Schilling Rudder (Fish Tail) Design for 65' T Boat

Hello,

I have the opportunity to add wedges to the rudder of my 65' Army T Boat and would like to know if there is a basic formula.

The rudder is 53" high x 33" wide, hydrodynamically shaped, area forward of the ruddershaft center line is 7"

How long should the wedge be and at what angle to the center line of the rudder?

Should this form a curve?

Information on this subject will be greatly appreciated.

Many thanks,
Hal

 

I doubt there's a simple formula for a Schilling rudder shape. You can find information in the aeronautical literature on divergent trailing edges. These tend to look like faired in Gurney flaps and extend something like 1% of the chord from the camber line.

Of course, it does no good to put on a divergent trailing edge if the rudder stalls at the leading edge. So you have to design the whole shape for high lift if that's your objective.

Here's an example section I designed. It has a thickness of 25% and XFOIL predicts a maximum lift coefficient of 2.0, which is really high for a symmetrical section. I then added a 5% thick gap at the trailing edge to give it a Schilling shape.

The key to designing a high lift section like this is to have a flat rooftop pressure distribution to get as much lift as possible with a given maximum velocity, and then use a very steep concave pressure recovery to get the most out of the boundary layer before it separates. Here's the pressure distribution at the two-dimensional design angle of attack 17 degrees:

The only way you're going to do this is with a computational CFD program like XFOIL.

Here's the predicted performance:

The flared trailing edge does extend the maximum lift somewhat, but it also doubles the minimum drag. So the question is, "Is the increase in max lift worth the extra drag when you're not making hard maneuvers?"

 

p.s. Here's the pressure distribution for the flared section at the same conditions. Notice how much better the actual (colored) pressures match the design pressure (black) over the front part of the section. This is because the flare has reduced the boundary layer thickness at the trailing edge.



This is really where the improved lift is coming from. Paradoxically, modifying the trailing edge has actually been felt at the leading edge.

So the message is, there's no simple formula for slapping trailing edge wedges on an existing rudder. It's not the wedges alone that give the Schilling rudder high lift.

 

Dear Mr. Speer,

Thank you for that excellent information.

In this case the vessel is a 90 ton gross, small ship operating at a hull speed of 10 knots. The propeller will have a large protective cage around it as well.

I wanted to add wedges to the trailing faces of the rudder for more effective turning ability at slow speed, drag is not really an issue.

We were looking at the rudder today in the shipyard and were wondering if an angle bar of 4" x 2" might be the right size to weld to the rudder on both sides. This looked a bit short and maybe to much of an angle at about 45 deg.

Maybe something like 6" on the flat side and 2" on projection might be better. I am just not sure. There is probably a good median.

I look forward to your thoughts.

Hal

 

The Gurney flap is a 90 degree angle piece added to the trailing edge, and it's effective. So you can use quite a steep angle. What will happen if it is too steep is the flow will separate ahead of the wedge but reattach farther back on the wedge, forming a separation bubble that effectively changes the contour to something more fair. There's a drag penalty to pay for this, of course. But the rudder effectiveness may not prove to be all that sensitive to the precise shape of the wedge.