T-Rudder

Discussion in 'Sailboats' started by SuperPiper, Dec 9, 2007.

  1. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    Somewhere, possibly on this forum, was a reference to Americas Cup campaigns exploring the advantage of a t-wing rudder. The rationale was that this contraption made the waterline "appear" longer, IIRC.

    Can you help me find this thread or article?
     
  2. westlawn5554X
    Joined: Aug 2006
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    westlawn5554X STUDENT

    Sure... there a search button inbetween MOST RECENT and QUICK LINKS, u cant miss it... then you type u lookin for ok... best luck:)
     
  3. SuperPiper
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    SuperPiper Men With Little Boats . .

    Yeah, been there. And Google, too.
     
  4. Doug Lord

    Doug Lord Guest

    AC t-foils

    I'll look to in a minute but in case you didn't know the concept arose from work Paul Bieker has done. His concept of "fooling the water into thinking the boat is longer" was first tried on I14's where it has worked quite well and is now standard on those boats. The "fooling" bit is only part of the reason the foils work well,though.
    Tests on boats that heeled weren't very satisfactory as I remember the story(and from my own testing experience in models). At 15+° angle of heel you start to get a bit of cross coupling between the t-foil and steering.
    Boats that sail flat-cool; boats that heel -not so good.
    ---
    Here's the only reference I could find to t-foils Paul Bieker,T-foils and the Americas Cup-about 2/3rds the way down:
    Why foiling has got the sailing world buzzing
    Address:http://www.sailingtalk.com/foiling.html Changed:5:18 PM on Monday, November 5, 2007
     
    Last edited by a moderator: Dec 9, 2007
  5. SuperPiper
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    SuperPiper Men With Little Boats . .

    Doug, thanks for the link.

    According to the article, the T-foil on Oracle Racing's rudder was attempting to lift the transom. My interest in the T-foil was just the opposite. In addition to pitch damping in waves, could the wings on the rudder be used to balance the pressure on the sail plan? Would it make sense to have a profile that would create negative lift at and above hull speed?

    Are there any references of this being tried?
     
  6. Doug Lord

    Doug Lord Guest

    t foils

    SP- the Moths before the Moth foiler used t-foils for years to prevent pitchpoling. The Bieker t- foil lifts upwind to allow the crew to move aft from what I've read. But it has negative lift downwind as needed(its controllable).
    The t-foils on the Rave hydrofoil lift at early stages of foilng then pull down as the boat goes faster. The Moth foiler rudder foil may do the same thing especally as the boat gets fast.
    The 16' Stealth catamaran has found that t-foils are a net positve esp. in reducing pitching and pitchpolng downwind. There is a thread under multihulls somewhere and you could google t-foils
    and/ or Stealth F16 for some discussions on catsailor.
    Are you wanting to use this on a mono?
    As to whether this makes sense in your application you'd have to look at the extra area you're adding, the lift you need and the total drag being created to see if it is a net gain. Or you could just try it....
    You may find some info here:
    Australian Moth Class Association; Index page
    Address:http://www.moth.asn.au/forum/index.php?sid=b2984b019982eb0292321c902664a9d2
    ------
    Also check the pix in the "Gitana secret weapon" thread under multihulls. There is a shot of the t-foil the guys added to that boats' stb ama experimentally. The made a rig like a retractable outboard bracket to to lower the foil just aft of the rudder.
     
  7. SuperPiper
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    SuperPiper Men With Little Boats . .

    Crew Weight Required To Balance Wind Speed

    The height of the sail plan's centre of effort above the waterline produces a bow-down moment on this little boat. As the wind speed (and the resulting boat speed) increases, more weight is required at the back of the boat.

    But what if there is a limit to the amount of weight available? Could a foil be attached to the back of the boat that would create a downward force equivalent to the weight of 1 additional crew member at boat speed = 6 knots?
     

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  8. SuperPiper
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    SuperPiper Men With Little Boats . .

  9. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    Section Change

    Is the section profile (NACA 0012) maintained across the entire span of a tapered foil? Or is the profile at the tip different from the profile at the root?

    Phil of Phil's Foils commented that one of his tricks is to go thinner where a transom-hung rudder breaks the water's surface. For example, he may neck down to a NACA 0008 at the waterline of a regular NACA 0010 or 0012 rudder. Or, is there a reason to be thicker at the tip and thinner at the top or vice versa?
     
  10. SuperPiper
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    SuperPiper Men With Little Boats . .

    Guide Me Through This

    The formula for lift:

    Lift = 1/2 x (density = 62.4lb/cu.ft.) x (velocity = 6 knots = 10 ft/sec)^2 x (foil area, sq.ft.) x (Cl = Coefficient of Lift)

    On page 6 of the Foiler Design thread, Tom Speers shows a high camber NACA 5313. It produces positive lift for angles of attack greater than negative 5 degrees. At neutral trim (angle of attack = zero), the lift coefficient is 0.5.

