Easy to build, cheap, foils?

Discussion in 'Multihulls' started by Rolf Nilsen, Jul 19, 2007.

  1. Rolf Nilsen
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    Rolf Nilsen Junior Member

    I know this have been discussed earlier, but searching dont give any conclusive facts so I risk starting this topic over again..

    We are to build three sets of daggerboards and rudders for three F-16 catamarans. The consortium are made up of three non-professional boatbuilders (computer admin, bean counter at a bank and videophotographer). We have done wood/epoxy projects before, so we know a bit about what we are doing. Most of the project have been tought out, but the foils remain. This post is a cry for inspiration on how to build no less than 6+6 foils easily resulting in fast and stiff boards. I think we will go with a rectangular planform to make them easier to build.

    Option one is hand-shaping foam by sanding with a template or hot-wire and use carbon as skins. Some fiddling need to be done where the foils enters the hull to take compression/buckling loads there or overbuild. Relatively time consuming and expensive (carbon).

    Option two, hand shaping wooden boards. Takes time and the boards will be heavy. Result not guaranteed to be of NACA standard and will vary between boards.

    Option three, buy the foils. $$$.. not an option, 'nough said.

    Option four, fold plywood and use a shaped stringer at the front, cover with glass. Not easy to shape and might have structural issues.

    Option five, do plugs and pull off some moulds. Lay up carbon skins with shear web and a stringer. Expensive and time consuming.

    Option six, build a female mould in plywood and vacuum veneers into shape. Put in shear web and a stringer before gluing and glassing. Should be cheap, stiff, light and strong. I heard this is how Marstrom do his Tornado foils.

    Other opinions and options from creative minds?
  2. Guest625101138

    Guest625101138 Previous Member

    Moulds seem the obvious choice but it will take a bit of time to get six of each done. The curing time can be a dampener on progress obviously.

    Light gauge aluminium can be formed up very quickly for making the plugs. It gives nice smooth finish with no effort. Just cut it with a cutting disc rather than shears so the edges are not stretched. I have even used aluminium sheet folded over aluminiun tube or flatbar for making test foils. It can be bent around wooden or foam formers and epoxied or just glued to the formers. Clamp and glue the trailing edges together.

    Other ideas to throw into the mix -

    You can stiffen up a moulded glass or CF shell using stainless tube and epoxy loaded with microbubbles. Weld tabs on the tube so the epoxy grips it.

    I have attached an image of the grind lines I use for hand shaping a prop foil from stainless flatbar. There are two grind lines, one along the front face 12mm from the leading edge and the other 17mm from the trailing edge. You can see that it is possible to get most of the material removed as flat surfaces for this prticular foil. It is just a matter of smoothing the curves after forming the flat faces. This section has very high L/D at low angle of attack.

    Making 6+6 foam formers means you can have them all on the go at the same time rather than 6 by 2 in series if you make two moulds. The foam can be easily dissolved if you want and replaced with epoxy/microbubble and stainless siffener.

    You should aim to get a reducing chord at the end as this improves lift to drag. No idea of the class rules but here are some foils for inspiration.
    These look better than I make.

    NACA 0020 sections make powerful rudders if they are not ventillating. You can use about 1/3 of the area of a stern hung rudder that ventillates. Again no idea of the rules and it means the rudder cannot be folded up if it is mounted under the hull. (I don't think many people realise how ineffective a foil becomes once ventillating)

    Rick W.

    Attached Files:

  3. Rolf Nilsen
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    Rolf Nilsen Junior Member

    If we do moulds, we would only do one set for daggers and one for rudders. There is no rush as this is something we do in weekends and evenings.
    An F-16 is 16'4" long by 8'2" wide with a min. weight of 107Kgs. Stainless steel except for fittings etc. would be way too heavy. Wood composite looks attractive, but a way to produce them is the challenge. It is very tempting to set up a female mould and build a test foil in strip+internal ribs and stringer like an old fashioned airplane wing.

