Wing Foils Below Waterline

Discussion in 'Hydrodynamics and Aerodynamics' started by BobBill, Apr 22, 2012.

  1. BobBill
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    BobBill Senior Member

    Not sure if this is proper forum.

    I am a dunce, when it comes to fluid and aero dynamics...but I have "gut" feelings and read the general, non-engineering material on the topic, that I see here and elsewhere.

    Now, the idea. I am lofting a build for an old Malibu Outrigger design, no longer a viable class, and making some changes.

    The pic below is hull, bottom up, without bottom. Notably, very narrow and very flat, with slight fore/aft rocker.

    The boat sails flat, generally.

    I have wondered, if one were to extend the sides of the bottom (just ahead of the dagger board slot, where the rocker begins) to form short wings on either side of the boat, if that would increase lift...just a hair?

    If so, would the lift and drag offset?

    The small wings would have the added benefit of foothold when boarding, if one was to take a dip...

    It would be a very simple matter to cut the bottom to the appropriate shape and fair the "wings" to form aerodynamic shape...

    I understand, it is a weird thought, but wondered what you might think about it.
     

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  2. philSweet
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    philSweet Senior Member

    Here's how you can answer that question for yourself. It takes a bit of time though. Download a free boat program such as Freeship that has the ability to predict drag. You can also use Leo's Michlet program with a chine input format. Once you have your hull, change the displacement by a few pounds and compare the drag prediction of the heavy and light versions at a few different speeds. You can then calculate the change in drag as a function of a change in weight. Any foil would need to produce a lift to drag ratio that is greater than the marginal effect that an increase in weight has on drag. And it needs to overcome this by enough to justify the build and the annoyance of having bumps on your hull. This is pretty crude, but its better than a WAG.

    The short answer is it is hard to do. It gets easier to justify the greater the weight that is born by the foil; but by then there are serious control issues to deal with.
     
  3. BobBill
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    BobBill Senior Member

    Hey! Very good. Thanks. Will mess with it. Probably nutty idea, but if it does not work, I can always remove the "wings," methinks. Fun too.

    I read about the Bruce foil but was guess it would be too complicated for this job.
     
  4. upchurchmr
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    upchurchmr Senior Member

    BobBill,

    The Bruce foil would be very simple, but it does not lift the main hull.

    AYRS had a booklet on hydrofoils which showed a small T foil attached to the front of a Pheonix catamaran. It would not lift the complete hull, but raised the front bow considerably (due to the fwd position). The idea was to prevent pitchpolling and increase speed. There was only the single picture in the book. This foil was retractable for slow speeds.

    The same thing should work, but when you are not going fast enough to develop appreciable lift you are still adding drag. Slow speed gets slower, higher speeds may be possible.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    BB, there is a foil system in use on some monohulls that extends a foil out to leeward(DSS) to increase RM and reduce wetted surface. It has proven successfull on boats designed specifically for it. The foil-anywhere from 4-6/1 aspect ratio and using a low reynolds number section- can produce a great deal of lift-RM can equal or exceed that of a canting keel in some cases.
    I'm using a version of it experimentally on my new tri* to allow a very small ama-the foil sticks out on both sides of the ama. It also is a very shallow draft foil designed to operate at least 1 chord below the surface with a 4' span on a 8" wide 16/1 L/B hull.
    On your boat, a small foil used as a step that is real low aspect may not help that much-but it won't hurt that much either and the step is a pretty good idea if you can't board easily from the transom. To get appreciable lift the foil would have to stick out sideways quite a bit.
    You might be able to make the foil on each side removable so you can test easily. Good Luck!

    * http://www.boatdesign.net/forums/multihulls/dream-flyer-fl-18-daysailing-trimaran-42340.html

    Sketch showing ama foil:
    click on image---
     

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  6. BobBill
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    BobBill Senior Member

    The ama is planned to be 12 feet abeam the hull. Make a diff?
     
  7. Doug Lord
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    Doug Lord Flight Ready

    Couple of more tidbits: a Moth foil at 7-8/1 aspect ratio lifts about 175lb. per sq. ft. at about 7 knots+(63412 section); 1.1 sq.ft foil supporting 80% of the load. Tom Speer a few years ago gave this rule of thumb for "foil assist": the wetted surface reduction by use of the foil should be about four times the planform area of the foil. So you need to know the pounds per inch immersion of the main hull and you could roughly calculate the area of foil that would be beneficial.
     
  8. BobBill
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    BobBill Senior Member

    So, Doug, at near 300 lbs, hull weight, the foils need to be x=(300/144)1.1 per foil...thinking they would extend like shark pectoral fins??? Or, too oversimplified, have to remember, I am clueless.

    Since the bottom is one long flat run, was going to incorporate the "foil" as integral, instead of bolting on later...seemed easier and less stressful on hull.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    BB, if you try this type of foil it has to extend out from the side so that at maximum lift it will still be 1 chord below the surface. If you can use a simple T-foil, it would be a lot less experimental-using a "DSS" foil in this way is very experimental! I'll have some results in a few months on my new boat that may help. Do you know the approximate area of the waterplane of your hull?
    The foils would stick out from the side of your main hull as they do on the ama in the sketch I posted earlier(on the left side).
     
