Skeg or Spade?

[Stumble]

Originally Posted by 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.

Bad design is bad design, if the spade fell off then there is something seriously wrong with the design. But there have been a lot of skegs fall off as well. In many cases these days the skeg is a purely cosmetic affection. It doesn't carry any real strength because the rudder shaft was designed to carry all the load. Then the skeg is glue to the skin after the hull is made. It is litterly a design cue that imparts the appearance of strength, but no actual strength.

As an example, the Oyster bandied about here is I believe using a rudder shaft around four times the minimum strength that the Lloyds Special Serivice Rules require for Cat 0 (any passage, any time) certification. It is litterly four times stronger than it needs to be to survive a hurricane at sea. By comparison the Dashew's on their boats designers for two times the LSS rules.

The Oyster rudder isn't going anywhere. But then they add a skeg to it. Why? Their customers wanted it, so it's there. Now knowing Oyster it probably is a massive grid tied load bearing appendage. But is it necessary? I doubt it. The rudder itself is already far stronger than it will ever need to be.

[K_V_B]

Originally Posted by jtsailjt

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.

You can design a spade rudder so that it can bear quite some loads without issue. There are quite a few yachts that use their rudder(s) as support when drying out.





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[Ericson38]

Originally Posted by K_V_B

You can design a spade rudder so that it can bear quite some loads without issue. There are quite a few yachts that use their rudder(s) as support when drying out.





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Rudder post bearings and load calculations on the post moments (bending) are designed for high radial loads, not axial loads that would tend to drive them further into the boat. That, plus torsional (twisting) loads.

Looking at S10 for rudders, sole pieces, and skegs (horns)...

http://www.iacs.org.uk/document/publ...v2_pdf1330.pdf

...there is no treatment for supporting the weight of the boat at the stern by the rudder. If a boat were supported that way, you could end up with a wet armature and leaking rudder skin seal shortly.

Are you talking about laying over on its side, with a load that is partially side (transverse) and partly vertical in to the boat ?

[valhalla360]

Originally Posted by Ericson38

Rudder post bearings and load calculations on the post moments (bending) are designed for high radial loads, not axial loads that would tend to drive them further into the boat. That, plus torsional (twisting) loads.

Looking at S10 for rudders, sole pieces, and skegs (horns)...

http://www.iacs.org.uk/document/publ...v2_pdf1330.pdf

...there is no treatment for supporting the weight of the boat at the stern by the rudder. If a boat were supported that way, you could end up with a wet armature and leaking rudder skin seal shortly.

Are you talking about laying over on its side, with a load that is partially side (transverse) and partly vertical in to the boat ?

Most boats aren't designed for this (or to sit on their skegs for that matter) but it's not hard to design it to handle the loads.

[Jim Cate]

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.

Can't speak authoritatively on the others, but my 1974 Yankee-30 had a full length skeg on the rudder. Typical S&S shape to keel and rudder appendages...

Jim

[Ericson38]

Originally Posted by Jim Cate

Can't speak authoritatively on the others, but my 1974 Yankee-30 had a full length skeg on the rudder. Typical S&S shape to keel and rudder appendages...

Jim

Remember the SS Yankee 38 which Catalina bought the mold from in 1979 timeframe ? That was the one that came to mind, with the tumblehome.

Well-Yankee made it with a skeg !

http://sparkmanstephens.blogspot.com...yankee-38.html

Catalina dropped the skeg to match the rest of their lineup (excluding Morgan)-

https://www.bing.com/images/search?q...eo0&ajaxhist=0

[Jim Cate]

Originally Posted by Ericson38

Remember the SS Yankee 38 which Catalina bought the mold from in 1979 timeframe ? That was the one that came to mind, with the tumblehome.

Well-Yankee made it with a skeg !

Sparkman & Stephens: Design 2094-C2 - Yankee 38

Catalina dropped the skeg to match the rest of their lineup (excluding Morgan)-

https://www.bing.com/images/search?q...eo0&ajaxhist=0

I do indeed remember that model, for i had lusted after a YY-38.

It would be instructive to have a face off between a Catalina 38 and one of the original YY-38s. If otherwise similar (kinda unlikely, that) it would shed some light upon the skeg vs spade speed issue. Doubt if that could be organized, for there are not many YY38s left nowadays.

Jim

[Snowpetrel]

Originally Posted by Ericson38

A 8500 lb side load is with a 10 sq ft rudder with 10 knot velocity of water flowing over it, with a lift coef of 3.0. A 50 ft sailboat of 45,000 displacement has that size rudder plane. This is what it is capable of generating before stall, in terms of lift to windward.

On the example attachment with 1 inch fiberglass skeg layup to the hull layup, with a 12,000 lbs/sq inch yield strength (or breaking), lets look at this.

