how to shape centerboard into foil?

[antipas]

Currently building the Romilly spv,the designer calls fr both tiller and centerboard to hv the underwater sections shaped into aerofoil(section offsets given)
Both hv alrd been constructed fm ply laminations the tiller has a trapezoid shape and cb triangular.(with the inner layer being of mild steel)
Without knowing any better hv cut 2 templates (in essence a pc of ply with half the foil section cut out fr a given width)to be used as a guide in combination with a straight edge.(so as to enable checking both vertical and horizontal shape/dimension)
Somehow though not convinced that this method would result in achieving a symetrical and dimensionally correct cb therefore the question to the forum fr any alternative suggestions.

[Hwyl]

You're doing it the way I would.

[JimD]

I just eyeballed my plywood rudder. The glue lines in the ply show up as contours when you fair it and they provide are very handy guide.

[Tom Lathrop]

You are correct in thinking that trying to work toward a curved foil shape from the beginning is not going to be easy. The best way to get to the proper foil shape in wood is to plane flats on each side that are tangent to the correct surface that you draw on the cross section. The edges of these flats are easy to see and make symetrical. After you are satisfied that the flat surfaces are correct, symetrical and only touch the foil cross section at one point. make a pencil line along the point of the flat that is tangent to the correct final arc. Now round off the sharp edges of the flats without touching the pencil line. This will get you real close to the shape you want and any further work is just touch up.

[paladin]

I have built rudders and centerboards to 3 1/2 to 4 inches thickness using the plywood lamination lines as contours and it seemed to work perfectly....then glassed over.......

[merlinron]

i guess i would use the ply lamination lines method, but would also use a template for this reason.....
the ply lines will tell you if you are keeping the foil shape even or parallel as the shape tapers, but the lam lines won'rt really tell you anything about the actual profile of the foil. that's what the template will do. lay out a few stations along the foils length to check your shaping progress with the template and watch the lam lines to make sure the foil's contour is running along it's length evenly.

[George Ray]

The lamination lines of plywood give you an INCREDIBLE view into the nature of the curves. The template and the lamination line will get you there.

[antipas]

Thank u all will attempt to decipher Tom's suggestion failing which will revert to the basic method.

[JimD]

Thank u all will attempt to decipher Tom's suggestion failing which will revert to the basic method.

I think he's saying start by making a chined/faceted 'foil' and then knock off the corners.

[Lazy Jack]

At sailboat speeds, is precise foil shape really important? Could one actually TELL the difference between the 'correctly' shaped foil and one that was close when it comes to performance?

I shaped my daggerboard (approx 7/8th inch thick) into a foil section by drawing a line 1/3 the chord length back from the leading edge. From 1/2 inch aft of the max chord thickness, I planed a straight taper to the trailing edge, and from the leading edge to that max chord line, I approximated a parabolic curve fairing it up over the max thickness and into the straight taper aft. GOOD ENOUGH!! Were it the perfect NACA eleventy-seven-B profile instead would I go faster and point higher?

I seriously doubt it

[Thorne]

The consensus here seems to be that yes, it is important -=- just as important as sail shape.

A square blue tarp on a post will get you sailing, and a squared chunk of wood will act as a centerboard -- but not as efficiently as a properly-made sail and shaped centerboard. They both act as 'wings', one 'flying' through air and the other through water, but shape is important for both.

[Lazy Jack]

The consensus here seems to be that yes, it is important -=- just as important as sail shape.

A square blue tarp on a post will get you sailing, and a squared chunk of wood will act as a centerboard -- but not as efficiently as a properly-made sail and shaped centerboard. They both act as 'wings', one 'flying' through air and the other through water, but shape is important for both.

I would agree, any amount of shaping is probably more 'effient' and certainly more satisfying than the slap of wood and the blue tarp but I wasn't considering that extreme. I'm thinking more towards a middle ground and more specifically about the centerboard...how could you measure the performance difference between somewhat close (like I described above) and the precise in the context of so many other confounding variables? I suppose I could build another dagger board with the same profile as my shaped board but as a slab with quarter rounded edges and see if I could detect a difference in sailing performance.

Actually, I won't bother...I already know the answer

So why did I go through the effort of shaping the first one? Not sure. I probably wouldn't bother next time, not for a 5 knot dory...

