Need help with Rudder Designs

I'm trying to figure out how to solve for the Doefficient of Lift of an inboard rudder. All of the formulas and macros I have are for Aircraft wings, and therefore don't take into account the effect the hull has on the rudder's performance. Ideas?

 

A small gap between the top of the rudder and the hull surface should reduce distortional forces on rudder, depending on laminar flow characteristics of the particular hull.

 

Typically I install the rudder with about 1/2" between the top of the rudder and the hull.

That being said, I was reading through David Gerr's Boat Mechanical Systems Handbook and he seems to suggest that the interference of the hull increases the efficiency of the rudder and can dramatically affect the CL calulations, is that the case or does the gap nullify the effect?

 

When in doubt, I've always used the paper
The performance of High-Speed Rudders in a Cavitating Environment
by
Gregory & Dobay. It is a bit old, 1973, but I've used it for many many boats for many many years. Always does the job...simple hand calc's too.

 

Excellent Ad Hoc. I'll look for it.

 

From the constructive point of view, a root gap is almost unavoidable.
But from the hydrodynamic point of view, the root gap is detrimental to both lift and drag characteristics of a rudder.
It puts in direct communication pressure and suction sides of the foil at the root, allowing the waterflow to leak and create vorticity (and hence to waste energy and increase drag) and to diminish the pressure difference between the two sides (and hence to decrease lift).
In other words, it diminishes the effective aspect-ratio of the rudder - with all the related and well-known consequences to rudder lift and drag curves.
Just to give you a numerical example:
You said that you have 1/2" on your rudder? If your rudder's mean chord is (for example) 25", the effective aspect-ratio will be some 10% less than the aspect-ratio of a rudder with no gap. It means more induced drag and less lift for a given deflection angle.
The positive side of the story is that your rudder will have a delayed stall, which can also be a desireable feature.
So if you cannot avoid to have a root gap you should try to keep it to a minimum necessary. If you need to minimize the loads acting on the rudder stock, or if you need to have a more stall-resistant rudder, then you have other more efficient ways of reaching the goal, like changing the rudder area, shape, foil sections etc.

I have not read that book so I don't know what was Mr. Gerr referring to in particular, but I'll tell you how I see it.
The hull "interference" acts in two ways, basically:
1) the hull boundary layer slows down the flow around the rudder root
2) the hull volume displaces and distorts the water flow around it, accellerating it (in the region outside of the boundary layer) to speeds which are somewhat higher than the free-flow velocity.
Knowing that, a higher inflow velocity to the rudder means that you can have the same lift force with a smaller angle of attack. A smaller angle of attack means less induced drag. That's why the efficiency increases. Of course, this influence will greatly depend on where the rudder is placed below the hull.
The effect of the gap will be as seen above.

 

Thank you, daiquiri. That was very enlightening.

 

Ok, so a rudder really does have a lot in common with a wing (the cross-sections can be quite different, so I wasn't sure) . Makes sense.

The next question is what program is out there that would allow me to test different rudder designs performance?

 

No, Daiquiri - i believe that Gerr referred to the hull acting like a top-plate to avoid spillover and vortex and increase efficiency. At least the way I remember...
Why aren't rudders, as a matter of course, designed to be as close to the hull (even touching) as possible?

 

Gerr referred to the hull as having a "mirroring effect" on the rudder, roughly doubling the effective aspect ratio.

This would be akin to a winglet on an aircraft. It reduces tip-generated vortices and thus increasing efficiency (reduced drag, etc) without increasing span.

The only reason I have for wanting a separation between the hull and rudder is friction and wear. After reading the responses here I see no reason to intentionally design a meaningful gap between the hull surface and the rudder.

 

Apart some Navier-Stokes solver, you mean?
Good question. Please let me know when you get the answer.
Just dont forget to always put the word "validated" before the word "program".

I think that few good books with foil and rudder test data, together with the aeronautical calculation methods (strip theory, lifting-line theory, slender body theory - depending on the type of rudder you're considering) are the way to go if you don't have a reliable and validated CFD software at hand.

 

Like I said - I haven't read that book, so if he reffered to what you said then it falls into the case of root gap, as described before.
But I remember that Dave Gerr was a member of this forum once, if he still is then maybe he might notice this discussion and reply himself...?

 

I mean anything, and yes, a vlidated program is what I am wanting. i currently have access to RhinoMarine and COSMOSFlowWorks. I really don't know that much about either (I am completely self-taught ).

@ AdHoc: Tried finding the paper you referred to but was unable. However, I did come across a book titled "Marine Rudders and Controls Surfaces" that looks like it holds promise.

 

pamarine
I would scan in my copy for you, but it is an old photocopy of a photocopy, scanning it would make it even less readable. It was originally published in SNAMEs Marine technology. May be worth searching their database, or contacting them directly. Have you also treid your local library, see if they can get a copy for you?

 

So far I've just used google.

I'll have to check the local library this weekend.