rudder alignment?

I am inexperienced and ignorant...so please forgive any incorrect nomenclature or "clearly self evident" facts.

I am building a rudder for a 20ft sailing canoe. I am using a NACA0010 foil.
What are the pro/cons of a swept back rudder ie 20degrees as compared to a vertical postioned rudder (parallel to stern). I recognize a few characteristics that will be different...a) that the rudder swept back will present an effectively different foil shape to the water flow (ie stretched longer)....b) that the swept back rudder will produce greater force with the equivilent degree of turning (longer lever etc)......more sensitivity? But what other benefits/detriments can be expected (taking for granted that all other factors would be the same ie same surface area)
Thanks
Steven

 

Sweep will affect the span-wise lift distribution, and consequently the tip-vortex. Angle of sweep needs to be developed in conjunction with the taper- ratio and general plan-form. You also have to consider the distance between axis of the hinge to center of effort for weather-helm.

Read up on the subject thoroughly or consult a naval architect.

PS. keep whatever foil-section you use paralell to waterline.

Yokebutt.

 

rudder sweep

The biggest potential problem is that sweep back will cause you to have to exert more force to hold and move the tiller.
In my opinion, a vertical rudder is always better because it keeps the center of the rudder area nearer the hinge point.
In fact,among the best solutions is a rudder designed with a small amount of area forward of the hinge point-say 15-19% of the total rudder area.This is a "balanced rudder"-but not too balanced: it still allows good"feel" to the tiller without unnecessary fatigue . This will work with a kick up type rudder but not with one where the blade rises vertically.
Good rudder discussions in Steve Killings book "Yacht Design Explained" which should be retitled Sailboat Design Explained. Also Pierre Gutelles book "Design of Sailing Yachts".Both have many diagrams and reference small sailboats. Neither book assumes too much of the reader and presents the material in a clear and consise manner.

 

Lorsail. Have you read a old British designers book " High Speed Small Craft " by Peter Ducane? It details about Balancing. Then describes tear drop versus thin streamline rudder airfoil. Drawings are provided. Also covered is how deep down into the water a rudder should be to generate lean in into a turn. Shapes and areas are also discussed. Are all these areas important in a human efficient boat?

 

rudder books

No, I haven't read it but it's on my list now-thanks!

 

Ditto on what Doug said but there are a lot of factors. If the designer intended for the rudder to provide some lateral resistance, then some balance area will certainly be needed to offset the force.

Then the size of the boat is a factor. Larger boats (generally) take more steering effort and will need more balance area forward of the axis. On small boats I like to "feel" the rudder and this means more aft sweep. Also helps in "sculling" around a mark or tacking in very light air. Of course, we would never do that.

A lot of fast multihulls and sportboats use a vertical rudder that slides up and down in a case (variable immersed area) which allows no balance area at all.

As an aside, there are a few boats with balanced rudders and a propeller just in front of the rudder. Under power, these boats are very difficult to steer because the propwash hitting the part of the rudder forward of the axis makes the rudder jerk around in your hand. Very tiring. One solution is to remove part of the forward area from the upper part of the rudder and add it near the bottom, keeping the total "balance" area the same. This has cured the hunting problem on several boats.

 

Tom,
Please allow me to offer you a minor correction, you can indeed balance a cassette rudder.

Last year we built an emergency rudder for an offshore racer. We wanted to use a cassette system to make deployment possible in a seaway. We also didn't want an unwieldy tiller half the length of the boat.

So, what we did was to incline the hinge-pin relative to the rudder. That way, the axis of the hinge passed the rudders center of effort at the right distance to produce a nice amount of weather-helm with a tiller of a comfortable length.

Yokebutt.

 

cassette rudder

Great thinking; great solution!

 

Like most of "my" great solutions, I stole it, that one came off of an F-25c.

Yokebutt.

 

As my wife would say, Yokebutt, "I stand approved". That is a neat solution. Since I have never dealt directly with a cassette rudder, or even thought much about one, I jumped to a false conclusion.