Results 1 to 29 of 29

Thread: NACA- what?

  1. #1
    Join Date
    May 2010
    Location
    Port Townsend, Wa.
    Posts
    861

    Default NACA- what?

    I apologize because I know the forum has covered this before, but it was never when I needed to know, :-) please advise where to look. An article in the Gougeon "epoxyworks" magazine seems to recommend a NACA - 12 foil for a rudder or centerboard. In broad terms this means the maximum thickness of the board is 30% aft of the leading edge, and is 12% of the fore / aft length of the board. This makes a fatter 'board than I am accustomed to seeing. For example, a board that came out of a slot at 1 1/8" thick, would only be 9 3/8" wide fore to aft, I would expect more. I get more confused in the condition as the board is raised and angles up into it's trunk. In this case the board gets much longer fore and aft, making a slimmer section than the article, and if one tapers the board in thickness as it gets deeper, the section could get thinner still. Please point me to existing wisdom.... If it matters, this is a displacement-speed-only sail boat 20' oa.


    Ken

  2. #2
    Join Date
    Aug 2000
    Location
    Bay of Islands,N.Z.
    Posts
    29,562

    Default Re: NACA- what?

    It's common on keel boats to have a range around 9 in a keel, 12 in a rudder.

  3. #3
    Join Date
    Jan 2004
    Location
    Norwich,United Kingdom
    Posts
    8,827

    Default Re: NACA- what?

    I have never seen a small boat centreboard that approached 12% thickness to chord and my own is a mere 6%.I suspect the ratio as applied to keelboats is related to the need for enough ballast and a more slender keel would be a bit lacking.There are plenty of instances of a 1/4 inch steel plate working quite well and that pushes the limits a lot if you pay attention to the pundits.

  4. #4
    Join Date
    Apr 2012
    Location
    Vermont
    Posts
    1,613

    Default Re: NACA- what?

    When I built the boards for my Jewell, the plans called for a NACA 0015 foil for the CB, and 0012 for the rudder. Because the centerboard doesn’t hang vertically, I chose to shape it along the red line, with a longer chord than the shortest line from leading to trailing edge:
    656C51B5-DC4F-4C10-BDF9-418C0981EBC4.jpg
    I made a template from a pattern:
    1A0BA8D1-8085-49BD-BD86-5A1DA5E1403A.jpg
    and here is an image before encapsulating the shaped board with glass and epoxy. It’s pretty fat:
    DAB46943-9ADE-47E6-95DC-E83D0E5C30A5.jpg
    It’s a new enough boat to me that I’m still getting to know her, but I’m well pleased with her performance so far.

  5. #5
    Join Date
    Sep 2010
    Location
    Bainbridge Island WA
    Posts
    4,459

    Default Re: NACA- what?


    I am building an 0012 rudder right now, that seems to be the "standard" profile for this appendage.

    ETA: Alex Zimmerman has a great write-up on developing foils in the July 2020 Small Boats Magazine.
    Last edited by stromborg; 12-12-2021 at 07:31 PM.
    Steve

    If you would have a good boat, be a good guy when you build her - honest, careful, patient, strong.
    H.A. Calahan

  6. #6
    Join Date
    Jul 2020
    Location
    Norwich, Norfolk, UK
    Posts
    1,019

    Default Re: NACA- what?

    Whilst a NACA is preferable over the width / full length of the board, it is common practice to just round the leading edge and taper the aft end.. This is most common in dinghies but many much larger boats have done this..

    Otherwise for a comparatively low speed boat, you are needing a very wide centreboard box and a thick centreboard both of which will increase drag..

    On my mini fixed Keel boat I would have ended up with a 6 inch thick keel on a 16ft boat.. So what I did was choose the NACA009 shape for a 3 inch thickness keel then cut it at the widest point, inserting a Flat between to get the width of keel I needed..
    I suggest you choose the NACA for the thickness of centerboard you need at the narrowest plan width of board and do the same widening the flat as it goes up to the hull...

    Note, shallow draft waters here, required a wider keel to get the necessary area to balance the sails..
    Last edited by The Q; 12-13-2021 at 04:56 AM.
    Just an amateur bodging away..

  7. #7
    Join Date
    Jun 2021
    Location
    Decorah, Iowa, USA
    Posts
    85

    Default Re: NACA- what?

    This page http://www.boat-links.com/foils.html has the equations for the curves for "parallel-sided foils" by Neil Pollock "which, for low speeds, is nearly as good as the NACA 0004 section." These can be applied to any plank. The leading edge is faired over a length of 4 times the thickness and the trailing edge is faired over as long as possible, both to the curves given on that page.

