Which rudder?

[Fidippide]

Hi everybody, I studied 6 naca profile with xfoil: 0008, 0010, 0012, 0014, 0016, 0018 at various Re (from 100000 at 1000000). What do you think should be best choice for a fast sailboat? Thanks.

[daiquiri]

What studies did you preform with Xfoil, in particular and what are the results? And then, the main question - what are the goals you need to obtain? ;)
Generally speaking, I wouldn't go neither over 12-14% nor below 10% thickness for several reasons:
- thicker foils have a higher drag at high speeds
- lift-curve slope decreases as thickness increases above 10-12%
- maximum lift coefficient decrease both below 12% and above 14% thickness
- stall angle is maximum in 12%-14% range
- thin airfoils are prone to the formation of a leading-edge separation bubble
- thin airfoils are prone to the leading-edge stall.
So, as a first hint, a NACA 0012 could be a good compromise between all of the above.
Bear in mind that choosing the airfoil is just a part of the job. The size and the shape of the planform has a huge impact on the final hydrodynamic characteristics of lifting surfaces like rudders and keels.

[Fidippide]

So, as a first hint, a NACA 0012 could be a good compromise between all of the above.
Bear in mind that choosing the airfoil is just a part of the job. The size and the shape of the planform has a huge impact on the final hydrodynamic characteristics of lifting surfaces like rudders and keels.

I totally agree with you. Thanks.

[jehardiman]

While an approximate 0012 2D shape is going to show an optimum at speed (actually its about 0011), a 0018 shape is a much better choice as the whole reason for a high lift rudder is to control at LOW speed. Almost any shape with a thickness under 20% (including a rectangular block with 5% of the corners knocked off, go check the data) will work at high speed. But a rudder needs to control across the speed range and therefor should have as high an angle of stall as possible. That leads to thicker sections with lower aspects.

FWIW, I wouldn't use a NACA 00XX airfoil shape for a hydrofoil such as a rudder. It is too fine aft. I would recommend a DTMB EPH or at least a hyperbolic trailing edge. Also, sweep and taper effects on spanwise AOA will greatly overshaddow any difference between a 0012 or 0018 2D data.

[Paul B]

FWIW, I wouldn't use a NACA 00XX airfoil shape for a hydrofoil such as a rudder. It is too fine aft. I would recommend a DTMB EPH or at least a hyperbolic trailing edge. Also, sweep and taper effects on spanwise AOA will greatly overshaddow any difference between a 0012 or 0018 2D data.


With a comment like this I doubt you have any experience with fast sailboats.

[jehardiman]

With a comment like this I doubt you have any experience with fast sailboats.

All sailboats are slow compared to the moving foils I usualy design for. But correct, I really haven't built any rudders for sailboats over 20 knots. High speed (20+ knots) sailboats just don't interest me, too limited in their ability to be useful...shrug...

FWIW, I don't think the modern trend to high aspect airfoil type rudders is correct but just a fashion, they look like they were designed by someone using some of the aero CFD code out there, not hydrodynamists. Then again, the whole idea of "high speed" sailing is racing where failure of a foil to either provide control or structurally just means more money for the designers and builders to do it over. That is a luxury I do not have with operational foils.

Honestly though, there is more than enough CFD, test, and full scale data to support my position (see chapter 9, PNA V3). Sail or power it makes no difference. It must be remembered that rudders are not keels nor wings, and therefor are selected for different criteria. They should not be selected for L/D at a low fixed AOA, but rather the force necessary to maintain command control against the course stability index across the speed range, vessel motion, and environmental conditions.

[Paul B]

FWIW, I don't think the modern trend to high aspect airfoil type rudders is correct but just a fashion,

I hope you are kidding here, otherwise...


they look like they were designed by someone using some of the aero CFD code out there, not hydrodynamists.

You don't have any idea what you are talking about.

[jehardiman]

I hope you are kidding here, otherwise...

