Skeg or Spade?

[robert sailor]

There are a few utube circulating that show boats sinking after a spade rudder failure but in many cases it's the support system for the rudder and not the rudder itself. This is generally the outcome of boats built with very skinny safety margins and has nothing to do with skeg or spade rudders. Bob Perry said in many cases some of the skeg hung rudders actually supported the skeg. It's a difficult detail to build properly. I personally have only owned one offshore boat with a spade rudder and 3 with skegs. The spade gave me problems and I have owned skeg since but I do know that spades can be built very strong but it costs a few bucks to do it right and don't expect that type of construction on boats built down to a price.

[Stumble]

Originally Posted by Kenomac

I don't need to look at a"line drawing," the boat is on the hard, so I'll take a picture of it in the morning for posting while I service the MaxProp. There's actually quite a massive hunk of bronze on the tip and bottom protecting the rudder.

I dont believe that a single round piece of stainless steel rod can be anywhere near as strong as what's hanging below our boat on front of the rudder.

Well then you are absolutely and utterly wrong. Your skeg is X strength, any number of spade rudders are stronger. Skegs aren't magic, they don't impart some bizarro world extra strength they are a simple mechanical structure of a given strength. Now a particular spade may not be as strong, but so what, any given skeg may be far weaker.

Steve Dashew on the subject - "We can tell you from our own experience, and the feedback from owners who have tested our theory, that a properly engineered spade rudder is stronger and more damage tolerant than skeg or keel hung fins."

Bob Perry - "Are spade rudders stronger? I wouldn't generalize like that. Some are and some aren't."

Fundamentally the system is as strong as it's designed to be. Regardless of the method the designer chooses to achieve that strength. Personally I will take a monolithic carbon spade that can't suffer from corrosion, or water intrusion by preference.


And in order to stay on topic.... Dodgers only keep you dry when beating, if you are sailing down hill you still get rained on. If you want to stay dry go below and drive from your iPhone linked to your autopilot.

[malbert73]

I had friends with a classic tartan 37 which hit an uncharted rock somewhere in FL and the whole Alex ripped off the bottom. They reflected that they would rather have lost a rudder. And I believe T37s to be somewhat well built, but without a massive skeg (like bristols or pacific seacraft which also encapsulate the prop as they're so massive) don't see a major structural advantage. Many skegs these days are skinny to try to preserve rudder effectiveness which makes me doubt that they would stay attached. A full skeg precludes balancing the rudder. A half skeg allows some balance but doesn't protect bottom of rudder so to me defeats purpose.
I used to be a full keeler but I'm such a convert from the sailing perspective and strength of a well engineered spade


Sent from my iPhone using Cruisers Sailing Forum

[Kenomac]

Originally Posted by Dockhead

Sure, but in what way would the result have been different, if you had had a spade rudder of strength equal to your skeg? What difference would it have made?

Have a look at the line drawing of your boat -- how strong does that skeg look? Neither you nor I is an actual engineer, but how hard would it be to design a rudder shaft with the same strength? Not a problem, I think.

OK, here's my "line drawing" taken this morning. To put things into scale, most of the skeg is approximately 12inches wide and the rudder 18 inches wide, maybe closer to two feet at the top. The structural support for the skeg is carried forward all the way to the prop support mount approximately 12ft.

I can only post one photo per post, so two more photos to follow.

Also, it's not an either or kind of issue, the skeg and rudder act as one to take on a possible impact. In this set up there is the structural strength of the skeg along with its support combined with the rudder shaft strength, which I might add is the same dimentional stainless steel as the 50ft spade rudder on the boat next door.

[Kenomac]

Another view of Oyster skeg and rudder.

[Kenomac]

Final view of top.

[Jim Cate]

Ken, while that looks impressively stout, the real issue is how well the skeg is integrated into the hull structure, and you can't tell much about that from external photos. It is quite likely well done... but there was a recent issue with a newer oyster that says that folks don't always get such things right!

Laying up structures like skegs is not simple... deep, hollow fins with limited cross sections... they all look good from the outside, but some are quite weak. Was there not a Moody that sank when a strike on the skeg ripped the whole thing off the hulll? The details escape me, but I think it went like that; DH likely remembers!

Jim

[Kenomac]

Originally Posted by Stumble

Well then you are absolutely and utterly wrong. Your skeg is X strength, any number of spade rudders are stronger. Skegs aren't magic, they don't impart some bizarro world extra strength they are a simple mechanical structure of a given strength. Now a particular spade may not be as strong, but so what, any given skeg may be far weaker.

I may be "utterly wrong" but I find more comfort in placing my faith in 12 inches of structural support combined with a three inch stainless rod over the stainless rod hanging there all by itself.

All skeg hung rudders don't appear to be created equal, I've seen many different designs.

[Kenomac]

Originally Posted by Jim Cate

It is quite likely well done... but there was a recent issue with a newer oyster that says that folks don't always get such things right!

Jim

Yes I'm aware of it, and that's the primary reason why I cancelled my order for an Oyster 82 with a six foot extention and modified weight distribution..... and opted to stick with our 53.

[Dockhead]

An interesting discussion moved from another thread.

[Dockhead]

Originally Posted by Jim Cate

Ken, while that looks impressively stout, the real issue is how well the skeg is integrated into the hull structure, and you can't tell much about that from external photos. It is quite likely well done... but there was a recent issue with a newer oyster that says that folks don't always get such things right!