    I'm planning to use those curves to make templates for fairing the rudder and leeboard on a plywood skiff.

  8. #8
    Join Date
    Apr 2018
    Location
    Bay Area, CA
    Posts
    205

    Default Re: NACA- what?

    I had the same question about profile and board width. See here for many helpful suggestions:

    http://forum.woodenboat.com/showthre...15#post6230115

  9. #9
    Join Date
    Jan 2009
    Location
    Narragansett Bay and Approaches
    Posts
    280

    Default Re: NACA- what?

    There are lots of options and actually finding the best section for your boat is a bit of an adventure.
    12 % sections are a good compromise of strength/weight/ hydrodynamic efficiency. That being said Paul Zander is correct that thinner sections are less critical and you can be less fussy with your shaping....but be symmetrical and careful with the shaping and finish of the leading edge.
    There are plenty of Airfoil resources on line so with a printer you can build really good templates. I prefer the “63” series over the “000” series. The 63010 is my default if I don’t have an obvious better answer. This is a 10% section with a slightly easier front half than the 000 foils.
    SHC

  10. #10
    Join Date
    Dec 2003
    Location
    Portland, Oregon
    Posts
    78,258

    Default Re: NACA- what?

    See Michael Storer's writings for more on the efficacy of 'near-NACA' (parallel sides) foils. He now specifies them for all his small boats, which makes fabrication a bit easier.
    David G
    Harbor Woodworks
    https://www.facebook.com/HarborWoodworks/

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  11. #11
    Join Date
    Sep 2008
    Location
    Victoria, BC, Canada
    Posts
    1,632

    Default Re: NACA- what?

    In all my reading about foils for centreboards and keels, the research that I found said that thicker foil sections tend to have higher stall angles, albeit at the cost of some increased drag due to extra surface area. While the benefit of high stall angle for a rudder is obvious, at first glance you might think it would be less critical for a CB or a keel.

    However, I am convinced that keels also benefit from thicker sections because the effective angle of attack moves around a lot as the water surface gets rougher. It is difficult for the best of helmsmen to hold a dead straight course in rough water, with the result that the boat wanders back and forth some. Add the pitching and heeling into the picture and I think the angle of attack can be quite variable well off the ideal.

    My own limited experience with a perfectly NACA-shaped but thin foil was that it worked very well in flat water and allowed the boat to point fairly high (for a gaff rig, that is) but lost 10-15 degrees of pointing ability as it got rough.
    Alex

    "The fishermen know that the sea is dangerous and the storm terrible, but they have never found these dangers sufficient reason for remaining ashore.
    - Vincent van Gogh

    http://www.alexzimmerman.ca

  12. #12
    Join Date
    Sep 2010
    Location
    Bainbridge Island WA
    Posts
    4,459

    Default Re: NACA- what?

    What I remember reading was that the 12% (thicker) foil works well for rudders because of that higher stall angle, makes sense, right? You need to be able to push the helm over to turn and having the blade stall isn't going to be very helpful!

    Centerboards on the other hand can be thinner (8-9%) as the board "sees" an angle of about 3 degrees as the boat moves through the water. As Alex points out, that angle changes a lot once the boat starts moving around. There is a lot of chaotic swirling around under our boats, it's no wonder NAs are still messing about with underbodies.

    The 0012 rudder I'm building for Marianita is an experiment, I don't think the result can be any worse than the one on there right now. As Iain drew it, most of the rudder sits squarely in the shadow of the hull, a 2" thick slab-sided lump that acts more like a brake than a trimming surface. The new blade will separate from the keel at the waterline and match the bilgeboards in draft so there will be a solid couple of feet working in clean water to move the boat. Looking forward to trying it out next season.
    Steve

    If you would have a good boat, be a good guy when you build her - honest, careful, patient, strong.
    H.A. Calahan

  13. #13
    Join Date
    Aug 2000
    Location
    Bay of Islands,N.Z.
    Posts
    29,562

    Default Re: NACA- what?

    Same with sails , the harder or more exotic the material and flatter they become the less groove there is, easy to fall out of it. Cruising boats tend to have fat forgiving foil sections on rudders particularly for those reasons.
    I like the 63 sections, I've always thought you can trick a rudder into thinking it has more balance than it has by shifting the chord back to 35%.
    Thats in my round tuit file when I take mine off our ketch. It's between 11 and 12 % chord and close to 00 sections.

  14. #14
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    27,835

    Default Re: NACA- what?

    The other issue is aspect ratio. Low aspect foils stall at lower speeds. For a given width of centerboard case, a longer chord length will stall at a lower speed, even if the board is proportionately thinner.

