Need help with FEA of a rudder

Split lines can in some cases help the study to make the meshing more reasonable, they act as kinda help lines for the meshing process (esp, if you have some thin walled features somewhere too). Reduces the calculation time on the computer too....

 

You guys know a good reference for SW Simulation Suite/Cosmos?

 

Ehm... You have excluded lateral force from your analysis, or did I misunderstood your description here? If you did, why did you?

 

the force is applied as if the boat was running straight at speed and the wheel was flung hard over.

So, that means the force of the water would be parallel to the longitudinal axis of the boat, but be acting at an angle to the rudder blade, forcing it both back and sideways.

There is a seperate force on the bearing in the tiller arm pulling the rudder back towards center, and the forces on the Rudder stock imparted by the two bearings in the hull

 

This is a bit off topic, but when I try to imagine that scenario I see the boat starting to turn longe before you cane turn the wheel or tiller that far..
Why not calculate for the maximum lift you can get from water before cavitation ?
I realize this doens't help with your FEA question, but maybe it's easier to apply jyst a huge preassure sideways, lift will always be many times higher than drag.

 

Pamarine, I understand your reasoning but I agree with RaggiThor - your scenario sounds pretty unrealistic. You discard inertial forces acting on the hull, which will tend to relieve the stress on your rudder.
But, on the other hand, it also sounds pretty conservative which is not necessarily a bad thing, since it will lead you to an oversized rudder structure.

 

I'm going for conservative, which I was trying to test worst-case. (not sure if I did or not).

Yea, if anyone was dumb enough to throw a rudder hard-over at planing speeds they almost deserve what happens next.

Also, this is kinda a learning experience for me as I'm new to COSMOS. I really want to figure out how to use FloWorks for Hydrodynamics so I don't have to keep exporting to Rhino to check everything.

 

pamarine

Have you tried the cosmos/SW website???

Your loading scenario, as mentioned above is not correct, nor realistic. But if you want conservative answers you'll get that
You may change your mind once you have the result...!!

 

I just found the SW Simulation site that has some info on it.

What would be a realistic loading scenario? I'm a bit confused because all of my formulas are based on full deflection at speed (for rudder area, stock dia and material, bearings, etc)

 

"...What would be a realistic loading scenario?.."

Welcome, your first FE lesson learnt!

FE only does what you tell it. Therefore you need to still require and understanding of what you are modelling, how it should be constrained and what loads to apply and where. Just as with doing a simple hand calculation analysis. It is like me saying can you work out the bending moment of these two beams....er..what beam...er...what size are they, how are they fixed, where are they fixed, what load is applied etc...without knowing more then just the statement of intent, any structural design is somewhat difficult.

Ok..first, think what is actually happening?..you have a boat at 55knots. A rudder is used to turn the boat. So, what is a rudder??..it is a lifting surface. This is used to generate a force to turn the boat. The rudder may be at an inclined angle to the flow, does this mean the force is at this angle?...well, what happens with an aeroplane wing?..is the strength calculation derived from the inclined flow directly onto the surface at that angle, or the circulation and hence the lift on the wing, owing to the inclined flow?

So, image the angle to be 90degree, ie square on...what happens, no lift, just drag..so the flow is creating the force/bending moment, but no lift!. If at 0 degrees, no lift no force. If at say 5 degrees, what happens...lift is generated. The flow of water must go around the surface otherwise there is no lift. If the water just impacted on the surface, there would be a simple transfer of kinetic energy but no lift, ie like hitting a solid wall, just at a slight angle..

So, you need the lift/drag characteristics of your rudder first. You can work out what lift (from the CL at an angle), also the classic 1/4 chord moment on the foil itself. All this will determine your rudder size, shape, and the stock and where the bearing are best located too.

