Aquiring LIFT in model yachts

Discussion in 'Boat Design' started by dale.frahm, Mar 29, 2010.

  1. dale.frahm
    Joined: Feb 2010
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    dale.frahm Junior Member

    Hi there ,

    I'm designing and building a model racing yacht for a Uni assignment .
    It is a rather radical and unusual design . I am testing to see if it is possible to breach its hull speed, and I'm designing for lift .
    I want to create lift in order to increase my LWL (as my bow and stern are reversed)(ie: under-hangs instead of overhangs), so as my boat speeds up, it lifts more, increasing its lwl, potentially getting faster.

    It measures 75cm long, sail area 5000cm2, 2.8kg displacement, with a 1.4kg bulb, giving a ballast ratio of 0.5 .

    It will race in a large pond (in Gosport), associated wind speeds vary 5-8 knots , ie: light wind conditions .

    Now, before I go get too carried away with this, I want to know how much lift is actually possible with a 75cm model yacht in such conditions .

    Leading me to the question of model aircraft lift (proof that scaled down foils do give lift), how much faster do model aircraft have to go, compared to real aircraft to obtain lift? (speed to size ratio) And how much slower to model yachts go compared to real ones, and hence will any efficient lift from keels and hydrofoils be possible ?
     
  2. Doug Lord
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    Doug Lord Flight Ready

    From a practical standpoint I can give you some facts relating to the multihull hydrofoil in the picture below:
    --LOA 56"
    --Beam 71"
    --SA 1900sq. in., reefable
    --weight 7.5lb
    --Three foils ,each 17 sq.in.
    --The two main foils supported 80% of the load(6lb.)
    --At lowest takeoff speed(5mph/4.3 knots) the main foils had a foil loading of .176lb per sq. in.; the rudder foil at the same area had a loading of .088lb. per sq. in..
    --The foil system developed all the RM for the boat so the main foils were loaded past three times the initial loading. Its important on a model to realize that you may have to reef the rig two or three times thru 20 knots of wind.
    From approximately 1900sq. in. to 600 sq. in..
    --The foils were max 9% T/C ratio with a 12in span and 7/1 aspect ratio. They are symetrical sections-don't worry too much about a precisely shaped section-planform, taper and aspect ratio are more important. A short mid-span flap was chosen because the foil is highly loaded in stronger winds and since it is very thin I was concerned about flap lockup if a full span flap were to flex.
    --The two main foils used "wands"(surface sensors to control altitude and generate RM.
    --Top speed was close to 18-20 knots in a 20 knot wind. In a 5knot wind the boat would do about 10 knots.
    Good Luck!
    PS- a foiling fixed keel,self-righting monohull is also possible with a Trapeze Movable Ballast System that moves athwhartship and fore & aft. It is extremely complex so I'd recommend you try a multifoiler first with either fully submerged wand controlled foils or using surface piercing foils. For speed at model sizes the fully submerged ,wand controlled foils have the edge because they also generate righting moment-and that makes capsize or pitchpole almost unheard of.


    Lester Gilberts site may also help:
    http://onemetre.net/index.htm
     

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  3. Munter
    Joined: Jul 2007
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    Munter Amateur

    Really Doug - your model could do 18 kts? That's quite a claim!
     
  4. Doug Lord
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    Doug Lord Flight Ready

    ==========
    Yeah, it is isn't it. Its quite a boat..... Over a measured 300' course in flat water and lots of wind,fully reefed.
     
  5. anthony goodson
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    anthony goodson Senior Member

    Abroad recently I saw what I assumed from the sail was a Moth on foils,I just wondered ,how simple/complex is the method used to control the ride height ,they appeared to be fully submerged foils.
     
  6. Doug Lord
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    Doug Lord Flight Ready

    Here is a control animation done for the Rave-but it illustrates the wand used as a surface sensor on the Moth. Go to the bottom of the enclosed page and click on "control animation". The wand has a bungee cord attached to it to pull it "against" the water. On the Moth the wand is usually located forward on the hull with a cable or pushrod connecting it to the flap pushrod. It is a simple system designed by Dr. Sam Bradfield and refined for its application on the Moth by John Ilett:


    http://sites.google.com/site/hydrosail/HydroSail-Home
     
  7. anthony goodson
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    anthony goodson Senior Member

    Thankyou Doug Lord for the link to that most interesting site.
     
  8. dale.frahm
    Joined: Feb 2010
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    dale.frahm Junior Member

    Thanks Doug , pretty fascinating stuff .

    I'm just wondering, cause my model is a monohull, what options i have for foil action .
    It has to comply to a class rule, and hence I'm limited to a rather heavy displacement - 2.8kg, with a 1,4kg bulb (ballast ratio 0.5)
    If after the race , I am able to remove ballast from the hull, and decrease my displacement , hence increase my ballast ratio to around 0.6 - 0.7 , I'll have a lighter , stiffer (wind stiff) boat .
    I have this idea of incorporating a foil as my keel . also adding hydrofins to the hull . Is possible to do something with no moving ballast ?
    I will post pics once I have my rig design sorted and properly rendered .
     
  9. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    ---------------------
    Dale, you might consider a DSS foil. It can be setup with one on each side pivoting like a centerboard or a single foil that slides athwhartship. See the Harken article-the DSS site is (temporaily) overwhelmed at the moment.
    Hugh Welborn, the inventor, used an RC model for the first tests -if you PM me I'll give you his e-mail. He seems like a first class guys and may be of some help. The advantage of the DSS system is that it sticks out to leeward and lifts up creating RM. We know already that foils at model scale can generate RM (as a result of lift) because thats how the F3 foiler above works.
    Without knowing the exact rules I would go with the narrowest hull with the least ballast and most SA you can. At model scale narrow hulls are almost always faster than wider hulls and with DSS that ought to be even more true.
    And in light air the foil retracts.....
    Good Luck!

    http://www.harken.com/Interviews/DynamicStability.php

    pix: DSS foil from Ocean Navigator; Welborn25 DSS from SA(see foil sticking out of port side)
     

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  10. sottorf
    Joined: Sep 2007
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    sottorf member

    Dale, coming back to your original question of how to generate lift on MODEL sized foils, you need to consider that these foils will be operating in partially laminar flow and perhaps in totally laminar flow. This means that the model foils will give you 30-50% less lift than their large full size counterparts. Similarly, the drag will be roughly double. All this has to do with laminar seperation on the upper surface of the foils... The effect is however that the foils wont lift the boat as well and you wont get the same speed so consider full scale results caefully.

    If you use the correct profile and avoid laminar separation then the drag will be very low and you will probably get quite good results.
     

  11. messabout
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    messabout Senior Member

    Dale; you are committing the classic screw up if you try to get too creative with your model which I presume is an RC. Beginning modelers are famous for being so clever that they shoot themselves in the foot.

    Just build a very conventional boat, probably similar to an International One Meter. IOMs are all over the place in England. That is the most competitive class in the world. You will not be able to outdesign the people who have been successful in that class. The IOM does not use foils except to the extent that they put a tiny positive incidence angle on the ballast bulb. Not for lift but for the fact that the boat may press the bow down slightly due to sail force moment.

    And here is the gospel! An almost universal agreement among RC sailors is that for every hour you spend on the model,you must spend at least four hours practicing with it. The fastest RC sailors everywhere agree with that dictum. SO ! Build the damned boat and keep it simple. Then practice practice practice. If you do that you are almost assured that you will defeat your competitors even if they have a superior boat. For what it is worth, the same principle applies to full sized dinghy racers too. I have first hand knowledge in both the model and the dinghy cases.
     
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