Skeg or Spade?

[valhalla360]

Another option is a kick up spade rudder.

On our old cat, it would still steer with the rudders kicked up but you just slow down, pull the rudder back into place and you are on your way. Worst case the cam cleat is locked too tight and the downhaul line breaks but with two rudders, you still have excellent steering even if you can't get one back down.

No reason this system couldn't be applied to monohulls.

It eliminates the need to withstand a heavy impact.

[tkeithlu]

Every time i ground (more frequently than I would like) I'm reminded that my rudder is protected by my keel, the bottom of the rudder foot is the first point to touch, and that the propeller is protected by the keel as well. Getting off a soft grounding is easy. In the case of spade rudders, I don't think that it is the strength of the shaft that is critical, it is whether the weight and momentum of the boat is such that a grounding will rip the shaft mounting out of the hull. That's a matter of the strength of the rudder mount, not the shaft.

[Dockhead]

Originally Posted by tkeithlu

Every time i ground (more frequently than I would like) I'm reminded that my rudder is protected by my keel, the bottom of the rudder foot is the first point to touch, and that the propeller is protected by the keel as well. Getting off a soft grounding is easy. In the case of spade rudders, I don't think that it is the strength of the shaft that is critical, it is whether the weight and momentum of the boat is such that a grounding will rip the shaft mounting out of the hull. That's a matter of the strength of the rudder mount, not the shaft.

It's a good point, but even more important than this, is the depth of the rudder in relation to the keel.

If there is a decent difference, you should never hit the rudder in a grounding, unless you reverse onto a steeply pitched shoal or something.

This is another really big tradeoff, because for the rudder to be optimally effective, it should actually be as long as the keel -- as you see on most race boats. This is unacceptable on a cruising boat in my opinion, and you have to make a compromise on hydrodynamics here in favor of safety. And I think as some others here have said -- this question actually doesn't really change much, whether you've got a skeg or not.

[Kenomac]

I shortchanged myself in an earlier post. Our rudder shaft is actually a six inch diameter stainless rod. I was looking at the 50ft Hanse next door and assumed ours was about the same when I wrote three inch. The Hanse is actually closer to a four inch rudder post on the spade rudder.

So I'm going to go out on a limb and reason that our six inch rudder post plus the 12 inch wide skeg is a little stronger than the spade rudder alone on a similar sized boat.

Now, I really haven't heard of any rudder failures on Oyster yachts, so maybe with the combo, they must be doing something right. We don't race, so safety is more of a concern for us and our boat isn't slow compared to other cruising yachts.

[Kenomac]

Here's another day I was glad to have a rudder protected by a skeg. Poop happens, on this day we had an electrical fire which took out all electronics and the engine.

We draw nearly 8ft, follow the track line. The chart markings are in meters.

And for those people who think that this sort of stuff only happens to idiots like Ken, well.... You're either sitting behind your computer at home and don't get out sailing much, or your turn will come. There's been so many times when suddenly we see a submerged rock, that isn't where it's supposed to be on the chart.

[Dockhead]

Originally Posted by Kenomac

I shortchanged myself in an earlier post. Our rudder shaft is actually a six inch diameter stainless rod. I was looking at the 50ft Hanse next door and assumed ours was about the same when I wrote three inch. The Hanse is actually closer to a four inch rudder post on the spade rudder.

So I'm going to go out on a limb and reason that our six inch rudder post plus the 12 inch wide skeg is a little stronger than the spade rudder alone on a similar sized boat.

Now, I really haven't heard of any rudder failures on Oyster yachts, so maybe with the combo, they must be doing something right. We don't race, so safety is more of a concern for us and our boat isn't slow compared to other cruising yachts.

Sounds like you could just saw off that skeg, Ken

[Stumble]

Originally Posted by Charlie

What I have gained from this discussion is that a spade rudder is a better foil and that a skeg is not necessarily stronger depending on attachment. So if I could have the boat designed to my standard I would go for an over engineered spade rudder. I would guess that on most production boats the spade is not over engineered. For that reason I like the skeg.

You would guess wrong. It is hard to design and build a skeg, and it is a very difficult and tricky laminating job, which is why most of them are two pieces then glued together, then bolted to the hull. In most cases the skegs are litterly being held on by the rudder.

