# How is the size of a rudder determined?

Discussion in 'Boat Design' started by Sunny Parab, Aug 18, 2014.

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### Sunny ParabJunior Member

How the size of a rudder is determined??? Can anyone help me please

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### daiquiriEngineering and Design

What type of boat?

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### PoidaSenior Member

I was going to say, "measure it," but it wouldn't be funny.

Poida

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### daiquiriEngineering and Design

Still early to say, but I have like a suspect that we might have some fun by the end of this thread...

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On average, take approximately 2% of the underwater profile area.

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### daiquiriEngineering and Design

Well, the exact percentage will depend on what boat we are talking about here, what speeds etc. I would really like to see more info about the boat in question before giving any number.

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### jehardimanSenior Member

It must be large enough that Ndelta dominates the stability derivatives such that N is always negative for delta not equal to zero and positive for delta identically equal to zero for the hull geometry and all speeds selected. See PNA, Vol 3, Chpt IX of the 1989 edition.

Really, your question is far too complex for a simple answer, though as Ad Hoc notes, there are rules of thumb for given vessel size, shapes, and speeds.

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As JEH notes, the stability indices are required for a more "exact" solution. But that requires model testing.

Naval Architecture is not a series of absolutes, it is all about trends. After model testing endless different types of hull forms and range of speeds (not pleasure vessels I may add), the general rule of thumb we have established (a trend) is circa 2%. This value is sometimes + or sometimes - the 2%. That is a judgement call based upon previous data and hull shape.

If the OP looks at GAs in journals/magazines of the same type and size of boat he is looking at, then do the same. Measure their underwater profile area and the area of the rudder, and see what their percentage is....beyond that, he needs testing. That's what we have done in the past when designing a vessel that was new to us, to to get a feel for what seems about right.

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### gonzoSenior Member

There is a huge difference between a harbor tug and a high speed powerboat. As Ad Hoc says, comparing your calculations to similar boats is always a fast way to know if you are within the norm.

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### CloudDiverSenior Member

I have had the same question when considering changing a design from a single deep foil rudder to twin rudders for the purpose of reducing draft with a lifting keel. I haven't given serious thought to the math since I am sure that I won't build such a thing until a few years from now.
I have defaulted on the concept that the total area of the two rudders should at least equal the area of the original single. Of course I know that kind of guess is not necessarily 'sound'. I need to understand the hydrodynamic principals involved when the boat is heeled making the leeward rudder 'less efffective', while the other is closer to perpendicular making it 'more effective'? I also know that there is a general consesus that the hull will manuver better at slower speeds under Aux engine power when a sigle rudder is directly in-line with the prop-wash, but there are plenty of twin rudder designs out there doing just fine.

Not wanting to hijack this thread so no need to post answers to my question, I'm sure I will find other threads related to twin rudders. But I do want to thank the folks who provide answers from a much deeper knowlege base, I have learned a great deal from you so thanks.

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### daiquiriEngineering and Design

Perfectly ok for the type of ships you are designing, AH. But it is a number which cannot be given as a generally-valid rule, as you know better than I do.

For example, a sailboat or a motorsailer with a 2% rudder would get out of control in some situations, and would have a high induced drag in others. Sailboats are normally in the 10% range, which can go up to 30% for high-end over-canvassed racers, like a 49er. Motorsailers are in the 4-5% range, just like motor boats which need a low-speed or reverse-thrust manuverability but don't have bow-thrusters or twin engines for thrust steering.

That's why I have asked for more info about what kind of boat is the OP talking about. But his silence tells me that we might not hear from him again.

Cheers

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### daiquiriEngineering and Design

In the case of a single-prop twin-rudder configuration, the total rudder area should be somewhat increased, to account for the absence of the beneficial prop wash. This increase will generally be in the 20%-25% range, as a rough guide.

For the rest, take a look at this thread: http://www.boatdesign.net/forums/boat-design/when-two-rudders-necessary-50989.html

Cheers

P.S.
The above is valid for motor boats. In case of sailboats, the rudder area is so abundant that no correction to the total rudder area is necessary.

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Didn't say it was. It is applicable for the type of boats we design.

Also, as noted, check simple GA layouts to establish a 'feel' for type %'age, in the absence of model testing.

One has to start somewhere. If it doesn't work, make either bigger or smaller...simple. The actual boat then becomes the "model" for testing

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### TANSLSenior Member

For monohulls up to 24 m in length, no more than 65 tons of displacement and about 12-16 knots, the formula: A = Lwl * D / 30 is more appropriate, in my opinion.
A = area of rudder
Lwl = waterline length
D = draft
Do not know which boats Ad Hoc concerns but must be surely very fast boats.

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