering correction became too large. This would slow the boat down. A stall reduces boat speed to about 1/2 on the E38.
any deflection of the rudder slows you down, this is why properly balancing the sail plan is important for speed regardless of the rudder design.

High aspect has high lift, and narrow stall tolerance. On a broad reach it was worse, and I would depower the main (reef and flatten) just so the head sail would pull the boat, rather than pull and main sail push.so ya, balancing the sail plan eliminates the issues with a high aspect foil. It also means you go faster.

I think the partial skeg (not full skeg with rudder completely behind) with some rudder at the bottom being ahead of the rudder post arrangement is better in that there are three bearings, neutral steering, and a bit more span on the whole foil (spade section and leading skeg foil) that results in a much more stall tolerant appendage when the going gets rough and large seas will knock you off course when they occasionally reinforce each other. A stall (even partial) results in a slow or fast round up, and a lot of chaos as the sails flog.

The skeg isn't the determining factor of the stall angle it's the aspect ratio of the foil. You could just as easily design a low aspect spade with a very high stall angle, it would perform equally badly with the skeg rudder. But the more efficient option is to design a high aspect spade, then balance the sail plan to match. On the other hand low aspect foils that don't penetrate far below the surface can be more prone to ventilation due to heel angle than deep spade rudders. I have actually watched someone heel over and lift the entire rudder out of the water then complain about stalling the rudder later at the bar. In this case it wasn't stalled, it was just completely out of the water.

For the strength to be better than a spade though, the skeg has to really be built to take loads, not go for a ride and look good. The Bill Dixon Taswell (and maybe Oyster and Moody) rudders have skegs that run about 3/4 the way down the rudder span.

all that is required to design a stronger skeg is to design and build a stronger skeg. Once you have a 'stronger skeg' I can then just as easily design a stronger spade.

This is really a reduction to the absurd. Once the strength of the foil has been selected a particular design option either can or cannot meet that minimum. Since either a skeg or spade can meet any reasonable strength requirement there is no meaningful strength difference. It's like arguing which is longer a 1/2"piece of string or a 1/4" piece of string.The information given simply does not contain the necessary criteria to make that determination. If you want to know which is stronger, measure them.

The u-2 spy plane, at max alt, with very high aspect ratio wings, flies at the point of stall of the main wings to reach max alt and hold it. The local windflow AOA 'window' is so sensitive to get to this balancing act, that the skill can't really be taught in a simulator.

I think part of the issue here is that people tend to think of skegs as an additional reinforcing element designed to help support the rudder. But that simply is not the case. Rudders wether spade or skegs are designed elements intended to provide a certain amount of strength. So in a particular case let's assume we want a rudder to be able to withstand 10,000 lbs of force. That is what the engineer says is the maximum needed to safety handle the loads of the boat (including hitting submerged objects and worst case scenarios). So the designer goes back and has the option of either designing a spade capable of withstanding a 10,000lb load, or designing a skeg capable of handeling a 5,000lbs load and an attached rudder that can handle an additional 5,000lbs load. At the end of the day the two rudders are equally strong.

Now the skeg designer MAY choose to design a skeg for 7,500 and the rudder for 7,500. And a legitimate argument could be made that the system is stronger than the spade that is designed for a 10,000lbs load, and you would be right. But the spade could easily be strengthened to handle a 15,000lbs load as well.

This is simply a truth about how to engineer to a given required strength.

What can not be designed around is that the skeg rudder will always have more drag than the spade, if they are designed for the same load and turning force. This is just a consequence of the skeg and the way water flows over it. And cannot be designed away. If you want a given AR, and X amount of turning force, and a Y stall angle, then the skeg must have more drag.

