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Old 10-06-2016, 05:06   #121
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Re: Skeg or Spade?

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Originally Posted by Dockhead View Post
That's what Dashew says -- he says that skeg rudders are MORE vulnerable to damage than spade rudders are, because they jam when any damage occurs.
He didn't learn that from Admiral Lütjens..
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Old 10-06-2016, 05:47   #122
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Re: Skeg or Spade?

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There are no numbers as far as I know, because you just don't see the same boat built with both types of rudder. But if you want to settle it by racing for money, two similar boats one with a skeg and the other with a spade, I would be game

The difference is very clear on the racing course, where no racing boat has been designed with a skeg rudder in at least 30 years. The hydrodynamic superiority of spade rudders is not controversial, and the difference is not minor ("hundreths of a knot").
This discussion is a mix of theory and practice. I am not interested in racing boats because they are clearly very different from a cruiser. Of course getting the best speed, pointing higher, tracking, and so on are issues for all sail boats.

There are cruising boats with spade rudders and similar hull form and perhaps sail plan and displacement as some with skegs. My point was more about the real world benefit or draw backs for an "optimized" skeg design on a cruiser and an optimized spade.

I also don't think a race is the way to settle this as engineers can compute polars and so forth and so it would seem the real numbers can be put on the table. That's what I would like to see.

As far as a skeg being more vulnerable to a spade... I would again point out it depends on the build and the engineering. You can't have it both ways... GRP can't be strong and stiff when you want and then flimsy when it suits your argument. People do tend to frame an argument and choose their "evidence" to support it. As far as I know... and I respect Dashew and have read his books... he's not an professional engineer. If he is I stand corrected... as I know he's "designed" several boats and sailed a few hundred thousand miles!

Both a skep and a spade are cantilevers for sure. The skeg clearly has a lover aspect ratio related to its support than the spade. I suspect the two together work synergistically structurally... rather than the skeg making the rudder weaker and more vulnerable. And spades have been known to drop and not so with with skeg supported rudders. Airplane wings are more like skegs than spades... aren't they?
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Old 10-06-2016, 06:35   #123
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Re: Skeg or Spade?

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Originally Posted by Sandero View Post
As far as a skeg being more vulnerable to a spade... I would again point out it depends on the build and the engineering.
Yep, agreed 100%.

A tough strong rudder is an important characteristic of a cruising boat. You really need to dive into the lazarette, look at plans and ask questions about the layup etc.
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Old 10-06-2016, 07:09   #124
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Re: Skeg or Spade?

So I did some digging... The Oyster skeg mentioned earlier, it's almost completely stylistic. So far as I could run the numbers, and I could be wrong I am not an engineer, the rudder stock is far stronger than it needs to be.

Doubling the Lloyds special service (serious weather and ice) numbers for a much larger, faster, and heavier boat would lead to a rudder that is 5" diameter. The Oyster has a 6" shaft. So at a minimum that rudder shaft is probably four times the necessary design strength of what it would need to be to be classed for litterly a worst case scenario.

The skeg could be glued on with bubble gum and it wouldn't effect anything.


Sandero,

If you won't accept the determination of Dashew, Perry, Farr, or RP, then I doubt anything I could possibly say would change your mind. I don't know which if any of them are PE's and frankly I couldn't care less they have all designed hundreds of boats, including many of the most respected designs ever built for off shore cruising. Professional licensing only matters if you don't have a proven record of doing exactly what you say you can, if you can prove your are capable I don't care what license you have (in much the same way no one cares that MIT has an unaccredited engineering school, because they are MIT).

From an engineering standpoint there simply is no gain to a skeg any more. Sure when the best material out there was wood there may have been a real justification. Without modern metal fabrication, and materials it may have been necessary. But we now have reliable stainless manufacturing, or even carbon monolithic capability. Rudders don't need the extra strength because they can be made stronger than needed without them.
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Old 10-06-2016, 07:15   #125
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Re: Skeg or Spade?

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So I did some digging... The Oyster skeg mentioned earlier, it's almost completely stylistic. So far as I could run the numbers, and I could be wrong I am not an engineer, the rudder stock is far stronger than it needs to be.

Doubling the Lloyds special service (serious weather and ice) numbers for a much larger, faster, and heavier boat would lead to a rudder that is 5" diameter. The Oyster has a 6" shaft. So at a minimum that rudder shaft is probably four times the necessary design strength of what it would need to be to be classed for litterly a worst case scenario.

The skeg could be glued on with bubble gum and it wouldn't effect anything.


