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Stumble · 30-03-2016, 14:52

Quote:

Originally Posted by Kerry1

"Sure it's possible to design a skeg with enough lateral strength, but it's just as easy to design a spade with enough lateral strength. Th skeg is just superfluous."
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The problem with the above statement and others like it, is that there is a vast difference between what is possible and what actually occurs. I have a spade rudder and can appreciate in certain conditions it is a weakness. Production boats are designed with a certain level of safety margin vs cost. I think for anyone who is contemplating using boats with spade rudders for extended travel and so exponentially increasing the risk of encountering extreme conditions will, (or should) be thinking in terms of a jury rig in case of rudder loss/steering failure and plan accordingly. I notice (in recognition of their inherent design weakness) that cars still carry spare wheels.

Kerry,

So your argument is that because a skeg could be built stronger that means engineers will build it stronger? This is just silly. Engineers determine how strong the rudder needs to be then design to this number.

So if it needs to be able to handle X,xxxlbs of side force, that's the number it is designed to meet. Skeg or spade doesn't change the fact that the engineer has met his requirements.

Now you might be able to argue that a skeg rudder doesn't suffer from as much side force and therefore the X number is lower, but that just means the engineer reduces the strength of the rudder. Where the spade rudder has to be strong enough to withstand 2x. But so long as they are built to the appropriate strength then one isn't any better than the other.

If anything you could argue that the spade rudder is less subject to impact damage because it was designed to be overall stronger in the first place.

Assume for a moment that our engineer is good at his job, and properly predicts the sailing loads that need to be designed for, along with appropriate safety margin reserves. And that the side force of the skeg hung rudder needs to be designed to handle a side force of 3,000lbs with a 7:1 safety margin the skeg has an ultimate strength of 21,000lbs. Where our spade that requires a a side load capability of 6,000lbs (double) needs a load capability of 42,000lbs.

Now you get hit by a tree trunk at speed. The spade rudder has a reserve double that of the skeg. Which one is more likely to be damaged?

Sailor Doug · 30-03-2016, 16:01

Was your boat built to the design requirements? No way of knowing. A poor employee or financial stress can produce boats not built to the design. I remember a decade ago of skegs braking on a series of high end boats.

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**Don C L** · 30-03-2016, 16:19

If one really desires having a more effective rudder without the weaknesses of the spade, one can design it to hang it on the transom with a kick-up shear pin and if worst comes to worst, it is easier to rig a spare rudder there. Yes, not all designs lend themselves to that, but a hull like a Peterson 44 could be fitted with a rudder on the transom and that is no dog of a boat. (and yes, you'd have to add some kind of long shallow skeg to correct the center of lateral resistance too if you were modifying that particular hull.)

**Jim Cate** · 30-03-2016, 16:19

Quote:

Now you get hit by a tree trunk at speed. The spade rudder has a reserve double that of the skeg. Which one is more likely to be damaged?

Your arguments about strength are valid, Greg, but do consider that most skegs have a sloped leading edge... some more, some less, but this feature has at least a chance of deflecting the debris downward and thus reducing the loading on the structure. And I suspect that even the weaker skeg designs are fairly well supported in the fore/aft direction.

Our partial skeg is essentially vertical and does not offer this advantage, but it is built very stoutly indeed. So far, so good!

Jim

44'cruisingcat · 30-03-2016, 23:23

Quote:

Originally Posted by Stumble

Now you get hit by a tree trunk at speed. The spade rudder has a reserve double that of the skeg. Which one is more likely to be damaged?

We have hit a log about 9" diameter at >9 knots. Aside from a snapped sacrificial rudder dowel, no damage whatsoever.

valhalla360 · 31-03-2016, 07:52

Quote:

Originally Posted by leftbrainstuff

Any cantilevered design, rudder or otherwise, will have much greater bending moment induced loadings on the rudder shaft and bearing surfaces.

This will translate to much higher shear and tensile loadings at the location of maximum bending moment when compare to a skeg hung equivalent.

For a rudder this is primarily where it meets the hull.

We engineers abhor cantilevered designs. A double shear design is far superior in terms of durability and transfer of loads.

A spade rudder has three primary benefits over a skeg hung rudder:

1) lighter
2) more efficient hydrodynamically
3) cheaper to manufacture

For a race boat 1 and 2 are important. For a yacht builder who prioritizes 3 I would expect to suffer lower durability.

