Skeg or Spade?

[Sandero]

Originally Posted by Stumble

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.

Would you kindly explain this statement... Thank you!


How about this: http://www.usna.edu/Users/naome/phmi...ts/Rudders.pps

[Ericson38]

Originally Posted by Dockhead

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.

You missed the point I was making. Dixon spent more time-material doing the skeg with 3 bearings than simply up sizing the two bearings, the armature, and the shaft. But he gained surface area, and tolerance to stalling in heavy sea states. Being a partial skeg, there is still rudder ahead of the stock, and therefore the steering cables-quadrant aren't loaded excessively, which also wears things out. The Taswell 49 skeg goes about 1/2 way down what could be considered the rudder's span length.

[Dockhead]

Originally Posted by Ericson38

You missed the point I was making. Dixon spent more time-material doing the skeg with 3 bearings than simply up sizing the two bearings, the armature, and the shaft. But he gained surface area, and tolerance to stalling in heavy sea states. Being a partial skeg, there is still rudder ahead of the stock, and therefore the steering cables-quadrant aren't loaded excessively, which also wears things out.

Yes, that's the type of rudder I have, and designed by the same man. The three bearings, and the shaft, can all be smaller than they would have to be for a spade with two bearings, as you say, but the result is the same, so who cares?

And what does the skeg have to do with the surface area? You can make a spade with any aspect ratio and any surface area you like.

I certainly agree with you about the unsuitability of thin racing rudders for strong weather cruising, and I also agree with you that they are likely to be actually slower in some conditions.

[Ericson38]

Originally Posted by Dockhead

Yes, that's the type of rudder I have, and designed by the same man. The three bearings, and the shaft, can all be smaller than they would have to be for a spade with two bearings, as you say, but the result is the same, so who cares?

And what does the skeg have to do with the surface area? You can make a spade with any aspect ratio and any surface area you like.

I certainly agree with you about the unsuitability of thin racing rudders for strong weather cruising, and I also agree with you that they are likely to be actually slower in some conditions.

To be neutral, here is a good example from a another designer (H&R)

https://www.bing.com/images/search?q...2o0&ajaxhist=0

Here is the Taswell arrangement

http://www.jordanyachts.com/wp-conte...2/SCAN0069.jpg

On your question of who cares..

Lets say the boat is going straight (not sliding more than 10 degrees or so) through the water, in other words not crabbing like a plane on approach in a side wind.

Now let have the boat yaw to leeward 10 degrees or so due to high wind from another direction, combined with the odd wave knocking the boat around in yaw (vertical axis). The boat reached a crest, and the wind had a different direction up there than down in the troughs.

With a spade alone, the rudder is going to be turned, say 10 degrees in the opposite direction to counter this rotation and get the boat back on the general course. The net AoA is 20 degrees for the spade in the water.

A 20 deg AOA for a spade is momentary, but for a high aspect ratio spade, will get close to the stall angle, depending on water speed, surface roughness, thickness, NACA foil # and the like. A stall means that water is not attached and flowing normally on both sides of the rudder. Drag goes up, lift goes down. Then there is the recovery.

The leading skeg has attached flow on both sides during this time, as its own aspect ratio is rather low, and the 10 deg boat yaw does not result in a skeg stall.

The skeg presents this laminar flow to the rudder plane which has a 10 deg turn angle now (the helm course correction).

The rudder does not stall but is able to work in this environment since the initial flow to it was initiall conditioned (flow AoA) by the skeg.

Its kind of like an enhanced leading edge for the movable rudder plane, allowing for control authority to be maintained in more difficult situations.

[DumnMad]

Ericson38
Thanks for your posts - explains a lot.

[Dockhead]

Originally Posted by Ericson38

To be neutral, here is a good example from a another designer (H&R)

https://www.bing.com/images/search?q...2o0&ajaxhist=0

Here is the Taswell arrangement

http://www.jordanyachts.com/wp-conte...2/SCAN0069.jpg

On your question of who cares..

Lets say the boat is going straight (not sliding more than 10 degrees or so) through the water, in other words not crabbing like a plane on approach in a side wind.

Now let have the boat yaw to leeward 10 degrees or so due to high wind from another direction, combined with the odd wave knocking the boat around in yaw (vertical axis). The boat reached a crest, and the wind had a different direction up there than down in the troughs.

With a spade alone, the rudder is going to be turned, say 10 degrees in the opposite direction to counter this rotation and get the boat back on the general course. The net AoA is 20 degrees for the spade in the water.

A 20 deg AOA for a spade is momentary, but for a high aspect ratio spade, will get close to the stall angle, depending on water speed, surface roughness, thickness, NACA foil # and the like. A stall means that water is not attached and flowing normally on both sides of the rudder. Drag goes up, lift goes down. Then there is the recovery.

