Shaping foils

[TJH]

Here's the approach I used to shape the rudder and centerboard for my Welsford Pathfinder. It's an approach intended to be simple to implement yet result in reasonably close approximation of a NACA symmetrical four-digit foil.

The approach, like in spar making, is to layout the stock so that material can be removed in a controlled, measureable, repeatable way. Because each cut is planer, progress is easily assessed and the completion of a particular cut is easily verified.

This works well regardless of the thickness-to-chord width ratio and regardless of whether the plan form or thickness is regular or varying.

STEP 1 – SQUARE OFF STOCK
Start by making the stock for the board rectangular in cross section. The simplest boards will be rectangular in plan form. But, if varying in width, mark and cut the board to its plan form shape now, though keep all lines straight until final shaping. Also, if the board is to vary in thickness (this is typical when the chord varies), taper the board now, but keep it rectangular in cross section.

STEP 2 – MARK CENTERLINE, ALL EDGES
Mark the centerline on the leading, trailing, tip, and top edges of the blank. These lines need to remain until final sanding, so mark them clearly.

STEP 3 – MARK CUT OFFSETS
Refer to the FoilCuts sketches for the cut offsets. Mark both the first cuts and second cuts marks on the tip and top edges. You may want to use different colored markings to help avoid confusing the two sets of cuts. You can also mark the second cuts offsets after you make the first cuts, if you like.

STEP 4 – EXTEND CUT OFFSETS TO FACES AND LEADING AND TRAILING EDGES
Extend the cut offsets to the faces and leading and trailing edges of the board. These are the marks to which you plane or sand off the corners.

STEP 5 – FIRST CUTS – PLANE TO CUT OFFSETS
Using a plane or a sander or some other means, cut off the four corners to make the board eight sided.

STEP 6 – SECOND CUTS – PLANE TO OFFSETS
Now, remove the next set of corners to make the board sixteen-sided. You may want to restore the second cuts lines on the faces of the board – some will have been removed by the first cuts.

STEP 7 – PLANE REMAINING CORNERS
Once the second shaping cuts are made, the board, with its sixteen faces, will be very close to the final shape. It may be worthwhile to use a hand plane to knock down the most prominent remaining corners along the leading edge.

STEP 8 – SAND SMOOTH
The final shaping is done by sanding. Round all break points, corners, etc.

DEALING WITH VARYING SECTIONS
If the board has a varying plan form, the method still works, but the layout becomes a bit more complex. For example, for a board that tapers from 12 inches at the upper end to 9 inches at the (lower) tip, mark the offsets using a chord of 12 inches at the upper end and the offsets for 9 inches at the lower end, and connect the marks by drawing a straight line on the faces. If there is a distinct break point somewhere in the plan form, add additional layout marks based on the chord width at the break points.

Keep in mind that if the width of the board varies, then the thickness must vary proportionally if you want the same section throughout the foil. For example, if the board varies width from 12 inches to 9 inches, the board should be 1 ½ inches at the 12 inch wide end and 9/12 x 1 ½ inches, or 1 1/8 inches thick at the 9 inch wide end to keep a constant thickness to chord proportion of 12.5%. Any such required tapering should be done prior to making the first cuts (while the section is still rectangular).

Keep all tape lines straight and break points sharp angles until you get to the final sanding stage.

EDGE TREATMENTS
Leading Edge:
The leading edge should be smoothly rounded.

Trailing Edge:
Don’t feather the edge to a sharp knife-edge – doing so will increase its susceptibility to damage. The layout lines leave the trailing edge about 8 percent of the thickness of the board. Simply round the trailing edge corners just enough to make the board easy to finish and handle. For a centerboard or leeboard, consider beveling the trailing edge somewhat. It will minimize oscillating eddies that setup vibration in the board. This is particularly useful with thin boards.

Tip:
Resist temptation to round the tip of the foil section beyond a slight rounding of sharp corners. Rounding will only facilitate the eddying that occurs at this point, reducing the overall effective board area.

[bennie]

a few years ago, i tried this without such a good set of offsets
and ended up shaping and checking endlessly.
the teak wooden rudder foil (from scrap strips) came out nice,
but it kept me from making the next foil.

i printed this set and text and can imagine doing this again,
in a much more straightforward way.

thanks for sharing,

Bennie
(Holland)

[Mike]

Hi Bennie

Nice to see your first post. Welcome aboard the good ship BBB.

Shaping centreboards and rudders into NACA aerofoils is really only of value if you are into speed as measured in fractions of a knot.

For those who simply wish to enjoy sailing as well as boatbuilding, a rough approximation of the shape is more than good enough, in my opinion.

I suspect TJH is something of a perfectionist. However, his build was a John Welsford designed 'Pathfinder' which, superb cruiser that she undoubtedly is, was not, I believe, designed purely for speed. She was built for good looks and great comfort as I recall and she certainly achieves those objectives in spades.

What I do like about TJH's post above, and his generously imparted advice, is that it does simplify the task for anyone who feels driven to achieve more speed from what is already a fast hull. For the vast majority of us, such an exercise is a little like putting 'go faster' stripes' on a Ford KA. Looks good but achieves almost nothing worth talking about.