No, Classic "paper water" problem. The analysis was caused by control problems in chop after delivery. In the analysis the outer third of the foil was basicly non-functional due to wave orbital and pitch effects even though the foil was "high speed", and noisy to boot. The fix was to shorten the foil, increase root cord, increase the TE, increase taper, and decrease sweep. This resulted in a lower aspect foil around 4-5 with less area and drag than the original.

You don't have any idea what you are talking about.

LOL, not even going to ask why I think that? :D

[MikeJohns]

..............I don't think the modern trend to high aspect airfoil type rudders is correct but just a fashion, ....................the whole idea of "high speed" sailing is racing where failure of a foil to either provide control or structurally just means more money for the designers and builders to do it over. ..................


And both insurance and efficient rescue services play their part in non-robust design. A typical rudder failure rate for offshore racing events is illustrative. Also the jury rudder systems people put so much faith in after a smooth water coastal test are usually inadequate in a boisterous seaway with a bit of a blow.

Classic ship theory usually points to a Naca 0015 foil, but I think many racing boats will be using something like a Eppler generated shape and Dave Vacanti seems to have a good niche in the foil design software market.

I'm never sure about those nice lift drag plots once the vessel hits a seaway it's probably all a bit superfluous. I have often see frustrated helmsmen with stalled rudders and wondered if they might have faired better with more of a barn door.

But the planform can definately be a fashion statement :)

[Paul B]

No, Classic "paper water" problem. The analysis was caused by control problems in chop after delivery.

I guess guys like you and Johns will always believe that those who earn a paycheck related to workboats know much more than those who design pleasure craft. Just like those who work on diesel rigs know more than the folks that work on the F1 teams.

The people at the top of "fast sailboat" design earn millions of US dollars. With the expertise you have, maybe you and Johns should team up and dominate that field and become rich. Can't be too hard seeing as how you two have so much more on the ball than the folks who dominate there now.

[Paul Kotzebue]

I've had the opportunity to work for and with some of the top sailboat designers during my career. The guys at the top have degrees from places like the University of Michigan and Webb Institute. They are in a different league than the self taught correspondence course educated cruising boat designers. The competence gap in sailboat design is huge .... much more than the general public is aware of.

The top sailboat designers are serious about winning and don't care much for fashion statements. The "performance cruiser" designers will copy what they do without any meaningful engineering analysis. The public has no way to really tell the difference.

[MikeJohns]

……..The "performance cruiser" designers will copy what they do without any meaningful engineering analysis. The public has no way to really tell the difference.

Yes definitely we have seen that throughout history and a lot of problems both structural and performance related have resulted from this .

Racing boats are not the exclusive domain of low drag high lift foils of course and many naval architects around the globe can produce a very good foil just how good will depend on the depth of the pockets of the person who wants the foil.

Racing boat technology definitely has it’s fashions and fads and I don’t think foils are immune from this, also if you have worked in the field you’d be well aware of just what conditions a foil is optimized for in the testing phase. Sometimes it takes some guts for a designer to drop a current fad and they can then start a sensible trend. If that has the added advantage of increased reliability in an often inherently poor structural form then everyone might benefit.

A naval architect will look at the benefit performance tradeoff but a racing designer wants anything that he imagines can shave another second off a racing course. In reality how much real world lift drag data exists for those optimized foils in a seaway? Sometimes chasing minimal wetted surface and going for that last 1% foil drag in ideal flow has it’s detriments and structure is definitely one of them.

Parameters which in the tank or in the CFD package see a foil operating nicely in the low drag bucket can vaporize once the boats is in a seaway. Much of the real flow is anyone’s guess and it’s then that the helmsman finds that foil lacking no matter how much time and money was spent developing it.

Suggesting that 3d flow fields are an exact science that can be learnt at university is probably best summed up by Leo Lazauskas

“CFD is, of course, an enormous advance
because we can now display our almost complete ignorance in full colour!”

There is no superior knowledge where you can sit down solve a system of pde’s and out pops the perfect foil and planform. In the real world there’s observation, tank testing and CFD and then there’s always a current trend.

