Skeg or Spade?

[malbert73]

Yup- the nose down feel of wedgies is a turn off to me when heeling and sailing powered up...


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[Dockhead]

Originally Posted by Ericson38

. . .One of my acquaintances has a new 2014 Oceanis 45, and with a spade single rudder, but almost full 14.5 ft beam brought to the stern. They have broached in the bay several times, and wish it had twin rudders. When heeled over, the fore/aft trim shift is so much that the rudder is rotated close to the surface, as the boat is close to wedge shaped and floats nose down when heeled down.
Twin rudders would be a great upgrade for that condition, I think.

The cheap mass produced boats which imitate the racing wedgie style of a real performance boat like a Pogo, generally do not succeed in my experience. They are shaped that way because the cost to build per cubic meter of interior volume is less -- all the cabins and extra heads you can stuff into the massive aft sections, plus everything you can stuff in behind the vertical topsides and high freeboard -- and because they remind the naive buyers of the latest racers. A good commercial formula. Having nothing to do with sailing, however, in my opinion.

Properly designed and extremely light, like a Pogo, however -- a real planing hull -- that is something completely different.

[Stumble]

Ya the Beneteau 50 and the Pogo 50 may have some similar styling cues, but they are radically different boats. One goes looking for high wind and heavy sea to dance on, the other doesn't.

It's like saying all front engine cars suck... Well my Hyundai isn't much of a performer, but that doesn't say much about a Porche.

[robert sailor]

Originally Posted by Stumble

Ya the Beneteau 50 and the Pogo 50 may have some similar styling cues, but they are radically different boats. One goes looking for high wind and heavy sea to dance on, the other doesn't.

It's like saying all front engine cars suck... Well my Hyundai isn't much of a performer, but that doesn't say much about a Porche.

Aren't Porsche 's known for rear engines or mid engines??

[robert sailor]

Originally Posted by Dockhead

The cheap mass produced boats which imitate the racing wedgie style of a real performance boat like a Pogo, generally do not succeed in my experience. They are shaped that way because the cost to build per cubic meter of interior volume is less -- all the cabins and extra heads you can stuff into the massive aft sections, plus everything you can stuff in behind the vertical topsides and high freeboard -- and because they remind the naive buyers of the latest racers. A good commercial formula. Having nothing to do with sailing, however, in my opinion.

Properly designed and extremely light, like a Pogo, however -- a real planing hull -- that is something completely different.

Yup for sure but you do have to give points to Benni's marketing team. The "chine" and wide beam aft is just so sporty and buyers believe that these lower cost production boats are sorta race boats. Truth is that upwind the Pogo is heeled quite a bit to reduce the wetted surface but aBenni's should be sailed much flatter especially with a single rudder as it will ventilate the rudder which will require way more helm which slows the boat down and may even cause the rudder to stall. Other than looks these two boats have nothing else in common.

[jtsailjt]

Originally Posted by Dockhead

Correct.

Distributing the load to another point like a skeg does could be good. But it harms hydrodynamic properties of the rudder, and thin skegs tied into the hull structure in a questionable way are not doing much. Why Oyster persists in making rudders like this is beyond my understanding.

Scaling up the rudder shaft to provide the necessary strength is not rocket science. Likewise adding on a sacrificial end to the rudder. A well built spade rudder presents no more risks than even a very good skeg, and will be far superior hydrodynamically.

As Stumble said: "A spade can be built just as strong as a skeg. But a skeg cannot be designed to be as good a rudder as a spade." A very apt description.

Simply not true that it "harms the hydrodynamic properties of the rudder" to have a skeg. It would if you put a skeg in front of a spade rudder but fortunately, that's not quite how sailboats are designed. The rudder combined with the skeg creates more lift on one side of the rudder/skeg combo and, if designed properly, it has NO problem creating enough lift to steer the boat just fine. Almost all airplanes continue to have similar appendages to a skeg hung rudder that are used to control both yaw and pitch. Even the wing of the plane works this way to control roll. You don't rotate the whole wing of the plane when you want to roll left or right, you just lower or raise the aileron on the back of the wing.

A skeg can easily be designed to be potentially load bearing during a grounding without harming the moving parts, and it doesn't need to be so massive that it interferes with the flow along the edges of the rudder. It's much more difficult to design and build a design when you are asking one part of the boat to have both load bearing capability AND be a moving part. Also, this moving part is a rather long cantilever so when its tip hits something immovable, its length acts as a moment arm which results in tremendous concentrated forces to be exerted on the hull where the rudder shaft passes through the hull and where it's supported at its top.