The leading skeg has attached flow on both sides during this time, as its own aspect ratio is rather low, and the 10 deg boat yaw does not result in a skeg stall.

The skeg presents this laminar flow to the rudder plane which has a 10 deg turn angle now (the helm course correction).

The rudder does not stall but is able to work in this environment since the initial flow to it was initiall conditioned (flow AoA) by the skeg.

Its kind of like an enhanced leading edge for the movable rudder plane, allowing for control authority to be maintained in more difficult situations.

OK, now I finally understand what you're saying. You're saying that the skeg has a beneficial HYDRODYNAMIC effect. That's really interesting. I've never heard that before, and I'm somewhat skeptical, but the question is way above my very modest level of knowledge in hydrodynamics/aerodynamics. I happen to have a crew on board who is a real aeronautical engineer -- I'll ask him about it. Very interesting.

A bit of googling turns up this:

"From "Yacht Designed Explained" by Steve Killing, Douglas Hunter

“The skeg, which sits solidly in front of the rudder, performs two functions--the first hydrodynamic, the second structural. Because the skeg is mounted on centerline and is at a constant angle to the incoming flow, it never sees a very high angle of attack, and the likelihood of stalling is greatly reduced. When the rudder is turned, it forms with the skeg a shape approximating that of an efficient, cambered foil, which can produce high lift values."
From “Ted Brewer Explains Sailboat Design” by Ted Brewer

“The benefits of a skeg ahead of a rudder are numerous. The skeg raises the stall angle at which the rudder will lose its lift; it provides a greater control surface area thereby adding to directional stability; it offers better control than a spade rudder when there is a high flow angle of attack, i.e., in case of a serious broach."

http://www.caliberyacht.com/Features...g_Stalling.htm


True? Or BS? Inquiring minds want to know. This is directly relevant to the original and main topic of this thread, so very good that this has been brought up.

[Sandero]

From what I just read on the www... the properly proportioned skeg can perform better in certain conditions... and the net "improvement" is there is any for a spade for a cruising boat is deemed negligible. "strength" is a red herring...

I suppose how fast can the same hull turn with the two rudders may be important for a race boat... but for a cruiser on a 500mi passage?

Again I am unclear that a spade will cause the boat to sail higher (lift)... and if so how would that impact a cruiser who typically will avoid sailing to windward if they can wait.

One would need to compare apples to apples wouldn't they... same hull and sail plan with two different rudders sailing sail journey.

Finally spade rudders are known to drop off and I have read of the same for skeg hung.

No?

[Dockhead]

Originally Posted by Sandero

. . .Again I am unclear that a spade will cause the boat to sail higher (lift)... and if so how would that impact a cruiser who typically will avoid sailing to windward if they can wait. . . .

For any cruiser who would rather wait for a different wind, than sail upwind, then the rudder type is not going to be very important.

But if you're sailing long distances against the prevailing wind, you won't have that choice. I got keenly interested in my boat's upwind performance when I had to sail 1500 miles upwind in one month -- that was two years ago. Gives you a whole different perspective on upwind sailing.

[cal40john]

Originally Posted by Dockhead

OK, now I finally understand what you're saying. You're saying that the skeg has a beneficial HYDRODYNAMIC effect. That's really interesting. I've never heard that before, and I'm somewhat skeptical, but the question is way above my very modest level of knowledge in hydrodynamics/aerodynamics. I happen to have a crew on board who is a real aeronautical engineer -- I'll ask him about it. Very interesting.

A bit of googling turns up this:

"From "Yacht Designed Explained" by Steve Killing, Douglas Hunter

“The skeg, which sits solidly in front of the rudder, performs two functions--the first hydrodynamic, the second structural. Because the skeg is mounted on centerline and is at a constant angle to the incoming flow, it never sees a very high angle of attack, and the likelihood of stalling is greatly reduced. When the rudder is turned, it forms with the skeg a shape approximating that of an efficient, cambered foil, which can produce high lift values."
From “Ted Brewer Explains Sailboat Design” by Ted Brewer

“The benefits of a skeg ahead of a rudder are numerous. The skeg raises the stall angle at which the rudder will lose its lift; it provides a greater control surface area thereby adding to directional stability; it offers better control than a spade rudder when there is a high flow angle of attack, i.e., in case of a serious broach."

Caliber Yachts Rudder Skeg Combination


True? Or BS? Inquiring minds want to know. This is directly relevant to the original and main topic of this thread, so very good that this has been brought up.

I've seen this written in other books on sailboat design so if it is B.S. more than one yacht designer believes it, but Killing is the reference I brought up back in post #53

[salticrak]

This may be a bit off topic,but here goes. I am an avid kayak fisherman who launches thru the surf and back on most of my trips. I have a 'glass fishing kayak with a skeg style rudder. It protects the rudder from heavy impacts with the sand.It also helps to minimize line tangle.
I used to use a spade ruddered kayak.The rudder could not stand up to the force of getting knocked back in the surf onto the sand.The rudder shaft would bend and crack the fibreglass housing.