[Paul B]

I've had the opportunity to work for and with some of the top sailboat designers during my career. The guys at the top have degrees from places like the University of Michigan and Webb Institute. They are in a different league than the self taught correspondence course educated cruising boat designers. The competence gap in sailboat design is huge .... much more than the general public is aware of.

The top sailboat designers are serious about winning and don't care much for fashion statements. The "performance cruiser" designers will copy what they do without any meaningful engineering analysis. The public has no way to really tell the difference.

Well said. I know some of the folks you've been involved with and I have no doubt their abilities far exceed some of the loud talkers here.

This guy Johns is always spouting off about things like "frustrated helmsmen with stalled rudders", but he is always too cowardly to name the boats/designers that are so lacking. It is a ploy he and his ilk use all too often.

Of course he is pretty much a failed "designer" whose agenda is to pontificate to local authorities until he can hopefully push through some sort of law that will bring him some income as a required "review" of design/builds in his country.

Based on his earlier comments here it is obvious he knows little about how modern "fast" sailboats are designed and built.


Then you have this comment from JEHardiman above: "Then again, the whole idea of "high speed" sailing is racing where failure of a foil to either provide control or structurally just means more money for the designers and builders to do it over."

This is just a horrible thing for someone to say. The insinuation is disgusting.

[jehardiman]

Then you have this comment from JEHardiman above: "Then again, the whole idea of "high speed" sailing is racing where failure of a foil to either provide control or structurally just means more money for the designers and builders to do it over."

This is just a horrible thing for someone to say. The insinuation is disgusting.

Well, as a king once said "Honi soit a qui mal y pense", and my statement is not an insinuation on anyone’s character though if you wish to make a shoe out of it and place it on your foot while stating that it fits....so be it. It is a fact that the failure of a racing boat just means that the sponsor will likely throw more money at it and try again, an option that I do not have in my job.

If modern high speed sailboat design was as good as you imply, then all the boats of a given design brief would look the same and would not fail underway. A quick survey of bleeding edge "high speed" sailing will show that designs are not standardized and that over the course of an ocean race 10-30% of the racers (depending on hull type) will suffer some form of failure (go read this thread from 3 years ago for a discussion and some data on high speed sailing failures analysis from the Route du Rhum http://www.boatdesign.net/forums/multihulls/canting-keel-monos-vs-multihulls-13511.html). We are left with only two options after accepting those truths, and I will assign the sins of vanity and omission over deliberate evil. If you feel I have insulted anyone professionally, you have options available in this state as I do not hide who I am.

In the 25 years I've been doing hydrodynamic design there are some truths that I have learned.

1) You spend 90% of the hydrodynamics money to get the last 10% of the answer. And in the overall scheme of things, that 10% performance can easily be eaten up by environmentals. Like someone suggesting a "laminar flow" hydrofoil for open water use with a span greater than 4 inches, crossflows are just too great for that to ever meet full performance.

2) Most analysis fails to consider environmental effects properly. Generally it is a schedule vs time vs money issue because many codes do not support oscillatory cross-flow or rigid-body motion interaction with flow. But sometimes a project just makes a poor decision on how to manage that part of the analysis. When a NG Ocean Systems project could not pass acceptance testing, a review I was involved in found out that they assumed that there were no waves with a period greater than 7 seconds in the required open ocean Sea State.

3) The person who makes the final decision is rarely the hydrodymancist and rarely is the final decision based solely on engineering performance. More often the customer who makes the decision is presented differing and often conflicting criteria. Ok, all designers raise their hand if the customer has told you "I want it done this way" and you did it even though there was a better way.

4) Fashion sells. This is true if you are spending 10k or 1 billion. Nobody wants to be too far from the crowd so they (customers and designers) all bunch up around a design that may not be the best but is currently winning. Often a good helmsman who had a good race sets a design trend when people believe that it was a piece of hardware that beat them, not that they themselves were out performed. And the design brief and/or rules plays into that also. Australia II's winged keel was not hydrodynamicly more efficient than Liberty's but the manipulation of the chain girth measurement gave it a sail area advantage. Same thing with IOR bustles; they didn't make the boats go faster.