    The dissolving foam trick + stainless stiffener would probably be overkill in this application.

    I think we will use NACA0007 for daggers and 0012 for rudders. Easy and simple as that. If we do a more advanced mould we could build a plug with nice -low drag- planform. With a rectangular foil we would be done with just one mould. How large is the increase in drag to lift with a rectangular planform compared to a more rounded tip? Is it worth it for homebuilders? Would having 'winglets' on the end of the rudder change the equation, as we intend to do so.

    Phills foils is excellent, but buying and shipping is expensive. It's also more fun to do it yourself.

    Thanks for the opinions and tips. Just what I was hoping for!
  4. Otter 33
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    Otter 33 Junior Member


    Easiest way to make more than 1 foil is to make a mold. Fairing and finishing the foils will take up most of your build time and thus why not put that into one mold instead of duplicating the effort over and over.

    You probably don't want to hear this.... but the method that will make the most economical sense (if you consider your time more valuable than $5 an hour) would be to CNC cut female molds. You could have elliptical planforms cut from Baltic birch plywood or mdf (elliptical takes the same amount of time on a CNC to cut) and surface the molds yourself and come out way ahead in the long run.

    I have to admit that I make foils professionally and have a CNC machine... not trying to sell my services here... just have been down the hand build road one two many times.
  5. Hobiestoke
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    Hobiestoke Junior Member

    the mold idea sounds best if you have to manufacture the foils yourself.

    I have a friend that made foils out of foam then clothed/glassed it but they ended up being way too bouyant. I made the rudder blades for my cat from shaped/epoxied maple and spent alot of time shaping/glassing them.
    But after initial testing, found out that even though they looked good and seemed to function ok from a technical standpoint, I was having tacking problems unless I had a great deal of speed already built up when heading into a tack. I ended up thinking there was too much 'slop' in the rudder system I fabricated even though everything seemed to be looking/functioning properly.

    I Finally solved this problem by purchasing some Hobie 16 rudders/tillers etc. off of ebay for $100 and cut and altered the rudders/hardware to fit my cat - the difference was amazing once I tested the cat with Hobie blades on it. No more tacking problems, it felt like I had been towing a cow before and now the boat is much faster, tack and performs much better.
    good luck!:)
  6. Hobiestoke
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    Hobiestoke Junior Member

    Heres A link to a Pic from the woods designs website of a 14ft Pixie utilizing 2 rudder/tiller setups from a Topper dinghy. A Topper is like a laser, very popular and manufactured in the UK. if you look at the pixie page on the site the second photo down shows the dinghy blades being used....

  7. Rolf Nilsen
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    Rolf Nilsen Junior Member

    We have buildt several rudders and centerboards for our tornados and have some experience with hand shaping wooden foils. We also have lots of experience in breaking them :) Problem with hand shaping is getting the sectional shape perfect and the foils straight. We have learnt the hard way to NOT USE PLYWOOD in foils. Even with heavy tri-axial glas on both sides.
    Poor performing rudders are often either not a god foil (uneven, soft/bendy, poor sectional shape, not straight etc), or set up wrong. Getting the right Ackermann angle helps in turning.

    I liked the CNC mould shaping tought. I'll do some drawings and send to a local CNC shop to get a price. If we manage with just one mould for the rudders and one for the daggers it could be economical viable. Doing a mould with ribs and veneers like for a beam or mast would probably be OK.
    We would definately like to build the foils ourself.

    Is there any experience here with hollow wooden foils? Any scantlings?
  8. Guest625101138

    Guest625101138 Previous Member

    The benefit of an eliptical planform is a function of the lift coefficient and the foil aspect ratio. This gives the relationship:
    For a low speed foil working at moderate lift an eliptical planform will have say 10% benefit with an AR of 10:1 and 20% benefit with an AR of 4:1.

