new rudder needed?

Looks like the sail might be a bit too far forward too. Does your boat have weather or lee helm? It should have a little bit of weather helm. That means if you let go of the tiller, it should steer into the wind slowly. If it steers away from the wind, that's your first problem. A balanced sailboat should steer into the wind if you let go, and so you have to 'hold the boat away from the wind' to go faster.

I've sailed on a boat with some lee helm, and the steering and maneuvering is much better with no jib sail, which fixes the lee helm. The boat doesn't sail well with the rudder fighting the boats balance.

 

Theo,
what you have is an aluminum plate -rectangle profile, plenty strong, with a nice kick-up. What you wish you had was a better 2D profile like a NACA 0012 and a little larger area, but it would be a lot of work to make the blade and the new kick up to hold it, for a relatively small improvement. I pose to you that 70% of the improvement is in the first 20% of that profile, so that is all you need to make and bolt on to the front of the aluminum blade. As far as dimensions go I would start with about a 1x4 inch pc of wood, cut a 2" deep slot up the middle for the aluminum. Print the NACA 0012 nose to the appropriate scale and cut it out of cardboard for a gauge, then go to town on the wood with a plane and sand paper. Mount it on the front of your aluminum blade with through screws (one tight others slot the aluminum) with some bedding caulk.

some pics on
http://cpyoa.geekworkshosting.com/forum/index.php?topic=3106.15

Wood nose with foam on metal plate -reply #26

 

What you say is correct as a generic info, and indeed at high Reynolds numbers (fully turbulent flows) the effectiveness of L.E. serrations or vortex-generators depends strongly on their geometry and size relative to the boundary layer thickness.

But at the speeds of interest in this case (Re=100.000-500.000, transition zone) the serrations are an effective mean of pushing the stall angle beyond the possibilities of a clean-foil, while at the same time keeping the drag low. And, above all, the rudder with LE serrations continues to generate considerable lift even past the stall angle of attack, which is an important point for a good rudder.

I understand that the OP wants a simple mean of increasing the rudder effectiveness in light breeze. Imo, this modification is one of most cost-effective and labor-effective ones one can suggest. That aluminum piece is going to get wasted anyways, so why not first trying to see if it can be enhanced and reused?

The work required for making an airfoil-shaped rudder is by far more demanding than that. The configuration in my previous drawing is not the optimum one from the hydrodynamic point of view, but is the one which is easiest to make with manual tools, as I assumed that it could be a point of concern here. If Theo has a possibility to bring the 10 mm quote down to 5-6 mm, while maintaining the 25 mm width, the final hydrodynamic characteristics of the blade could be even better.

The ultimate choice is Theo's, of course.

Cheers

 

I still have not seen why this works an would appreciate any good links. All I am sure of is that it will make a turbulent leading edge so that you will not have laminar separation. I have a kayak with a plate rudder I might experiment with if I knew dimensions. I wonder if a sheet metal clip on could be made for easy comparison.

 

As about WHY it works, it is still a field of debate between scientists. Some relate it to the interaction with the boundary layer, and in particular with the splitting of the large laminar bubble behind the leading edge into a series of vortices which dissipate less energy, some explain it with macroscopic vortex and lift distribution effects of the local sweep-back of the serrations (considered roughly as tiny delta wings).

As for the numerical data, you can check, for example, this work of Selig et al. at low Reynolds numbers: http://m-selig.ae.illinois.edu/uiuc_lsat/Low-Speed-Airfoil-Data-V5.pdf
Or this NASA technical memo for high Reynolds: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720023342.pdf
Or the attached pdf files
And several others, many of which are unfortunately not available for free reading.

Cheers

 

thanks again: so, the bullet nose is slotted onto the leading edge. how would you suggest the trailing edge of the bullet nose finish- tapered down towards the surface of the blade?- I tried to illustrate what I am thinking-
and thanks again for the thoughts.

 

Slavi, I fall into the vortise generation camp on this debate, at least with my tests this seems the results. I also agree the scalloped leading edge is a simple thing to do to this plate, though I don't think the gain will be justified or quantified by the net results on this boat. Stall angles and flow may be improved, but given the ranges it'll work through, likely not something that will be significantly noticed underway.

