Proas are strange

Discussion in 'Boat Design' started by Anatol, Oct 2, 2015.

  1. Anatol
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    Anatol Senior Member

    I mean that in a good way :)

    I'm designing/building a proa and I'm coming up against a set of basic design questions which I'd appreciate some guidance about. My question is this - with a style of boat whose behavior is so different from conventional bilaterally symmetrical tacking craft, what aspects of conventional wisdom are irrelevant? For instance:

    1. If you don't have to tack through the eye of the wind, steering capability under sail does not require the capability of a quick 90 deg turn. In which case the maneuverabliity of skeg keel ad rudder are not advantageous. Tracking like it was on rails may be a good thing. ie justifying a deep V.

    2. Uniquely, a proa has to have a reversible but balanced sailplan. But a modern proa should shunt as easily as a modern tacking boat tacks. What is the best strategy to achieve this goal?

    3. helm balance - Is the taboo of lee helm a taboo on an (atlantic) proa - it seems that weather helm would be a liability.

    any thoughts appreciated!
    thanks
     
  2. Skyak
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    Skyak Senior Member

    1. I would not say that maneuverability is something you would be willing to sacrifice on a proa. I think the defining point is that rudder=leeboard in size because they alternate duty on each shunt. This is a big change from board>>rudder on typical sailboats and it will lead to a tendency to have a very powerful rudder and likely a powerful weather helm. Lee helm always indicates that the rudder is less efficient than it could be -always bad.

    The base drag is a multiple of wetted surface area. Because of this a V bottom has more drag than round or elliptical, or even flat for that matter. Resisting leeway is best done with the fins and the hull is best optimized for low drag. About that wetted area -it will lead you to a more even loading of your foils because it is less efficient to drag that area you don't need. If you can balance the load on the fin acting as the dagger (depends on the sail) you might be able to retract the foil acting as the rudder to reduce wetted area.

    You don't mention it but proas have a unique bunch of choices to make when it comes to foil symmetry -symmetric section or asymmetric? It always lifts in the same direction so asymmetry would be a benefit. Rotating or reversing? Reversing foils are a big compromise.
     
  3. Anatol
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    Anatol Senior Member

    Skyak - thanks

    > Lee helm always indicates that the rudder is less efficient than it could be -always bad.

    always? or poor sail balance.

    > The base drag is a multiple of wetted surface area. Because of this a V bottom has more drag than round or elliptical, or even flat for that matter.
    Resisting leeway is best done with the fins and the hull is best optimized for low drag.

    understood, induced drag from lack of a top cap on leeboards also worries me - not sure of relative quantities though.


    > If you can balance the load on the fin acting as the dagger (depends on the sail) you might be able to retract the foil acting as the rudder to reduce wetted area.

    nice idea

    > proas have a unique bunch of choices to make when it comes to foil symmetry -symmetric section or asymmetric? It always lifts in the same direction so asymmetry would be a benefit.

    right, I'm aware of this. The general wisdom seems to suggest asymmetric hulls are not an asset, how would you shape an asymmetric lifting foil - not a 'foiling foil'

    > Rotating or reversing? Reversing foils are a big compromise.

    right. Though again, the increased simplicity may mitigate increased drag - again, I don't have a seat of the pants sense of relative quantities.

    There are so many possible placements of foils/rudders, all with tradeoffs of efficiency and linkage complexity, etc. I'm looking for sweet spots between efficiency and simplicity. So this is another design question. I'm leaning toward a leeside rotating board with anti-vortex plates top and bottom, and kickup, with stem-hung rudders, fwd one lifting.

    I want as much as possible to occur automatically in a shunt. The board may weathercock around and be locked with a line/camcleat on each position. Rudders will demand a lifting and dropping maneuver.

    It seems rudders serve two different purposes - for maneuvering in close quarters at slow speed. And as trim tabs underway. I'm thinking of decoupling these functions - adding a steering oar or steerable outboard motor for in-harbor maneuvers.
     
  4. messabout
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    messabout Senior Member

    Skyak: one argument here. Flat bottoms with hard chines will invariably have more wetted area than a vee, ellipse, or round of equivalent volume. Interestingly you can manipulate a bevel chine, with flat bottom configuration to minimize wet surface almost as well as ellipse or round and better than a vee. There are other advantages to the bevel/flat configuration ........ structural advantage and reduction of chine turbulence.
     
  5. Skyak
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    Skyak Senior Member

    Lee helm is always less efficient than weather helm because the rudder lift is in the wrong direction so all of it just increases the force the dagger must lift and induced drag is increased. This is why balancing the sail plan is important to speed.

    The way I would make asymmetric boards that reverse would be to take the front half of a good asymmetric profile with max thickness at the center (say NACA a 2510) and mirror the profile so it has two fronts and no aft profile. The plan form would be elliptical at the bottom and the very top could be straight plan form so you could lift it to trim.

    I can't picture what you are describing in words -I get conflicting pictures
    -my view of anti-vortex plates conflicts with 'kick-up' which rotate
    -lee side (the way I would go) conflicts with 'stem hung'

    At any rate I don't feel the need to tell you about what is best because there is someone already doing everything I would -and he generously contributes here. See "Bucketlist" on

    Harryproa.com

    Mess. -A chined hull would certainly be a simplification that could be done with little reduction in performance -good for the lee hull. I would just go flat on the windward hull and make it plane or fly.
     
  6. Anatol
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    Anatol Senior Member

    Interesting. What are the proportions of this 'bevel'? Effectively its a mini-chine, no? Point me to a drawing ?
    As for the reduction of chine turbulence, there is a also an argument for chine-runners, which make what seems to be the opposite argument - reduce trailing vortices by preventing the flow lee-windward, under the hull. I"m interested in this idea.
     
