Balanced rudder behind a keel?

This is a long shot.

How close to the LE of a balanced rudder can you fair the Trailing edge of a full keel? see diagram below.


If the keel and rudder are close with the rudder at 0 degrees, then the slot opens with rudder angle, at well trimmed sailing attitude with say 5 to 8 degrees of rudder it is effectively unbalanced with no gap , but with larger angles when maneuvering, say at 30 degrees will I have enough slot to make a balanced rudder lighter on the helm?

None of my knowledge bases touch this, probably down to tank testing again, seems that the thicker the foil section (NACA shown ) the lower the drag at small angles with this sort of set-up. Whether the relatively small gap will still lighten the helm enough to make the extra turbulance worthwhile Is the question.


The vessel is a 40 ton sail boat full keel deeply immersed rudder not a racing machine .


Any reference, experience ideas or opinions welcome, thanks.

If I tank test i'll report.

 

An interesting question, but not one with a definite answer. I suggest you find yourself a Naval Architect with either a good CFD package, or at least MSES, or some similar multi-element solver.

Tim B.

 

What you want to know is the pressure distribution on the rudder to work out where your pressure poit is. Then you can calculate how much your rudder is balanced. The problem is similar to aeroplane wings with a flap. I think there is data out there for NACA profiles with a flap.

I think you want the gap to be as small as possible to avoid water flowing from the high pressure region through the slot into the low pressure region. Real aeroplanes usually have a big slot but the axis of rotation is forward of the flap leading edge. That is mainly to avoid stalling of the flap while landing. The air flows from the underside of the wing onto the top surface of the flap and therefore reduces the chance of flow separation. Maximum lift is more imortant than minimum drag. There is lots of engine power to burn during landing.

Do you want maximum lift on the rudder or minimum drag with the rudder deflected?

 

The aeronautical literature has lots of information about the hinge moment of balanced surfaces, but there aren't many configurations where the leading edge of the flap is as exposed as you've diagrammed it. Usually the thickness of the wing ahead of the flap is about the same as the leading edge of the flap so the flap doesn't form a forward-facing step.

You might look at these NACA reports:
http://naca.larc.nasa.gov/reports/1951/naca-tn-2495/naca-tn-2495.pdf
http://naca.larc.nasa.gov/reports/1948/naca-rm-a7l02/naca-rm-a7l02.pdf


You may also want to consider:
http://naca.larc.nasa.gov/reports/1942/naca-wr-l-464/naca-wr-l-464.pdf

In general, hinge moments are notoriously difficult to predict accurately, so they are usually based on test data for a specific configuration. I wouldn't be surprised to see your rudder being over-balanced at moderate deflections as the leading edge of the rudder extends more into the flow.

If you really want to have an effective slot when deflected, you might consider putting the hinge line ahead of the rudder leading edge, and using a horn (area at the tip that projects ahead of the hinge line) to balance it.

 

Thanks for your responses.
The rudder is a retro fit on an existing vessel called "Mahe" an old steel German navy training schooner from pre-WW1 , the owner wants to ease the helm and reduce power usagae for autohelm. Anything will be an improvement over his existing flat plate barn door. Doing the sums it will be cheaper to design with easy adjustment allowed for in the hinge placement than to model and tank test. Otherwise I have a new hull to tank test early next year and we'll try the rudder configuration on that model.

Thanks again for your responses

 

On many aircraft there is a filler strip , think of heavy broom bristles to close such gaps.

Hard to do submerged but 2 fairingstrips could easily be atached to the keel to slow/stop any water flow at extrime angles.

Have you considered the OTHER option , install a linked or powered trim tab to take 90% of the work out of helm movement?

The linked tab is an install & forget , but powering a trim tab with the AP would use even less power.

FAST FRED

 

To reduce power consumption of the autohelm, also do some research about a trim tab attached on the trailing edge of the rudder blade. Google for "anti servo tab" or "Flettner rudder". The idea being, that the autohelm will be actuating the anti servo tab, which in turn moves the much larger rudder blade.

take care,

Hans

 

Hi Tom,

thanks a lot for the url's to these documents. These are very useful for a non-hydro project of mine.

Hans

 

Ed Monk Jr. did it on the Skookum 34 back in the early 70s, just before he died. It worked fine.

 

I seems to me that if you are going to a balanced rudder for lighter load on the
steering, the balanced part of the rudder needs to be effective.

Without looking at any programs or math, the balanced part of your sketch does not look like it will operate like you need it to since it is hidden behind the keel. There needs to be some separation to get the rudder out of the effect of the keel.

 

The sketch is just an illustration of the question, not the final design, I was hoping someone had actually looked into this or knew of a study. The question was :

"How close to the LE of a balanced rudder can you fair the Trailing edge of a full keel? " ie what separation is actually required for it to be effective ?

I think this is another tank test project (to join the other 12) , in the mean time the job is done, I simply hinged it at the LE rather than risk an unpredictable helm.

 

In the comparisons of boat rudder to aircraft control, the Elevator and Rudder, not the Flaps, on an airplane is the closer comparable control. Usualy the balance on an elevator or rudder, is called the Horn, and only comprises part of the length of either control.

 

Would it be feasible to create an aperture in the trailing edge of the keel, either with or without a propellor in place, and extend a portion of the leading edge of the rudder forward into the aperture for balance, leaving a sizable gap? This would be similar in effect to the partial skeg configuration on some sailboats but would retain the full protection and support of the keel for the rudder.