I won't discount the satisfaction that comes with producing precision by hand and a beautifully cut sail set just right is second to nada!

[JimD]

Google 'naca foil' and there is tons of info. Many homemade rudders and daggerboards suffer from being too thin. I believe the general rule is that thickness should be around 10% of the chord:

http://www.vacantisw.com/images/Articles/Foil%20Design/Figure1.gif
http://www.vacantisw.com/images/Articles/Foil%20Design/Figure2.jpg

If its not then you may be going to a lot of effort for little result. Here is one of many articles on the subject: http://www.vacantisw.com/foildesign.htm

[Chan]

I think it all revolves around how fast you want to be or better yet how competitive you want to be...designers used slab sided barn door centerboards for centuries, granted they were not as fast as they could have been but they did get the fish to market fresh.

[Audasea]

I've built and shaped about half a dozen boards/rudders....so I'm no expert, but do have some newbie experience. Most NACA foils assume the trailing edge goes to a knife edge....or at least that is how the math works out.....but you want to leave a slightly squared off edge don't you? I leave mine no less than 1/4 inch. That throws off the foil by half an inch or more. Do you add that in and let the widest point move back, or leave it where it is and have the widest part of the board forward of where it is supposed to be? Not a big issue, but an issue.

Second. I thought building CB's and rudders out of plywood was bad? That plywood doesn't handle the cyclic stress and eventually fails. Mine are built out of solid Doug Fir or laminations of "softwoods" and covered with fabric. I have a commercial built one of laminated mahogany. Nice.

But with solid wood, I like Tom Lathrop's method of doing several flat sides, then rounding those off. Easy method to keep both sides the same. I also trace the profile of the foil I want on the bottom and cut out plywood shapes to fit up and down the sides to check my progress. But even with all that, it takes me less than an hour to shape one once the chips start to fly. A fun job.

[John Meachen]

If you are to stand any chance of achieving accuracy,one point needs to be made.At the outset of the job it is imperative that you mark a centre-line from top to bottom of both the leading edge and the trailing edge.You need to define the centre plane of the foil in order to determine symmetry,regardless of the shaping process chosen.It is also good practice to shape the sides in stages if you use a solid or built up board.To do otherwise opens up the possibility that the internal stresses in the wood may cause cupping.At the risk of causing offence,I would advise that you will not be able to achieve an accurate and symmetrical foil section in an hour.A beginner at shaping and a board the size of a Golant Gaffer's might absorb a couple of days.You will produce a bigger pile of shavingsthan you would have believed possible. If you were to glass sheath and fair the board for the ultimate finish allow another couple of weeks for cure times and so forth.Wassa's illustrated posts show what can be achieved.

[Lazy Jack]

Google 'naca foil' and there is tons of info. Many homemade rudders and daggerboards suffer from being too thin. I believe the general rule is that thickness should be around 10% of the chord:

That makes sense to me empirically. Fast aircraft have thin wings (and therefore generate less lift for any given airspeed and angle of attack) than slow fat wings. My daggerboard being only about 7/8 thick and 12 inches wide is definately on the skinny side for a slow boat (5 knot hullspeed)

[Philip Maynard]

I sometimes hear that that trailing edge should not come to a point but should be squared off. Birds, fishes, airplanes do not have squared off blunt trailing edges, it just does not make sense. I always bring mine to as fine an edge as I can without it getting to thin as far as being easily damaged.

[leaotis]

An engineer friend of mine had a 20' plastic something sloop. For years he sailed with a club of the same boats. One winter my friend made the dagger board and rudder into NACA foils. He reported that boat wasn't faster but he could point several degrees higher and became the "fastest" sailor in the club.

[antipas]

The question is not really if a foil shaped cb would result in better performance,although I strongly believe if nothing else, steering should be more efficient if the rudder sits behind a nicely shaped foil.
Since the design calls fr foil shaped rudder and cb one might as well attempt to do it.
In the case of Romilly the cb thickness is 10% of chord length at worst and more towards the bottom as it is triangularly shape.
strength should not be an issue as the cb embodies a steel plate and will be sheathed with 2x270gsm fiber.
The trailing edge is 6mm or abt 1/4"thick .
drawing cl's fm top to bottom is doable but one is only left with one permanent line (the one at max thickness)which in this case is at 30%chord fm leading edge.
thank u all fr input.