The advantage of a spade is greater maneuverability at very low speeds and since it can be smaller than a skeg hung rudder, it will generate less drag than the combined skeg and rudder combo. But this additional drag is a very small part of the total drag on a cruising boat, but much more noticeable on a racing hull. This greater low speed maneuverability comes at the cost of greater vulnerability to the rudder and consequently the whole boat, and less longitudinal stability, especially in the event of an issue such as a snapped steering cable. It's a tradeoff and which pros/cons are more important depend on how the boat will be used and what your priorities are.

I can see the advantages of a spade for a racing boat where the primary object is removing as much drag as possible and the boat will never beyond range of the crew being rescued in the event of a grounding or hitting a submerged object or for a coastal cruiser. They're also great for making really tight turns at very low speeds such as pulling into a slip in a marina. But for a cruising boat that will be depended on to cross oceans while keeping its crew safe and proceeding towards its destination I think that a skeg hung rudder offers advantages that a spade rudder doesn't offer and that's why most serious cruising boats continue to be designed and built this way.

[Stumble]

Originally Posted by robert sailor

Aren't Porsche 's known for rear engines or mid engines??

Yes, yes they are... And I need to go figure out what my wife slipped in my breakfast.

I stand by the point, but the example was all kinda stupid.

[robert sailor]

Originally Posted by Stumble

Yes, yes they are... And I need to go figure out what my wife slipped in my breakfast.

I stand by the point, but the example was all kinda stupid.

I do the same thing sometimes, you've got lots of company

[Dockhead]

Originally Posted by jtsailjt

Simply not true that it "harms the hydrodynamic properties of the rudder" to have a skeg. It would if you put a skeg in front of a spade rudder but fortunately, that's not quite how sailboats are designed. The rudder combined with the skeg creates more lift on one side of the rudder/skeg combo and, if designed properly, it has NO problem creating enough lift to steer the boat just fine. Almost all airplanes continue to have similar appendages to a skeg hung rudder that are used to control both yaw and pitch. Even the wing of the plane works this way to control roll. You don't rotate the whole wing of the plane when you want to roll left or right, you just lower or raise the aileron on the back of the wing.

A skeg can easily be designed to be potentially load bearing during a grounding without harming the moving parts, and it doesn't need to be so massive that it interferes with the flow along the edges of the rudder. It's much more difficult to design and build a design when you are asking one part of the boat to have both load bearing capability AND be a moving part. Also, this moving part is a rather long cantilever so when its tip hits something immovable, its length acts as a moment arm which results in tremendous concentrated forces to be exerted on the hull where the rudder shaft passes through the hull and where it's supported at its top.

The advantage of a spade is greater maneuverability at very low speeds and since it can be smaller than a skeg hung rudder, it will generate less drag than the combined skeg and rudder combo. But this additional drag is a very small part of the total drag on a cruising boat, but much more noticeable on a racing hull. This greater low speed maneuverability comes at the cost of greater vulnerability to the rudder and consequently the whole boat, and less longitudinal stability, especially in the event of an issue such as a snapped steering cable. It's a tradeoff and which pros/cons are more important depend on how the boat will be used and what your priorities are.

I can see the advantages of a spade for a racing boat where the primary object is removing as much drag as possible and the boat will never beyond range of the crew being rescued in the event of a grounding or hitting a submerged object or for a coastal cruiser. They're also great for making really tight turns at very low speeds such as pulling into a slip in a marina. But for a cruising boat that will be depended on to cross oceans while keeping its crew safe and proceeding towards its destination I think that a skeg hung rudder offers advantages that a spade rudder doesn't offer and that's why most serious cruising boats continue to be designed and built this way.

Sorry, but there are a number of errors here. Reading the real science would be profitable: http://eprints.soton.ac.uk/43260/1/001-78new.pdf

Spade rudders on the contrary give much better responsiveness at low speeds. Because they require less rudder angle to produce the same force. And they are more effective and at lower speeds, going in reverse. So they are much better for maneuvering in marinas.



As to the hydrodynamics -- skeg rudders produce a lot of drag, which is very bad when you're sailing, and very noticeable even on a cruising boat. The difference to airplanes is that the foils on airplanes are cambered, and for normal level flight don't require any or much angle of attack to generate lift. So the extra drag from having part of the control surface fixed rarely comes into play. The exception is the vertical stabilizer which might need to generate varying amounts of lift to trim the aircraft according to its load -- and for that reason, most airliners have "spade" stabilizers -- that is stabilators, where the whole surface moves like a spade rudder. Sailboat foils are symmetric, so have to be used at a certain angle of attack to generate lift. Rudders with part of the surface fixed -- that is, skeg rudders -- are very poor for this hydrodynamically.


