Balanced spade rudders a liability?

[ObiWanSand]

I just finished Rick Page’s book “Get real, get gone!” Excellent, thought provoking and enjoyable book, highly recommended! I found much ground for rethinking some of my attitudes! I am however still into cats (not the furry ones…) despite of Mr. Page’s hard arguments to the contrary. In the end it seemed that his prime objection was their cost, which might be a fair one given his angle in the book.

At any rate, I have spent a fair amount of time contemplating his aversion to balanced spade rudders. Looking at the pics below he has some fair points!



Admitting a rather perfunctory knowledge of physics, and naval engineering it still seems to me that a rudder constructed this way has at least some serious issues;

· Very exposed, lacking protection from a nearby keel or a skeg
· The length of the working arm must place tremendous forces on the fully cantilevered rudder/hull connection
· Could the additional hydrodynamic forces from heeling further aggravate the wear and tear of this set up?

I am concerned about safety of course, as we are planning to sail as a family on a cat, and there are very few other rudder choices to pick from! I found some more pics though;


This is the L42 (yes it is presently on our short list!) and while looking at this pic, as well has having inspected it myself up close in Dusseldorf, I have the following thoughts;

• The rudders are still fully cantilevered, but they are far shorter/lower aspect ratio. The arm is therefore shorter and the forces on the connection point should be far less.
• The keel is located directly ahead, possibly close enought to afford some protection. I can picture a log e.g. being pushed out of harms way be the keel.
• Could the fact that cats heel very little, also be a positive factor in reducing stress on the rudders?
• Redundancy. I believe you can still control the vessel on a single functioning rudder.

I did a few searches, and found quite a bit about rudder failures. But not much relating directly to rudder failures on cats. So here is my appeal to the ladies and gentlemen of the forum;

1. Are you familiar with the loss of rudders (spade rudders in particular) as a recurring issue with cats. Especially Lagoon, as we are presently most interrested in the L42?
2. What do you think of the reasoning above? Does it hold some water?

[sailorboy1]

Originally Posted by ObiWanSand

I did a few searches, and found quite a bit about rudder failures. But not much relating directly to rudder failures on cats. [/LIST]

And what would that suggest to you?

[44'cruisingcat]

If you look hard enough, you'll find reasons to be anti-anything. The fact is, the vast majority of production boats (mono or multi) have spade rudders. Incidents happen, but as a percentage they'd be rare.


Catamaran rudders generally don't need to be as big as mono rudders - as you say, cat's don't heel, so they don't have the amount of weather helm. So they're somewhat less vulnerable. There's also the fact that there are two, so some redundancy there.


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

[Southern Star]

While spade rudders will not have the protection/strength of rudders mounted aft of a keel or well-engineered skeg, if well engineered/constructed they should survive just fine in all but extreme circumstances. Poorly engineered skegs (some Roberts designs, as I recall), undersized carbon rudder stocks (the Fastnet disaster), and under-spec rudder heads/stocks (Alpha 42, Hull #1) can lead to catastrophic rudder failure without striking objects, but for the most part, rudder failures are quite rare in any well engineered/built boat. Really, as with so many things in yacht design, rudder design is a trade-off: You gain 'power assist' to your steering with a balanced spade rudder, which reduces loads on the steering mechanism/autopilot, but do so at at a slightly increased risk that your steering could be disabled if you are unfortunate enough to strike a large solid object that is not deflected by the keel.

Brad

[leftbrainstuff]

Originally Posted by ObiWanSand

I just finished Rick Page’s book “Get real, get gone!” Excellent, thought provoking and enjoyable book, highly recommended! I found much ground for rethinking some of my attitudes! I am however still into cats (not the furry ones…) despite of Mr. Page’s hard arguments to the contrary. In the end it seemed that his prime objection was their cost, which might be a fair one given his angle in the book.

At any rate, I have spent a fair amount of time contemplating his aversion to balanced spade rudders. Looking at the pics below he has some fair points!



Admitting a rather perfunctory knowledge of physics, and naval engineering it still seems to me that a rudder constructed this way has at least some serious issues;

· Very exposed, lacking protection from a nearby keel or a skeg
· The length of the working arm must place tremendous forces on the fully cantilevered rudder/hull connection
· Could the additional hydrodynamic forces from heeling further aggravate the wear and tear of this set up?

I am concerned about safety of course, as we are planning to sail as a family on a cat, and there are very few other rudder choices to pick from! I found some more pics though;


This is the L42 (yes it is presently on our short list!) and while looking at this pic, as well has having inspected it myself up close in Dusseldorf, I have the following thoughts;

• The rudders are still fully cantilevered, but they are far shorter/lower aspect ratio. The arm is therefore shorter and the forces on the connection point should be far less.
• The keel is located directly ahead, possibly close enought to afford some protection. I can picture a log e.g. being pushed out of harms way be the keel.
• Could the fact that cats heel very little, also be a positive factor in reducing stress on the rudders?
• Redundancy. I believe you can still control the vessel on a single functioning rudder.

I did a few searches, and found quite a bit about rudder failures. But not much relating directly to rudder failures on cats. So here is my appeal to the ladies and gentlemen of the forum;

1. Are you familiar with the loss of rudders (spade rudders in particular) as a recurring issue with cats. Especially Lagoon, as we are presently most interrested in the L42?
2. What do you think of the reasoning above? Does it hold some water?

A balanced spade rudder has several benefits when compared to a barn door:

1) lightweight
2) low steering loads
3) small wetted area therefore less drag
4) chord length and overall length not constrained by a skeg
5) shape can be optimized for performance

The tradeoffs, compared to a barn door are:

1) poor durability when subjected to impacts and strikes
2) unprotected but more responsive with better hydrodynamic behaviour
3) single shear rudder shaft attachment resulting in very high shear and bending moment loads

Like everything there are tradeoffs.

