Foil planforms

Discussion in 'Multihulls' started by Rolf Nilsen, Jul 20, 2006.

  1. Rolf Nilsen
    Joined: Jul 2006
    Posts: 24
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: tack

    Rolf Nilsen Junior Member

    As a complete amateur on fluid dynamics..

    What is the real life and theorethical difference between an elliptical planform and a rectangular planform for foils? Most of what I have read say that an elliptical planform will make the pressure differential more likely to be elliptical and waterflow at the tip will go over the foil (where we want it to go) instead of heading down towards the tip.
    As a beachcat sailor, I notice that the Tornados are limited to elliptical planforms in their class rules, while the formula and A/C-class boats have either rectangular planforms on their foils, or a slight taper to the aft edge. Which is 'best' for a small racing cat? Is there any free software like XFoil is to foil sections for looking at planforms and pressure distribution (and why is an elliptical pressure distribution best??). I would be grateful for inks or pointers to litterature..

    If you add T-foils at the tip of a rudderblade to stabilize the platform when going trough waves, will the T-foil stop/lessen the pressure leaks?

    Finally, word of advice on T-foils on rudders for 16-20 foot beachcats? Is the improved stability of the platform/rig worth the extra drag?
  2. jehardiman
    Joined: Aug 2004
    Posts: 2,536
    Likes: 225, Points: 63, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Propaganda....there are other thing going on such as induced flow...

    The one that works best for your situation....:D

    See Hoerner "Fluid Dynamic Lift"

    T foils/fences are specifically for non-elipitically loaded planforms. That is wholly another subject.

    How well designed is it to the conditions encountered?...These are the things of small precentages.....If it was "silver bullet" everyone would be using them
  3. Rolf Nilsen
    Joined: Jul 2006
    Posts: 24
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: tack

    Rolf Nilsen Junior Member

    I just checked the availibilty of Hoerners "Fluid Dynamic Lift". It's available trough amazon both in the UK and the US, but at ridiculous prices from US$100 to US$300 on the used market..

    I was thinking about a T-foil at the bottom of the foil, like a winglet. You said earlier that elliptical planform loading was propaganda. As I am trying to find out the difference in elliptical planforms and more rectangular planforms, it would be very nice if you could elaborate. Why is a rectangular planform with a winglet a wholly another subject than elliptical planforms vs rectangular? Like I said, I am an amateur..

    Well, how should it be? The rudderblade is a surface piercing foil while the T-foil is a foil operating at smaller angles of attack in the wake behind a daggerboard. Most beachcats are sailing at speeds between 8-20 knots with most of the time spent between 10-17 knots.
    I see several benefits of T-foils, and some drawbacks. They help stabilizing the platform, thus increasing the rigs efficency. They also help with preventing nosedives. If they also work as winglets and stop pressure leaks over the tip when turning, it sounds like a "good thing". I have heard positive reports on T-foils on beachcats, especially smaller ones who need the extra longitudional stability.
    Drawbacks are obvious. Increased drag (but how much, especially induced drag?) and loss of kick-up abilities (why do you need kick-up rudders if you have deep daggerboards and can raise/lower the rudders at need?).

    I seem to have more questions after your reply than I started with. Like what is the difference in induced drag from an elliptical planform vs. a more rectangular planform?
  4. jehardiman
    Joined: Aug 2004
    Posts: 2,536
    Likes: 225, Points: 63, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    What I said is correct, a T-foil is specifically not designed to be ellipitically loaded, that is for foils without devices such as "winglets", "T-foils" (actually just bi-directional winglets), and flow fences. With such devices, the loading (root to tip) is more trapizodial, sometimes approaching rectangular. That is why I recommended Hoerner...they were originally called "Hoerner Winglets" for a reason....

    Do not confuse the shape of a foil with the loading of a foil, i.e. both an ellipitical planform (i.e. ellipitically shaped wing such as on a Submarine Spitfire) and a "rectangular" tapered planform (such as the wing of a North American P-51 Mustang) may be ellipitically loaded and both could be refered to as ellipitically loaded planforms. As I read it, you asked about the shape of the planform bering superior, not the loading, and my response was an ellipitically shaped planform was not inherently better than rectangular one.

    I'm missing something here...You specifically asked about T-foil rudders, not "a foil operating at smaller angles of attack in the wake behind a daggerboard". That is not a rudder, but more of a flap. Again that is wholy another issue as it is effected by the daggerboard and is considered along with it.

