# Designing a Rudder

Discussion in 'Boat Design' started by UoNMEng, Mar 4, 2012.

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### UoNMEngNew Member

Dear all,

My group and I are working on a human powered speed boat for a student yearly project.

I am trying to find a simplified set of equations from a reliable source either a journal or article, for referencing terms, for the design of a rudder placed at the stern.

The boat we are designing its a hydrofoiling cayak, but we are in need for directional control through the use of a rudder from 0 to 5m/s.

I plan on using a NACA0012(S) with the bearing joint and centre of pressure at quarter chord length for stability.

My plan of action was to calculate the force generated for a selected angle of attack using the Lift Coefficient found through JavaFoil/Xfoil for the foil shape.

F=0.5*p*A*V^2*Cl

This force will create a torque/moment around the central axis of the 5.3m cayak which should (hopefully) be the centre of gravity at 2.65m from the stern.

My question is, how do you find out how much torque/moment force is required? I understand it basically needs to be larger than the rotational drag incurred from the hull during movement and I read somewhere on this forum that this was 5% of hull side profile but I can not use a forum for referencing.

Does anyone know a reliable source for these approximations? or a better solution using formulae?

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### daiquiriEngineering and Design

Hi and welcome to the forum.

First, the pivot point of the rudder should be placed ahead of the 1/4 chord, because you want to have a positive force feedback from the rudder and because the center of pressure will move forward as the aspect ratio (AR) gets smaller than 5-6 (up to 15% of chord L.E., for AR approaching 2). The effective AR of your rudder will be in that (low) range because I understand that you plan to use an outboard rudder, so the water surface will shorten the effective span.

I would suggest you to read the article on rudder design written by David Vacanti for Professional Boat Builder magazine ( http://www.proboat.com/ ) few years ago. I would also warmly recommend downloading and reading through the following document by A.F. Molland from the University of Southampton: http://eprints.soton.ac.uk/43260/ titled "Rudder design data for small craft". You will find a lots of useful data and corrective coefficients for various types of rudders inside that paper.

Cheers

Last edited: Mar 5, 2012
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### UoNMEngNew Member

Hey,

Thanks for the document, exactly what I was looking for. Much appreciated

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I addition to the excellent advice from D, i use this paper frequently for all types of boats.

"The performance of high-speed rudders in a cavitating environment", by Gregory D.L. & Dobay G.F., SNAME publication 1973.

Don't be put off by the "cavitating" part in the title. I've used it for slow speed SWATHs before too, works very well, simple too.

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### SamSamSenior Member

This is purely speculative thinking powered by the fact that a human powered vehicle needs all the help it can get, and that this vehicle is to be an experimental mental exercise, but for all you who might know better, would there be any advantage to having a bow mounted rudder?

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### tom28571Senior Member

The fact that this is a hydrofoiling kayak means that it will operate both in the water and above the water surface. Is that so? A rudder would then have varying amounts of immersion unless it is mounted low on a foil strut. Have you considered an air rudder that would always have the same insertion? Of course, a hydrofoil strut can be made steerable either at the bow or stern and avoid an extra foil altogether. That appears to be the lowest drag option.

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### daiquiriEngineering and Design

Another option:
You can diminish the rudder area (and drag) by installing either spoilers (http://en.wikipedia.org/wiki/Spoiler_(aeronautics)) or hydrodynamic brakes (http://en.wikipedia.org/wiki/Air_brake_(aircraft)) on the foils. They would be retracted inside the foil shape during the straight run, thus minimizing the drag, and would pop up only when rudder is activated, helping it's action (left spoiler/brake would activate for a left turn, and vice versa).
The negative side is that they have a weight cost, which has to be carefully evaluated.
Cheers

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### SamSamSenior Member

Or achieving steerage by redirecting the thrust of the propeller, like an outboard does.

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### rwatsonSenior Member

#### Attached Files:

• ###### humpback_whale.jpg
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Views:
777
Last edited: Mar 6, 2012
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### Leo LazauskasSenior Member

Thanks RW.

I proposed investigation of tubercules as a mech. eng. student project at
Adelaide Uni now that we have a decent wind tunnel. One keen student made a PhD out of it!
Here's a link to one of the papers that came out of the study.
http://www.mech.unimelb.edu.au/people/staffresearch/AFMS site/17/329_Paper.pdf

Leo.

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### rwatsonSenior Member

"The results suggest that the tubercles may function in a similar way to wing-fences in that they reduce the extent of the span-wise flow, hence delaying tip stall and reducing the strenght of the wing-tip vortices"

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### daiquiriEngineering and Design

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### Leo LazauskasSenior Member

Yes, there many "devices" in nature that serve dual purposes.

That's why it's important to use sound theory and to conduct good
experiments with fixed, inflexible wings to asess how well the natural
systems translate into boating applications. Hansen's PhD thesis
and other studies at Adelaide Uni show the complicated flow-field in the
humps and hollows on the wings. (I've no doubt that there are many other
studies at other universities too.)

14. ### FrostyPrevious Member

Have you noticed that the Airbus 380 has turbercules around the trailing edge of its engine cover.

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### rwatsonSenior Member

No - have you got a picture ?

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