Foil shapes

[Howard]

Hopefully, this thread will be the jinx buster.

Any good way to get an even foil shape on rudders and centerboards, other than carefully eyeballing them?

[Barry Pyeatt]

I've found this site to have good information. Some of it can become a bit more involved than we need for good performance since it is more technical and oriented to racing foils.

That being said, the shaping techniques and foil shapes are sound and somewhat applicable to our use with high aspect rudders.

Think of a symetrical airplane wing.

http://mothboat.tripod.com/CMBA/Building/foils.htm

One thing for certain, a foil shaped rudder works more efficiently than one that isn't shaped with these criteria as a part of our consideration.

Rounded leading edge tapering to widest portion of the rudder about a third of the way back from the leading edge and then tapering to a thin trailing edge that is squared off rather than extremely sharp.

In racing foils the trailing edge is ideally as sharp as possible, but in reality it doesn't work out. Any dings or dents can create differences in the flow patterns and change the efficiency.

For out use, that isn't quite as high a criteria. Square off the taper at about 1/8" to 3/16" thickness.

[Dave R1]

Howard, you could cut templates from cardboard of the appropriate shapes at given distances from the end. Shape until the templates fit both sides.

You could also use a router guided along the outside edge to cut steps and then plane the corners off the steps.

[Barry Pyeatt]

I found that I could use my table saw with an extension built up on the fence to support the rudder blank on edge. I had marked out the foil shapes on the top edge and the lower edge of the blank. ( I didn't cut the rounded bottom edge to shape; I left it as a squared off blank, until after planing down the foil shape.) Then setitng the angles on the blade of the saw I nibbled away at the leading edge until I had the same amount removed from either side and close to the line of the foil shape. It doesn't take much to get there. Then did the same with the trailing edge. This took off the same width cuts on leading and trailing edges and left a flat blank between them but it was a start that was closer to being even and constant from top to bottom.

Then I made a couple of masonite templates of the foil shapes at the top of the rudder and the smaller chord at the bottom edge of the rudder and tacked them onto the blank and using the sureform or a jack plane brought the side surfaces down across the sides to close to the template outlines. This only took a few strokes of the plane on either side of the rudder to bring them close. I used a long straight edge to sight along the surface to see that I wasn't cutting too deep or in the wrong areas.

Once that got close to the finished outlines, I used a scraper to remove most of the remaining area and then a long, large surface sanding block to get it to the finished shape. Then rounded off the bottom edge and finished it.

Entire process probably took an hour to an hour and a half.

[JeffM]

Good grief, Barry! I thought I'd done mine cleverly, but I took days for my rudder and a good week for my cb! The CS rudder is supposed to be laminated from 2 pieces of 3/8 ply. I started with that, then cut 1/4" pieces to the limits of the 3/8 mark on my foil template and glued those the appropriate places either side, then added 1/8" ply (cut to the 5/8 mark of the template, ) on top of that. Then I used a grinder to cut down the "steps." The ply layers help as they emerge in grinding to keep both sides even. Even so, I have some ugly places in an otherwise pretty rudder where I filled some over-zealous grinding with thickened epoxy. For my cb, I started with a 1-1/8" thick blank and just ground at it, holding my template against it to find the high spots. It isn't quite symmetrical, but I hope that won't effect performance too much.

[Barry Pyeatt]

I have to admit, this isn't my first rudder or centerboard for that matter. Been doing this for 50+ years. So working at how to get things done a little quicker is sort of second nature to me. Now then finishing it does take a bit longer than fabricating it from a solid wood blank.

If I were making it from plywood laminated to create a thicker blank, I would still approach it the same way most likely. Although I would seriously consider using a router to actually shape the foil with a jig that allowed me to follow the foil templates and run the length of the rudder on a sliding rail system. Not difficult to fabricate or use for other curved surfaces for that matter. Takes a bit longer to make up the jig, but actual shaping work would go very quickly and be completely symetrical. And then ready for glass.

This is the latest one for my Weekender. From a vertical grain Doug Fir blank a bit more than 1 5/8" thick. This is very tight grain Doug Fir, very unlikely that it could even be found these days. I've had the blank kicking around in my wood pile since 1968, it was a sign for a business and I was looking for some use that would recycle it.

[Howard]

Ok....now for some math. If I understand this correctly, A NACA foil is defined as % of chord at it's widest point, which I think is 30% aft of the leading edge. As in a foil with a 10 inch cord, 1 inch thick is NACA 0010, with the 1 inch being 3 inches aft of the leading edge. A table of offsets are used to calculate the other positions. With the units above, I'm looking at a CB that is 3/4 inch wide and 11 3/8ths fore and aft (chord length). that is 6.6 % of chord. My rudder is a 1/2 wide with chord of 10 inches, or a 5% foil. These are essentially flat plates. They won't generate much lift at the speeds these boats operate in, unless the boat planes. So do I really need to bother with much other than rounding off the nose and tapering the tail a bit?

[capt jake]

So do I really need to bother with much other than rounding off the nose and tapering the tail a bit?

Well, that all depends. You see, when I am not sailing, I take the rudder off and fly it around the back yard, just for fun. :roll: :roll: LOL I am KIDDING. I just shaped it until 'it looked good!'.

[Barry Pyeatt]

Quote:

So do I really need to bother with much other than rounding off the nose and tapering the tail a bit?

Probably not! From what I've seen on the Weekenders that I've sailed thus far is that the rudder works provided it has some shaping done to it. It stops working and starts dragging if it pivots more than about 30 degrees either side of dead center. And it doesn't do much to grab the water unless it has some shaping done. Round works better than sharp for the leading edge and the other way around for the trailing edge.

Just remember the angled edge is the leading edge.

[JeffM]

Barry, I recall that you were going to compare your stock and foil rudders for stall angles, etc. Any clear difference yet? I went into foil shapes myself partly as a result of reading your old posts.

[Barry Pyeatt]

That is still the plan. Got to get the boat in the water first though.

[PAR]

The foil shapes for non-planning craft should be the NACA 00 series. These (typically 0012) have a rounded leading edge, the rounding is a percentage of it's cord length and work very well at the speeds small sailboats move. They make a big difference in the ability of a rudder or centerboard to perform it's job. Squaring of the trailing edge is also important, rather then drawing it out to a feather edge. Besides the fragileness of the thin edge, the flow properties are a little better with the squared edge. Personally, I make the foil length 5% longer then necessary and lop off the last 5% with a saw, making a square trailing edge (a 10" cord rudder blade would be made 10 1/2" then the last 1/2" gets cut off).

On planning craft, you'll be better suited with a parabolic leading edge, likely one of the 63 series shapes will do a better job of clinging to the "groove" longer before it begins to generate a bunch of drag or stall. Another good site for a description of foils and shapes is http://www.vacantisw.com/foildesign.htm

I've used stainless steel rod and pipe (split) for leading edges with good success, as have half round bronze rub strips, which bend on easily and take to salt water nicely. I've seen some use PVC pipe (split) or flat Teflon stock, by I can't trust how is must be attached.

[PAR]

That "singing" the rudder made was the water coming off the blade in such a disturbed flow (ventilation) that it resulted in sympathetic vibrations in the metal blade (it can happen to wood or other materials too). At that point the blade is acting as a brake and the wake created by this disturbed flow is the actual rudder. The vibration in the tiller is a good indication of what the blade was doing - dancing from the pressure changes as the eddies cleared the trailing edge.