    I'm becoming confused by your train of thought. How in the world does a T-foil prevent rudder kick-up (poor design?) and why do you think T-foils increase longitudinal stability (i.e. better rudders decrease stability but increase control force)?

    "Induced" drag is a function of total lift, not loading shape. The more lift, the more induced drag. Lift and drag (including skin, form, and induced) of a foil are defined and developed perpendicular and parallel to the FLOW....Side force and driving force (or drag) are defined and developed perpendicular and parallel to the HULL. A high lift/low drag foil as hull speed increases with respect to leeway has a small AOA and reduces boatspeed because the resultant vector points aft. Draw a diagram

    You need to first get a good book on the design of foils, then work to the design effects of end plates, then work on the effects of foils on sailboats (such as Aero-hydrodynamics of Sailing by Marchaj).
  5. Rolf Nilsen
    Joined: Jul 2006
    Posts: 24
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: tack

    Rolf Nilsen Junior Member

    I suppose this must feel like what a chess grand master goes trough when he faces a beginner. The beginner moves random pieces on the board, and confuses the grand master who is looking for a pattern or strategy. I did warn you that I was an amateur on this..
    I am beginning to wonder if what I am asking about is too large a subject to delve into on an online forum? It probably doesnt help that I am not using the same terms as you (who know this stuff) to explain what I am asking for. Sorry..

    I tough the difference in properties for different planforms was easily explained. I'll have to eplain for my wife why I want to spend so much money for Hoerners old book and try to get a copy :)

    Supermarine Spitfire I assume.
    Thanks for the example, that is clear now. The load needs to be studied mathematically, and I suppose Hoerner does so. I tought perhaps there was a clear advantage to more rectangular planforms as most open beachcat classes are implementing them.
    A bit of trivia: During the war there was a phletoria of Spitfire versions. On one of them they clipped the wingtips to give it better manouverability (they also did other stuff to this version, which the pilots did not like). I think the pilots refered to the modified plane as "clipped, crapped and cropped".

    I am asking in a clumsy way. I mean that the winglet is operating at a small angle of attack, and that the water is probably turbulent as the daggerboard is relatively close to the rudder/t-foil. So, the question might be better if I ask what the general differences in drag is for elliptical, rectangular and rectangular with bi-directional winglet rudderblades?

    The _winglets_ dont prevent rudder kick-up, but you dont want the rudder to kick up if you hit jellyfish or other soft objects when sailing at speed.
    The idea of adding winglets is not to make the rudders more effective, but to dampen pitching when going trough waves. The idea is to do what can be done to dampen the rigs movement as the platform goes over/trough chop/waves and thus make the rig more efficient. Modern beachcat hull designs are moving towards more wave-piercing shapes, and it seems to pay off.
    An added benefit of winglets on the rudderfoil is less danger of pitchpoling when you push it downwind. Again, the platform I am thinking about is beachcats, but it seems like the ORMA60s also have tested and implemented this on some boats. The question is wether the added drag from winglets attached to the bottom of the rudderblade is larger than the increased power you can get out of your rig due to less pitching.

    Yes, which is why you optimize foils for the boat and expected speed.

    Can you recommend a good book on foildesign for me? I have "Theory of Wing Sections" by Abbott and Von Doenhoff on order at Amazon, but are not sure if this is a good choice. Funny that the most referenced works are from the 30's to the 60's..

    Thank you for taking the time to answer!
  6. Doug Lord

    Doug Lord Guest


    Rolf,if you haven't seen it already you might find important foil design and implementation information here under "Hydrofoils":
    Australian Moth Class Association :: Index
    The 10-11/1 beam to length ratio Moths used rudder t-foils way before they went to full flying foil systems and there is a wealth of info on this site -plus you could contact some of the Moth designers for more in depth info. There is also information available on the I14 website including the theory behind the rudder t-foil used on that boat. That particular application is slightly different than on the Moth.
    Again, you would also be able to contact class designers to get more in depth practical information. Good Luck!

  7. rayaldridge
    Joined: Jun 2006
    Posts: 581
    Likes: 26, Points: 0, Legacy Rep: 322
    Location: USA

    rayaldridge Senior Member

    This is probably a completely different tangent, but I've been considering the use of rudder fences in order to allow fixed low-aspect rudders on a beach-cruising cat. No doubt it would be faster and more efficient to use kick-up rudders, but these are annoying, and the design concept is anxiety-free cruising.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.