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As an engineer, I disagree unless you place the unrealistic limitation that all features must be built to the same size and strength.
Obviously, if you take a boat with a skeg hung rudder and just cut away the skeg, the remaining rudder will be far weaker. That's not how design is done.

Engineers don't abhor cantilever design, they take it into account during the design and if cantilever makes more sense, they use it.

Now if boat builders actually get an engineer involved is a separate question.

valhalla360 · 31-03-2016, 08:01

Quote:

Originally Posted by 44'cruisingcat

But if you're really worried about rudders, there are plenty of designers who draw boats with kick-up rudders. Not production boats though.

Actually, there are a few production boats with kick up rudders. Gemini has a thousand or so and TomCat has them. But as you say, if it's really important to you, I'm sure you can retrofit something similar. For cats with fixed keels, it's usually just not a big concern.

The cost issue gets complicated. We have two 34' catamarans (anyone need one?) both have significantly more interior room compared to a 34' monohull. When you get up into the 40' range with 20' beam it's even more pronounced. In addition, it used to be that cats easily won on resale value. 20-30yrs ago, there were a pleathera of used monohulls available but very few cruising multihulls and the ones that were available were all fairly new. Supply/Demand resulted in higher used prices.

Of course, you have to consider the design intent. An extreme performance cat will have far less living space than a cruising oriented cat but still probably similar to a cruising monohull.

Cats are still a little more expensive but the difference has shrunk to the point that unless you are at the extreme lower end on price, you can still consider a cat if you compare equivalent boats.

sailorboy1 · 31-03-2016, 08:33

Quote:

Originally Posted by valhalla360

Now if boat builders actually get an engineer involved is a separate question.

You must not pay attention. It is apparent in all these posts that if boat builders wanted things to be strong they would get faceless internet forum members involved!

mark_morwood · 31-03-2016, 12:27

Quote:

Originally Posted by ObiWanSand

...

Redundancy. I believe you can still control the vessel on a single functioning rudder.

This aspect is quite key to your risk analysis, which is I think what you are doing.

There are arguments that spade rudders are at higher risk of damage than skeg hung rudders. For the sake of argument assume this is true.
For a variety of reasons almost all production catamarans in the size range you are looking at have spade rudders.
However the mitigation of the consequence of the risk is very different for a catamaran versus a monohull because of redundancy.
It is certainly possible to lose both rudders, but most of the cases I have heard of, including ours, just one was lost. (A well publicised exception was the Alpha 42 that effectively lost 2 in the Atlantic in mid-winter in 2014)
I know of multiple cases of single rudder loss in catamarans where the boat was easily sailed with just one.

When we lost a rudder on our Catana 48 between the Galapagos and Marquesas the boat was very controllable with just one rudder. In fact I'm a little embarrassed to say I didn't realize that the cause of our "autopilot problems" was a lost rudder till we got to the Marquesas, as the impact was so slight when the lost rudder was to windward. We sailed the remaining 1000 miles to the Marquesas, and then cruised on through the Tuamotus with its many interesting passes with relatively few issues. We finally managed to have a replacement rudder built in Tahiti. We in fact built 2 replacements, as at 22 years old the shaft to rudder attachment on the remaining rudder was feeling suspect.

So bottom line, I wouldn't let the style of rudder be a factor in your choice of a general purpose cruising catamaran, because if you do you will dramatically limit your choice of boats, thereby probably making you compromise on other features that are much more important for you. Not to say you shouldn't consider the builders reputation for reliable construction as a factor, I just wouldn't focus on rudder style.

Mark.

**a64pilot** · 31-03-2016, 14:05

You can have your cake and eat it too.
Island packets all have balanced spade rudders, that are pretty well protected.
But seeing as how a Cat has two and I assume will do fine with one, that I would be comfortable with. You can be a little less risk adverse when there is a spare.
Cats from my view of no Cat experience are simply a completely different animal, you just can't think of them in mono terms, or so it seems. They have it would seem tremendous safety advantages with redundant almost everything.

Dock, that kink in the wing you speak of, is often called a flight control

there were many unattached flight controls in days gone by and still are a few in STOL designs as they give better control at low air speeds, but they are rare, the kink seems to work OK for most.

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