# How is the size of a rudder determined?

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

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### PetrosSenior Member

Tansl,

Interesting formula. is that formula the same for powered displacement boats as it would be fore cursing sailboats? It seems for very small boats (under 10 meters) it would make the rudder less than half the size of typical "rule of thumb" for sailboats (2 to 3 percent of sail area).

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### JSLSenior Member

Most books (Skene's, Gerr, etc) on boat design will give the ratio of rudder area vs lateral plane area. High speed power about 2%, sail about 8-10%. Others.... in between

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### TANSLSenior Member

Now you have a little more limited scope of your fórmuila, thanks. I have one doubt, how it relates the underwater profile area with the Froude number?
Now, for a boat of Fn = 0.5 and L = 20 m would be a speed of 7 knots. A rudder 2% of the area underwater profile, really be enough for so low speed maneuvering?.
Petros, you're right, as I stated in my previous post, this formula is valid for ships that are far from small boats and also comes to power boats, not sail.
As we see, and very rightly points daiquiri and JSL, you can not generalize.

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### daiquiriEngineering and Design

Actually,
Fn = V / sqrt( g L )​
a consistent unit system should be used (preferably SI).

So, Fn = 0.5 equals to 7 m/s, or 13.6 kts.

Cheers

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### TANSLSenior Member

You're right, daiquiri, I made a very stupid mistake, unbecoming of someone who wants to be as rigorous as I pretend. I must admit

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### rxcompositeSenior Member

If it is Fn dependent, is there a relation with rudder aspect ratio? Say a rectangular (horizontal) for low Fn number and a tapered one for high speed with a combination of both for all in between.

7. ### Jammer SixPrevious Member

This entire "answer" reminds me of why attorneys call Latin "billing language".

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### jehardimanSenior Member

It is the truthful answer. I've always said that any fool can design and build a ship or boat, but it takes a confident fool to convince someone to give him money to build one, and an educated fool to keep himself out of court by making it work. Naval Architecture is one of the few cases where it is better to understand what you don't know rather than just copy because there are no absolute answers. Every decision is a tradeoff.

9. ### Jammer SixPrevious Member

Speaking as a retired general contractor, it takes a Master to give an explanation to anyone who asks.

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### jehardimanSenior Member

Speaking as a Professional Engineer that does his own general contracting, house and building construction is dirt simple compared to building a boat or ship.

11. ### Jammer SixPrevious Member

I haven't been talking about building a boat or a ship.

That's the point you continue to miss.

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### PetrosSenior Member

Actually Jehardiman is using industry language not really intended for non-engineer or NA types.

His answer basically means the rudder has to be large enough to bring the heading back to the intended course after any perturbation that takes it off the intended heading. Same idea in an aircraft (but in all three axis of motion) or even driving a car down the highway.

For example, when you drive your car down the road, and presuming it is stable, any bumps or gusts of wind, etc. that pulls it out of its intended path (a straight line), it will come back to that intended heading without any input from the driver. If it was neutrally stable the driver would have to give input to the steering to bring it back on course. This is very tiring and requires constant attention. Unstable means it keeps veering off course and you are always having to give steering input to keep it on the road. i have sailed many small sailboats that behave this way, marginally unstable that requires constant tiller input to keep it on course. Not a desirable trait in a large powered vessel.

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### Ad HocNaval Architect

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### Sunny ParabJunior Member

Which type of rudder is more efficient and has better manoevuring of the ship??? Please compare the differences

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### daiquiriEngineering and Design

Sounds like a question for a university exam.
In that case, you should do some bibliographic research on your own.

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