A spade rudder however is dead simple to engineer and build. You just go up a size on the rudder stock.

Bob Perry has more than a few forum posts discussing how much trouble skegs are. Which is why he much prefers spades.

[cal40john]

I have no problem with spade rudders, my Cal has a spade rudder.

There is one advantage to the skeg rudder that I don't think has been brought up, which is a skeg rudder theoretically has a higher stall angle, so in those I really want the boat to go this way, but the boat really wants to go that way moments, the spade rudder can give up first.

This author thinks that many skeg rudders are so poorly done in the transition from skeg to rudder that they actually will stall earlier than a spade though.

https://books.google.com/books?id=Gl...0spade&f=false

[Cheechako]

A Spade Ruder is an "unsupported beam" a Skeg hung rudder is a "supported beam" . There is a huge difference in strength in the basic concepts.
Having said that, some skegs are not very robustly built.
For spades the problem is for the shaft in a spade to be really strong it gets much too big in diameter and that effects how well the rudder works also.
A spade definitely feels much better than a skeg hung rudder.
It's really surprising more spade type boats don't just hang a rudder off the stern.
-further aft/better control
-still can be balanced
-No shaft corrosion issues
-possibly less likely to sink the boat if the rudder is damaged.


I guess the issue is the complication of wheel steering with an aft hung rudder.

[Tortuga's Lie]

Originally Posted by TeddyDiver

So it wasn't skeg hung after all, instead a spade with a bottom hung foil infront of it..

I had a Tartan 37 C (Sparkman-Stephens) for 20 years and the skeg is indeed hollow and bolted on with the rudder pintle about half way down the rudder. The rudder/skeg is designed (theoretically) to shear off upon impacting something immovable. The rudder post was still a beefy 3.5" pipe but this stopped at the pintle and below this, the rudder was a fiberglass skin over foam. The boat used the skeg for tracking and it was faired beautifully to the rudder using flaps to smooth out turbulence.

My new boat is a Tartan 4100 with an elliptical spade and I can attest that the spade is a much better performer and this rudder seems to be built very robustly for a 41 foot boat, having a 4" post with 1/2" a wall thickness. The boat tracks and balances very nice under sail but under power she wants to pull to port. Perhaps if there were a skeg, it wouldn't?

[cal40john]

Originally Posted by Cheechako

A Spade Ruder is an "unsupported beam" a Skeg hung rudder is a "supported beam" . There is a huge difference in strength in the basic concepts.
Having said that, some skegs are not very robustly built.
For spades the problem is for the shaft in a spade to be really strong it gets much too big in diameter and that effects how well the rudder works also.
A spade definitely feels much better than a skeg hung rudder.
It's really surprising more spade type boats don't just hang a rudder off the stern.
-further aft/better control
-still can be balanced
-No shaft corrosion issues
-possibly less likely to sink the boat if the rudder is damaged.


I guess the issue is the complication of wheel steering with an aft hung rudder.

Once you have a foil that pierces the surface you now can have problems with ventilation. The rudder develops a low pressure area on one side when turning which can suck air down the blade at which point it becomes mostly ineffective.

I have had this happen on dinghies and beach cats, it's impressive how steering control just disappears. I don't think I've ever sailed a KB with a transom hung rudder.

Some dinghies like the Geary 18 go to the trouble of a cassette rudder to get the rudder off of the transom.

[Van Der Beek]

Originally Posted by Cheechako

A Spade Ruder is an "unsupported beam" a Skeg hung rudder is a "supported beam" . There is a huge difference in strength in the basic concepts.

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.

[Cheechako]

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.

Yep. Or just take a yardstick, fix one end on the table and see how much it sags. Now fix it on both ends.

[Stumble]

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.

Um, no this test just proves you shouldn't design a spade rudder with just one fixed point. Luckily every spade rudder ever built uses at least too. One at the top, and one at the hull penetration.

Because skegs are generally no wider than the rudder they are attached to they do not provide any leverage advantage to the system. At best they can divide the required strength amongst two points instead of one, but that's it. So long as a spade is as strong as the skeg+rudder the systemic strength is the exact same.

[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.