    This java applet is really good to play with if you want to get an understanding of foils:
    You can download the applet if you want so you do not need to connect to use it. It is much easier to use than Xfoil (IMHO). It is set up with air paramaters but these can be changed and do not make much difference for the same Re# anyhow. The options tab allows you to set the fluid parameters and you can also set the foil AR. You cannot set the Oswald efficiency. I gather it assumes this is 1 i.e. eliptical.

    You will see that the important part is the nose section. This is the part to get right for foils that need to work over a wide range. As I said I like a NACA 0020 for a rudder because it just keeps powering on and gives very high lift coefficient up to 1.7. If the rudder does not ventillate you can get tiny planforms that are still effective.

    I would be very interested to see the detail around the leading edge of the foils that Hobiestoke made. It sounds like they were too pointed and stalled out. I have experienced the same thing with a Bolly boat prop that has a sharp nose. A sharp nose has a very narrow operating range. You can see this if you use the circular arc option in JavaFoil. Compare the lift at say 10 degrees with the NACA 0020 and the arc foil.

    I do not believe it is difficult to shape a foil. You just need to know what is important. Some even have bumps to overcome a particular transition condition.

    Rick W.
  9. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    This is something so many forget when they try to change the planform.
    There is no use in an elliptical panform if the sections get out of shape!
  10. Rolf Nilsen
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    Rolf Nilsen Junior Member


    those were some very interesting hard numbers. A foil with elliptical loading (not neccessarily the same as planform?) working at moderate lift have an 10% advantage if the AR is 10:1 and 20% if AR4:1. Beachcats are definately closer to AR4:1 or at maximum AR5:1 on daggerboards. If we are to break down this advantage (which did not become instantly clear by reading the link you supplied, which discussed the benefits of lifting line theory vs. thin airfoil theory and AR) into components, would it all go to reduced tip vortices, or is it reduced tip vortex, better L/D ratios and..? What makes the rounded tip more efficient, and how do you design it? Reading about aero/hydro dynamics always leave me with two new questions for every thing I learn.
    Too bad there is just 2D design tools available for us amateurs who just want to use them sporadically. Access to 3D tools would help with these discussions and in others like the Spitfire wing vs. other designs.

    Great! I always found it to be hard, long and tedious work. Starting with rough sawing, then belt sander/plane and finishing off by hand sanding. Even when taking a lot of care it is hard to end up with a straight foil having correct sections. Do you have any practical tips and methods to recommend?
  11. Frosty

    Frosty Previous Member

    Im thinking of doing some thing under my boat. I was going use plywood but some has said its a no no.

    I have a picture of something I have in mind for under my cat. I was going to shape by hand but would have template at hand. Would it be so critical?

    A good foil in this situation would lift what per square foot? at 20KTS say

    Click on it to enlarge. the third down would be my choice of foil.

    Attached Files:

  12. Guest625101138

    Guest625101138 Previous Member

    I did not want to over qualify the numbers but take them as ballpark. The benefit of the end taper I gave you earlier was just taken off a program I made to design propeller blades. I tried two values for the Oswald efficiency number - 0.7 and 1 at the nominated aspect ratios. My understanding is that this is the range from a square edge to an elipse.

    The Oswald efficiency is one of the factors to take into account when you calculate induced drag as shown in the referenced formula. The eliptical planform loses less energy to the longitudinal votices so less induced drag. You also get less induced drag if the aspect ratio is higher. There is no induced drag if the foil is not lifting.

    There are other ways to reduce induced drag but for a symmetrical foil I have found that the simplest way is to just taper the tip. For a 4:1 aspect foil the benefit of a taper should be in a range that is noticeable.

    Playing around with different foils in JavaFoil will give you a good idea of the merits of various foil shapes. This, combined with some testing on a boat, should give you good insight into optimising the foil.

    To shape foils I use lines to mark flat planes as shown in the original post. With foam I use a large course rasp for rough shape close to the lines and then a finer pressed steel rasp to smooth out curves. You can make a template to check the shape as you do the final sanding.