[merlinron]

I sometimes hear that that trailing edge should not come to a point but should be squared off. Birds, fishes, airplanes do not have squared off blunt trailing edges, it just does not make sense. I always bring mine to as fine an edge as I can without it getting to thin as far as being easily damaged.

i think the rule of giving the trail edge of a foil some square-ending is to minumize damage to it as much as possible. a nice fine trailing edge that has been dinged up causes more eddies and will stall sooner than a slightly blunt, but clean trailing edge.

[John Meachen]

I would like to reassure antipas that a centre line on the leading edge and one on the trailing edge wil be visible after shaping.These lines may be invaluable in establishing that symmetry exists and can be used for the purpose only for as long as they are not covered or sanded off.

[Woxbox]

At sailboat speeds, is precise foil shape really important? Could one actually TELL the difference between the 'correctly' shaped foil and one that was close when it comes to performance?



Theoretically, at least, a poorly formed board will vibrate like crazy at higher speeds, while a good foil shape will not. I can attest to the fact that flat sided centerboards and daggerboards vibrate in their cases, but I've heard noise from factory-made boards that at least look like they have a good foil shape. Of course, if you have boys on board, they get a kick out of it when you're going so fast it sounds like someone started up a motor!

[merlinron]

it's common for the 1/4 steel plates used on lasers to vibrate. they do get going pretty fast. i would think reguardless of how thick or what the chord of your board is, the more exactly symetrical it is made the less the chance of vibrating. unless of course it is simply too thin and weak.

[kc8pql]

Here's a very good, easy to understand explanation of NACA foils, which series to choose for rudders, fin keels and centerboards, why you would go to all the trouble, and an easy to understand table showing how to lay out a true NACA foil. http://www.boat-links.com/foilfaq.html

[wscherf]

Northwest Marine Design, the people that sell plans for the K-16 daysailors, have a nifty method of accurately making foil-shaped boards or rudders. The structure is built up in 3 inch lifts, aligned by dowels. Each lift defined by a paper pattern glued to the top, and is cut out on a bandsaw. The lifts are then glued up in a stack, and worked to final shape with planer and belt sander. The method provides one accurate line every three inches. No templates or gauges required.

[Lazy Jack]

From the link posted above on NACA foils:

"As a leeboard, centerboard or daggerboard the difference between the plate and foil would be much less pronounced because the boat is usually moving "crabwise" through the water at a much smaller angle than you'd use at the helm when tacking. As long as it doesn't vibrate, a flat plate is probably as effective and a lot less hassle than a shaped foil section."

[merlinron]

i read that too, and it makes sence because it's actually a matter of "angle of attack" that makes the board do it's job.

[Chan]

Who is this NACA fellow anyway? And what makes him so smart???

[JimD]

Who is this NACA fellow anyway? And what makes him so smart???

NACA = NACA (National Advisory Committee for Aeronautics, NASA's predecessor)

[Steve Lansdowne]

http://www.duckworksmagazine.com/06/howto/foils/index.htm

[kc8pql]

Here's something else interesting to play with. It's a foil profile generator:
http://www.pagendarm.de/trapp/programming/java/profiles/NACA4.html

[Woxbox]

i read that too, and it makes sence because it's actually a matter of "angle of attack" that makes the board do it's job.


It's totally true that it's the angle of attack that makes a foil work. But having a good shape dramatically increases the lift/drag ratio.

This is a pet peeve of mine -- every science book or science museum display I've ever seen claims that a wing has to be curved and asymetrical to provide lift. Now every kid who ever played with a foam or balsa (my youth) plane knows that this isn't true. Those toys have flat wings and can fly.

And, of course, high-performance aerobatic planes have symetrical wing sections, and boat foils are symetrical. The angle of attack is the main thing, but the foil shape influences efficiency in a big way.

[epoxyboy]

If you are to stand any chance of achieving accuracy,one point needs to be made.At the outset of the job it is imperative that you mark a centre-line from top to bottom of both the leading edge and the trailing edge.You need to define the centre plane of the foil in order to determine symmetry,regardless of the shaping process chosen.