A skeg for a 50 ft boat may have a form circumference where it meets the hull of, say, 40 inches. That would be 40 sq inches for the tension side, and 40 sq inches for the compression side. On the tension side, 12,000 x 40 = 480,000 lbs.

Not sure where the deficiency is, with 1 inch layup.

I really like the way you have use real numbers intead of just statements, but to me something about these numbers don't seem right, but then I am no engineer, and I am not used to imperial units. But anyway playing with the numbers and using a whole bunch of aproximations that may be wrong...

8500x 2.5 feet (assuming the rudder is 5x2 feet in size) gives a torque of 21250 ft lbs

Lets say the root of the skeg is approx 5 inches wide (and lets assume its square to make the calcs easy), so the centre axis of the skeg material is 2 inches from the neutral axis. So 21250/(2/12) gives us 127500 lbs tension and compression on each side. This compares to the 480000 lbs of the section for a safety factor of about 4:1 not ideal for long term fatigue resistance, but since this is probably a very rare peak load its not likely to be too bad.

Interestingly I get about the same sort of safety factor (by my crude and possibly wrong calculations) for a 5 inch 316ss round stock on an equivalent sized spade rudder.

This is very much a back of envelope calc, any real engineers out there?

Fibreglass loses strength in water, and stainless suffers from corrosion. The fact that bent spade rudders are rare, and and cracked skegs are rare shows that both the loads rarely exceed the yeild strength. Most of the spade rudders that are lost seem to be due to internal corrosion, or inadequate internal bracing of the rudder top bearing.

[double u]

...only an unbreakable skeg will be all that much superior to a spade!

[valhalla360]

Originally Posted by Snowpetrel

I really like the way you have use real numbers intead of just statements, but to me something about these numbers don't seem right, but then I am no engineer, and I am not used to imperial units. But anyway playing with the numbers and using a whole bunch of aproximations that may be wrong...

8500x 2.5 feet (assuming the rudder is 5x2 feet in size) gives a torque of 21250 ft lbs

Lets say the root of the skeg is approx 5 inches wide (and lets assume its square to make the calcs easy), so the centre axis of the skeg material is 2 inches from the neutral axis. So 21250/(2/12) gives us 127500 lbs tension and compression on each side. This compares to the 480000 lbs of the section for a safety factor of about 4:1 not ideal for long term fatigue resistance, but since this is probably a very rare peak load its not likely to be too bad.

Interestingly I get about the same sort of safety factor (by my crude and possibly wrong calculations) for a 5 inch 316ss round stock on an equivalent sized spade rudder.

This is very much a back of envelope calc, any real engineers out there?

Fibreglass loses strength in water, and stainless suffers from corrosion. The fact that bent spade rudders are rare, and and cracked skegs are rare shows that both the loads rarely exceed the yeild strength. Most of the spade rudders that are lost seem to be due to internal corrosion, or inadequate internal bracing of the rudder top bearing.

I don't think I got my point across when I responded to the calculations post.

The whole calculation is a red herring.

You can have a million lbs of side force and if it's designed to handle it it's perfectly fine.

By selectively taking the part of the calculations, it leaves the impression the spade design is stressed to a much greater degree but that's only because the skeg design calculations stopped before running the calculations on the skeg itself.

It's basically an apples to oranges comparison.

[K_V_B]

Originally Posted by double u

...only an unbreakable skeg will be all that much superior to a spade!

Even an unbreakable spade?



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[double u]

...will be even more difficult to make ...& not desireable to boot!!! It would result in the same problems a skeg has: break it- & you tear out half the stern.

[jtsailjt]

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.

LOL, if you seriously think that reading a (almost 40 years "behind the times) 1978 article dealing with rudder design for "high performance small craft" is the last word on either rudder design OR the practical considerations involved in deciding what's best for a cruising sailboat, then I suggest that MUCH more studying is in order. FYI, my Bachelor of Science was in Civil Engineering focusing on structural design, and I've spent the last 35 years flying all types of high performance aircraft, from F-16's to most Boeings, so I think I'm reasonably well informed about how forces act on structures as well as how airfoils work, but thank you for your attempt to educate me about what you call the "real science."

We do agree that spades yield better low speed maneuverability but how important is this to a competent helmsman on a cruising sailboat? If your idea of "cruising" is parking your shorthanded boat in a marina slip every night, then I suppose the ability to make 90 degree bat turns is very high on your list of priorities. But I think most "real cruisers" tend to anchor out or pick up a mooring much, much, more often than they spend in a slip, so that the somewhat lesser low speed maneuvering that a skeg mounted rudder offers isn't something that they find limiting. Besides, I've had 2 skeg mounted rudder sailboats and have always managed to find my way into a slip when I needed to and I make no claim to being anything better than an average cruising helmsman. Also, most boats in the size range you are considering will be fitted with a bow thruster, which will allow any skeg equipped sailboat to turn 360 degrees in its own length if needed. How much more maneuverability do you need?

