T-Rudder

[SuperPiper]

Some NASA reports and other links from the Foiler Design thread discuss in great detail the reduction of drag from adding endplates or winglets. However, I don't specifically recall there being any comment about an increase in lift through the use of endplates or winglets. Is there any reference to reducing rudder or keel area by adding foils?

Can the rudder area be reduced if a T-Foil is added?

The foil in my project will need to be lower than the skeg on the bottom of the outboard motor. Otherwise, there will be a very dramatic interference while the motor is tilted down and running. The area of the existing rudder seems quite adequate so I was going to gamble and reduce it by 5-10%. Can it be further reduced by adding the foil?

[MalSmith]

Endplates only improve the aspect ratio and hence L/D of the foil. They do not effect the lift coefficient, which is a function of the section shape. So theoretically the rudder area should remain the same. However, in this application the T-foil helps considerably by increasing the effective rudder areas in situations where it is loaded up e.g. when broaching. I also suspect that because the T-foil reduces spanwise flow, it may also help to reduce ventilation. In practice, from my model experience it was certainly possible to reduce the span of an existing rudder by say 25% and still get improved control when the T-foil was added. How our model results translate to a full sized yacht however is indeterminate.

Beware that adding a rudder T-foil to a ballasted yacht adds to the complexity of the tuning problem by adding a third dimension to the excersize. There will be trial and error involved in getting it right but the results should be interesting.

Mal.

[SuperPiper]

The span of the new rudder will be dictated by the depth of the outboard motor. The area will be 10% less than the existing rudder. The resulting geometry will have a very high Aspect Ratio.

The bottom of the rudder will have a flared flange with bolting hardware in the bottom of the rudder. This will allow for some experimentation: symmetrical T-Foil, asymmetrical T-Foil with the camber down, a detachable bulb, etc.

Per Mal's suggestion, the T-Foil will start with an area about 1/2 the area of the rudder.

Weight is not a concern. The entire assembly should have neutral bouyancy (60 #/cu.ft.).

Drawings to follow.

[SuperPiper]

It didn't take long to abandon the constraints posted above.

It is early in the design. The next steps include:

- Choosing a T-foil span that will fit through the companionway.
- Reducing the span and the aspect ratio of the rudder.
- Concentrating on the rudder-to-foil junction.

Any comments or direction from this?

In case you didn't know you can make the junction a bit bigger(and stronger) by extending it forward sort of like a bulbous bow. The Bladerider guys have done it on the mainfoil/ board junction. I did it on an experimental windsurfer foil system. You can see mine(among other things) at : X21-T/aeroSKIFF
http://www.monofoiler.com/images/x21/x21-11.html
Merry Christmas!

[SuperPiper]

Doug, thanks for the recommendation. Good photos at the link you supplied.

If it was possible to see your bulb head on, what shape would it be? From the photo, I thought it looked oval or it looked rectangular with rounded corners. Do you have a good reference for this shape? Your wing is at the top of the bulb. What was the rationale for that? You must have had a reason for your configuration. I assumed that the wing would be centred on the bulb.

Details and thought patterns, please.

SP, the "bulbous bow" on my foil is bigger than it would have to be for a simple t-foil junction. That's because it also served to streamline the intersection of the tail tube-between the main and aft foil. There are some comments by Tom Speer on his site and in Foiler Design about these joints.
Try to find a picture of the Bladerider mainfoil junction....

[SuperPiper]

This project is close to the cutting-lumber stage.

The core will be laminated plywood and hand-shaped from templates. 0.10" of glass and epoxy will be hand-laid over the plywood. Obviously I am not worried about the glass-to-epoxy ratios. Then the finished profile will be templated again.

Per the Phil's Foil site, the section will be NACA0008 to NACA0010 where the blade breaks the surface. The rest of the rudder blade will be NACA0012.

I considered templating at varying angles to the chord because the flow at the top of the rudder could be approximately tangent to the bottom of the boat, 17 degrees from the waterline, and more or less horizontal at the bottom. But the difference in profile amounted to about 3-1/2%. Instead of a NACA0012, the profile would be NACA0012.4. Not worth worrying about because the 0012 was just a guess at the start of the design.

The T-foil is about 1/2 the area of the rudder blade as suggested by MalSmith. The T-foil will be attached to the flange on the bottom of the rudder with 4 or 6 flathead screws. The profile of the T-foil is still being researched. The default is NACA5313.

I tried to keep the aspect ratios of the rudder and the T-foil about the same. I don't know why that would be a requirement. Does anyone have experience with that?

[wet feet]

I've been following this thread with some interest as I have been making and using T-foil rudders for several years.If I have successfully attached the image file,you will see one that has had a few seasons use on a fairly fast trapeze dinghy.It was out of use for a few days while I made good some damage to the tips of the T-foil.Its a bit more complex than some because it operates as a lifting rudder (please don't mention the drag when its not fully down!) because on a shallow sandy coast a fixed rudder can bring you to a sudden and expensive halt.
The fine entry at the waterline is not too evident but I use the style that used to be favoured by the Roga 470's when they were at the to of the heap rather than trying to blend an entirely different foil section,much less a smaller cross section into the finished blade.