The last error is to say that "most serious cruising boats continue to be built this way" -- you are about 25 years behind the times! Which serious cruising boats still have skeg rudders? Besides Oyster? Not a single one that I know of. You find some semi-balanced rudders with a short skeg, like my boat, but hydrodynamically these are more spade than skeg. But most "serious cruising boats" have gone to full spade rudders, even getting rid of the stub skegs which were popular 20 years ago. Hallberg Rassy, Swan, Discovery, Contest, you name it. Even Hinckley and Morris now put spade rudders on all their boats, even the retro-style ones. I'm afraid the skeg rudder has gone the way of the full keel.

[Kenomac]

The spade rudder on our Hunter 450 was a handfull to contol with the helm, we always seemed to be fighting with it, it wore us out sailing just a few hours. Not the case with the Oyster, seems better balanced. Was it the boat design or the rudder design? I'm the same guy setting and balancing the sails.

[Dockhead]

Originally Posted by Kenomac

The spade rudder on our Hunter 450 was a handfull to contol with the helm, we always seemed to be fighting with it, it wore us out sailing just a few hours. Not the case with the Oyster, seems better balanced. Was it the boat design or the rudder design? I'm the same guy setting and balancing the sails.

That would be the design of the rig and the hull. No rudder can solve an unbalanced rig.

Also, the type of rudder, and the specific design of it, are two different questions. A spade rudder is a superior type, hydrodynamically, but if it's the wrong size, wrong position, or wrong aspect ratio for the particular use, or wrong chord profile, then it won't be a good rudder.

Your Oyster was designed by one of the world's greatest designers, so you can be sure that the execution of the design was first-rate.

[Sandero]

Thanks to all for this stimulating discussion. Not having literally helmed a boat with an attached rudder, or a balanced spade my experience is limited to the full skeg and a deep fin keel on my boat which I've done maybe 50k miles and 99% using AP. Turning radius is quite reasonable. I can only wonder what the difference would have been with a balanced spade... I've felt that the rubber was an under performer in a measurable way.. such as well cut and trimmed sails are or a slippery clean hull. Most race boats don't deal with hitting the bottom and are in deep enough course that this is not an issue. On the other hand I think I've read of spades falling off in some cases and that seems almost impossible with a skeg.

There appears to be a place between full keel with attached rudder and deep fin bulb keels with high aspect spades. I feel OK being in the middle.

[neilpride]

Originally Posted by Dockhead

Sorry, but there are a number of errors here. Reading the real science would be profitable: http://eprints.soton.ac.uk/43260/1/001-78new.pdf

Spade rudders on the contrary give much better responsiveness at low speeds. Because they require less rudder angle to produce the same force. And they are more effective and at lower speeds, going in reverse. So they are much better for maneuvering in marinas.



As to the hydrodynamics -- skeg rudders produce a lot of drag, which is very bad when you're sailing, and very noticeable even on a cruising boat. The difference to airplanes is that the foils on airplanes are cambered, and for normal level flight don't require any or much angle of attack to generate lift. So the extra drag from having part of the control surface fixed rarely comes into play. The exception is the vertical stabilizer which might need to generate varying amounts of lift to trim the aircraft according to its load -- and for that reason, most airliners have "spade" stabilizers -- that is stabilators, where the whole surface moves like a spade rudder. Sailboat foils are symmetric, so have to be used at a certain angle of attack to generate lift. Rudders with part of the surface fixed -- that is, skeg rudders -- are very poor for this hydrodynamically.


The last error is to say that "most serious cruising boats continue to be built this way" -- you are about 25 years behind the times! Which serious cruising boats still have skeg rudders? Besides Oyster? Not a single one that I know of. You find some semi-balanced rudders with a short skeg, like my boat, but hydrodynamically these are more spade than skeg. But most "serious cruising boats" have gone to full spade rudders, even getting rid of the stub skegs which were popular 20 years ago. Hallberg Rassy, Swan, Discovery, Contest, you name it. Even Hinckley and Morris now put spade rudders on all their boats, even the retro-style ones. I'm afraid the skeg rudder has gone the way of the full keel.