5) Pretty pictures sell even better. I watched GD-EB present a project to the US Navy with a manifestly flawed flow analysis...the PowerPoint pictures were pretty though. Same as the shark fin keels of a decade ago...the humpback flipper fins of today… or elliptical planforms. No real performance advantage, just look “better”.

And finally...

6) The only thing a Naval Architect has to sell is his opinion, because I can guarantee you that if you have two NA's analyze the same shape you will get two different answers....

[Ad Hoc]

"...What do you think should be best choice for a fast sailboat?.."

In addition to what has been said above...whatever works for you!..and define 'fast'.

I've designed some very very basic 'spade' type rudders on boats which would be considered "fast" by sailing the fraternity, ie flat plates. So long as the rudder provides the yaw/moment required, without being overly draggy, the final shape is almost irrelevant.

[Paul Kotzebue]

There is no superior knowledge where you can sit down solve a system of pde’s and out pops the perfect foil and planform. In the real world there’s observation, tank testing and CFD and then there’s always a current trend.

I agree.

[jehardiman]

"...What do you think should be best choice for a fast sailboat?.."

In addition to what has been said above...whatever works for you!..and define 'fast'.

I've designed some very very basic 'spade' type rudders on boats which would be considered "fast" by sailing the fraternity, ie flat plates. So long as the rudder provides the yaw/moment required, without being overly draggy, the final shape is almost irrelevant.

This statement is so full of win.

I concur about flat plate rudders for some "high speed" sailing vessels. Increasing speed is all about controlling as much power as possible while maintaing drag to a minimum at the speed you are currently going. For over powered vessels, fully cavitating rudders are a viable option and for those, area and location is all that matters

Given the ISAF Executive Committee decision to include kiteboards in the sail speed records, that opens the door to all sorts of high speed sailing "yachts". If someone with a depressor constrained semi-rigid wing pod sailer isn't doing 80 knts after 10 years, they just aren't trying hard enough.

[MikeJohns]

...........
This guy Johns is always spouting off about things like "frustrated helmsmen with stalled rudders", but he is always too cowardly to name the boats/designers ....... It is a ploy he and his ilk use all too often.

Paul B
I can never work out why you are so scared of the engineers/NA's here, you have made similar delinquent remarks to many of the professionals that contribute to this forum.
The same people would be happy to engage you in sensible dialogue if you tried being civil .

As for cowardly behaviour perhaps you could provide your own details. Your full name would be a start

[Paul B]

So many strange ideas, I wold guess you have never participated in high end sailboat racing and don't have the basic understanding of the princliples. First you said this:

It must be remembered that rudders are not keels nor wings

By this quote it seems you don't know what is going on under a modern fin keel/spade rudder monohull when going upwind.


Well, as a king once said "Honi soit a qui mal y pense", and my statement is not an insinuation on anyone’s character though if you wish to make a shoe out of it and place it on your foot while stating that it fits....so be it.

If you make the statement you should at least have the stones to own it.


A quick survey of bleeding edge "high speed" sailing will show that designs are not standardized and that over the course of an ocean race 10-30% of the racers (depending on hull type) will suffer some form of failure

Another laughable statement by you. Of course you can cherry pick disasters like the '79 Fastnet the like. But your assertion is not true, and you should be better at statistics than this.



The person who makes the final decision is rarely the hydrodymancist and rarely is the final decision based solely on engineering performance. More often the customer who makes the decision is presented differing and often conflicting criteria. Ok, all designers raise their hand if the customer has told you "I want it done this way" and you did it even though there was a better way.

At the top of the heap (America's Cup, TP52, etc) the designers don't have any "customer" making the decisions. The design team does. In fact, some top design offices routinely turn down work from owners who have their own "ideas". It isn't worth the time, effort, and the firm's reputation to deal with these people.


Fashion sells. Nobody wants to be too far from the crowd so they (customers and designers) all bunch up around a design that may not be the best but is currently winning.

The designs bunch up because they ARE the best at the time. Please list all the "fashions" that have been winners, but were not correct for the time/rule.


