Rudder Design

Hello everyone, this is Julio from Argentina.

I currently have a 30ft cruiser/racer boat. The class is Fayd 30. Upwind speed 6.5/7 knots and downwind generally 8/10 knots (maximum so far was 18 knots).

The problem I have is that the rudder stock diameter is 2.83in aluminium but it should have used 2in steel and the rudder is very thick. The profile is not very good and I plan to replace it.

Rudder thickness will go from 3.54 in now to 1.97 in.

I´m planning to use 0012 to 0014 naca profiles. (12.5% in the upper part and 14% in the lower). Jefa rudders uses the same but they have 14% in the upper part and 12 in the lower.

Is there any advantage on using a thicker profile? I've read naca 00 tests and only 0008 seems to lose lift very soon.

thank you in advance.

regards

 

This would be the design.

 

Welcome to the Forum Julio.

I don't know anything about the finer details of rudder design - however I am thinking that if your boat can sail down wind at 8 - 10 knots happily, and even reach 18 knots without the rudder losing grip / stalling, then it is doing a pretty good job really (?)
If you change the stock diameter then you will need new bushings as well in the stock tube, in addition to a new rudder.
Is it really worth it?

 

Welcome indeed Julio.

Is there anything else wrong with the rudder?
I would not go changing anything.
The overall shape of the rudder is more important than its width.
Is it the rudder the boat was designed to have?

 

Thank you Bajansailor and BlueBell for your responses!

The problem is that the rudder is very inefficient so I think I would benefit from the change and PHRF formula doesn´t take into account this change. If everything is correct I will have more lift and less drag with the new rudder.

If it was for cruising I wouldn't change anything but for racing 0.1/0.2 knot increase means being more competitive.

A friend of mine can do the stock labor so it won't be that expensive.

I´m finishing a keel mod and I´m adding running backstays so the boat will be a different one after all the mods .

 

Really? How inefficient is it?
And, how much more efficient is a new rudder.

You'd gain more benefit by removing weight from the boat.
However, this is your project and to answer your question you're going to need to look at lots of details as to what wing profile to choose.
Others with experience in your boat and differing rudders may be better off to comment.
Best of luck with your modification.

 

Using same rudder surface Area it seems I´ll have 10% more lift and 30% less drag (using naca tables). Are will be reduced 10%.

I don't even have an inboard engine to reduce weight

 

You say the rudder stock is very thick, but relative to what? How wide is the rudder at the top? How far down does the stock extend into the rudder? What is the existing cross section like? Where are you getting your airfoil data? It seems to me that a rudder should be sized and the foil selected by how well it works when the boat is going slowly. If I remember correctly, the data in Theory of Wing Sections only goes as low as a Reynolds number of 3 million, but I think that the Reynolds number on your rudder might be 1/3 of that or less when your boat is going slowly. Not to worry, the NACA 0012 will work down to much lower Reynolds numbers, but the relative virtues of different airfoils may not bet the same.

If the existing rudder foil shape is bad, then you could significantly improve the boat's handling with a better one.

 

Why will the lift be 10% greater with a thinner section?

For the same angle of attack the lift will be essentially the same as long as the flow is attached. The lift is proportional to angle of attack as long as the flow is attached.. And the usual behavior is the angle of attack at which the flow begins to separate is lower with a thinner section. So the maximum lift is higher with a thicker section.
(Note that lift of an airfoil section is scaled with the chord, not the thickness.)

 

I think, by 10 percent greater lift, he means that the foil won't stall until it's developing 10 percent more lift than the old section. Of course, since we don't know what the original section was, we can't judge this for ourselves. That 10 percent increase means a 10 percent increase in control power.

Xfoil seems to think that, at a Reynolds number of 200,000, even a NACA 00016 looks pretty good. Somewhat greater lift at stall than a NACA 0012, but slightly more drag at 0 lift.