    Therefore, to produce a downward force of 200 lb at a speed of 6 knots, the inverted foil only needs to be 0.13 sq.ft. or 18.5 sq.in.?

    Gee that seems awfully small. Where did I go wrong?
     
  11. Doug Lord

    Doug Lord Guest

    Lift formula

    SP, this formula was given to me by a reputable guy on foils.org:
    S= L divided by (F X V² X CL) where:
    --------------------
    S= area of foil in sq.ft.
    L=lift in pounds
    F=2.09
    V²= speed in mph squared(6 X 1.15=6.9mph)
    CL=Lift Coefficient
    It comes out to 4.02 sq. ft. for your foil.This formula corresponds nicely to actual facts on the water with a Moth and with my own boat.
    A Moth main foil(early)=1.09sq.ft.;rudder foil around 50-60% of that. Rave main foil=1.77sq.ft.-one on each side-total area 3.54sq.ft. rudder foil=50% of total area or 1.77 sq.ft..
    My opinion is that for your application this foil would produce more drag than benefit.
     
  12. SuperPiper
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    SuperPiper Men With Little Boats . .

    Acceleration Due To Gravity

    I forgot that lift includes 'g=32.2 ft/sec^2'.

    The 'F=2.09' in Doug's formula is the composite of the acceleration due to gravity, the density of water, and the conversion from feet per second to miles per hour. My corrected calculations now agree with Doug's.

    Doug is probably right to question the advantages of a T-foil on this little boat. This was also discussed on the Cat Sailor's forum. Here are the perceived advantages:

    A short waterline boat (Hobie 14 on the Cat Sailor's forum) could become a more stable platform with T-foil rudders. The rig may sway and pump less in a chop. The loading on the rig would be more constant. The angle of attack of the sailplan would be more consistent. The drive from the sailplan would also be more consistent.

    Some of the cat sailors considered that the winglets actually reduced the induced drag of the rudder tip.

    I am still exploring how the T-foil affects the apparent waterline length of a stout, little boat.

    In the case of my little vessel, I am considering a cambered foil to automatically pitch the bow upward (stern down) when the wind builds.

    More later.
     
  13. MalSmith
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    MalSmith Ignorant boat designer

    Regarding use of Rudder T-foils on keel boats, I was involved in a model yacht class that used rudder T foils to good effect. See http://members.optusnet.com.au/mal.smith1/750.html. The T-foils were used primarily to prevent nosediving. The class rules limited the hull length, but not the sail area, and the limiting factor for speed round the course was longitudinal stability downwind.

    An interesting side effect of the tee foil when sailing upwind is that then the boat heels, the T-foil will neutralise the the usual increase in weather helm. When tuned correctly, it was possible to have neutral helm from upright through to maximum heel angle. There is also a noticable change in trim as when heeled the rudder will normally lift the stern when counteracting weather helm. With the helm neutralised by the T-foil, this does not occur. So from this point of view it is probable that the T-foil improves upwind performance by improving the running trim and reducing rudder drag caused by counteracting weather helm.

    Mal.
     
  14. SuperPiper
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    SuperPiper Men With Little Boats . .

    That's way cool, Mal! You modeling folks are definitely cutting edge.

    Can you provide some detail on the proportions of your foil? Its area compared to the rudder area? The amount of sweep? The amount of taper? The section profile?

    Can you explain how the foil neutralizes weather helm?

    Mal, my boat is only 6x the length of your model. Some of the principles may be easy to re-apply.
     

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  15. MalSmith
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    MalSmith Ignorant boat designer

    I doubt that there are any hard and fast rules that can be applied accross the board. Our foils were developed by trial and error. We started off using foils that were about 2/3 the area of the rudder. The foils were assymetrical section with the camber on the underside. The foils were set at somewhere between 0 and 2 degrees angle of attack (negative, i.e. the foil is designed to pull the stern down). Later we reduced the size of the foils to less than haff of the rudder area and increased the angle of attack to compensate. It is useful to be able to vary the angle of attack of the foil fairly easily for tuning purposes. As for any foil, high aspect ratio is better than low aspect ratio. We sometimes used various methods to move the foil as far aft as possible e.g. mounting the rudder on a gantry, or mounting the foil on a boom as in the photo above. Another thing we did that was very effective was to have a variable foil angle connected to the sheeting system such that the foil angle increased as the rig was eased. This way we could reduce the foil drag upwind but get plenty of downforce downwind and the boat could be made to plane bow up when running square.

    I would suggest making your foil to suit your practical limitations e.g. limit the span to the width of the transom if possible. Design it so that it at least has a bolted connection to the rudder so that it can be easily removed for modification or tuning the angle of attack. As for area, I think you should be guided by your calculations, but err on the large side to begin with to keep the angle of attack within reasonable bounds.

    Regarding weather helm, viewed in profile the projected area of the keel and rudder reduce as the boat is heeled more, whilst the projected area of the foil (the centre of which is located aft of the upright CLR) gets greater. Therfore, as the boat heels, the CLR moves aft.

    Mal.
     
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