Laying up structures like skegs is not simple... deep, hollow fins with limited cross sections... they all look good from the outside, but some are quite weak. Was there not a Moody that sank when a strike on the skeg ripped the whole thing off the hulll? The details escape me, but I think it went like that; DH likely remembers!

Jim

Yes, this.

Ken's skeg looks pretty massive, and as an Oyster from the Richard Matthews days, I'm sure it's well built. But the flip side of that massiveness is even worse hydrodynamic performance of the rudder, compared an average skeg.

Jim is absolutely right that what skegs LOOK like and how strong they actually are, are two completely different things. It is very hard to tie skegs into the structure, unlike a rudder shaft which can be supported at different levels inside the hull.

Jim correctly recalled an old Moody which sank in the U.S. somewhere, when the owner backed into something and the skeg ripped off, making a hole in the hull.

Either a skeg rudder or a spade rudder can be made as strong as you like -- it depends on how they are engineered. To make a skeg really massively strong you would need to extend it up into the hull and tie it into a frame. You would have to build it like a keel. So it's probably actually harder to make a skeg really massively strong, than it is to make a spade really massively strong. Steve Dashew thinks skeg rudders are inherently harder to make strong and are inherently MORE vulnerable to damage, than spades. Here is what he says about it:

"Years ago the debate about the most effective rudder for steering was settled, and the cantilevered spade configuration was the winner. But what about hitting debris, running aground, and catching nets you might be thinking.

"First: structure. We can tell you from our own experience, and the feedback from owners who have tested our theory, that a properly engineered spade rudder is stronger and more damage tolerant than skeg or keel hung fins.

"There are three keys to this strength. The first is a substantial strong rudder shaft. Our norm is to use twice ABS and/or Lloyds Special Service rule as the baseline stiffness specification. Next comes sufficient hull strength and gusseting of rudder bearing carrier(s) where they connect to the hull for long life and abuse. The last item is a frangible rudder tip, so that it goes away before the rudder shaft itself is damaged, leaving the upper two thirds of the blade with which to continue on.

"The photo above is the rudder on the FPB 64 Iron Lady, which spent several hours pounding on a reef. It will give you a feel for how tough these blades are.

"Next comes catching things on the rudder. Here we have a swept leading edge to shed, and almost total protection from the full depth prop skeg.

"The alternative is a skeg hung rudder, or a spade with a beam from the trailing edge of the keel to the bottom of the rudder. The problem with both of these approaches is damage tolerance. If either is bent from pounding, then the rudder is jammed and you have no steering.

"We’ve cruised well over 250,000 miles now with spade rudders, and done our share of running aground, and we are happy to report we’ve yet to have a problem."

SetSail » Blog Archive » What is the Best Rudder Configuration – Spade, or Skeg Mounted?



My own boat has -- as has been popular for a couple of decades on high end boats -- a partial skeg. This spreads the load on the rudder like a normal skeg does, while allowing the rudder to be partially balanced, which greatly improves hydrodynamic performance. Best of both worlds?

Actually I don't think the partial skeg adds all that much strength, and it does impact the hydrodynamic performance. Why not just make the rudder shaft and bearings that little bit larger and do away with it? And the joint at the end of the skeg is a perfect place for catching ropes -- as I found out once off Sark.

So my next boat will have a Dashew-style massively built spade, not a partial skeg like I have now.

[Dockhead]

Originally Posted by Kenomac

Yes I'm aware of it, and that's the primary reason why I cancelled my order for an Oyster 82 with a six foot extention and modified weight distribution..... and opted to stick with our 53.

LOL!

I'm not sure everyone appreciates your dry wit, Ken.

I do!

[valhalla360]

I think the main difference is about marketing. There is nothing intrinsically stronger about a skeg hung rudder. It's probably more about old timers not understanding the physics involved along then pointing to budget boats with spade rudders...as a result companies targeting old timers, give them skeg hung rudders not because they are better but because that is what they demand...even then it's exceedingly rare to have a rudder failure for either type.

[Juho]

I already proposed to build rudders that break in a controlled way when they hit something hard. Another obvious thing to do would be to install them behind a watertight bulkhead, and make their support weak and local, so that thy do not destroy the whole boat when they fall off. The first method is better though. Or maybe I should have both. It is not common to lose a boat because of a lost rudder, but that's a nasty thing to happen at the open sea, so better be prepared. My rudder would maybe first lose its lowest section, then break a fuse pin and fold, and then rip the non-critical aft section of my boat.

[Dockhead]

Originally Posted by Juho

I already proposed to build rudders that break in a controlled way when they hit something hard. Another obvious thing to do would be to install them behind a watertight bulkhead, and make their support weak and local, so that thy do not destroy the whole boat when they fall off. The first method is better though. Or maybe I should have both. It is not common to lose a boat because of a lost rudder, but that's a nasty thing to happen at the open sea, so better be prepared. My rudder would maybe first lose its lowest section, then break a fuse pin and fold, and then rip the non-critical aft section of my boat.

Many rudders are designed with sacrificial lower parts of the rudder, including German Frers' Hylas 54, all of Dashew's boats, and probably many others.


I agree strongly with you about the watertight compartment or at least cofferdam for the rudder post. One thing I hate about my boat is that the lazarette, where the rudder post is, has a strong bulkhead in it, and would have been simple to make a separate watertight compartment, but it was left open at the bottom to the main hull volume . In order to save a bilge and another pump, I guess, and to make it easier to run cables and pipes, but at the expense of a lost opportunity to make the boat much safer.