  15. #15
    Join Date
    Sep 2018
    Location
    MO. USA
    Posts
    171

    Default Re: NACA- what?

    Ken,
    Most of my smaller boats wetted foils have a PMB "parallel middle body" section of various proportions on the profile of the Keel, Dagger, or Rudder. As David G touched on in post #10.
    My One Class 14' Jet just has a flat plate swing keel of steel.

    Untitled 3 copy 2.jpg
    Last edited by John Howland; 12-21-2021 at 10:43 AM.

  16. #16
    Join Date
    Jan 2009
    Location
    Narragansett Bay and Approaches
    Posts
    280

    Default Re: NACA- what?

    It's not as much stall angles as the low drag bucket.
    Anyone who has replaced a flat sided board with properly shaped board will tell you it makes a difference.
    Maybe not as much as having a smooth clean bottom, but it is worth the trouble.
    One of the conceits of this group is that we share an appreciation for finer sorts of boats and that we are willing to do the extra work to enjoy those boats. I apply the same logic to sailing performance, there is "OK," "Close to Good," "Good," and "Enjoyable." You can talk yourself into some sort of value proposition where you are happy with less because it was cheaper and or easier to achieve. But WTF if you are going to get the tools out and some pretty valuable wood, why not do the best job you can? It's actually fun.
    SHC

  17. #17
    Join Date
    Aug 2000
    Location
    Bay of Islands,N.Z.
    Posts
    29,562

    Default Re: NACA- what?

    Quote Originally Posted by SHClark View Post
    It's not as much stall angles as the low drag bucket.
    Anyone who has replaced a flat sided board with properly shaped board will tell you it makes a difference.
    Maybe not as much as having a smooth clean bottom, but it is worth the trouble.
    One of the conceits of this group is that we share an appreciation for finer sorts of boats and that we are willing to do the extra work to enjoy those boats. I apply the same logic to sailing performance, there is "OK," "Close to Good," "Good," and "Enjoyable." You can talk yourself into some sort of value proposition where you are happy with less because it was cheaper and or easier to achieve. But WTF if you are going to get the tools out and some pretty valuable wood, why not do the best job you can? It's actually fun.
    SHC
    I'd appreciate your thoughts on my my comment above about the 63/35% chord sections.

    By coincidence I was watching a video on designers and Naval architects that Gareth put me on to and a 63 series keel at 10.5% chord was discussed .

  18. #18
    Join Date
    Jan 2004
    Location
    Norwich,United Kingdom
    Posts
    8,827

    Default Re: NACA- what?

    In the late nineties I had read Frank Bethwaite's book and been convinced by his arguments and supporting evidence that a low drag section was the way to go.So I made a new centreboard and it was in the boat for the practice race of a regatta held on a shallow piece of water.Arriving at the windward mark first justified the work in my eyes and had we not capsized near the gybe mark it might have been possible to hold the position.Then while milling around waiting for the next race to start we hit something hard underwater and obviously had to race in that condition,the difference was noticeable.It really is an endeavour where near enough isn't good enough if performance counts.

  19. #19
    Join Date
    May 2010
    Location
    Port Townsend, Wa.
    Posts
    861

    Default Re: NACA- what?

    Ok, I am trying to summarize what I have seen here; I gather that NACA 12 is good for a rudder, but might be fatter than necessary for a centerboard. This is all for a Beachcomber / Alpha dory which will be a firmly displacement, not a surfing boat. I am constrained by a CB slot which limits the thickness to about 1 1/8" and the profile to about 13". However this board will never go further down than about 45 degrees. This means that the longest chord along the water flow path will be about 18 3/8" If I understand what I have read, it means that my thickness will be about 6% of flow path, and will decrease as the board is raised part-way. So my question is: should I keep the thickest point of the centerboard at 30% aft of the leading edge of the water path over the surface, move it to 50% as one of the drawings above suggests, or something else? No matter what else, I will probably want to keep the very lowest tip of the board slightly fat, backed up by 1/4" G-10, and sheathed in GRP against contact with rocks, yes?
    Ken

  20. #20
    Join Date
    Oct 2018
    Location
    Vigo, Galicia, Spain
    Posts
    555

    Default Re: NACA- what?

    "However, I am convinced that keels also benefit from thicker sections because the effective angle of attack moves around a lot as the water surface gets rougher"

    Exactly

    In this issue there are three variables that are sometimes forgotten: (1) wind tunnel data comes from perfectly manufactured profiles (2) cleanliness and fouling ... and as you rightly point out (3) water turbulence and sailboat movements

  21. #21
    Join Date
    Jan 2009
    Location
    Narragansett Bay and Approaches
    Posts
    280

    Default Re: NACA- what?