    The important things with a foil are the curvature of the leading edge, the sharpness of the trailing edge, fairness of the surface and the smoothness of the surface. Exact measurements are nor critical. For example if maximum thickness is at 40% rather trhan 35% it will not make a lot of difference. But if you bring the leading edge to a point then you will dramatically reduce the operating range.

    I have seen some foil designs with odd bumps to overcome particular conditions but they are exceptions.

    Rick W.
  13. Guest625101138

    Guest625101138 Previous Member

    It is not as simple as lift per square ft. But for a first approximation work on a Cl of 0.6. Your speed is say 10m/s so lift is:

    Newtons = 1025 / 2 * 0.6 * 10^2 * sq.m

    Lets say each one is 3ft wide and 8" long (.9 *.2 = 0.18 sq.m)

    Newtons = 30750 * 0.18

    Newtons = 5535

    kgf = 5535 / 9.8

    lbf = 564 * 2.2 = 1242

    Or you could go to this site:
    And produce a foil of roughly the desired shape. I prefer the top one in your images. It can be approximated with camber of 3%, thickness of 8% and set at an AoA of 3.5%. The attached image was a screenshot of it with these settings. I think this would be a better shape for improving lift to drag than the highly cambered one you have nominated.

    It provided a lift of 1819lbf. Note I did not correct for aspect ratio - it comes down a bit when you select this option.

    This is a substantial force and would take some managing. It would need to be solid and well supported. I am thinking not less than 1/4" steel plate for the skin of the foil. Think in terms of what you are using for a rudder.

    Rick W.

    Attached Files:

    1 person likes this.
  14. Frosty

    Frosty Previous Member

    Thanks for that very informative post Rick.

    I dont intend to lift the boat out,- but just say half to increase speed. I was thinking I would use a peice of 1 inch ply, 2 foot by 2 foot 4 inches shaped as a foil exept for a 4 inch section in the middle. This would be literally epoxied onto the 4 inch flat section that I have under the hulls. That would be it.

    At the figures you mentioned that would get the hulls quite a way out of the water, (14 ton boat) yet without loosing stability. some smaller ones to the rear may also be adopted. But basically I would try to lift just forward of the centre of the boat.

    Attached Files:

    • foil.jpg
      File size:
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  15. Guest625101138

    Guest625101138 Previous Member

    I may not understand what you are saying about the ply but if it is 2 foot long and 2'4" wide then it will not make a very good foil. It will be very hard to shape into a reasonable foil. Also the aspect ratio, width to chord length, is a factor to consider. You cannot work on area alone.

    I also doubt that the ply will stand up for very long. You are dealing with big loads from the lift plus bouyancy induced pitching could result in even higher forces. What happens if a hunk of ply breaks off at speed? How well guarded are your props???

    The pictures of the foils you have posted look more in line with the forces involved.

    I did a comparison using JavaFoil. The attached shows the shape of the two foils. One is the flat sheet with rounded nose and pointed tail. The other would be made by cutting the ply in halves and and stacking the two pieces together.

    Both foils have the same L/D at the their optimum setting of around 12:1. The fat foil has a Cl of arounf 0.5 at optimum while the flat board has a Cl of 0.1. Accounting for the reduced area of the fat one it still provides 2.5 times the lift of the flat one.

    The other advantage of the fat one is that it is a much stronger shape for the loads carried. You could sandwich some steel plate in the middle so that if the ply did crack up you could keep the pieces of wood in place. You could even run some steel flat bar in grroves along the top and bottom of the foil. This would add considerable strength.

    I good foil with a 4:1 aspect will give an L/D of around 30:1 so you can get an idea that you lose a lot by constraining yourself to the 1" ply that has limited strength.

    The L/D on a typical planing hull is around 8:1 so you get very little gain by adding to the stubby foil.

    Rick W.

    Attached Files:

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