  10. BobBill
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    BobBill Senior Member

    The bottom is flat at about 1700 square inches, not counting sides of hull...as below? Would not know how to figure sides in water...as it would vary a bit, and boat would generally sail flat anyway.

    As noted, there really is not much "bottom" at all. Sort of a narrow dory, narrow and long, with 12 foot akas and outrigger, which may or may not have rudder...as I am trying to update old Malibu Outrigger design, but retain its outstanding simplicity.

    I figured the "wings" would extend out from bottom, much like the wings on an F86 Sabre jet...with appropriate shape to provide lift, which I would customize by fairing over flat bottom.

    One reason for doing it this way it because I do not want to have a board on the ama and allow for easy beaching as well, when in the Great Lakes or if I take it out to SF area, or FL etc.

    I should add that I once thought to widen the bottom to gain higher sailing level, but thought this would do the same thing without altering the framing, as simple as it is notwithstanding.
     

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  11. Doug Lord
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    Doug Lord Flight Ready

    A) Assuming the waterplane area is 1700 sq.in.=11.8 sq.ft.,(Note: it is not clear to me whether you are referring to waterplane area or area of the bottom-THIS IS CRITICAL!) I'm assuming you mean waterplane area because thats what I asked for.........
    so 11.8 X 1"(.083')= .97cu.ft= 62lb per in. immersion. Note, that since the sides are highly flared this amount per in. will decrease and introduce an error in terms of how high the foils will lift. I don't have time to calculate it for the reduced lb.in immersion so this is just a rough approximation,
    -
    B) Tentative foil on each side = .4' X 2.5'= 1sq.ft. planform area
    Lift of each foil at 7.75 knots boat speed = 101lb or 202lb total,
    -
    C) Divide total lift of 202lb by Lb. per in. immersion(62lb)= approx. 3.26" of vertical lift at 7.75 knots boat speed. Remember , it will be somewhat more because of the angled sides.
    --
    D) So, 3.36" =.27' ; .27 X 36(both sides of boat)=9.32 sq.ft. reduction in wetted surface with two one sq.ft. foils,one on each side. Keeping in mind that Tom Speer said that the reduction in wetted surface should be 4 times the planform area of the foils: planform area of foils =2 sq.ft., Wetted surface reduction 9.32 sq.ft which equals 4.66 times the planform area and is good to go as a test.
    --
    E) Since the foils are projected to lift the boat 3.26"+, the foils ,with the boat not moving, should be at least 3.26" + one chord(4.8")= 8"+ below the surface with the boat at rest.
    -
    F) Foil section is important, angle of incidence is important-you should probably sail the boat w/o foils initially so you know what the speed is over the entire wind range you would normally sail in, then you can look at this again and determine the right angle of incidence for the foils on this boat.
    Hope this ballpark calculation helps......
     
  12. BobBill
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    BobBill Senior Member

    Thank-you, Doug. I am beginning to understand. The figure I gave was the bottom area only as it is all I have.


    At the same time it seemed more logical that the waterplane should include all wetted surfaces at the water line, but can extrapolate from there.


    My old brain is beginning to unlock.


    Very helpful and I am grateful to you.
     
  13. Doug Lord
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    Doug Lord Flight Ready

    =====================
    The waterpane area* must be on the plans? The problem is that it will be significantly larger than the bottom area which will change the lb.s per in. immersion which will change the total of the reduction in wetted surface potentially requiring larger foils. Now, using the rule of thumb by Speer you could calculate exactly at which speed the foils begin to be an asset. The rest of the time they are primarily drag. You have to decide if the light air penalty is worth the gain in stronger wind. The foils I tried above increase beam 5' and I don't know how wide your main hull is but I'd bet that any increase in foil area would have to be an increase in chord which reduces aspect ratio and changes the one chord measurement-lots of interrelationships. Just to add to your library and help you understand and calculate foil lift and other considerations for the practical application of hydrofoils you might consider getting Ray Vellinga's book "Hydrofoils Design Build Fly" it is excellent and written for the person actually doing experimental work-like you. Probably at Amazon.

    *waterplane area is the area of the hull viewed from top as sliced at the waterline when normally loaded. Does not include sides.
     
  14. BobBill
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    BobBill Senior Member

    Doug, the hull is very narrow, 26" at deck and about 12 half way between bottom and rails.

    I was going to use that area as the plane and go from there...Will have to unroll plans and see if on there. I figure if I go half way, I will be covered in light and gusty, with one or with crews...but figuring average as two...
     

  15. philSweet
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    philSweet Senior Member


    If you assume a parabolic waterline (or any other paramentric shape), that leads directly to the beam required at the waterline, with length being irrelevant. Hadn't ever thought about it that way.
     
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