But enough about impressing all onlookers as we squeeze our 50'+ boats into tight marina slips all by ourselves. What we all do for the vast majority of our time on the water is to sail, and a skeg mounted rudder performs just fine in this regime, both in a straight line and while tacking (no need to "wear ship") while offering greater longitudinal stability, especially in a "rudder free" situation, as well as offering a more stable hove to platform, and greater protection from unexpected obstructions or ground contact.

One interesting experience I had many years ago, while sailing my full keel Hinckley Pilot 35 downwind in about 20 knots of breeze and 6-8 foot seas, was to catch a glimpse of something just beside the bow followed by few seconds of a rumbling sound followed by a spinning telephone pole(!) surfacing just astern that I had hit broadside at 6+ knots. My boat suffered no ill effects, my current boat would probably have had damage to my maxprop and superficial damage to my skeg, but I probably wouldn't have lost the ability to steer. Besides possibly breaking your wrist as your rudder instantly snapped sideways, what do you think hitting a telephone pole broadside at 8+ knots would do to a cruise in your 50' spade rudder equipped sailboat? Real world situations such as this one weren't mentioned in that 1978 article that you keep posting a link to....

There's no doubt that spade rudders are popular among some sailors and some builders, but is what's "best" for cruising their primary motivation? For builders, it's a lot easier to not have to deal with a skeg, either in the hulls mold or bolting one on and fairing it in. Spades are "sexy" and can make almost any helmsman seem like Dennis Connor as he zips around the marina, which can be a selling point, but take that same potential customer out in 30 knots of wind with a bit too much sail area up and he may find himself wishing for the stability of his good old skeg hung rudder. Your list of builders who have switched to spades leaves quite a few out who are still using skegs, and includes a few that I don't ever seem to see doing much actual cruising out of sight of land.

I recognize the superiority of spades for tight maneuvering and for racers (BTW last Sunday I won my race by about 1/4 mile in my spade rudder equipped Sunfish in a steady 20 knot breeze!) where every bit of drag counts for a lot and rescue is never more than minutes away so the potential loss of the ability to steer isn't anything worth worrying about, but IMHO for a cruiser planning to be as self sufficient as possible in whatever conditions mother nature throws at that cruiser, he/she has other priorities that are better met by a skeg equipped sailboat.

Of course I realize that each of us have our own ideas about how to order our list of priorities regarding the perfect cruising sailboat and I'm completely OK with that. But I think you should be a bit more cautious about assuming that because someone disagrees with your opinion that it's due to a lack of knowledge or education about the "real science" or subject...on their part.

[valhalla360]

Originally Posted by jtsailjt

LOL, if you seriously think that reading a (almost 40 years "behind the times) 1978 article dealing with rudder design for "high performance small craft" is the last word on either rudder design OR the practical considerations involved in deciding what's best for a cruising sailboat, then I suggest that MUCH more studying is in order. FYI, my Bachelor of Science was in Civil Engineering focusing on structural design, and I've spent the last 35 years flying all types of high performance aircraft, from F-16's to most Boeings, so I think I'm reasonably well informed about how forces act on structures as well as how airfoils work, but thank you for your attempt to educate me about what you call the "real science."

Unless you are suggesting hydrodynamics have changed in the last 40yrs...it's pretty much the same situation and skegs have the same limitations and it's pretty much just as easy to build a spade rudder just as strong as a skeg hung (Another Civil Engineer here)

We do agree that spades yield better low speed maneuverability but how important is this to a competent helmsman on a cruising sailboat? If your idea of "cruising" is parking your shorthanded boat in a marina slip every night, then I suppose the ability to make 90 degree bat turns is very high on your list of priorities.

Not just low speed maneuverability but more efficient lift at low speed. Efficiency includes both lift and drag.

But I think most "real cruisers" tend to anchor out or pick up a mooring much, much, more often than they spend in a slip, so that the somewhat lesser low speed maneuvering that a skeg mounted rudder offers isn't something that they find limiting.

So we are back to the "real cruiser" debate. When your boat isn't as capable it must be because "real cruisers" wouldn't need that capability.

This isn't to say a skeg hung rudder can't provide reasonable maneuverability but if the goal is to select the best option, it's spade style.

Besides, I've had 2 skeg mounted rudder sailboats and have always managed to find my way into a slip when I needed to and I make no claim to being anything better than an average cruising helmsman. Also, most boats in the size range you are considering will be fitted with a bow thruster, which will allow any skeg equipped sailboat to turn 360 degrees in its own length if needed. How much more maneuverability do you need?