Often a good helmsman who had a good race sets a design trend when people believe that it was a piece of hardware that beat them, not that they themselves were out performed.

Please give examples of this. I know many designers who look at something that wins by "fluke", and they sure don't follow that line.


Australia II's winged keel was not hydrodynamicly more efficient than Liberty's but the manipulation of the chain girth measurement gave it a sail area advantage.

There was more to the AII keel than the chain girth advantage. While it is true it was probably less than a percent or so advantage, even the smallest advantage is something that is not left on the table.

I hope you realize the days of AII were still the dark ages of modern design and the process is not remotely the same today.


Same thing with IOR bustles; they didn't make the boats go faster.

The bumping of the IOR boats at measurement points did make the boats faster in relation to their rating. That was the game, and it took a great amount of skill to balance the bumping attributes vs deficiencies.


Same as the shark fin keels of a decade ago...the humpback flipper fins of today… or elliptical planforms. No real performance advantage, just look “better”.

Tell us what reputable design firm has used a "shark fin keel" in the last 10 years..or 20..or 25? What reputable firm uses the "humpback flipper fin?

If you don't think elliptical (not actually elliptical planform) keels were not a performance advantage in the days of IOR you clearly have no clue.

[Paul B]

Paul B
I can never work out why you are so scared of the engineers/NA's here, you have made similar delinquent remarks to many of the professionals that contribute to this forum.


This is hilarious, considering I am the VP of Engineering at my firm.

What I object to is someone with no practical experience in solving the problem pontificating about their abilities. Hell, you know so little about composite construction you couldn't come up with any way to transfer loads of a fin keel in a composite hull except to "add steel".

Last year I was interviewing for an open req. I was given one resume of a friend of a co-worker. He was a good engineer with years of experience in his field. However, I would not have interviewed him, except for the personal request.

He came in and during our chat I gave him a part and asked him how he would make it. The answer he gave was appropriate for the materials and processes he used in his previous positions, but it was totally useless for the materials and processes we use.

If it came down to my decision to ask about performance sailing I would trust great engineers/NAs like John Reichel, Marcelino Botin, or Rolf Vrolijk before I would think about asking someone who welds plate onto workboats.

I would never ask anything from someone who obviously looks the other way when presented with data that doesn't follow his pre-conceived, archaic ideas. This is the person you have proven to be.

[booster]

Jehardiman and Paul B!

Good comments there Jehardiman and Paul B. Regarding strength of composite structures, like rudders, some general purpose finite element codes (FE-codes) can handle it. The latest version of ABAQUS incorporates several fracture models for composite structures. As always the results are strongly influenced by the material parameters. These are not always easy to find. The same is true for the loads. Here CFD=Computational Fluid Dynamics can help. Who doesn't remember John Bertrand's come-back in the America's Cup. That fracture (and ultimately sinking) probably could have been avoided if some FE-calculations had been performed. One can analyze different load-cases such as accidental, when using the winch at the wrong side of the boat (introducing torsion). The same is true for the other America's Cup yacht which also got v-shaped (altough the load-case probably was different). ANSYS is another FE-code that as been used to solve these kind of problems. I believe the German team used it in their latest America's Cup campaign.

Regards,
Booster

SIMULIA develops the Abaqus suite of general-purpose nonlinear finite element analysis (FEA) programs for mechanical, structural, civil, biomedical, ...
www.simulia.com/


Welcome to ANSYS, Inc. - computer-aided engineering technology and engineering design analysis software products and services.
www.ansys.com/

[MikeJohns]

This is hilarious, considering I am the VP of Engineering at my firm.

If you actually read what I posted, it's not funny at all, that’s rather sad, maybe that’s why you are so frustrated. You respond so emotionally to every technical discussion and you are the master of the tactics of which you so vehemently accuse others.

What I object to is someone with no practical experience in solving the problem pontificating about their abilities. Hell, you know so little about composite construction you couldn't come up with any way to transfer loads of a fin keel in a composite hull except to "add steel".


Would you care to put a link to this creativity? Perhaps you’d like to consider who exactly is outside of their field. Just who is ‘pontificating’ Paul?

