# Rudder Size

Discussion in 'Boat Design' started by hillmaster, Feb 20, 2009.

1. Joined: Sep 2008
Posts: 6
Likes: 0, Points: 0, Legacy Rep: 10
Location: malta

### hillmasterJunior Member

Hi

i would like some help regarding the size of the rudder! what is the size that a rudder should have?
i assume that the size of the ruuder should be proportional to lift that should be generated in order to turn the boat! but how much lift should be generated?

thank you
Hillmaster

2. Joined: May 2004
Posts: 5,373
Likes: 252, Points: 93, Legacy Rep: 3380
Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

3. Joined: Oct 2008
Posts: 6,973
Likes: 917, Points: 113, Legacy Rep: 2488
Location: Japan

Calculate the lateral under water area, and take between 1~2% of this, and that is rough guide for a rudder size.

4. Joined: May 2008
Posts: 127
Likes: 7, Points: 28, Legacy Rep: 89
Location: Carentan (France)

### SViauNaval engineer / Designer

Another question, maybe stupid.... Sailing boat, or motor boat ?

5. Joined: Sep 2008
Posts: 435
Likes: 18, Points: 0, Legacy Rep: 318
Location: French Guyana

### TcubedBoat Designer

A number i use (i do not know if anyone else does this) is the coefficient of rudder volume ;

Rudder surface * distance between center of lateral resistance to center of area of rudder / Boat displacement.

This gives a number to compare boats by. Two things- the coeff for motorboats is always much smaller than for sailboats and the coefficient decreases for boats size. Look at a twenty foot sailboat rudder and then look at a tea clipper's rudder for example. The clipper has a very small rudder compared to its size. So you must compare coeffs between boats of similar size.

6. Joined: May 2008
Posts: 127
Likes: 7, Points: 28, Legacy Rep: 89
Location: Carentan (France)

### SViauNaval engineer / Designer

Good system to evaluate the power of steering developped for a rudder.
This is more or less the same idea behind balancing a sailing boat with sail surface vs anti-drift surface and center of actions.

7. Joined: Apr 2007
Posts: 2,164
Likes: 53, Points: 48, Legacy Rep: 575
Location: Florida

### mydauphinSenior Member

Help with Rudder Identification

I need to understand this rudder. The surface area numbers are confusing because some are in Inches and other is in feet. What percentage of Balance vs main area. What is this style of rudder called. I am trying to sell it and would like to know what size vessel it should fit. I have two and therefore a twin rudder vessel.

#### Attached Files:

• ###### rudder.jpg
File size:
88.7 KB
Views:
5,241
Last edited: Feb 26, 2009
8. Joined: May 2004
Posts: 5,373
Likes: 252, Points: 93, Legacy Rep: 3380
Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

Kudos for this, but I hope you're using volume units for displacement. In that case it becomes a pure number and can be used without corrective coefficients when switching between various unit systems.

It is very similar to the Tail Volume Coefficient used in aircraft design, defined as:
Vh = (Sh x Lh) / (Sw x MAC)
where:
Sh = horizontal tail area
Lh = distance from tail's aerodynamic center to the airplane's CG
Sw = wing area
MAC = the mean aerodynamic chord of the wing.​
Tail Volume Coefficient is a quick and efficient way to express the airplane tail effectiveness, because it relates the factors which cause the need for a tail (wing forces and moments) to the factors which influence the tail action (tail surface and position relative to the CG).

Something similar could probably be concieved for boat design, bearing in mind the difference between sailboats' and powerboats' rudder usage.

9. Joined: Sep 2008
Posts: 435
Likes: 18, Points: 0, Legacy Rep: 318
Location: French Guyana

### TcubedBoat Designer

Of course! that is the whole point , a dimensionless number.

I don't know if you've noticed but i always use metric or S.I. Feets and inches belong in the middle ages..... So displacement in liters is the same thing (almost, slight difference for salt water) as boat mass in kg. And then i would use decimeters for the rudder term as well, obviously.

And you guessed exactly right what made me create that figure. (i design light airplanes as well)

10. Joined: Feb 2006
Posts: 93
Likes: 4, Points: 8, Legacy Rep: 19
Location: Bulgaria

### dimitarpJunior Member

11. Joined: May 2004
Posts: 5,373
Likes: 252, Points: 93, Legacy Rep: 3380
Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

12. Joined: Apr 2007
Posts: 127
Likes: 6, Points: 0, Legacy Rep: 133
Location: Istanbul/Turkey

### ÇemberciSenior Member

An example is finding rudder size.

oktay çemberci
istanbul/turkey

#### Attached Files:

• ###### IMG.PDF
File size:
1,006.7 KB
Views:
1,974
13. Joined: Apr 2007
Posts: 2,164
Likes: 53, Points: 48, Legacy Rep: 575
Location: Florida

### mydauphinSenior Member

Guys, I know you geniuses can figure out this simple question. See the rudder in my previous post in this thread. What is rudder surface area? What is percentage of area in front of axis for balance and behind for main area? Also for what size boat would this rudder work? It is for planning boat like a sportfisherman but is it for 30 footer, 40 footer, 50 footer, etc...?

Thanks again, and you geniuses dont let me down.

14. Joined: Sep 2008
Posts: 435
Likes: 18, Points: 0, Legacy Rep: 318
Location: French Guyana

### TcubedBoat Designer

Your rudder surface area is simply (part of the surface in sq inches)/144 + (part of surface in sq feet)

somin like 3 1/2 sq ft..

15. Joined: May 2004
Posts: 5,373
Likes: 252, Points: 93, Legacy Rep: 3380
Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

I don't really get the meaning of that "you genuises", but since I don't like thinking negatively I'll take it in a most affable sense... We're probably short of geniuses here, they've been all taken by the Microsoft and are designing blue screens now.

As about your rudder over there, this is what I can tell you:
- the total area is 3.92 sq. feet
- the compensation area (area in front of the stock) is 0.98 sq. feet.
- the compensation area is therefore 17% of the total rudder area, which makes it quite suitable for a fast cruisers, semidisplacement or planing type. It's probably less suitable for slow cruisers, which may require as much as 25% of compensation area.
- an old rule of thumb says that, for twin rudders, the total rudder area should be 3-4% of boat's lateral plane area (which is basicaly the lateral projection of the underwater body). So the rudder you have is suitable for a boat with a lateral plane area of about 100-130 sq. feet (in a single rudder configuration) or about double that value (200-260 sq.feet) for twin rudder installation.
Hope that will help.

A (kitchen) Genius

Last edited: Mar 8, 2009
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.