[Dockhead]

Originally Posted by Stumble

I think part of the issue here is that people tend to think of skegs as an additional reinforcing element designed to help support the rudder. But that simply is not the case. Rudders wether spade or skegs are designed elements intended to provide a certain amount of strength. So in a particular case let's assume we want a rudder to be able to withstand 10,000 lbs of force. That is what the engineer says is the maximum needed to safety handle the loads of the boat (including hitting submerged objects and worst case scenarios). So the designer goes back and has the option of either designing a spade capable of withstanding a 10,000lb load, or designing a skeg capable of handeling a 5,000lbs load and an attached rudder that can handle an additional 5,000lbs load. At the end of the day the two rudders are equally strong.

Now the skeg designer MAY choose to design a skeg for 7,500 and the rudder for 7,500. And a legitimate argument could be made that the system is stronger than the spade that is designed for a 10,000lbs load, and you would be right. But the spade could easily be strengthened to handle a 15,000lbs load as well.

This is simply a truth about how to engineer to a given required strength.

What can not be designed around is that the skeg rudder will always have more drag than the spade, if they are designed for the same load and turning force. This is just a consequence of the skeg and the way water flows over it. And cannot be designed away. If you want a given AR, and X amount of turning force, and a Y stall angle, then the skeg must have more drag.

What this man said.


The point may be somewhat subtle for some.

Maybe I could simplify the argument a bit --

1. There is no inherent limit of the strength of either skeg or spade rudders.

2. A skeg rudder with a strong skeg (not like some) might require a bit less material to reach a given strength level, because of the beneficial effect of spreading the load. You could make the rudder shaft a bit lighter.

3. But so what? Just make the rudder shaft of the spade a bit larger, and you get the same result in terms of strength.

4. So why sacrifice the hydrodynamics of the rudder just to save a bit on the size of the rudder shaft? Doesn't make any sense.

[Ericson38]

Originally Posted by Stumble

I think part of the issue here is that people tend to think of skegs as an additional reinforcing element designed to help support the rudder. But that simply is not the case. Rudders wether spade or skegs are designed elements intended to provide a certain amount of strength. So in a particular case let's assume we want a rudder to be able to withstand 10,000 lbs of force. That is what the engineer says is the maximum needed to safety handle the loads of the boat (including hitting submerged objects and worst case scenarios). So the designer goes back and has the option of either designing a spade capable of withstanding a 10,000lb load, or designing a skeg capable of handeling a 5,000lbs load and an attached rudder that can handle an additional 5,000lbs load. At the end of the day the two rudders are equally strong.

Now the skeg designer MAY choose to design a skeg for 7,500 and the rudder for 7,500. And a legitimate argument could be made that the system is stronger than the spade that is designed for a 10,000lbs load, and you would be right. But the spade could easily be strengthened to handle a 15,000lbs load as well.

This is simply a truth about how to engineer to a given required strength.

What can not be designed around is that the skeg rudder will always have more drag than the spade, if they are designed for the same load and turning force. This is just a consequence of the skeg and the way water flows over it. And cannot be designed away. If you want a given AR, and X amount of turning force, and a Y stall angle, then the skeg must have more drag.

Skegs are used to get to a target bending force without deflection rating, depending on the design. It depends on how the designer wants to tackle this area. If there are 3 bearings with a skeg, then it stands to reason that the mid span point (or even lower) of the free rudder is supported part way down, not just all above it with the two bearing arrangement.

Skeg designs are also usefull in water flow angle induced stalls, or stalls that simply occur when the foil is given more steering input than it can handle.

Having owned 3 spade designs (all Ericsons) and one partial skeg (not full sjeg like a Hylas 49 or KP 44-46), I am really impressed by the partial skeg-rudder fingertip control in high winds (35 kts with full head sail, stay sail, and main). The higher the aspect ratio, the easier to stall the foil, air or water medium.

A stalling to stalled spade has way more drag than a skeg-spade in the same angle of attack regime that is not stalling. The ocean will yaw-rotate any boat in a strong sea, and course corrections have to be made with authority. I used to steer around seas that I knew were going to force the stern downwind, but I don't have to make those maneuvers now.