While bow thrusters are becoming more common, they are far from standard fare. Also bow thrusters have their limits. A friend had a quadrant fail (long story), and if they got over about 2kts, the bow thruster wouldn't provide any steering. As they were coming in with a crosswind, they needed some speed so they didn't slide sideways but then they lost steering.

But enough about impressing all onlookers as we squeeze our 50'+ boats into tight marina slips all by ourselves. What we all do for the vast majority of our time on the water is to sail, and a skeg mounted rudder performs just fine in this regime, both in a straight line and while tacking (no need to "wear ship") while offering greater longitudinal stability, especially in a "rudder free" situation, as well as offering a more stable hove to platform, and greater protection from unexpected obstructions or ground contact.

Sure they function but not as efficiently. Based on my watching of other cruisers, the vast majority of time the vast majority of cruising boats, motor 90% of the time. With the lower forces on a semi-balanced spade rudder, it's easier on the autopilot which provides plenty of longitudinal stability. Unless you get into true racing boats with extreme keels, it's just not an issue.

One interesting experience I had many years ago, while sailing my full keel Hinckley Pilot 35 downwind in about 20 knots of breeze and 6-8 foot seas, was to catch a glimpse of something just beside the bow followed by few seconds of a rumbling sound followed by a spinning telephone pole(!) surfacing just astern that I had hit broadside at 6+ knots. My boat suffered no ill effects, my current boat would probably have had damage to my maxprop and superficial damage to my skeg, but I probably wouldn't have lost the ability to steer. Besides possibly breaking your wrist as your rudder instantly snapped sideways, what do you think hitting a telephone pole broadside at 8+ knots would do to a cruise in your 50' spade rudder equipped sailboat? Real world situations such as this one weren't mentioned in that 1978 article that you keep posting a link to....

Ironically, just heard a story yesterday from a guy with a sistership, hit a submerged rock shelf at 8kts. He has about $3k in fiberglass damage to the leading edges of the keels and rudders but otherwise was able to sail on.

In the situation, you describe, I would be more worried about punching a hole in the hull. If the keel survives, it will likely deflect the object or the boat so the rudder doesn't take a direct hit.

Of course, you can play what if until you find a situation where the spade would fail, but we can reverse that and come up with a situation where the skeg fails.

There's no doubt that spade rudders are popular among some sailors and some builders, but is what's "best" for cruising their primary motivation? For builders, it's a lot easier to not have to deal with a skeg, either in the hulls mold or bolting one on and fairing it in. Spades are "sexy" and can make almost any helmsman seem like Dennis Connor as he zips around the marina, which can be a selling point, but take that same potential customer out in 30 knots of wind with a bit too much sail area up and he may find himself wishing for the stability of his good old skeg hung rudder. Your list of builders who have switched to spades leaves quite a few out who are still using skegs, and includes a few that I don't ever seem to see doing much actual cruising out of sight of land.

Sexy typically implies rare and new. Spade rudders are pretty much the standard, so I doubt many people are buying boats with spade rudders because they are sexy. If anything it's the opposite. Manufacturers still producing skeg rudders (often non-structural) to meet the expectation that skegs are salty and strong.

I recognize the superiority of spades for tight maneuvering and for racers (BTW last Sunday I won my race by about 1/4 mile in my spade rudder equipped Sunfish in a steady 20 knot breeze!) where every bit of drag counts for a lot and rescue is never more than minutes away so the potential loss of the ability to steer isn't anything worth worrying about, but IMHO for a cruiser planning to be as self sufficient as possible in whatever conditions mother nature throws at that cruiser, he/she has other priorities that are better met by a skeg equipped sailboat.

Comparing a sunfish to a cruising boat is just silly and has nothing to do with a spade rudder on a cruising boat. As has been repeated multiple times, there is no problem designing a spade rudder just as strong. Also for self sufficiency, a semi-balanced rudder has a huge advantage. If you need to connect a temporary tiller to the quadrant, the semi-balanced spade will be drastically easier to steer compared to a barn door hinged at the leading edge.

Of course I realize that each of us have our own ideas about how to order our list of priorities regarding the perfect cruising sailboat and I'm completely OK with that. But I think you should be a bit more cautious about assuming that because someone disagrees with your opinion that it's due to a lack of knowledge or education about the "real science" or subject...on their part.

So far I have yet to see a functional benefit to skeg hung rudders that can't just as easily be addressed a spade hung rudder when you compare apples to apples.

[robert sailor]

I don't think there will be any winners in this debate and I sorta see why. Guys with a decent built boat that have skegs or partial skegs know their boats have a better rudder system for cruising than the typical high production cruiser with a poorly built spade rudder system. I don't think that this is debatable as even the largest builder of sail boats has admitted that it builds very marginal spade rudder systems on many of their boats. Yes a well built spade is at least equal to a well done skeg rudder but there are not that many well built spade rudder systems being installed these days.