Sandero,

If you won't accept the determination of Dashew, Perry, Farr, or RP, then I doubt anything I could possibly say would change your mind. I don't know which if any of them are PE's and frankly I couldn't care less they have all designed hundreds of boats, including many of the most respected designs ever built for off shore cruising. Professional licensing only matters if you don't have a proven record of doing exactly what you say you can, if you can prove your are capable I don't care what license you have (in much the same way no one cares that MIT has an unaccredited engineering school, because they are MIT).

From an engineering standpoint there simply is no gain to a skeg any more. Sure when the best material out there was wood there may have been a real justification. Without modern metal fabrication, and materials it may have been necessary. But we now have reliable stainless manufacturing, or even carbon monolithic capability. Rudders don't need the extra strength because they can be made stronger than needed without them.
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.
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Old 10-06-2016, 09:00   #126
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Re: Skeg or Spade?

FWIW, the skeg or spade debate was active 32 years ago, see, for example, the discussion on ~ pg 116 in this 1982 Cruising World article:

https://books.google.com/books?id=oJ...=spade&f=false

Several Bob Perry (and other) skeg-rudder designs are featured. Of course, a lot has changed since 1984: CAD and carbon fiber construction for example.

Question for the experts: why aren't tabs on modern aircraft 'spade' mounted? Would an airplane with spade tail fins be more efficient than same plane with 'skeg' hung tabs? If so why don't modern aircraft (A380, 787) use them?

https://en.wikipedia.org/wiki/Stabilizer_(aeronautics)
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Old 10-06-2016, 09:02   #127
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Re: Skeg or Spade?

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Originally Posted by Sandero View Post
. . .

As far as a skeg being more vulnerable to a spade... I would again point out it depends on the build and the engineering. You can't have it both ways... GRP can't be strong and stiff when you want and then flimsy when it suits your argument. People do tend to frame an argument and choose their "evidence" to support it. As far as I know... and I respect Dashew and have read his books... he's not an professional engineer. If he is I stand corrected... as I know he's "designed" several boats and sailed a few hundred thousand miles!

Both a skep and a spade are cantilevers for sure. The skeg clearly has a lover aspect ratio related to its support than the spade. I suspect the two together work synergistically structurally... rather than the skeg making the rudder weaker and more vulnerable. And spades have been known to drop and not so with with skeg supported rudders. Airplane wings are more like skegs than spades... aren't they?
I'm not really what you're arguing about here --

Do you doubt that spade rudders work better than skeg ones do?

Or are you doubting that spade rudders can be strong?

Or what, exactly?

No one said that every spade rudder is stronger than every skeg hung rudder, and no one said that skeg rudders have no advantages. As you said yourself, and we all agree -- it depends on how it's built in all cases.

I think everyone would agree that a strongly built skeg can add strength to the whole construction -- the load is spread to another point, after all. The question is how much and whether it is worth it.

But besides hydrodynamic disadvantages, which I say again is not controversial, skegs also have structural disadvantages, which we've discussed. Are those fatal to the whole idea? Naturally it depends on the particular design. A skeg which is made just as a symbol of strength as some of them are -- a la Bob Perry's comment -- is not doing anything, worth the disadvantages.


Full skeg rudders are dying out -- very few sailboats are made with them any more, and less all the time. Does that tell you anything? It tells me that there is a pretty broad consensus among boat designers that there's not all that much to gain with that construction, for the average cruising boat. Why? Well, we explained it, in this thread. Contest Yachts, for example, one of Europe's great builders, has used only spade rudders for at least 20 years or so. Which is not to say that there is nothing to gain, or that full skeg rudders don't have any advantages at all.
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Old 10-06-2016, 09:05   #128
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Re: Skeg or Spade?

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Originally Posted by hodgmo View Post
. . .Question for the experts: why aren't tabs on modern aircraft 'spade' mounted? Would an airplane with spade tail fins be more efficient than same plane with 'skeg' hung tabs? If so why don't modern aircraft (A380, 787) use them?

https://en.wikipedia.org/wiki/Stabilizer_(aeronautics)
That's a great and interesting question.

We have some real aero engineers on here, and we should let them answer.

One thing I can say however is that some planes DO have "spade" stabilizers and rudders.
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Old 10-06-2016, 09:29   #129
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Re: Skeg or Spade?