 

Well that last part is what I thought. I'm thinking I need to distance myself from this builder. Against my advice, he has rushed head-long into building a boat without any testing or engineering to speak of other than his own experience and what I'm able to sneak in under his nose.

i.e. I had barely finished the hull when he was off building it. The rudder bearings, Helm, steering ram, prop, shaft, engine and transmission are all selected and installed without any engineering to speak of. Of course the whole time I'm saying "consult a naval architect before starting" but all I get is "It's fine, nothing new here to worry about" etc etc

Anyhoo, back the the analysis question.

Here's how it plays out in my mind. As the rudder is turned, the AoA of the blade is increased, resulting in an increase in lift (lateral force) and drag up to a maximum AoA where the blade would stall and no longer be producing lift. I don't think that stall is an issue with most rudder designs, but as speed increases, Stall can occur at lower AoA so therefore it is theoretically possible. Also, due to the density of the water there is a longitudinal force on the rudder deflecting the blade aft.

The Blade is supported by the bearing inside the hull, which not only provides a surface for the Rudder stock to rotate on but also holds the rudder stock to the deisred angle, resisting the deflections caused by lift and resistance.

The Blade is held at the desired AoA via the tiller arm which in turn is connected to a hydraulic steering ram capable of a maximum 1000psi. The tiller is kept from rotating on the Rudder stock by a 1/4" x 1/4" x 1.5" bronze key. The tiller arm converts the linear force from the steering ram into torsional force on the Rudder Stock. The Pin is subjected to shear loads across the mating faces by the opposing forces from the Tiller and the Rudder. This in turn will cause the Rudder stock to want to twist.

So the rudder assembly is acted upon by Torsional forces and bending force. The Tiller arm and steering system (and of course metal selection in the Rudder and Stock) counter the Torsional forces, and the Bearings (and rudder materials again) counter the Bending forces.

The possible failure points are deformation of the blade, torsional failure of the Shaft, or shear failure at the pin.

Now the next question I have is as the vessel turns, the force from the flow of water along the hull is or is not parallel to the Longitudinal axis of the hull? I am thinking that the lifting and resistive forces are acting at right angles to each other and the resistive force will always be on a line tangetal to the curve of the vessel's path. But this path will be something between the angle of rudder deflection and the CL of the hull due to slip as the boat travels around the curve.

 

Oh dear...sorry to hear.

Well, all you can do is then cover your ***. So, everything you provide has very large caveats, so you take no responsibility for anything other than what you are prepared to put your name against. Once you have done the calculations and if it shows that he is in big trouble, stick to it, do not deviate. If they wish to chnage it, fine let them, you have done your job professionally by telling them where it is wrong and why. It is then up to them to decide what to do with that advice. That is all one can do, and make sure ALL communication is in writing, not verbal. You do nothing unless in writing....it is the only way to protect yourself.

You can lead a horse to water, but you can't make it drink!

 

"..Now the next question I have is as the vessel turns, the force from the flow of water along the hull is or is not parallel to the Longitudinal axis of the hull?.."

Not 100% sure i follow you there.

From a distance, there are in fact two rudders. The rudder, located aft, and the hull itself. The rudder aft is turning the boat, so the flow of water will not be parallel. Imagine looking down from up high and seeing the boat. the direction of travel, when helm is applied, is not straight, so the water flow, from this view is not parallel to the hull.

But locally up close, like your head is against the hull watching the flow, the flow is parallel, owing to the shape of the hull, the water cannot go around it, like it can a rudder, so it flows along the hull.

However if your hull is a true planning hull (at 55knots, fair guess!), the water, locally on the hull, actually becomes almost transverse!

 

lol, yea. Got plenty of experience in the CYA department

So, what am I missing in the analysis posted above? Or a I making it more complex than it needs to be?

 

"...Or a I making it more complex than it needs to be?.."

Yup...massive over kill. (A rudder calc shouldn't take more than about 30mins or 1~2hours if you're not familiar)

Just establish the max lift and quarter chord moment, from the CoE, and the rest is simple 'static analysis'!