    Quote Originally Posted by kbowen View Post
    Ok, I am trying to summarize what I have seen here; I gather that NACA 12 is good for a rudder, but might be fatter than necessary for a centerboard. This is all for a Beachcomber / Alpha dory which will be a firmly displacement, not a surfing boat. I am constrained by a CB slot which limits the thickness to about 1 1/8" and the profile to about 13". However this board will never go further down than about 45 degrees. This means that the longest chord along the water flow path will be about 18 3/8" If I understand what I have read, it means that my thickness will be about 6% of flow path, and will decrease as the board is raised part-way. So my question is: should I keep the thickest point of the centerboard at 30% aft of the leading edge of the water path over the surface, move it to 50% as one of the drawings above suggests, or something else? No matter what else, I will probably want to keep the very lowest tip of the board slightly fat, backed up by 1/4" G-10, and sheathed in GRP against contact with rocks, yes?
    Ken
    You may find this useful
    http://www.paulzander.biz/centerboards.html
    However he does not provide the guidance on determining how large your centerboard has to be. There is no doubt that it doesn't have to be as large as what you have shown, but getting it right takes a few passes.
    SHC

  22. #22
    Join Date
    Sep 2010
    Location
    Bainbridge Island WA
    Posts
    4,459

    Default Re: NACA- what?

    I've been digging through my library trying to find something definitive on sizing centerboards and rudders but it is a subject of some debate. I've made my best guess based on what others have done on similar sized boats. Just waiting for a late spring haul-out to put it to the test.
    Steve

    If you would have a good boat, be a good guy when you build her - honest, careful, patient, strong.
    H.A. Calahan

  23. #23
    Join Date
    Jan 2009
    Location
    Narragansett Bay and Approaches
    Posts
    280

    Default Re: NACA- what?

    To figure out the size you need to know 2 things.
    1) How fast you go. Velocity is a second order function so it has a greater impact than anything else.
    2) How much side force you need to generate.
    Side force is a function of righting moment (Ft/Lbs or equivalent) divided by length of overturning arm ( distance between vertical center of effort sails and center of pressure centerboard.
    Lift = V (ft/sec)^2 * A ( ft^2) *Cl (coefficient of lift)
    The low drag bucket of most airfoils is usually at a Cl of less than .3 so for first approximations you can use that.
    Using approximate numbers from a 110.
    Righting moment is about 2000 ft/lbs @ 10 degrees heel with the crew on the trapeze and the helm hiking on the rail.
    Sail force is about 200 lbs, so the keel has to generate 200 lbs of lift.
    Upwind boat speed is about 5 knots ( 11.3725 ft/s)
    Your target maximum Cl is .3
    Your first approximation is 5.15 ft ^2
    If your boat speed is 6 knots, you need 3.73 ft^2
    A 110 keel is 4.95 ft^2 which is kind of in between the two.

    You refine this by repeating the process with increased information about the plan form, aspect ratio, and real section data as per the Paul Zander article above.
    SHC

  24. #24
    Join Date
    Jan 2004
    Location
    Norwich,United Kingdom
    Posts
    8,827

    Default Re: NACA- what?

    I read the Paul Zander article and my take is that the figures for a typical keelboat relate to the (very low)Reynolds numbers for such a boat.A smaller boat will be operating in a different range and may not have the need for a substantial amount of ballast to be incorporated within the volume of such a keel.A classic style of boat like a Beachcomber/Alpha would probably not have been outfitted for racing with the knowledge that we now have applied to it.I imagine it would have stayed within the boundaries of what was normal at the time.We now have much better sail cloth and the potential increase in drive that comes from it so it would be a shame not to capitalise on that extra drive by using an equally good set of surfaces underwater.I have a feeling that some gains may be had by allowing the board to pivot down a bit more and this might lead to a bit of thought about how to control it's position,or indeed where to locate the pivot bolt.

  25. #25
    Join Date
    Sep 2002
    Location
    On the river, Auckland, New Zealand
    Posts
    7,183

    Default Re: NACA- what?

    Quote Originally Posted by David G View Post
    See Michael Storer's writings for more on the efficacy of 'near-NACA' (parallel sides) foils. He now specifies them for all his small boats, which makes fabrication a bit easier.
    I use a very similar section to that which Mik uses, one of the benefits in a small, lightweight boat is that the higher stall angle of the thicker nose on that foil makes the boat much more reliably manoeuvrable at low speeds in rough water. As well that higher stall angle is very helpful sailing to windward in a chop.
    If I'm drawing a pure racer for a skilled helmsman, thats a different proposition and I have a range of foils for that, depending upon other characteristics of the boat.