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Originally Posted by hodgmo View Post
Question for the experts: why aren't tabs on modern aircraft 'spade' mounted? Would an airplane with spade tail fins be more efficient than same plane with 'skeg' hung tabs? If so why don't modern aircraft (A380, 787) use them?

https://en.wikipedia.org/wiki/Stabilizer_(aeronautics)
I believe this is mostly to do with propulsion method. Next time you fly on a commercial passenger plane with a Bermuda rig, I bet it will have a spade tailfin. I think some fighter jets do use spade style flight surfaces for extreme maneuverability. On a boat, the rudder functions not only to steer the boat but to provide lift to windward as a primary function in addition to steering the boat. On a plane, other than for a short distance at take off and landing to keep things under control, the rudder is mostly extra drag (yeah it induces some stability). Therefore the ability to form the tail smoothly into the fuselage, provides less drag by eliminating the gap between the fuselage and the rudder. They accept less turning capability because it's more important to reduce drag. Ignoring stunt planes, most wings are intended to only provide lift in one direction. If you wanted a boat that only needed to sail on one tack, you could form the skeg to fit in with the rudder and create an more hydrodynamic shape but most sailors want to be able to sail on either tack.
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Old 10-06-2016, 11:05   #130
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Re: Skeg or Spade?

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F
Question for the experts: why aren't tabs on modern aircraft 'spade' mounted? Would an airplane with spade tail fins be more efficient than same plane with 'skeg' hung tabs? If so why don't modern aircraft (A380, 787) use them?

https://en.wikipedia.org/wiki/Stabilizer_(aeronautics)
I'm not a expert at this stuff. (I'm an expert on other stuff that doesn't apply )


Modern fighter planes do use 'spade' mounted tail surfaces (elevator/Horiz stab for example). I'd guess it's a performance thing - you can get way better performance and the fly-by-wire will keep the aircraft pretty much flying. F-16's, F-18's, etc.. Easy to see the entire thing move in videos.

Most jetliners kinda do too - the elevator trim moves the entire horizontal stabilizer. (I would guess for efficiency - i.e keep the stabilizer and the elevators in the exact same plane - the one that trims the aircraft properly)
That's why the jackscrew failure on Alaska 261 was so bad - the control surfaces couldn't overcome the fact the entire stab was at the wrong angle.

I may be completely wrong here - I am not an expert.
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Old 10-06-2016, 11:35   #131
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Re: Skeg or Spade?

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Originally Posted by hodgmo;2140939

Question for the experts: why aren't tabs on modern aircraft 'spade' mounted? Would an airplane with spade tail fins be more efficient than same plane with 'skeg' hung tabs? If so why don't modern aircraft (A380, 787) use them?
[url
https://en.wikipedia.org/wiki/Stabilizer_(aeronautics[/url])
I can't think of many airplanes that do not have skeg mounted rudder except for some fighters which have 2 "spade like rudders" or ruddervators. You probably could not control a spade rudder without computers and hydraulic power.
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Old 10-06-2016, 11:46   #132
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Re: Skeg or Spade?

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I can't think of many airplanes that do not have skeg mounted rudder except for some fighters which have 2 "spade like rudders" or rudervators. You probably could not control a spade rudder without computers and hydraulic power.
It's a matter of manoeuvrability like in dog fighting. Airliners need more directional stability hence "skeg rudders". Both have hydraulics becouse of the forces involved. Computers are used in both but fighters are a bit different matter as they are aerodynamically unstable..

BR Teddy
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Old 10-06-2016, 12:09   #133
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Re: Skeg or Spade?

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It's a matter of manoeuvrability like in dog fighting. Airliners need more directional stability hence "skeg rudders"...

BR Teddy
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.
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Old 10-06-2016, 12:24   #134
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Re: Cockpits like Amel Super Maramu

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ID:	125911 We've had both and I must admit the skew hung rudder looks more capable of withstanding an impact.


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Old 10-06-2016, 12:29   #135
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Re: Skeg or Spade?

Ok, I can chime in here for a couple of posts a ways back. I have a Buchan 37, which originally had a keel hung rudder. It was built as a race boat and was pretty fast for it's day. The second owner removed the rudder from the keel and fitted a well balanced spade farther aft. And I'm told the performance increase was considerable. Sorry no numbers here either but I'll wager she will out perform any other Buchan still sporting a keel hung rudder. One advantage of a spade not yet mentioned is you need less angle to accomplish the same maneuver because the entire surface is turning the boat and not having to compete with a skeg or keel directly in front of it. My boat will practically pivot in place on axis which is very helpful in tight quarters.

My .02, carry on.
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