Skeg or Spade?

[jeepbluetj]

Originally Posted by valhalla360

Another option is a kick up spade rudder.
.....

Yup. I actually agree - a kick-up rudder would be great for reducing damage in a collision. And having two, like on a cat, gives a whole lot of redundancy.


I kinda had a similar thought earlier while reading this thread. It dawned on me that the safest kind of rudder is that kind on a Macgregor 26M - twin kick up rudders. (this is said somewhat in jest....)

[Dockhead]

Originally Posted by Van Der Beek

Do the broomstick test.

To get an idea of the forces involved you can take a broomstick and hold it at the top – with one hand - then stick it into the water and try hold it level as you go along. Then do the same - only this time use both hands - one at the top and one at the middle. The first example represents a spade rudder the second one a skeg rudder. I think you will find the forces and stresses involved to be far higher on a spade rudder than a skeg rudder.Holding the broomstick leveled with one hand proved tiresome and after a while I couldn't do it. Holding with two hands was a breeze.

This test gave me a clear representation and understanding of the forces and stresses involved between the two designs.

Sure, but you're ignoring one small point --

What's holding up the end of the skeg?

The skeg itself is a cantilever just like the rudder is. It's a broomstick held up by one end. So a skeg rudder is just two cantilevers instead of one. And the skeg is an inherently worse one, because it meets the the hull in only one place. Wikipedia (as usual) has a good article on it:

https://en.wikipedia.org/wiki/Cantilever


This shows the types of cantilevers:



A skeg is like the top example.

A rudder is like the second example, requiring less strength in the beam (and more importantly, in the attachment) to resist the same load.

[Dockhead]

Originally Posted by robert sailor

Some other posters have alluded to this but some of the issues between the 2 designs are brought on by marginal design work. Fast boats do not want a real wide rudder profile and therefore the post size has to be kept smaller in diameter, this is easily done but requires better built techniques. Cheaper built boats do not spend time and money designing well built and robust rudder systems giving spade rudders a bad name. If the spade is a thinner profile it will stall easier than a skeg hung rudder but that just gets back to design. Yes on a beamy boat with a single rudder with higher heel angles the top of the rudder will be out of the water and this will allow the rudder to ventilate and possibly stall. Skeg hung rudders are not easy to build well adding to the chance of failure. Spade rudders should be supported at deck level and at the hull, those built with the top bearings being supported by a plywood box glued together with Plexus are not the quality level for continued offshore sailing. It's just going to be a matter of time before skeg hung rudders go the way of the dodo bird but it's taking longer than it should because some builders continue to build down to a price and continue to give the best rudder a lousy reputation.

Exactly.


For anyone struggling with the engineering --

ALL rudders on boats with divided keel and rudder, skeg or spade, are cantilever structures, with a beam supported only at one end.

As someone above correctly pointed out -- the normal skeg is actually a worse cantilever than the rudder, since it is supported only at one point (the "one hand on the broom" thing), whereas a rudder is supported in at least two places along the shaft.

Any cantilever can be designed to any strength -- just needs the right amount of material in the right places.

The beam with an unsupported end is called on to work harder than the same beam, supported at both ends, but that is a normal engineering challenge which is not rocket science to deal with. Cantilevers have been used in structures for millennia. MOST bridges use cantilevers:

[Van Der Beek]

@Dockhead

Well, with fail rates and catastrophic failures being higher with spade rudders than skeg rudders, and with more skeg rudders out there than spade rudders, the skeg rudder seems to work better, even when more poorly designed.

[model 10]

Originally Posted by Dockhead

Exactly.


For anyone struggling with the engineering --
MOST bridges use cantilevers:

Oh come on "Bridges"? How about something like an an aircraft wing or stabilator. At least they move.

[Don C L]

Not sure if I can add anything here, but as someone who was crew on a Frers 65 that had its big spade rudder snap off after a week of pounding to weather I tend to eye them with suspicion. I am sure they can be massively built, but the rudder post diameter may end up being fatter than most designers prefer. Two other things, the depth of the spade relative to the anchor points is important, as well as taking into consideration the fatigue at one point the metal may experience by a steady rocking back and forth, as in beating to weather for a long time. That was our best analysis for the failure at the time, but I can't say what the cause was finally determined to be.

[Dockhead]

Originally Posted by Guy

Oh come on "Bridges"? How about something like an an aircraft wing or stabilator. At least they move.

Yes!!

[Dockhead]

Originally Posted by Van Der Beek

@Dockhead

Well, with fail rates and catastrophic failures being higher with spade rudders than skeg rudders, and with more skeg rudders out there than spade rudders, the skeg rudder seems to work better, even when more poorly designed.

Actually there are far more spade rudders "out there", than skeg. Big volume producers, both in America and in Europe, abandoned skeg rudders decades ago.

The fail rates boil down to one single company (I won't name names) -- the same company which builds eggshell bows made out of a thin single skin of GRP which shatters on impact. If they had chosen to make all their boats with skeg rudders, there would not have been a lesser failure rate.

If you exclude the rudder failures of that one company, then the number of rudder failures of either type is vanishingly small. As it should be. Rudder falling off should be as common as airplane wings falling off. And the engineering required to exclude this is more or less the same, so we know it's not rocket science.

[tkeithlu]

Those spades scare me. But, then, so do some of those skegs. I'll have to confess that I didn't realize that they build skegs that are not part of the keel. If it's not intergrated into a keel, they both look terribly vulnerable to breaking the hull if they hit something solid. That's a lot of leverage to put on a fiberglass shell. Mine has a keel in front of it that can support the entire weight of the boat. Old fashioned, not fast, but damned strong in a grounding.