I quite agree with you, but the fate of skegs in the last years come from hull design in the last years to, I mean to properly built a skeg most builders opt for split hull construction and this actual days most production boats are single hull mold construction, is quite expensive and time consuming to add a skeg in a hull , looking at how the actual hulls are designed and built , don't make any market sense to think in skegs anymore, there is still few builders building hulls with skegs , a example could be the Belliure 50s Endurance, they modified the keel and the hull but they keep the skeg for the new models, and few others , most customs boats , saying that,, skegs don't have anymore any useful purpose in the actual flat planning light displac hulls,,, like full keels as you mention...
Welcome to the era of double rudders and aft beamy light hulls.

[robert sailor]

Originally Posted by neilpride

I quite agree with you, but the fate of skegs in the last years come from hull design in the last years to, I mean to properly built a skeg most builders opt for split hull construction and this actual days most production boats are single hull mold construction, is quite expensive and time consuming to add a skeg in a hull , looking at how the actual hulls are designed and built , don't make any market sense to think in skegs anymore, there is still few builders building hulls with skegs , a example could be the Belliure 50s Endurance, they modified the keel and the hull but they keep the skeg for the new models, and few others , most customs boats , saying that,, skegs don't have anymore any useful purpose in the actual flat planning light displac hulls,,, like full keels as you mention...
Welcome to the era of double rudders and aft beamy light hulls.

Good points

[Ericson38]

Originally Posted by Dockhead

Sorry, but there are a number of errors here. Reading the real science would be profitable: http://eprints.soton.ac.uk/43260/1/001-78new.pdf

Spade rudders on the contrary give much better responsiveness at low speeds. Because they require less rudder angle to produce the same force. And they are more effective and at lower speeds, going in reverse. So they are much better for maneuvering in marinas.



As to the hydrodynamics -- skeg rudders produce a lot of drag, which is very bad when you're sailing, and very noticeable even on a cruising boat. The difference to airplanes is that the foils on airplanes are cambered, and for normal level flight don't require any or much angle of attack to generate lift. So the extra drag from having part of the control surface fixed rarely comes into play. The exception is the vertical stabilizer which might need to generate varying amounts of lift to trim the aircraft according to its load -- and for that reason, most airliners have "spade" stabilizers -- that is stabilators, where the whole surface moves like a spade rudder. Sailboat foils are symmetric, so have to be used at a certain angle of attack to generate lift. Rudders with part of the surface fixed -- that is, skeg rudders -- are very poor for this hydrodynamically.


The last error is to say that "most serious cruising boats continue to be built this way" -- you are about 25 years behind the times! Which serious cruising boats still have skeg rudders? Besides Oyster? Not a single one that I know of. You find some semi-balanced rudders with a short skeg, like my boat, but hydrodynamically these are more spade than skeg. But most "serious cruising boats" have gone to full spade rudders, even getting rid of the stub skegs which were popular 20 years ago. Hallberg Rassy, Swan, Discovery, Contest, you name it. Even Hinckley and Morris now put spade rudders on all their boats, even the retro-style ones. I'm afraid the skeg rudder has gone the way of the full keel.

I know you are running with that 1978 report, but C.A. Marchaj has another view of the partial skeg design in his Sea Worthiness, the Forgotten Factor (1996). Look on page 309 if you can find a copy and you will see his plot for rudder force versus incidence angle with and without partial skeg, and the stall points. It is much different than that in your 1978 report, figure 11, which is based on full skeg versus spade (in generalities as the two designs are not formally presented that are compared).

Even in the report you cited (thanks for the link), note the integral of the function (visually) being the area under the two curves in figure 11. Since the functions are mathematically derived and the math is too complex when past the peak lift prior to stall, the author uses dotted lines to suggest how the lift forces may degrade once into the stall, with further increases of AoA. CFD analysis or real tank testing would fill these points in, but again the dimensions and conditions would have to be included in the report.

The 1978 report does not analyze the partial skeg design, as the author noted in section 6. By 1978 there were really three approaches being built from the earliest fiberglass boats in the late 60s to the 1978 report, being spade rudder with separate keel, the full keel with attached rudder, and the skeg mounted rudder with detached keel.

The spade was used by many builders in the early 70s, including Islander, Ericson, Catalina, Hunter, Cal, C&C, Yorktown, Newport, Sabre, Columbia, CS, Coronado, J-Boats, Yankee. It was a very popular design even then. But it also had control problems. In the cases of my Ericson boats, it wasn't a trim issue, it was a control authority issue in heavy air and higher sea states. Fast boats, would outrun (about 1 knot faster) same length boats with larger underwater appendages, with the right winds and sea state, but I knew my limitations and picked trips to the Channel Islands (San Miguel and Santa Rosa) carefully.

Tayana, Caliber, Island Packet, Passport, and Hylas use full or partial skegs currently.