Skeg or Spade?

[Kenomac]

Originally Posted by Sandero

It seems to me this is a hill of beans. The measurable improvement to the cruising yachtsman has not be demonstrated to me at least. And it seems to me lift will cause the stern to move to lee assuming it's present (lift).

Not having seen the Oyster required support at the lower end... because you are turning a simply supported beam into a cantilever.

Well, here you go. Now you've seen it.

[neilpride]

Originally Posted by Dockhead

If that were true, we would see skegs on race boats. But the last skeg on a race boat disappeared decades ago.

Actually there are very clear and very objective reasons why spade rudders work so much better upwind. There is no controversy about it among boat designers. It doesn't mean you can't go upwind at all with a skeg rudder -- it's just that it's quite a bit less efficient. Very much like a full keel -- we all know classic full keel boats which go like freight trains (Jolie Brise!), but you don't see full keels on race boats, do you?


Spade rudders are of no benefit when sailing off the wind, though, which is why most cruisers don't really care. That's because -- like an airplane's vertical stabilizer -- there's no angle of attack in normal conditions.

The rudder does need an angle of attack, however, when sailing upwind -- just like an airplanes horizontal stabilizer. That's why practically all large airplanes have movable horizontal stabilizers, even if they also have elevators. That's because if you operate an elevator continuously to produce lift to trim the plane, you create a lot of drag with the skeg-rudder type configuration. So modern large aircraft move the whole stabilizer for that purpose, and use elevators (if they even have them) only for maneuvering.

This is very much how sailboat rudders are used -- turning, and producing lift upwind, are somewhat different functions. Actually there's no reason why you couldn't build a sailboat rudder like an airplane's horizontal stabilizer -- the whole rudder moves within say 5 or 6 degrees, and anything more than that, you use a separate movable surface (like a skeg rudder) at the end. That would probably have a few advantages over any of the rudder types we actually have -- probably better stall resistance, better feel, and easier to make really strong. But I guess this would probably be too complicated and expensive to be worthwhile.

Dock, this is like a Polux post, no offense, i say it again, the turn in the last years in hull design evolution send the skeg rudders to the trash bin, for the average cruisers able to sail to weather in comfort at a reasonable speed i guess its all what they need, i dont mean they perform better than a spàde, no, spades are the best in terms of pure perfomance, but if you weight the pros and cons in production boats i reach the conclusion that spades in most production boats are just a fashion trend.

[DumnMad]

Originally Posted by neilpride

Dock, this is like a Polux post, no offense, i say it again, the turn in the last years in hull design evolution send the skeg rudders to the trash bin, for the average cruisers able to sail to weather in comfort at a reasonable speed i guess its all what they need, i dont mean they perform better than a spàde, no, spades are the best in terms of pure perfomance, but if you weight the pros and cons in production boats i reach the conclusion that spades in most production boats are just a fashion trend.

Or maybe the spade is just a cheaper option marketed as a better alternative.

[neilpride]

Originally Posted by DumnMad

Or maybe the spade is just a cheaper option marketed as a better alternative.

Cheaper for sure, they just need to drill 2 holes and slip the blade in the cheaper market spectrum.. Skegs in the other side are a feature of split mold hull construction where they can layup the skeg properly , i see a bunch of bad skegs just because they dont follow a proper layup in the skeg or because they simply try to laminate a skeg with a single mold hull obviously with the difficulties added, they add weight, they are complex to built properly , and they are expensive compared with a single spade rudder constr... and what they are looking for the actual builders ?? the opposite.

Light, faster, cheaper... and the spade rudder fit the bill, and to sad they dont pay attention or care to rudders in this matter because spade rudders can be strong enough and well fitted for offshore work like some builders have shown,, give me any good strong skeg rudder anytime v a weak flimsy spade rudder or give me a strong probed spàde rudder v any skeg rudder out there... pick one by one ....

[Dockhead]

Originally Posted by Sandero

It seems to me this is a hill of beans. The measurable improvement to the cruising yachtsman has not be demonstrated to me at least. And it seems to me lift will cause the stern to move to lee assuming it's present (lift).

Not having seen the Oyster required support at the lower end... because you are turning a simply supported beam into a cantilever.

Not a "simply supported beam", but two cantilevers. We've been through this in this thread already.

[TeddyDiver]

Originally Posted by Dockhead

If that were true, we would see skegs on race boats. But the last skeg on a race boat disappeared decades ago.

I said earlier in this thread it's becouse of the aspect ratio. High aspect rudders are bend by the lift they create. Obviously there are limits how much you can bend skeg hung rudder without either compromising the hydrodynamics or preventing to turn it at all. Geometry you know. So with high aspect rudder there's no question which one to choose.

[Dockhead]

Originally Posted by neilpride

Dock, this is like a Polux post, no offense, i say it again, the turn in the last years in hull design evolution send the skeg rudders to the trash bin, for the average cruisers able to sail to weather in comfort at a reasonable speed i guess its all what they need, i dont mean they perform better than a spàde, no, spades are the best in terms of pure perfomance, but if you weight the pros and cons in production boats i reach the conclusion that spades in most production boats are just a fashion trend.

Certainly not a Polux post! I'm not a fan boy. I just like facts and truth and real engineering and hydrodynamics, as opposed to prejudices, and especially, as opposed to people just blindly defending whatever it is they have, which in my opinion is really stupid (I have a skeg on my own boat!).

Spade rudders in production boats are a whole different conversation. We've seen that they tend to be underbuilt in some of them. Everything in high production boats is driven by cost, first of all, and an underbuilt spade rudder is undoubtedly cheaper to build than anything. It goes well with the generally light construction, which is also cheap, but both also have as very positive side effects better performance.