[Van Der Beek]

Originally Posted by Dockhead

Actually there are far more spade rudders "out there", than skeg. Big volume producers, both in America and in Europe, abandoned skeg rudders decades ago.

This is true but boats are still sold with skeg rudders and boats previously sold with skeg rudders are still out sailing, most sailors don't change boats like most people change cars every two-three years.

So, I think you will still find that there are more skeg rudders out there than there are spade rudders. Also, many of the boats with skeg rudders will be far older than its counter parts with spade rudders.

Originally Posted by Dockhead

Rudder falling off should be as common as airplane wings falling off. And the engineering required to exclude this is more or less the same, so we know it's not rocket science.

The engineering may or may not be the same but the maintenance isn't.

[poiu]

I think some of the discussion concerning the way skeg rudders are supported is wrong, along with the analogies of poles and boards and cantilevers The Oyster skeg rudder shaft is supported in three places. The bottom of the rudder by the skeg, then the top of the rudder by the bearing where it penetrates the hull and third by the bearing in the deck. The rudder shaft is so well supported, it is not the weak link in the structure.

[Cheechako]

Originally Posted by Dockhead

Sure, but you're ignoring one small point --

What's holding up the end of the skeg?

The skeg itself is a cantilever just like the rudder is. It's a broomstick held up by one end. So a skeg rudder is just two cantilevers instead of one. And the skeg is an inherently worse one, because it meets the the hull in only one place. Wikipedia (as usual) has a good article on it:

https://en.wikipedia.org/wiki/Cantilever


This shows the types of cantilevers:

Attachment 125737

A skeg is like the top example.

A rudder is like the second example, requiring less strength in the beam (and more importantly, in the attachment) to resist the same load.

Yes the skeg is a beam with only one end fixed too. However, the high stress is at the junction with the hull in a spade, and the skeg is Y shaped to distribute load on the hull... kinda like the Eifell Tower or the Space Needle. The spade is essentially "fixed" once inside the hull. Rarely does a spade rudder post tear the upper portion in the hull apart, most damaged spades I've seen were simply bent sideways at the hull. :>)

[model 10]

Originally Posted by Van Der Beek

The engineering may or may not be the same but the maintenance isn't.

If one did fall off some model, there would probably never be another one. Changes would be made. Loosing your rudder is not allowed.

[Stumble]

Originally Posted by robert sailor

Some other posters have alluded to this but some of the issues between the 2 designs are brought on by marginal design work. Fast boats do not want a real wide rudder profile and therefore the post size has to be kept smaller in diameter, this is easily done but requires better built techniques. Cheaper built boats do not spend time and money designing well built and robust rudder systems giving spade rudders a bad name. If the spade is a thinner profile it will stall easier than a skeg hung rudder but that just gets back to design. Yes on a beamy boat with a single rudder with higher heel angles the top of the rudder will be out of the water and this will allow the rudder to ventilate and possibly stall. Skeg hung rudders are not easy to build well adding to the chance of failure. Spade rudders should be supported at deck level and at the hull, those built with the top bearings being supported by a plywood box glued together with Plexus are not the quality level for continued offshore sailing. It's just going to be a matter of time before skeg hung rudders go the way of the dodo bird but it's taking longer than it should because some builders continue to build down to a price and continue to give the best rudder a lousy reputation.

Really fast boats are actually getting away from even using tubes as rudder stock. A much better design is to use a carbon fin with the entire fin entering the hull at a ring bearing. Basically you just shove the entire top of the rudder into the bottom of the boat and attach it to to massive bearings.

First this allows a monolithic structure so you don't have any seams that can fail. Then you don't have the stainless webbing and post that can corrode. It's probably the strongest way to build a rudder at the moment.

Except that a lot of the fastest boats have shifted the rudders to gantries on the back of the boat. This has all the strength of a monolithic rudder, plus they can be kick ups, and replacing one at sea is pretty easy (most boats doing this have twin rudders anyway).

[Dockhead]

Originally Posted by Cheechako

Yes the skeg is a beam with only one end fixed too. However, the high stress is at the junction with the hull in a spade, and the skeg is Y shaped to distribute load on the hull... kinda like the Eifell Tower or the Space Needle. The spade is essentially "fixed" once inside the hull. Rarely does a spade rudder post tear the upper portion in the hull apart, most damaged spades I've seen were simply bent sideways at the hull. :>)

The Eifell Tower has four legs

If you could design a skeg like that, then it would be very easy to make it very strong.

And of course you're right that speading the load fore and aft between a skeg and the rudder shaft, does allow one of the two to act in tension while the other is acting in compression -- making it a little easier to make it strong -- but only with regard to forces fore and aft.

But beams also work in tension and compression within themselves. So it doesn't take all that large a rudder shaft, to be as strong as a skeg which spreads the load a few centimeters at the point where the rudder meets the hull.

You are right that rudders bend where they meet the hull, but I don't know what conclusion you're drawing from that. If you support the beam further up, like at the deck, then you use the strength of the beam more effectively. So the same size beam will be stronger supported that way, than it would be if it were supported only at the very end, as a skeg is. The Wiki article on cantilevers explains this.

So it's not that hard to make a spade rudder as strong as any skeg.

And then there are boats like poiu's -- which seem to have massively strong rudders shafts AND massive skegs. The rudder is like an airplane wing -- it should be designed to NEVER fail, so I guess it can't be too strong, and poiu's rudder sounds admirable. But I guess it's not stronger than Dashew's spades, and Dashew's spades will have incomparably better hydrodynamic performance.