Kind regards

[bennie]

hi BBB readers,

not commenting on TJH's caracter or intentions,
let me explain my intentions for shaping a wooden rudder:

i had a steel plate rudder
(which must have had less drag than the foil shaped rudder i have now)

but i broached, in semi mild conditions, swamping boat, friends and gear
which was a sobering experience, and a lesson to learn
not my idea of "enjoy sailing",
and it kept me from asking friends to come along.

i tried to understand what caused it, what i did, and what would work next time. Part of 'problem' was the rudder, in my opinion.
rough and coarse and flat.

i intended the new rudder to be able to steer the boat, even in a broach or any disaster in the making, where intuitive reactions overrule anything.

i am fully satisfied with the way my foil rudder translates handling into boat "yaw" and "turn", even with large rudder angles / angles of attack, even with unexperienced friends at the helm.

When shaping the rudder, my main considerations where a smooth form and a smooth surface and a squared trailing edge. Which i think are crucial for the waterflow to follow the shape of the (over-)angled rudder.

a rough aproximation of a smooth form and surface, whould not do for brisk steering (opinion).

Besides, i enjoy boat building too.
which means for me: only a smooth form and surface will do.
and the plan form (the sillouette) is important, for looks.
another part of the joy: it only cost glue,
since I made it from found hardwood scraps,
and reclaimed hardware (pintles etc.).

an unintended effect:
All these things get noticed, people even come to put their hands on it (never happened with a steel rudder). And they consider it worth talking about. Every aspect i mentioned, attracts attention.
but nobody ever started about NACA or foil or speed.

i did try to get a "reasonably close approximation of a NACA symmetrical four-digit foil", as TJH intended (and why not, when i put in the effort anyway).

i fully agree with Mike about "what i do like about TJH's post above, and his generously imparted advice, is that it does simplify the task"
which i tried to say in my 24 october post.

greetz,

Bennie
(Holland)

[Mike]

i had a steel plate rudder
(which must have had less drag than the foil shaped rudder i have now)
................

i tried to understand what caused it, what i did, and what would work next time. Part of 'problem' was the rudder, in my opinion.
rough and coarse and flat.
...................

i did try to get a "reasonably close approximation of a NACA symmetrical four-digit foil", as TJH intended (and why not, when i put in the effort anyway).Hi Bennie

An interesting post, if I may say so? Your comparison between a flat steel plate rudder and one with a 'foil' shape with regard to drag is a little wide of the mark, I feel. To explain, from an aviator's viewpoint, drag is not merely related to profile or cross sectional shape. You need to consider fluid dynamics too. The problem, as I see it, with a flat steel rudder would be in its eddy drag. As the water splits to move either side of the leading edge, it tries to stay close to the rudder surface and, because that surface starts with a 90 degree bend around the leading edge and is then flat, eddies are formed across the surface. These eddies cause considerable drag.

On aircraft, the idea is to maximize what is called 'laminar flow'. An aerofoil shape goes some way to achieving laminar flow - at least until the relative angle of attack to the air parcel becomes too high (somewhere around 15 degrees relative), at which point, the laminar flow is destroyed and eddies form at the wing surface, causing increased drag. This is what happens when an aircraft wing 'stalls' and all lift is lost.

Much the same sort of thing happens at a rudder, depending of course on its shape and the relative flow of water across it - relative to the flow of the surrounding water. To test this, try slamming your rudder over hard and very fast. It will act like a brake. Move it over gently and steadily and you will maintain the laminar flow of water around it and its force will remain smooth and positive, achieving maximum control.

So, while a flat steel rudder blade will present less profile to the water and therefore less profile drag, there will be considerable loss of laminar flow and the overall result is usually that the flat plate will have far greater drag - in total - than a foil.

Hope I have explained that in a way that you can follow and that it helps with your understanding.

As to the simplicity in TJH's explanations, I totally agree that they are both useful and helpful, which is why they remain on the site. :)

Best wishes

[bennie]

hi Mike,

thanks for your post, which nicely supports what i tried to say:
that a perfectionist approach for foils is of value for boat control,
joy in building and for attraction, rather than speed.

and you explain, how that is going to reduce drag. I can follow that.

i like your compliments, to TJH at the end, and to me at the beginning.

kind regards,

Bennie
(Holland)

[Mike]

Hello Bennie

My pleasure to add something to this thread. Tom (TJH) is an interesting character with many good ideas. It's a pity his contribution here was so limited and he preferred to go elsewhere - but we can't win 'em all. I've been following his current build myself and he really does work hard to achieve a good boat. He has a lot of ability, that's for sure.

As to your own posts, I am greatly impressed with your English. I've often found that people in Europe can read and write our language better than many of us Brits can. Well done to you.

Glad you followed my attempt to explain a little about drag and fluid dynamics. And you are right about the difference a foil shape makes. The contribution to overall boat speed is usually very small in the overall scheme of things. Important, of course, to those who race. They are always looking to improve boat speed, even if only in increments of a fraction of just one knot.

For the rest of us, the foil shape is perhaps more about positive control and elegance, as you rightly say.

Regards