Powerboat rudder design

I am brand new to this forum and have a question for discussion,
My question is about single screw powerboat rudder design that I don't seem to be able to answer from the books I have available to me. I speak of the contraguide rudder. In the several books in which I have found it discussed, the writers seem to be in disagreement as to it's purpose. Some correctly say that it's design counters (straightens) the propeller race spiral by aligning the upper and lower segments of the rudder against the spiral and in doing so one gains propulsive thrust. Others say (and picture) the opposite, the upper and lower segments of the rudder aligned with the propeller race spiral, gaining efficiency through decreased resistance of the rudder itself. The latter was my theory and the way I built my rudder. I reasoned that any thrust gained by straightening the spiral would be imparted to the rudder itself and transmitted back to the vessel as an equal retardation whereas having the upper and lower rudder segments aligned with the spiral would both decrease the retardation of the rudder and benefit the steering performance as well. However, I still have nagging doubts because of the contrary (no pun intended) literature, some quoting efficiency increases of as much as 15% on "Liberty" ships.
Any and all comments on the aforementioned and directions to good literature on the subject and welcome.
John

 

I would love to see a picture of the rudder style. Can you e-mail a member who would post it?

 

Hi Richard,
No, I don't know a member who can post a picture but most naval architectural texts show and discuss this type of rudder.
Picture the 11" edge of a std. sheet of paper (rudder) taped to a dowel (rudder post); cut the rudder horizontally in the center; hold this creation in front of a slow fan with the horizontal cut at the center. The two parts of the rudder will diverge at some angle "maybe 10 degrees". Now if you made up a solid rudder (flatplate, wedge, or airfoil) to match the divergent paper angle, you would have a rudder the upper and lower parts of which are more closely aligned with the propeller sprial. (and maybe steers better and has less resistance)
Finally, picture the same rudder with the segments reversed against the propeller spiral; now you have a contraguide rudder... The theory here is that by straightening the spiral, more propeller thrust is developed for the same power.
This is the short form, the slit in the rudder has a horizontal plate in it and the stern post has matching angles faired in it as well.
John

 

The business of straightening the propflow is a proven idea. It is used in all ducted fan flow model airplane engines. Thrust improvements of 18 % are actual. Tugboats use a form of it also. Tube around the prop exhaust to confine the flow to straight back. Mounted in the pipe are 1 to 6 rudders. Tug owners do not waste money. In some designs, the entire pipe turns, ala Kort Nozzles. None use true flow straighteners. Water may not respond as well as air at 200+ mph. Turbulance is wasted power.

 

Richard, I understand that if the "twist" could be taken out of the propwash (air or water), and it all went straight back, thrust would be increased by some number; thus the countrguide rudder.
But my original question was 1) how well the rudder would end up working with its segments each turned say 20 degrees (10+10) from alignment with the propwash sprial. 2) isn't the energy imparted to the rudder/s in correcting the flow spiral transmitted to the vessel through the rudderstock as a retardation.
IMO Kort nozzles, although shown to increase bollard pull and therefore useful in towing or pushing, have never been shown to be useful in open water work. Otherwise every vessel in the world would have a nozzle. Steerable nozzles or multiple rudders in a nozzle, as used in tugs, although a boon to steering and pulling, are a big drag for open water work also.
John

 

You are right. At about 15 to 20 mph the Kort becomes drag. Per tug people. I know this for a fact as I called a Kort company and asked them.

 

15 to 20 MPH is VERY optimistic. I'd say 8-9K on the size vessel we are talking about here (30-40' LWL) and then, only produces increased pull but not increased speed.
BTW The body of a Kort is a wing in cross section and it "works" by this means not by confining the propwash per se.

 

You are probably right, as I was asking about a 17', 350 hp speed boat. Would help me higher up.

 

The short Kort Nozzles work that way . The newer hybrids are closer to a Kort internal airfoil to elimanate cavatation in shallow waters. With the longer tailpipe to increase thrust using the same principals as jet engine tailpipes do without a afterburner. Much different Reynolds, but very similar in logic.

 

Well, enough about Korts. What I need is some good reliable information on contrguide rudders.

 

I found one very good web site with a explanation of it and who owns the rights to it. Web search " contraguide rudder " . Read the first site of www.oldriverbillzumwalt.members.ktis.net

 

After looking at the rudder I can see how it will reduce steering effort. I really can not say if that converts into efficency. But if they put it on several boats? Also, during sharp turns, additional rudder area may be needed. I assume the rudder must be matched to each prop. As prop pitch changes so would the splay of the rudder halves. I think it would also be self centering and require extra effort in reverse. Emperical positioning is required. It is also possible to cause a cavitation problem in sudden full reverse. But considering the foward towing efficency, I would use it.

 

Richard,
Yes, that is what I have been talking about and exactly what the rudder I built for my boat, 20 years ago, looks like. I actually really did arrive at the angle between the two halves by using the paper (actually cardboard) and fan method that I mentioned above. But I didn't build a contraguide rudder, I built the opposite, a rudder with it's segments aligned with the flow... Sooo, back to my original post, data, what I need is data. It's a German patent from the '30's and I don't read German. Besides it will take a lot of work to reverse the segments on my rudder (which works well by the way) and to test it against the rudder I now have. Someone has certainly done this so I don't want to reinvent the wheel. As I mentioned, a few of the liberty ships had these rudders but I don't know where to find the testing information.
John

 

Contra Guide rudders...

Hello...

See http://oldriverbillzumwalt.members.ktis.net/rudders-korts.htm ...

Ah - nevermind - Petersen beat me to it I see...

SH.

 

Sean,
Thanks
John