So unlike fat sterns and flat bottoms, spade rudders in inexpensive production boats are not a "fashion statement", but a rational decision about cost and performance. Strength in hard ocean conditions is not high on the list of design criteria -- and that's also reasonable, because that's not how those boats are typically used. So they are not just spade rudders, but lightly built spade rudders. In more expensive boats, you also find spade rudders, but they are much more strongly built.


So spade rudders are not fashion, but real progress, just like fin keels were real progress, compared to full keels. The main difference there, however, is that full keels are very expensive to build, and also, have some real and significant benefits. But despite certain real benefits of full keels, they are in the dustbin of history.

Unlike the case with full keels, I don't see any real benefit of skeg rudders. You can make a spade rudder just as strong as any skeg rudder, as we discussed in a lot of detail above. It's just a question of design. Hydrodynamically the spade is far superior. And not just for racers, but for any sailboat going upwind. You can vary the aspect ratio and therefore vary the stall characteristics, if you want a more forgiving behavior in an ocean cruising boat, without going to a skeg rudder.

So on the contrary it's the skeg rudder, which is built to suit fashion, rather than function, in my opinion, if it's built at all, which is becoming doubtful. But again -- all that being said -- there's no difference off the wind in the hydrodynamics of both rudder types. Maybe in some cases it's cheaper to achieve a certain level of strength with a skeg; I don't know. But the lack of skeg rudders in almost any new boat design seems to say that even this is not a benefit.


Unlike some people on here who are big fan boys of fashionable hull forms, I am also not trying to tell you what to like. If you like skeg rudders despite all of the facts against them -- that's your privilege. You should love your boat, so no one can tell you what you should like. Buy an Oyster before it's too late! After Brexit, it will be 10% to 20% cheaper!

[neilpride]

Ahh yes, i see the news today, so whats the future in the short term for UK builders out of the EU....?

[Dockhead]

Originally Posted by TeddyDiver

I said earlier in this thread it's becouse of the aspect ratio. High aspect rudders are bend by the lift they create. Obviously there are limits how much you can bend skeg hung rudder without either compromising the hydrodynamics or preventing to turn it at all. Geometry you know. So with high aspect rudder there's no question which one to choose.

What you say about the limits of aspect ratio with skeg rudders is true, but if you will read any of the basic information on hydrodynamics, you will see that this is not the only difference in the hydrodynamics between skeg and spade rudders. They produce far more drag, to generate a given amount of lift. Drag is the enemy of going upwind. This is not controversial among boat designers.

[Dockhead]

Originally Posted by neilpride

Ahh yes, i see the news today, so whats the future in the short term for UK builders out of the EU....?

God only knows. Someone in the Financial Times this morning wrote that it's a "leap in the dark". Sounds right to me.

In the short term, not too much probably will happen, because it will take at least two years and more likely, five years, for the UK to actually leave the EU. But after that it's anyone's guess. A lower pound and reduced regulation could actually be beneficial. Non-tariff barrier, thought, could be fatal. I dunno.

[Sandero]

Originally Posted by Dockhead

Not a "simply supported beam", but two cantilevers. We've been through this in this thread already.

Wrong...

The skeg is a cantilever... the attached rudder is simply supported skegs appear to have more wetted surface and friction.

[malbert73]

Boarding a 737 as we speak. Tried to take a picture but it's true as dock head says: fixed keel (wing) and spade rudder (tail wings) 😀
No skeg there.


Sent from my iPhone using Cruisers Sailing Forum

[Dockhead]

Originally Posted by DumnMad

Or maybe the spade is just a cheaper option marketed as a better alternative.

Yes, as NeilPride and I both wrote -- a lightly built spade is definitely cheaper, and that is certainly the main driver for inexpensive production boats.

But the improved performance is not a joke, which is why now virtually all expensive cruising boats also now have spade rudders, except Oyster and a handful of low production makers.

A really strong spade rudder may or may not be cheaper than a really strong skeg rudder -- I don't know. I don't think a strong rudder of either type is cheap. As NeilPride said, a lot of skegs are badly made and add little strength.

Actually I think it is probably much harder to build a strong skeg, than it is to build a strong spade. The skeg is also a cantilever, but you don't have the high support point you have with a spade, so you have to work harder to tie it into the boat's structure, which as NeilPride says, is not always done.

With a spade, the engineering is more straightforward -- just a question of right shaft diameter, right bearings, and right structure, and the sacrificial bit at the end.

Steve Dashew's spade rudders are my ideal, and the way my next boat will be built. Dashew claims that a well-built spade is actually stronger than any skeg rudder. His rudders are outrageously overbuilt (I like that about Ken's Oyster as well) -- the way rudders should be, if you can afford it.

[Snowpetrel]

Originally Posted by malbert73

Boarding a 737 as we speak. Tried to take a picture but it's true as dock head says: fixed keel (wing) and spade rudder (tail wings) 😀
No skeg there.

Also they have some pretty funky bustle, Just like an IOR pintail!

[Dockhead]

Originally Posted by Sandero

Wrong...

The skeg is a cantilever... the attached rudder is simply supported skegs appear to have more wetted surface and friction.

Sorry, but unless the rudder has no shaft, then the rudder itself also supports the structure -- so a skeg rudder is indeed two cantilevers tied together. You are selling skeg rudders short! It is actually a small advantage of skeg rudders that the rudder shaft and skeg both function structurally, and spread the load somewhat.

Skeg rudders do have more wetted surface and friction (i.e. drag), but that's not the main disadvantage -- the main disadvantage is that they produce a lot of drag when the rudder is turned, like when you're going upwind. That's the main disadvantage. Drag is your enemy when you're trying to get upwind. Lack of balance is another.