Yes spades are faster,but skegged rudders are stronger for sure.

[Jim Cate]

Yes spades are faster,but skegged rudders are stronger for sure.

What you must be saying is that IN YOUR CASE, on a non-sailing boat of completely different design and purpose, the skeg rudder is stronger for sure.

Has absolutely no relevance to the subject of discussion IMO.

Jim

[Stumble]

I really wish we had an actual engineer that really understands this stuff. Because it's complicated and we are past what I really feel comfortable trying to explain. But here goes anyway.

First there are competing issues here.

Strength of the system
Stall angle
Lift
Drag
Yaw force

The issue is that they are all related, or inter-related.

So as an example, the drag generated by a spade vs skeg in a strait line has one answer, but it has another at say 4 degrees of turn. But a skeg does not generate as much turning force at 4 degrees as a spade does. So what you really need is to set the yaw force then compare the lift/drag at that point. Because a spade requires less AOA to generate the same yaw you don't have to turn the blade as far. At the same time since you aren't turning the blade as far you have less drag, more lift, and a far more efficient system.

This is why despite the lower drag per angle of deviation a boat with a spade rudder comes out of a tack with more speed.

Dealing with all of these dependent variables is beyond my engineering back ground. At best I can work around the edges. But once strength is set aside, it's pretty unavoidable that every single designer worried about speed uses a spade. It is possible I am wrong about the particulars, but not the result.

This of course also effects the stall point relative to desired yaw force. Let's assume that a spade generates twice the yaw at a given AOA compared to a skeg (I doubt the difference is this much) but stalls at half the AOA of a skeg (again I doubt this is true). Which rudder stalls first? Obviously the spade will stall at a lower AOA, but you have generated the same amount of turning force at the point of the stall. I don't have an answer here, and I suspect it has a lot to do with the speed of the boat at the time, and the aspect ratio of the foils. But it's an interesting question I have no answer too.

[Snowpetrel]

Really now the conversation seems to be between a half skeg, or no skeg, straight Spade rudder, due to the fact that the full skeg rudder has no balance.

The half skeg has a advantages of possibly being better supported and prehaps having better stall characteristics.

But the half skeg suffers from an ugly gap at the point where the skeg stops, this has to hurt hydrodynamicly, and who knows how this is really going to effect its stall mode. With the bottom being a spade section and the top being skeg half it might stall which is nearly as bad as it all stalling.

This gap is prone to catching ropes and weed in a very awkward place. A spade rudders gap is further up, and often protected by the propeller.

Another issue with the half skeg is the heel bearing is an awkward thing to build, align and fit. It either needs to be split, or built in by some cunning method that then makes servicing interesting.

With three bearings it is very hard to align them all and ensure that they are all sharing the load nicely. Inevitably one pair of bearings do more of the work than the other, so the load gets concentrated on them, potentially accelerating wear and fatigue.

Cleaning and antifouling around the gap beween the skeg and rudder is awkward. The gap really should have "flaps" over it to streamline it further complicating things.

Overall an oversize, overstrength spade is simpler. With a bit of extra size, and a good wide stall resistant section the thing is never going to get close to a stall condition if you sail the boat properly. The extra width from say a NACA 0015 or 0020 section over a skinny NACA 0012 section gives more room for a larger, stronger rudder stock.

Its interesting how the surface roughness increases the chance of stall. Well worth doing a nice job of fairing the foil and its antifouling.

However I think both types done well are fine. But they need to be done right. There is little margin for sloppy engineering or build on any modern rudder or keel designs Unlike traditional long keel designs with attached rudders where they could be easily built on the principle that if it looks strong enough it probably is strong enough.

I do think some sort if auxiliary rudder probably is a good idea with most modern rudders, as is a cofferdam ahead of the stock.

My ideal boat would have a transom hung spade. Real simple and very strong, though more prone to ventilation.

[double u]

I used to think "full skeg only" was the way to go - I am not so sure any more:
1. practical reason: extremely limited choice of boats (particularly secondhand)
2. structural considerations: no matter how strong, a skeg can be broken, & then one is really "up the creek". on a metal boat it would bend & jam the rudder which would make the boat totally unmanageable. Quite likely a spade will be destroyed before that, BUT: if cleverly made the rudder can be dropped (or pushed out) & one could continue either on the auxiliary rudder of the windvane (Hydrovane, Windpilot or Mustafa) or have pintles in place on the transom & use some floorboards (prepared) as jury rudder.
worst-case-szenario would imho be to have too strong a rudder & then rip out portions of the hull /(=no boat any more)

[double u]

how about this:
http://www.boatdesign.net/forums/att...ket-12-077.jpg
http://joecoopersailing.com/wp-conte...Jersey-011.jpg