  7. Anatol
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    Anatol Senior Member

    Skyak -
    " The way I would make asymmetric boards that reverse would be to take the front half of a good asymmetric profile with max thickness at the center (say NACA a 2510) and mirror the profile so it has two fronts and no aft profile. The plan form would be elliptical at the bottom and the very top could be straight plan form so you could lift it to trim."

    Resident Foil expert Tom Speer has offered a reversible foil plan, but in correspondence he seemed to suggest - if I recall correctly - that he didn't think it would have good lift/drag characteristics, so I kinda gave up on that idea.
     
  8. tspeer
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    tspeer Senior Member

    You should consider where you will be sailing from and your destinations. The steering control power may be dictated by the ability to maneuver at low speed in close quarters. A boat that is well balanced and sails well can still be hazardous in a marina when you're trying to get to the fuel dock or have to land at the Customs dock.

    The performance of a section with a rounded trailing edge is critically dependent on just how the boundary layer separates on each side. The computer may say the section will perform well, but I wouldn't expect it to work all that well in practice. I think the evils of a sharp leading edge are fewer than the evils of a rounded trailing edge.

    I'd be more of a fan of conventional sections that are rotated through 180 deg for each shunt.

    A good design criterion to pay attention to is wetted aspect ratio, depth^2/total wetted area (including the hulls).
     
  9. Anatol
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    Anatol Senior Member

    Apologies for my silence - real world got in the way ;)
    Tom, could you pleas unpack that criterion?
    thx
     
  10. rael dobkins
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    rael dobkins Senior Member

    Hi there, been messing with designing and building Crystal Clear for 3 years now....

    My opinion only...

    1. A proa will never shunt as fast as a bermuda rigged mono will. impossible.
    So, my plan is to learn what she does best and what not. The shunt may be slower, but needs much less room then a tack as she stops and heads back. This is an advantage, if your'e in confined situations heading towards a dead end.

    2. Deep V with a single chine to windward and a touch of dead rise on the bottom is the easiest to build and has fare volume. If you wonna race, make it round, otherwise single chine is the most logical. As you are presenting a pretty big vertical wall for grip on your lee side, minimizing the need for lee boards, as they are 1 more thing to mess with when shunting and 1 more complicated thing to build, that adds weight and weakens the over all structure.

    3. Helm balance is just pure total balance. If your CE is way ahead, if your CLR is way back, what ever helm you have will be minimal and a big enough rudder will handle it no problem. On Crystal Clear we can get our CLR way back by moving our drinking water aft. I do believe that on close reaches she will sail herself and on broad reaches we will use our retractable rudder, that we pull out and move to the new stern. When shunting in such conditions or in close quarters. Under those fast shunting conditions all weight water and gear is totally balanced for and aft, keeping CLR in the middle, but crew and helmsman move aft at every shunt.

    Finally as simple as you try to keep it, it will get complicated at some point... That's what happened to me over and over again and again... But hey, Proas are proas and they are just so cool.

    Good luck and All d best....
    From Balkan Shipyards
     
  11. Skyak
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    Skyak Senior Member

    I agree completely the rotating profile will outperform but I was asked how I would build a reversing foil and I would venture to say it would beat using a deep sharp hull against leeway.

    Before we leave the subject can we finish it? It doesn't come up often and I fear it is disregarded before it is considered and quantified.

    Tom how would you shape a reversing foil? A link to your conversation will do.

    Do we know the performance of our best try reversing relative to the rotating foil?

    I have one other wild idea -Take the front end of a high lift profile, one with a hollowed pressure side, make the leading edge radius smaller but not sharp, then attach a plate 60-70% of the cord wide to a hinge down the center of the pressure face. The plate flops to the aft end providing the sharp trailing edge and a good camber angle. I can sketch this if needed. I can't help thinking this would make a very close approximation to high lift profiles -certainly close enough to match performance at the low angles of attack of a dagger board.

    Anatol, it's about time you told us more abut your application. How big? Crew? Race? Cruise? Off shore? Windward leeward? We can't speak to superiority without knowing function.
     
  12. Doug Lord
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    Doug Lord Flight Ready

  13. tspeer
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    tspeer Senior Member

    Depth is the maximum depth of the board/keel & hull. Square that.
    Divide by the total wetted area of everything in the water - board(s), keel(s), rudder(s), hull(s)

    The skin friction scales with the wetted area, so dividing by the wetted area is aimed at minimizing the skin friction. Less wetted area is better.

    The induced drag due to side force is inversely proportional to the square of the span of the lifting surface. So increasing the depth will reduce the induced drag.

    When you increase the wetted aspect ratio, you are doing something that will reduce either the skin friction or the induced drag.

    In aircraft design, the wetted aspect ratio is probably the most useful single indicator of an airplane's lift/drag ratio. Boats have wave drag in addition to skin friction and induced drag, but the latter two are often the biggest sources of drag. So wetted aspect ratio is a useful design metric for boats, too.
     
  14. Anatol
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    Anatol Senior Member

    Rael,
    thanks for your email - is there online material about Crystal Clear ?

    there are two issues here - hull(s) and sail plan. Are you saying a proa with and conceivable sail plan will not shunt as fast as a mono will tack?

    agreed on the KISS strategy. I'm working on two hull designs, one is a persuaded narrow Sharpie-like hull, the second is fully persuaded with a 120 degree keel angle (pics coming).

    clever. Is that automated?


    Very true on both points!
     

  15. Anatol
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    Anatol Senior Member

    30' daysailer/weekender. First I want to explore design possibilities of an Atlantic, built in ply/glass. Looking at the whole enchilada - hull shape, foils, sail plans.

    Trying to find a sweet spot in the design space defined by -
    1. ease of build
    2. low price
    3. speed
    4. windward performance
    5. ease of handling
     
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