Couldnt agree more - it's also the best opportunity you will have to minimise twist in the finished foil. I tacked a thin batten along the marked centre lines at the top and bottom of the CB blank and sighted along them to check they were parallel - you can almost guarantee that your blank will have some degree of twist to begin with, or will develop twist as you remove material. If you dont mark out these lines as John suggests, you are almost flying blind.
I shaped my CB with a plane using these lines and a "half foil" cardboard template. It is close but not perfect, but then its not a racing boat so I dont really care.

Pete

[antipas]

Varna quote
You will need a few jigs for a tapered foil but this approach the jigs were easy to make.

John thanks would certainly need few jigs as the cb tapers fm a max 17" to abt 8" it is also imperative to hv a has been suggested cl's as guides.
Since my cb has a steel plate embodied at the centre of laminations another way to keep some control would have been to drill holes at certain chord lengths and measure the half breath height using a caliber,but such is life that my steel plate is a slightly bend (fm cutting torch)so this method although laborious is not workable in this case.
As fr the rest hull is completed and being painted before it's turned right side up for interior completion etc.
On the masts (apart of the cb)have some dilema as designer's preference fr this boat is carbon fibre mast abt 25ft L which if build in wood would add 77 pounds in weight (not exactly where u need it most) and is also frightfully expensive if ordered,but will cross that bridge later.

[Chan]

So what happens when the naca foil centerboard is only partially lowered?

[John Meachen]

For Varna's consideration.epoxyboy describes how the centreline at each edge can be used.The position of maximum width can also be marked once the taper in the section has been checked,as it will remain after shaping.You may be correct in your belief that perfection is not critical as the boat will still sail.The differerence may be that an efficient foil will make it possible to return to the home mooring when the wind is dying and the tide is about to turn rather than spending a night anchored in an out of the way spot with not much left to eat or drink after a spell away.The comparison with aircraft becomes more relevant if you take into account the different densities of the fluids they operate in.
The question about a NACA section being partly raised would only really come into effect when the boat is in shallow water.At other times,there is the likelihood of better performance due to the better lift/drag characteristics of a enuine foil section.I would be extremely interested to learn where Varna has found evidence of a NACA section foil allowing more leeward slip than a flat plate.

[Nicholas Carey]

Currently building the Romilly spv,the designer calls fr both tiller and centerboard to hv the underwater sections shaped into aerofoil(section offsets given)

Given that the original poster is building ROMILLY SPV, a design by Nigel Irens (http://www.nigelirens.demon.co.uk/) and Ed Burnett (sometime contributor to this august body), I suspect the builder has got a perfectly good foil design.

Nigel Irens' multihulls hold several world records, including Ellen MacArthur's non-stop solo circumnavigation (71 days, 14 hours 18 minutes) in the Nigel Irens' design B&Q/Castorama -- average speed 12.66 knots. Ellen MacArthur also set (and holds) the trans-Atlantic women's record with B&Q/Castorama, making the crossing from Ambrose Light (NYC) to Lizard Point in Cornwall in 7 days, 3 hours 50 minutes (average speed 17.09 knots). A number of other Nigel Irens designed multihulls have set and hold speed records in various other categories.

Another Nigel Irens multihull, the catamaran DOHA, (http://www.doha-2006.com/gis/menuroot/spirit/Aroundtheworld/TheBoat.aspx) which was built in 1984 as an 80-footer and has since been upgraded to 102 feet is poised to challenge MacArthur's circumnavigation record (they're aiming at coming in at under 70 days).

[Wild Wassa]

I get some fairly desperate bits to refinish.


http://www.imagestation.com/picture/sraid215/p85c84a75401ffc5d5ed77dcaf4f1cfda/ecdabdbe.jpg


Tasar CB trailing edges


http://www.imagestation.com/picture/sraid215/p7a88f96fea02f210f15f359b9951d5ce/ecdabda3.jpg


Above, stripped, CPES'ed, epoxied and refilled ... now for making true.

I just couldn't get by without my trusty tungsten carbide scraper ... for bringing foil shapes back to life.


http://www.imagestation.com/picture/sraid224/pd0bb5b765ba6f0c988743670cbcf4d8e/eaadb382.jpg


Warren.