With higher performance, single shear attachment and when optimized in terms of manufacturability you sacrifice reliability, durability and toughness.

Sent from my SM-N900T using Cruisers Sailing Forum mobile app

[UNCIVILIZED]

The strength, or lack of, in any rudder, comes from the design. For example, either Kurt Hughes or John Shuttleworth designs his rudders to be turned 45 degrees to the water flow at 25kts, & then adds in a substantial safety factor. In addition to their kick up feature.

Or on the Dashew's boats (monohulls), the rudder posts are 8" solid aluminum, & project well down into the rudders. And several of their boats have been tested via severe groundings, & come away with "only" cases of reef rash.

Generally, with any rudder, water intrusion. And what that does over time, is the big killer.
But if you find a boat that you like, get the design specs on the rudder, & run them by an engineer of your choosing. For peace of mind.

Keep in mind, too, that it's rarely tough to build them, or rather re-build them, to much stronger than OEM spec, if you so desire.


What boats are you considering, BTW?

[Brob2]

Originally Posted by ObiWanSand

I did a few searches, and found quite a bit about rudder failures. But not much relating directly to rudder failures on cats.

Originally Posted by sailorboy1

And what would that suggest to you?

They never leave the dock?

[Stumble]

There is absolutely no reason a spade rudder can't be as strong as a attached rudder. The only question is what are the relative strengths of the design. Hung rudders may be stronger, and it may be easier to design them to a given strength, but cantilevered beams (i.e. Spade rudders) are well understood design problems other than doing the math it isn't difficult.

[Don C L]

So I am curious, one of the great advantages of multihulls is the ability to beach the boat when needed. Does the cat in question with its spade rudders still afford this option?

[44'cruisingcat]

Most do. The boat will usually sit on it's minikeels and rudders.

[ObiWanSand]

This sure is a fine forum! Compliments to all for sharing your experience and competence

I am rather well read on sailing, but lack the milage...the miles under the keel as the saying goes! The living on board for months at the time...indeed the goal is years but that remains to be seen. The family will have to enjoy the lifestyle, should that come to be the case. That's why I have adopted Mr. Page's slogan; "Smiles on deck, not miles under the keel!"

So it seems that in this case, as with every other in this reality, it is back to tradoffs again. One feature will give you this, but will cost you that. I will try to summarize how this discussion applies to us;

• We can't afford a one off. Hence the possibility of demanding rudder design criteria is off limits. We are stuck with the options provided on production boats.
• As with most things, certainly aviation where I come from, it is a risk assessment game. Altough in this case it may just be accept whatever it is you get, and do the best you can!

• I think 'Leftbrainstuff' summarized the pros and cons rather neatly. So I will simply add Amen to that!
• Our goal is cruising the tradewinds, with a risk aversive attitude, paying close heed to wx forecasts and avoiding hurricane seasons! This should tip the risk assessment scales in our favour...even for our balanced spade rudders I hope.
• As far as the other main risks are concerned; fatigue/man over board and collision with floating objects, I believe the multis have the upper hand on most other sailing solutions.
• Concerning financial risk, I find the Lagoons a fairly safe haven. I believe a new model of reasonable size will be fairly easy to sell again...should we wish to.

For us it seems we are simply down to what model Lagoon we will buy. And at the risk of repeating myself, I do find the L42 to be a rather fine sailing vessel.


Brdgs,


Oddvar


PS! A quick scan on Google Maps will reveal what type of boats are presently out there cruising. Check out typical cruising hang outs like Cartagena, Nuku Hiva, Hiva Oa or Fiji, just to name a few. You will quickly find that the statement: "Multis stay mostly at the dock", has very little to do with fact.

[ObiWanSand]

BTW!

At the Dusseldorf show three nice cats where placed right next to each other; L42, FP40 and the Bavaria/Nauticat Open 40.

The were all sitting nicely on their keels, only steadied by an extra support just in front of their rudders. That support, I was told, was just to account for the uneven loading as several tens of people were roaming the boats!

[ObiWanSand]

PS! again...

We are really curious about the rudder issue, and once again the appeal to all you (lucky buggers! out there cruising is, have you heard much about this;

1. Are you familiar with the loss of rudders (spade rudders in particular) as a recurring issue with cats. Especially Lagoon, as we are presently most interrested in the L42?

Brgds,

Oddvar

[denverd0n]

Originally Posted by 44'cruisingcat

If you look hard enough, you'll find reasons to be anti-anything.

Ditto this. And, of course, you'll also find reasons to be pro-anything. There are a lot of self-professed "experts" out there who insist that their way is the only way.

When I read things from these guys I just remind myself of the definition of an expert... X is an unknown factor, and a spurt is a drip under pressure!

[captstu]

Cats carry a 'spare' rudder. One will generally sail just fine.

My three rudder failures in a lifetime of sailing were not related to post failures.

One quadrant failed with corrosion, leaving me to sail home with backup manual steering. Annoying, but no real risk. It was a spade monohull with conventional Edison wire steering.

A big cat on an offshore delivery has a wire snap after a lightening strike weakened a bad design.

The third and most serious was a Gemini with a terrible concentric cable design that failed shortly after survey when a cable snapped hidden within its sheath. Only turned left. Had to kick up both rudders, disconnect engine steering an hand steer the engine while sitting in the water of the engine well.

In my experience, rudder failures aren't as much of a problem as the steering gear.

Does your boat have workable backup steering that will take you half way there?