I also think the same is true of the bulbous leading edge idea, though if coupled with some fairing (as shown) an improvement will be seen, but again with limited usefulness underway on this particular boat, given the amount of effort required.



Another simple approach might be to round over the leading edge of the plate and taper the trailing edge as much as practical. Some time with a grinder and slight improvements, for minimal investment.

 

Theo,
the picture on the left is what I have seen and was describing with the slight tweek that the surface of the trailing edge of the wood is parallel to the plate. The idea behind this is that a vortex will develop behind the wood that pulls the flow around to reattach to the plate. This plan has the option of adding the full NACA0012 profile with foam or sheet metal later. If you ask around RC model makers you will find someone with a hot wire foam cutting rig that would be happy to show you how well it cuts your profile.

The tadpole shape on the right might be fine as well. It is like a a mast and sail profile -the drag and lift well known but a little more work.

As to the question of significant improvement this is what I think it will achieve;
-It adds to the profile area ~10% and it is close to the axis so it will not increase tiller load -might reduce it if the leading edge is past the axis.
-a flat plate reaches about a 0.8 coef. of lift at about 7 deg then drag just increases with no added lift. With this nose maximum lift of 1 to 1.2 can be achieved at 10 deg before it begins to stall and drag over 12.

So for a boat that needs more rudder force =1.1*1/.8=1.375 37.5% increase min. This should be a noticeable improvement and a decent return on a couple hour project.

The down side is that on balance (zero lift) drag will increase. If the boat has proper lee helm this should not be a net negative but it is a little more drag in a tack.

By the way, I agree a sharp trailing edge is well worth the effort.

And take a look at "yuloh sculling oar". I think you will find it a desirable addition to your boat. It stores easily and can be used in seconds so you never have to sit in irons or worry about the wind dying.

Still reading about serrated leading edge -not impressed so far. Selig document does not show drag which is a big question for me. The lift data is mixed with improvement limited to specific speed/geometry. I find it most informative that standing block flow interrupters dramatically outperform any of the leading edge cuts.

 

Bullet profile it is then- with parallel tapering off. the plate is only about 4mm thick so no rounding or tapering is practical. just finally- the fixing to the plate.
you recommend the top hole is fixed, and that the subsequent fixings down the edge are slotted, but presumably still very tight, the slot being there to allow for minute movement?- should the slots run up/down or fore aft?
cheers Theo

 

Clearance up and down to allow for differing expansion/contraction of aluminum and wood. I would sure like to hear how the result works after you have sailed it for a while.

My Jet keel plate is only 5mm thick and it has a rounded leading edge and a sharp tail -and it's big and steel (lots of work, less gain). The sharp tail helps against stalling.

PAR, I am also wondering, what do you think of my estimated lift improvement?

 

Try tacking more like this. https://www.youtube.com/watch?v=UTccA37dbzw

 

I don't think the flow over this rudder will be fast enough to get the eddies to reattach, with just a nose job. If the plate was foamed and sheathed into a 00 series, some improvements would be seen, but again at these speeds quite marginal at best, though it may "feel" better on some points, mostly just an exercise in foil making.

 

just checking I'm calculating right:
chord length is from the nose of the foil to the trailing edge of the aluminium. The 'bullet' essentially will make the rudder wider (if you know what I mean - i.e. sticks out further)
the rudder is 210mm so with the nose on the chord length would be about 250mm.
I enter the 250mm into the foil generator and then plot the foil. the csv datapoints then match the size I am making without the need to scale?? i.e. I have a maximum of about 15mm thickness on one face, so the total thickness will be 30mm. looks right when I plot in Excel anyway!

 

It all depends on what impresses you.
Here is another good document, which might bring a better light on why I had suggested LE serrations for this particular problem: http://eprints.soton.ac.uk/343276/1.hasCoversheetVersion/write_up.pdf
But ok, let's see how will the bullet-shaped LE behave.

 

thanks Daiquiri- a very interesting discussion all round. I suspect with my boat the difference would remain a theoretical discussion- too many other variables in play, most significant being my actual sailing experience