beam windage

Discussion in 'Multihulls' started by Samnz, Feb 11, 2011.

  1. Samnz
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    Samnz Senior Member

    Does anyone have a calculation for windage drag?

    My new 8.5m tri has box beams 300mm high (tapering down to 250mm at the ends) and the front edge is flat at the moment. The loose plan is to build a fairing onto the front edge of the front beam but this adds weight, and im close to (if not over) our 8.5 class weight 900kg min anyway. The middle 1.6m of the beam is covered by the cabin and the beams are 6m long each.

    Other question is the shape of the fairing, should it be elliptical or a shape to defect water downwards?

    What about the aft beam?

    any feedback would be great

    (the boats not going to be pink!)
     

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  2. Brorsan
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    Brorsan Junior Member

    Are you really going to paint it pink?
    (j/k)

    I am by absolutely no means expert, not even a multihull sailor (yet) But i think that the front edge's form of the beam is of less importance considering drag, the aft edge of the beam is much more important considering drag (i think) but still, my humble guess is that is not worth adding weight for that purpose only. To gain effekt the beams should be made with that in consideration from the beginning. But like i said, that is just guesses.
     
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  3. Samnz
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    Samnz Senior Member

    Hi Brorsan

    Thanks for replying, I have been thinking along similar lines, whats the point in a front fairing but no back fairing? However most big tris (orma 60's etc) do seem to have a front edge beam fairing with no back edge beam fairing, maybe its just for water not wind?
     
  4. Gary Baigent
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    Gary Baigent Senior Member

    Brosan is dead right, Sam - and the after wing section of your beam doesn't have to be able to take the hammering that a leading section has to handle. You could even make a double sided trampoline (or soft sail material) attached to top and bottom of your beams to achieve a better aerodynamic shape. Not as good as a true wing shape but far better than the ORMAs and most other trimaran designs, which must create drag and turbulence with their chopped off after beam sections.
     
  5. captainsideburn
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    captainsideburn Junior Member

    don't they fair the leading edge more for protection from heavy water/waves? I seem to remember reading someones acct of ostar or something where the beams were damaged because of wave action on them?
     
  6. Munter
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    Munter Amateur

  7. catsketcher
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    catsketcher Senior Member

    Complex

    There are two reasons to fit fairings - air drag and wave drag. The problem is that you have to make brave assumptions about the direction of the apparent wind if you are to produce a helpful fairing.

    You can't assume the wind is head on. Going upwind you will probably have apparent coming at you at 30 degrees. So your beam is being crabbed through the wind. Also the whole boat is wiggling up and down and you may (almost certainly) get some turbulent interaction between the amas and beams which will set up some vortexes as well.

    All this means is that it will be very problematic to work out where the wind is coming from. You could stick a woolie on a stick and go sailing. That's probably the best. Or you could copy clever people like Irens.

    The fast tris I have seen have fairings - usually only on the front of the beam. I would presume that the much more important job of the fairing is to reduce wave impact drag. The good thing is that you can usually work out where this is coming from. On my Twiggy I spent a good amount of time watching the fairings do their stuff on a good work and reach. Most Twiggy's only had fairings on the outer 4 foot of the beam. There was no need for any more.

    As to fairings on the back I would remind you that the AC tri also got rid of the tramp so I think they were going to the ultimate. Also they are a much smoother platform sailing at a higher speed so drag is a much bigger proposition than for a slower tri.

    If you can get a hold of Rob James "Multihulls Offshore" you will find that on Great Britain 3 (I think it was 3) the fairing was bashed by waves and slowed the boat as it deformed. Also some Farrier sailors don't like getting wet with the spray from the aft beam which has no fairing. Farrier's new shape beams are shaped in a way that deflects water downwards. They have a low frontal area otherwise.

    The only other literature I remember was by John Shuttleworth. He said that fairing for wind should not be done on aft beams as the air is so turbulent as it reaches the aft beam that skin friction is the greatest component of drag. That was in the 80s though.

    My vote - small front mounted fairings 1-1.2 metres from the beam ends made from extruded polystyrene foam and 2 layers of four ounce cloth over. Lasted fine on the Twiggy. Light as and easy to change. Oh as to shape - make them almost flat on the bottom and straight from the top of the beam to the leading edge - a wedge - a la Verbatim. Bugger the back- tape your ears back instead.

    cheers

    Phil
     
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  8. Gary Baigent
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    Gary Baigent Senior Member

    Yes, well castsketcher, what you have suggested is pretty much the same as the those weak fairings that got bashed to distortion on Rob James tri. I also call BS on the chopped off half wing beams - because they are obviously draggy. I mean a wing is a wing that has been around for millenniums - they are a perfect design through evolution and you don't see half beams on bird wings, fish or sea mammal fins or flippers. It is so obvious ... that it is almost too bloody obvious. And fashion and conformity plays a huge role in the design of yachts, especially conformity. As Jim Young says about design, "Like sprats they all rush off, then come back to the same place."
    On the point of speed, a bird doesn't fly much faster (or much slower) than a good trimaran at speed - and everyone knows that to increase performance with a limited driving power, that streamlining and airfoil shapes is the only way to achieve this. To achieve performance a couple of percent above what is considered near maximum, double to amount of power is required. And, lets face it, yachts are great tangle of draggy shapes and protrusions.
    But a new era is here. A quiet prediction: yachts are going to be (soon and also right now) designed and shaped to be almost aircraft-like. BMWO showed the way to go, cleaned up their beams to wing shapes, trampolines cut back to a minimum, full wing rig, neat fairings wherever drag could be reduced, and so on. Welcome to the future, coming right now.
     