Post technical facts, quotes from previous posts and lets discuss them technically. Not this emotional leap to the grand racers viewpoint that you are so keen on.

He came in and during our chat I gave him a part and asked him how he would make it. The answer he gave was appropriate for the materials and processes he used in his previous positions, but it was totally useless for the materials and processes we use.

And that qualifies you for what Presumably from your previous statements you are not an engineer ?

If it came down to my decision to ask about performance sailing I would trust great engineers/NAs like John Reichel, Marcelino Botin, or Rolf Vrolijk before I would think about asking someone who welds plate onto workboats.

You know I’m not a welder anymore than you are. As for this ‘trust’ you would trust them to produce an optimal performance foil for the racing you would engage in, that’s not quite the issue we touched on a raw nerve of yours here is it.

I would never ask anything from someone who obviously looks the other way when presented with data that doesn't follow his pre-conceived, archaic ideas. This is the person you have proven to be.

What data? You have never offered any that I can recall. In fact you contribute nothing in that regard but prejudiced opinion. I keep asking you to post facts and data ….please .

To suggest that others do this to you is a joke. Your substitute for data tends to be vitriolic diatribe. Like here in this thread.


This thread should be about discussing rudders, discuss the strengths and weaknesses, the hydrodynamics of the foil shape and plan-form, the structural reliability, materials and likely modes of failure.

Here’s something I’d like you to consider :

A few high profile racing boats have sunk recently because their large diameter CF rudder shafts have snapped at the max BM inside the vessel, are all the designers rushing to retro-fit a partial bulkhead or enclosure to remedy this? Should a broken rudder shaft sink a boat quickly? What’s really wrong with adding the extra 10kg to prevent this happening in an offshore vessel? Will this ever happen without regulation when winning and robust conservative design are such uneasy bed partners?

[MikeJohns]

......... Regarding strength of composite structures, like rudders, some general purpose finite element codes (FE-codes) can handle it. The latest version of ABAQUS incorporates several fracture models for composite structures. As always the results are strongly influenced by the material parameters. These are not always easy to find. The same is true for the loads........

How to account for matrix aging, micro crack propogation and the service limits for such structures is less predictable.

There's also a few gremlins lurking in composite layups that can bite when highly stressed, even under lab conditions with ideal layup and best practice there is often quite a large scatter from test coupons.

[Ad Hoc]

"...even under lab conditions with ideal layup and best practice there is often quite a large scatter from test coupons..

Exactly and not to mention that FEA is also significantly affected by the mesh size and the element choice too. So when using FEA, especially with composites, a database of "reality fudge factors" is required to be taken into account; the more coupon tests the better. Simply because what is shown on the 'design' drawing, is very rarely achieved when actually made by the laminator, in terms of 'as designed strength'.

[Paul B]

This thread should be about discussing rudders, discuss the strengths and weaknesses, the hydrodynamics of the foil shape and plan-form, the structural reliability, materials and likely modes of failure.

Er, no.

This is the topic of the discussion:

Hi everybody, I studied 6 naca profile with xfoil: 0008, 0010, 0012, 0014, 0016, 0018 at various Re (from 100000 at 1000000). What do you think should be best choice for a fast sailboat? Thanks.


Your psychological issues prevent you from seeing that not every discussion is about your crusade to save us all from ourselves.

After reading the things you write it is no wonder you don't have a full order book.

[booster]

Hi!
Well, the discussion starts to resemble that canting-keel issue:

Er, no.

This is the topic of the discussion:




Your psychological issues prevent you from seeing that not every discussion is about your crusade to save us all from ourselves.

After reading the things you write it is no wonder you don't have a full order book.

On my 2.4mr (4 knots max speed) I have a NACA manyzeros12. I tested a manyzeros15 and the boat got easier to hanldle in gusts, but the general feeling was negative. The plane-profile is elliptic, which is not so sensitive to stall-out. On the other hand, it introduce vertical flow. I tested a square plane-profile that stall-out more easily, but the vertical flow is minimized. By placing the rudder out-balanced, I got the same good non-stall-out feel as on the elliptic plane-profile. I seems that the out-balanced rudder place the rudder in relatively more non-turbulent flow behind the keel. Perhaps it compensates for the stall-out sensetivity.
Regards,
Booster

[booster]

MikeJohns and Ad Hoc!
Yes, matrix aging is difficult to take into account. Mesh-density dependence is avoided by adaptive meshing or the old-fashioned patch-test. Several fracture models are mesh-density in-sensitive, like the Hillerborg fracture model which is mesh-density independent. The softening region of the stress-deformation relationship must be taken into account, not the stess-strain relationship. The coupled fracture modes can roughly be taken into account by old fashioned expressions similar to combined tension-, buckling- and shear. More sophisticated methods exist as well.
Regards,
Booster

[Ad Hoc]

Booster
you need to be careful. All 'decent' FE codes must pass the patch test, at the very least you can do this yourself to ascertain the limits of the elements. But the mesh density I'm referring to, is not how the element 'works', I'm referring to how the whole model is working to produce a realistic result. Having a mesh density from 1 to say 5, 50, 100, and then up to 1000 elements in a corner region, for example, doesn't mean with increasing density from 1 through to 1000 the accuracy will improve the result, quite the contrary in fact!. The model doesn't know intuitively that it is working correctly. It needs real world input to validate the model and the mesh and boundary conditions.

Many papers have established that a mesh density of around 1/2t (t=thickness of 'material') provides acceptable results when doing detailed analysis for hot-spot stress, for example.

[booster]

Ad Hoc et al!

Yes, absence of radii in the CAD makes use hot-spot stresses even more important. Sometimes one finds engineer rookies using the p-method on very coarse meshes. A lot of red in the results! However, Ad Hoc, I presume you were talking about the h-method. Some regulations like DNV specify methods for this. Often the traditional tangent method, using the path towards the singularity, can be used. Maximum principal stresses are plotted vs. the path. The traditional 20% criteria can be used. As always the extrapolation of the results from the integration points to the nodes can be confusing. Here, the new C3D10I-element of ABAQUS is an important improvement. The analysis of carbon-fiber rudders is important. How doesn’t remember the rudder failure of the Tony Castro design Justine. If some FE-calculations had been performed the failure could have been avoided. Justine was happy to get a new rudder from a bigger boat having the same rudder-stock diameter. The managed to win before a Ron Holland design (Vento perhaps was her name), impressive. The event started a trend towards rudders with bigger area. In some sense
keel area was transferred to the rudder. Especially the French designers were adopting this.

Regards,
Booster

[Ad Hoc]

booster

"..If some FE-calculations had been performed the failure could have been avoided..." I'm not overly familiar with the history of this vessel. However, i would not make such a bold statement, as this implies a total reliance upon computer software to problem solve rather than a good solid education and training.

My calculator, which i might add is 30 years old, works a treat, has more computing power in it than Apollo 13 had. It wasn't technology per se that got men to the moon...!!!

Slight digression and off topic, sorry.

[booster]

Ad hoc et al!
Yes, good engineering knowledge is a must. Regrading Castro's Justine:

www.freebase.com/view/en/tony_castro

"Tony Castro, is based on the South Coast of England in Southampton and has been designing sailing and motor yachts for 25 years, with about 6,000 boats launched to date that include custom boats and One-design production models. After leaving Ron Holland’s design team in 1980, Castro's first significant racing yacht design was Justine, the first and only yacht to win the One Ton Cup with five straight firsts."

Regards,
Booster

[booster]

Hi!
How can a Naca and Wortmann-profile be combined? I found an article on the site below. It is in Swedish so I will try to translate (but first in Swedish!). The rudder- and keeldesign of the Omega42 is discussed by Peter Norlin on the site:

http://www.omega42.org/Historien%20om%20Omega%20422.asp

"Rodrets profil kände vi sedan tidigare. Det är en konventionell NACA 0012. Kölprofilen har däremot varit ett frågetecken som Peter nu rätade ut då han inför sitt framträdande sökt i sina arkivgömmor och till slut hittat hemligheten om Omega 42:ans kölprofil - en unik N-profil:

"Det är en kombination av en NACA 64010 (Anm.: NACA=National Advisory Committee for Aeronautics, en federal amerikansk myndighet som främst under 30- och 40-talet utförde forskning om effektiva vingprofiler) och en Wortmann (Anm.: Professor F.X. Wortmann var under många år chef för tyska Institue of Aerodynamics and Gasdynamics i Stuttgart och är världsberömd för sina fina vingprofiler för främst segelflyg). Det är NACA 64010 i början och från halva profilen bakåt är den gradvis övergående i en Wortmann. Jag lade dom här två på varann och så gjorde jag en egen profil, en N-profil. Den är lite fylligare akter om där den är som bredast - den går inte in lika snabbt, och är alltså mer lik de här laminärprofilerna som man faktiskt använder idag. Vi valde en sådan profil för att vi skulle ha så mycket volym som möjligt. Men det rör sig inte mer än om några millimeter på vardera sidan, så jag tror att ytfelet kompenserar alltihopa...

Procentuellt är den (Anm.: i bredd av kordan) 9,33 procent upptill och 12,2 procent nertill, 10 cm från botten, vid raka generatriser. Om man säger att det är en kombination mellan en NACA 64010 och en Wortmann 10 procent så hamnar vi rätt. Jag har tabellen på den, men det är svårt att hitta det exakta ursprunget. Det rör sig hur som helst bara om millimeterskillnader mellan de två. Lite tokigt var det kanske att vilja göra något eget, men resultatet är helt OK."

Translation:

"The rudder design is well known. It is a conventional NACA 0012. The keel-profile, on the other hand, has for long been a questionmark that Norlin now straightens out. Deep in his archives Norlin found the drawing of the keel-profile of the Omega42. It is a unike N-profile.

It is a combination of a NACA 64010 ( NACA=National Advisory Committee for Aeronautics, a federal American authority that mostly during the 30- and 40-performed research on efficient wingprofiles) and a Wortmann ( Professor F.X. Wortmann was under several years manager of the German Institue of Aerodynamics and Gasdynamics in Stuttgart and World-famous for his fine wingprofiles for mainly airoplanes). The keel of Omega42 is a NACA 64010 in the front and from half the profile and backwards it is a gradually transformed into a Wortmann. I put these to profiles on top of each other and made a profile of my own: an N-profile. It is little more volymnous after its maximum beam. It does not go that fast and is more similar to the laminar-profiles that one in fact uses today. We choose this kind of profile to get as much volume as possible. But it doesn't differ more than some millimeters on each side. Procentually is it (beam divided by corda) 9,33 percent in the upper part and 12,2 percent in the lower part, 10 cm from the bottom of the keel, using straight generatrices. If one says that is combination of a NACA 64010 and a Wortmann 10 percent, on gets it OK. I have got the table of it but it is hard to find its exact origin. However, it does only differ millimeters between the two profiles. Perhaps it was a little bit strange to do a profile of your own, but it works OK."

Regards,
Booster

[Petros]

Interesting thread, but no one has answered the question.

Does any one have a foil section they would recommend for a rudder?

My inexperienced preference would be the 12 percent NACA profile, and I would not go too high an aspect ratio.

The AR issue was brought up early, and I have often wondered why they would use such high aspect ratio rudders. They stall too easy and are vulnerable to damage. I have had bad experiences with stalled rudders. It seems basic and stupid simple to me, especially on a cruiser, that they would put such a high aspect ratio rudder on a sail boat. The difference in rudder drag can not be that large compared to the keel and hull. And in rough seas, the stall resistance of the lower AR seems to be would be an advantage.

[fng]

I used naca 64-012 for both the keel and rudder on my last yacht with fantastic helm from 8 - 19 knots, My mates sportboat has an eppler 836 rudder which is also great - the data shows the eppler to be a more forgiving section and more lift. I've sailed on boats with 0012 rudders and they truely are a barn door in comparison