Rudder design question

Hi everyone,

I have a quick question about rudders. It seems that rudders that have a little flexibility are slow. Why does that make the sailboat slow? What is the advantage of having such a stiff rudder?

Does anyone know of any papers, articles or books that would document this issue?

Thanks for your time,

Matt

 

Seabird, define "slow". Slow to turn? Have more drag?
It makes addressing the point easier (as there may be a different answer depending on context)
Steve

 

See, that's the thing, I've never seen a rudder with any kind of bend. Would it affect the drag? Would it affect the turning response?

I'm interested in any performance related issues that could be encountered with having certain flexibility in the rudder.

Matt

 

Matt,
I would imagine that too much bend would affect the helm response, but not a _great_ deal. Most rudders deflect somewhat unless skeg-hung (but I'm assuming we're talking dinghies here anyhway?)
The best way to find out (and probably the cheapest as well) is to build two rudders, identical in every respect but flexibility. Go sail, try it out. You need more excuses to sail? ;-P
STeve

 

Any foil that bends without twisting will be barely perceptible, unless the flexing is excessive, but most foils that bend will twist.

Depending on which way the foil twists, you will either increase the angle of attack of the foil or decrease it, more or less. This makes for sloppy control, and could be dangerous in high speed conditions.

For what it is worth, a flexing foil also dissipates energy out of the system, and hence may not be helpful either. You want the energy of the water to turn your boat, not be wasted in bending your foil.

There are trade offs: flexibility has it's advantages. You may want some twist in your foil. Take Steve's advice and experiment on the water.

 

The discussion about rudder flexing so far has been concerned with performance and control, but I wonder if flexing could also be a structural and longevity concern.

JR Watson wrote an article years ago in EpoxyWorks on building a new centerboard for his Searunner trimaran. He had to do this because the constant flexing of the old fiberglass covered, plywood core board had caused the plywood to delaminate due to rolling shear. The fiberglass skins were still intact, but the board was "floppy."

His new board was a long grain cedar core with unidirectional carbon "sparcaps" for bending strength, wrapped with fiberglass cloth for torsional rigidity. Since the elongation to failure for both carbon fiber and long-grain softwoods are about 1%, his new board would presumably last longer.

Other than being able to absorb more energy if you hit something, would there be other reasons for having a "flexible" rudder?

 

Small elongation to failure = high fatigue resistance. :idea:

I can't see any advantage in excessively flexible foils either; (All foils flex, its just a matter of degree).

Nevertheless, perhaps, just as you need some twist up high to spill wind, and due to faster and more streamlined (less turbulent) air, you could do with some twist down low for exactly the same reasons. The water is "faster" the deeper you go, and long high aspect rudders need not contribute to heeling moment as much as they will.

Just a thought.

 

i could see where oscillation would slow the boat.

 

good point Tohbi

 

Dionysis -

I'm not an engineer, but I'm not sure low elongation to failure necessarily = high fatigue resistance.

But matching a core with skin laminates that have similar elongations-to-failure would be optimal from a weight standpoint - your core won't fail well before your skins, and vice-versa. SInce a cedar core and carbon skins have similar ETF, assuming matching fiber orientation, then they are well matched.

Unidirectional E-glass laminate may have almost as a high an ultimate strength as a carbon laminate, based on similar fiber and resin volume, but the E-glass would strain much more before failing. When paired with a wood core, the core would fail well before the E-glass skin. The way around that is to increase the E-glass skin thickness to match the strain of a carbon laminate at the same loading, but then your skins would be way too strong (and heavy...)

 

Neither am I Chris, and your are right, look at wood - same fatigue resistance as carbon, completely different strength and stiffness. Fatigue has more to do with the interatomic bonds, and would be particular to each material.

Yep, wood and carbon are a good combination as far as fatigue is concerned. Just remember that the wood is there primarily to support the carbon against local buckling, not to resist primary loads.

Hmm...you would have to run the numbers, but ud e-glass laminate weighing around the same as ud carbon laminate, is a little less than half as strong in tension and compression, a quarter as stiff and three times less fatigue resistant.

As far as wood is concerned stiffness is a little more than e-glass, and a third less strong.

If you want stiffness and strength, then carbon is the way to go, with some wood for the purposes of reinforcing the carbon locally. This is good ideal fatigue wise.

To gain the same stiffness with e-glass, and minimising the weight penalty, you are better off designing for a larger moment of inertia, ie, going for a bigger section. Strength wise you are ok. You are not doing yourself any favours going heavier, you end up with greater inertias, and hence have to design in more weight to counter, which leads to greater inertias..a bad spiral.

I hope this helps

 

dionysis -

Here's a link for graphlite carbon fiber rod as used by Jim Marske in ultralite sailplane spar caps. It has info on typical strengths achieved for hand layups of carbon fiber and e-glass:

http://www.continuo.com/marske/carbon/carbon.htm

Hope the attached GIF table comes through too. These numbers are supported by other technical literature I have on hand regarding composite construction for homebuilt boats and aircraft. If nothing else, it reinforces the point you really need to run test samples of a laminate if you're striving for minimal weight and minimal safety factors. Stephen was right in recommending build two rudders and test them both! So much depends on a builder's skill with materials, and I'll bet most homebuilt composite laminates probably fall well below theoretical design limits that assume perfect fiber orientation and resin/fiber volumes.

I've taken this off topic, so I'll look at starting a new thread on composites in the Materials forum to see if anyone bites.

 

Thanks for the link Chris. It just goes to show that you have to be careful about quoted laminate properties, just as you say.

 

Foil Design

Hi guys,

Thanks for all the input and comments on the topic of rudder design.

Chris: you were mentioning something about literature on composite construction for homebuilt boats and aircrafts; I would be extremely interested in getting a hold of this literature.

I think I need to explain the context of my situation a little bit for all of you. I am a Mechanical engineering student at McGill University, this semester I have to find a design project that will be worth 3 credits towards my degree (112 credits). I am writing up my project proposal that is due this coming Monday, and I have chosen to investigate foil design for small dinghies.

I am presently doing research for a professor in the domain of Fluid-Structure Interactions and he will be my design project supervisor along with a post-doctorate professional that works for ANSYS in Montreal as well as another professor whose research interests lie in composite materials. Basically I have all the tools at hand to design a CAD model of a rudder or daggerboard, convert to a finite element software (ANSYS), and investigate different shapes and composite materials when applying loads due to fluids.

A stiff board makes the numerical analysis slightly easier since the fluid does not affect the shape of the structure and hence we are looking at a Fluid Dynamics problem only. On the other hand, intruding flexibility into the board creates an interaction between the fluid and the structure and the problem becomes much more complicated but at the same time more interesting. ANSYS is a commercial software and hence it has limitations which will most likely be explored with this project if it goes through.

I feel extremely lucky to be able to combine my passion for sailing with my academic knowledge. I hope this is the first step towards a career that has been a dream for me since my early teens. I’ve been a race coach for Laser 2 sailors for the past 6 summers but know that I’m graduating, I need to find a real job, and hopefully I will be able to remain close to my passions.

Again, I really appreciate all input and comments. I will try to keep you up to date with my work. If any of you have good yacht design references it would be greatly appreciated since I need good documentation to back up my final paper.

Sincerely yours,

SeaBird

 

thanks chris, any website that shows how to build a mast of this graphlite carbon fiber?