Design Question

[catamaran29]

This is my first time, so be gentle.

Is there a "rule of thumb" regarding what percentage of total lateral surface should be rudder. Or, is that the wrong approach for sizing the ruddder.

I'm messing with a PROA design. According to formulas provided on sites around the net, I have ample Lateral Surface Area for the sail area planned. Now I'm trying to solve the rudder issue. I need to know how big it should be. Then, I'm going to work on how I plan to solve the "shunting" issue.

Hope someone out there has an idea.

Thanks in advance, and have a good'n

[Eric Sponberg]

One decent rule of thumb is that the total rudder area should be about 5% of the Lwl x draft (which is the underwater profile area). Generally, you don't want to be less than this, and you don't need to be much more than this. Other factors come into play, for example, you want to make sure the rudder is deep enough so that it stays in the water when the boat pitches. Others may have other guidelines and concerns.

Eric

[SailDesign]

I have always used 1% to 1.5% of total sail area, depending on aspect ratio, proposed use, etc.
If twin rudders, make each rudder 55% of what a single rudder would have been (with the proviso that the leeward rudder should be totally submerged at 10 degrees or so.

Steve

[terhohalme]

Do you have a centerboard too? If not, you need bigger rudders for your proa. At least double the previous recommendations.

Terho

[SuperPiper]

OK, so I used the 5% rule and calculated that my rudder may be on the large size. Perhaps 2x too large.

What are the classic symptoms of a too-large rudder?
What are the classic symptoms of a too-small rudder?

On my stout little pocket-cruiser, it is very easy to initiate a turn but it takes some significant force on the tiller to get the boat to straighten again.

[Skippy]

too-large rudder? extra drag. The wetted surface will slow the boat down.
too-small rudder? hard to turn. That may? also mean extra drag when you're going in a straight line, since the rudder will have to be turned farther to compensate for the boat's slight weather helm.

[SuperPiper]

Extra drag? How can I tell? I'm not sailing by polars with a vpp running on my laptop. You will need to tell me how the tiller will feel or how the boat will respond.

Is the tiller heavy? Is the tiller light?
Is it easier to round up or easier to bear off?
Does the boat point better or worse?
Is the boat more stable while reaching? While beating? While running?

If I chose to reduce the area of the rudder, I would likely leave the length the same (the distance down into the water) and I would shorten the cord. However, I would not consider reducing the thickness of the slab. Therefore, the rudder would become more fat and round. I would be afraid that this may actually increase the drag. . .

[Ssor]

Catamarin29, Check out this site http://www.wingo.com/proa/flyproa.html
They don't obviously use a rudder.

[Skippy]

They used a steering oar.

Originally Posted by SuperPiper

Extra drag? How can I tell?
If I chose to reduce the area of the rudder, I would likely leave the length the same (the distance down into the water) and I would shorten the cord. However, I would not consider reducing the thickness of the slab.

The boat just goes slower. I suppose you could measure the force on the pintels holding the rudder, but you wouldn't feel that on the tiller.
You probably want to keep the aspect ratio (depth/chord) and thickness/chord at least approximately constant. At least nothing too extreme.