    John Welsford
    An expert is but a beginner with experience.

  26. #26
    Join Date
    May 2010
    Location
    Port Townsend, Wa.
    Posts
    861

    Default Re: NACA- what?

    I am very appreciative of all this information and am still listening, so thanks and please keep it coming if you have the time. Re: John Meachen, above, The Beachcomber & Alpha were competitive racing classes in their day, but I can't recall seeing any small boat designs of that era which paid much attention to careful design of the elements we now call appendages. Perhaps this was a missed opportunity at the time? The boat I am reworking and Gardener's drawings show a superficial rounding of the centerboard and rudder. However, the boat is for the Northwest Maritime Center and will be used in teaching / livery programs, so it makes sense to me to build some "teachable moments" into it, such as the cross-sectional shape of the board. Hence, the board will be hinged at the top with a removable trunk-cap so it can be easily pulled out for 'show-and-tell' and to clear possible gravel in the case. Hinging it at the top and the resultant change of shape moved the geometric center of the submerged board forward a couple inches, so we have lengthened the trunk and board aft as much as it seemed prudent considering the rowing position there, and it was decided to keep the trunk ID 1.25". Those choices became the constraints for the rest of the design. The current status is that I have a centerboard blank about 52" long, x 13" x 1.125" thick, which will only drop downward to 45 degrees due to the top hinge, so it will become a much finer section in use due to the angle. I have embedded strips of G-10 in the leading and trailing edge. This for durability in the leading edge, and for strength in feathering the trailing edge. We haven't started to shape the cross section yet. However I took the NACA 12 drawing from early in this thread, and stretched it to different amounts horizontally and vertically in photoshop to make it usable as a pattern for the stock I have created.
    Are there any other ideas, suggestions, critiques, etc?

    Ken

  27. #27
    Join Date
    Sep 2010
    Location
    Bainbridge Island WA
    Posts
    4,459

    Default Re: NACA- what?

    Ken-
    It might be a bit late for this now, but when Clint Chase developed the Deer Isle Koster I built he put a fixed pin in the centerboard and corresponding tracks in the CB trunk so you could remove the cap and pull the board out. Nice way of eliminating a couple of holes in the trunk at or below the waterline like they so often are.
    Steve

    If you would have a good boat, be a good guy when you build her - honest, careful, patient, strong.
    H.A. Calahan

  28. #28
    Join Date
    May 2010
    Location
    Port Townsend, Wa.
    Posts
    861

    Default Re: NACA- what?

    Quote Originally Posted by stromborg View Post
    Ken-
    It might be a bit late for this now, but when Clint Chase developed the Deer Isle Koster I built he put a fixed pin in the centerboard and corresponding tracks in the CB trunk so you could remove the cap and pull the board out. Nice way of eliminating a couple of holes in the trunk at or below the waterline like they so often are.
    Yes, and there is also historic president on the Great Lakes for having a rod down to a pivot on the lower- forward end of the board in addition to whatever rig lifts the aft end so the whole business could be pulled out that way. On my own boat I ran a piece of heavy bronze track down the forward end of the trunk with a pivot that rides on it. On the dory however I was worried about the solution you describe from Clint due to questions of how to glass-and-googe that guide slot sufficiently on a boat that will be often wet, going from barely on the beach to water a lot. Anyway, I am committed to the current pivot pin and geometry, but maybe someone else who reads this will benefit.

    Thanks,

    Ken

  29. #29
    Join Date
    Jan 2004
    Location
    Norwich,United Kingdom
    Posts
    8,827

    Default Re: NACA- what?

    Thanks for the name checked response, my comment about the racing part of the Beachcomber/Alpha heritage was meant to apply to the use of synthetic sailcloth and should probably have been clearer.As a great fan of the work John Gardner did in documenting the heritage of small boats in his home waters,I had gathered that the dory racing era saw some keen competition.Given that the sails are almost guaranteed to be more efficient than those of the past,I can appreciate the desire to apply good foils to the hull.The big decision revolves around how much contemporary knowledge do you wish to draw from?A true NACA section that would fit in the width of the case and have an effective chord of perhaps 26 inches would be rather delicate.Is a possible solution a more vertical leading edge?It may not sit well with the traditional metal rod for lifting and lowering the board,but it would work better and move the thickness/chord ratio a bit nearer the current norms.

    For the pivot arrangements,I have often recommended the use of an oversize hole in the board to accept a bush that is a tight fit in the case as it permits the use of a bolt that uses s/steel washers to bear on rubber washers on the outside of the case, without the risk of distorting the case or pulling the sides form the bedlogs.



Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •