Rudder configuration

[AdrienJousset]

Hi everyone,

I would like to have a discussion regarding two different rudder configurations on a 7.9m sailing sport boat. The aim is to develop a one design class that can also race in GP26 class. We are pretty far in the design process and so far are really happy about the design.

Anyway here is my question...

How much a rudder standing behind the transom would affect the performances compare to a fin rudder located under the hull? I am not sure about the name for those two configuration so I hope you will understand me.

I strongly believe located under the hull the rudder will have better performances. However we would like to have a lifting rudder for multiple reasons. Here is an example of solution for this configuration.



How heavy such a mechanism could be? how expensive?

A removable rudder located at the transom is for sure, simpler, cheaper... less efficient???

I thank you for your advices, talk and ideas...

[daiquiri]

The rudder in the pic looks like the one Eric Sponberg (http://www.boatdesign.net/forums/pro...-sponberg.html) has used in one of his projects. He will surely be able to tell you all the details regarding costs, weight etc.

Regarding the efficiency of the outboard (transom-hung) vs. inboard (or spade) rudder, the second one will nearly always be more efficient. The reason lies in the fact that the outboard rudder pierces the water surface.

When the tiller is turned to some angle, a pressure distributon is set around the rudder blade (foil), which results in a lift force. Without entering too much into details of this pressure field, it can be simply summed up as: low pressure over the dorsal face of the foil, high pressure over the ventral face. The difference between these two pressures (times the rudder surface) gives you the lift force.

Now, consider the inboard rudder. Down at the tip, the pressures on the dorsal and ventral side of the foil must equalize, because the tip is one single point for the waterflow and only one pressure value can exist in that point.
So, looking at the rudder in a spanwise direction, what you have is that the lift force distributon will go from some finite positive value at some mid-span point, to zero at the tip.
At the root, if there is no gap between the rudder and the hull a finite pressure difference can exist between the dorsal and ventral side of the foil, so the lift can be bigger than zero at the root.

But, if there is a gap, you have the same situation seen at the tip - the lift distribution will have to go to zero at the root because pressures will have to equalize between the two sides. Technically speaking, the aspect-ratio of a rudder with a root gap becomes half the aspect ratio of the rudder with no root gap, with the consequent increase in induced drag.

You can read more about root-gap influence here: Need help with Rudder Designs

In the case of an outboard rudder (transom-hung), you have surface-piercing rudder. Down at the tip side, all the considerations done before remain valid.
Along the line where the blade pierces the water surface you have again the lift going to zero, plus another phenomena hapening: the ventilation. The low pressure zone on the ventral area below the water surface sucks down the air and creates a hollow in the water surface shape - which creates an additional wave drag component.
The lift distribution in the spanwise direction becomes similar to the lift distribution of the inboard rudder with a root gap.

I have made the attached drawing which will hopefully explain the things better than words.

Cheers!

[messabout]

An inboard rudder has some advantage if compared to a transom hung rudder of identical size and shape. Transom hung types are almost always surface piercing units which may be somewhat less efficient than totally submerged units. BUT....Submerged units will be placed forward of the transom and nearer the CLA so there is a moment arm difference that favors transom hung types. From a practical viewpoint, the transom hung rudder is the hands down winner. Get some sea weed, trash bag, or pot warp on the TH rudder and you can clear it in seconds. With the underbody type you will get out the scuba gear. If I interpret your picture correctly, you could clear this rudder pretty quickly too. Although if clearing it underway you would be rudderless for some interval that is larger than the TH interval. TH wins the decision when grounding too. It's not a question of whether you will ever ground the rudder but when you will ground it. The TH kickup type is not immune to damage but is far less so than the alternative type.

Given the complication and expense involved, your clever inboard layout is not my cup o tea. I would have reservations about buying a boat so equipped.

[tspeer]

The rudder on my Chris White designed trimaran has a cassette that gives it some of the characteristics of both the inboard and transom-hung rudders.





It is inboard, but can still kick up. When kicked up part way, it is still steerable, because the pulleys on the steering cables are aligned with the kick-up axis, and there is a fixed skeg at the top of the rudder that is the distance between the kick-up axis and the end of the sugar-scoop.

The fixed skeg contributes to stability, but not maneuvering. There will be some maneuvering force contributed by the skeg due to favorable interference with the rudder (it acts like a winglet), but nowhere near as much as if the skeg itself were part of the rudder.

This particular rudder has all of the taper in the leading edge, with the trailing edge parallel to the axis of the rudder. The axis is located so there is enough area ahead of the axis to balance the hydrodynamic moments, making for a very nicely balanced helm when sailing. However, when under high power, the portion of the rudder on which the prop blows has proportionately more area ahead of the axis than does the rudder as a whole, making it grossly over-balanced under power. It can be a real hand-full until you get used to it. Much better just to put it on autopilot when motoring!

[Paul B]

Originally Posted by AdrienJousset

Here is an example of solution for this configuration.

Both the Henderson 30 and Melges 32 sportboats have used this type of inboard cassette. I believe thay Henderson/SOCA had a patent on it. I don't know if it has lapsed.

In order for this to work you have to have a FLAT in the hull shape(longitudinal and athwartships) large enough for the drum. If you have any curvature in this area the drum will not align with the hull as it rotates.

[AdrienJousset]

Thank all for your comments... this system is indeed patented by Soca.
http://www.patentstorm.us/patents/5791277.html

Still protected, probably for another 10 years.

Great idea. Pricy though