[SuperPiper]

wet feet, you are the indisputable expert. You have already built and sailed on T-foil rudders. So the obvious question is: "What are the advantages of a T-foil?"

Does your foil create lift? Are you trying to lift your transom or sink your transom? Or, are you just damping the pitching motion?

Intuitively, I wanted to build a low aspect ratio T-foil like yours. But the "rules" that have been published in the various threads of this forum suggest that high-span/low-chord is more efficient. How has your configuration performed for you? Has your shape evolved through the various builds over the years?

I am interested to know the construction details. And, I don't quite follow your description of the fine entry at the waterline. Please elaborate.

Your opinion is definitely credible.

[wet feet]

There is no doubt that for foiling Moths and similar applications a high span is the way to produce lift.I initially tried a T-foil with the intention of improving the efficiency of a shorter rudder by reducing the induced drag at the tip.Does it work?I have no test figures but I have noted the number of aircraft currently built that feature winglets.Boeing and Airbus probably do have figures for their applications but obviously they are operating in a compressible medium and at different Reynolds numbers.
The tip foil is set to neutral incidence so that if the trim of the boat changes any force generated will be trying to restore the equilibrium.I have tried not to use too great a span as the original intention was to reduce drag and more frontal area,and indeed wetted surface would be of no help.I would add the note of caution that my experience of T-foils is limited to trapeze dinghies which need to be sailed upright.If you are proposing to apply them to a boat which sails at a heel angle for much of the time,you may be causing more problems that a new rudder will solve.
The structural aspects of retaining a T-foil are quite challenging,more so if you use a high aspect ratio tip foil.There is the need to be aware of the risk of encountering debris in the water as well as coping with the loads imposed by simply sailing.I use an elliptical planform,partly for drag reasons and partly to encourage weed to slide past.Initially I used an epoxy/microballoon fillet to attach the tipfoil.After a couple of breakages due to encounters with solid objects I added some strength by cutting a couple of slots in both parts of the rudder and inserted pieces of cured carbon sheet which had been shaped to lie just beneath the surface,uniting the parts.The slight gap was filled with the epoxy/microballoon mix and the area re-coated with light glass cloth in epoxy.There has been no trouble since.
The easy way to visualise the finer entry at the waterline is to imagine taking the curved face of a half round rasp to the first inch and a half of the section to reduce the leading edge to something like quarter of an inch between the corners that appear.Blend the leading edge into a sharper curve and ease away the ridge at the aft end of the channel that has been created.

[SuperPiper]

Originally Posted by wet feet

The tip foil is set to neutral incidence so that if the trim of the boat changes any force generated will be trying to restore the equilibrium.

My plan was to use a section that produced negative lift. What you are suggesting makes more sense. The attached graph from a tspeer post shows that the proposed 5313 produces lift in the neutral position that doubles with just a few degrees increase in angle of attack. Even a symmetrical section would produce lift at a few degrees of pitch. And, it would work to restore equilibrium in either positive or negative attitude.

More reasons to make the T-foil detachable & interchangeable.

[wet feet]

It happens that since I posted to this thread last week I have been able to make further progress with a rudder that has been under construction for some time.One of the attached photographs shows the finer entry at the waterline and the other shows the tip foil having the centre line of the foil section marked at one of the reference points-visible as pencil lines.The section of the rudder itself is an SD8020 and the tip foil is NACA64A010.

[SuperPiper]

Excellent photos!

It is not every home shop that has a granite flat. You work to a great degree of precision.

Your fine surface entry is visible in the 2nd photo. Thank-you.

Excuse my stupidity. Is the flow over the T-foil parallel to the laminations or perpendicular to the laminations? I am confused by the shape and profile of the foil in the photo.

We seem to have differing philosophies about the strength of our rudders. Your strength seems to be in the laminated lumber. The glass covering is probably just to protect the wood. I have purchased some 6mm exterior ply to make the low-strength core. The strength for my rudder will be in the 0.100" glass covering.

I am now in the midst of drawing the 16 templates in 0.1" chord increments. That is a bit of a chore! 3 done, 11 or 12 to go.

[nflutter]

that bit of granite is pretty impressive.
http://bp1.blogger.com/_FjFuQZDXcp8/.../s400/DSC04190
heres my moth t-foil that i built to replace an old one that fell off. old skiff moths (before foilers) all had t foils - they would kick in as you nosedived and suck the stern down and save you. the boats are very hard to sail downwind without the foil, which is only about 200 - 300cm^2, at about neutral angle of attack. most people use a symmetrical section for reduced drag when the boat is stable. as far as i know it has no effect on the flow over the hull, and it is not designed to reduce the induced drag of the rudder, though with a bit of CFD im sure it has that potential. I14s use different adjustable foils that are very interesting and more along the lines of what i think you are after.. being a certain (small) distance from the surface they effect the flow over the back part of the hull. they are about the width of the planing surface of the hull and they are sometimes adjustable in pitch. http://www.philsfoils.com/Designs/images/I14tfoil4.jpg
http://www.i14nsw.org/res/481.jpg