  9. oldsailor7
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    oldsailor7 Senior Member

    Yeah. Gary +1.
    Norm Cross already went that way in his later designs.
    Upside down aircraft wing sections.
    Curved side down, (Bernoulli).
     
  10. catsketcher
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    catsketcher Senior Member

    I won't call BS on anything as I haven't go the time or money to do the two boat testing but birds and dolphins all use their wings to produce lift so it is reasonable that evolution has equipped them with lovely high lift and low drag shapes.

    A crossbeam has to do lots of things. It has to be easily made, stiff, light, able to be walked on, hard to fall off, resistant to torsion and low drag. The old fashioned box beam is the ultimate in weight saving in usual load conditions. It gets a great second moment of area for lowest possible weight.

    As soon as you get away from a box (or more correctly an I beam but that's worse in torsion) you need more weight for the same stiffness. You also get a harder build and maybe a beam that is easier to slip off than one with a flat deck and a toerail. The engineering gets much harder if you try and go away from box beams (Ocean Emu was an example of a designer stepping away from a box beam and getting fingers burnt)

    Interesting note. When Frank Bethwaite designed the Tasar mast (I have raced them a fair bit) he did some real wind tunnel testing with different sections. He found that the Tasar mast with its cut off square back was LESS drag than his typical wing shape section. It seemed that turbulence was causing problems. Read his very interesting book on High Performance sailing for more. I don't want to get into a reference bashing flame with anyone but wind flow is very interesting and assumptions made have been seen to be incorrect in the past and may be so in this debate.

    cheers

    Phil
     
  11. Doug Lord
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    Doug Lord Flight Ready

    ===============
    Bill Beaver did a study of the foiler Moth and found some very interesting facts:

    " At 15 knots, Table 3 shows an aerodynamic drag of 17lb. This is about 70% of the total hydrofoil drag, so it appears that the aerodynamics of the boat are now nearly important as the hydrodynamics."
    --
    He also found that the helmsman was 42% of the total aerodynamic drag!
    ---
    So when considering speed on a fast boat you MUST consider aerodynamic drag as much as you do hydrodynamic drag. I think this applies to all fast sailboats.....
     

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  12. Samnz
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    Samnz Senior Member

    I think this is the key. I actually looked at the boat yesterday and realised that with such massive floats, when sailing upwind with 25 to 30 degrees of apparent wind at 20 degrees heal angle (thats the angle the main hull just pops out) The wind would not be able to 'see' the beams, theyre blocked by the Floats and mainhull, you can see what I mean in the photo if the boat was heeled a bit more...

    The boat will almost always be raced in the sheltered Auckland area so waves arent really a major concern for me.

    So would there be any measurable difference between a faired and unfaired beam in wind drag on this type of vessel?
     

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  13. Samnz
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    Samnz Senior Member

    Hi Gary, I edited your post for you ;)

    my boat has to do more than just racing so will have tramps and a cabin etc, I know you disagree but....:)
     
  14. Gary Baigent
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    Gary Baigent Senior Member

    Catsketcher. Good comments about square beams being simple and strong, no argument. But Frank Bethwaite's study of Tasar rig ... the Tasar rig is FIXED, doesn't rotate - and if it a tear dropped spar is fixed, well, yes, of course there is greater drag and turbulence because the slightly larger chord tear drop spar, being fixed, is crabbing sideways through the air. But a fixed rig is very old world, you know, a mast of today HAS to rotate.
    Point taken, Sam - but your attractive curved beams on your three hulled catamaran are still up there in the air; making a trailing edge shallow, horizontal V wouldn't do any harm, would look cool too.
    I'm on your side, Doug, not always but definitely with the Bill Beaver study. Getting the crew down is basic common sense. Even the old guys on A Class keelers here, had the crew lying down on the high pressure zone of the windward rail.
     

  15. catsketcher
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    catsketcher Senior Member

    Nice tri

    Looks like you have a nice boat there Sam. Even if Gary and I disagree on air drag I would like to see fairings for water and waves on the beam fronts. The polystyrene originals are still there 28 years after my Twiggy was built. The good thing about cheapies is that you can quickly surform them up so that you can throw them away if needed. After you have settled on a shape you can make some nice carbon ones up on a cheap mould. A lack of fairings will slow the boat down as water is so heavy. If your boat has to accelerate every wave the beam interacts with then it will slow down.As for sailing in the harbour - well it doesn't matter about offshore or onshore. It is not the big waves that hit your beams - its the chop and you get that anywhere. Your boat will pitch to the swell but will blast through the chop. The beams will blast through too and need to have a wedge to help the leeward beam push the water that will sweep over the deck when pushed hard. (Okay I sailed a 120% float tri but I would lose that bow often in a wave)

    Gary - the Tasar mast is an over rotating section. One of the - no the absolutely worst thing - about the otherwise lovely boat is that the mast doesn't rotate by itself. The poor crew has to do it as they tack.

    What was happening was that the separation bubble (and hence drag of the mast/sail combo) was bigger behind the total wing mast. The cut off section energised the boundary layer and re-attached flow to the mainsail much quicker. Until someone does the wind tunnel testing it is possible that with all the turbulence set up by the floats, crew, rig, main hull and beams of a small tri you want to get that bubble back down again ASAP rather than try for streamlining.

    Maybe Sam should be putting golf ball dimples on his beams as well. Mythbusters did a good show on this. They dimpled a car and it was less drag than without dimples. It was a nice shape too. They tested at about 40mph.

    cheers

    Phil
     
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