Assessment of sail center of pressure

Discussion in 'Multihulls' started by patzefran, Mar 25, 2015.

  1. patzefran
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    patzefran patzefran

    Most people use baricenter of projected sail area as sail center of pressure assessment (cardboard method !!!!). Old designer used empiric factor of correction in percent of LWL to balance the boat, to account for the assessment error and other factor, as leeboard displacement of the center of pressure for a heeling monohull (giving a strong windward moment). For a trimaran, the hydro drag axis goes to leeward when the outrigger is pressed, which induce a leeward moment and need a different correction factor.
    This very crude approximation for center of pressure assume uniform pressure on the sails. At least on modern multihulls rotating rig this lead to wrong center of pressure, real one being well ahead at low angle of attack upwind. On old fixed mast rig one can understand the rulle applies better owing to the unattached lee flow behind the mast.
    Machaj give a simple rule to assess center of pressure :
    For mainsail , use quarter chord point behind the mast ad mid height chord between the sea level and the tip of the sail. Same thing for other sails, and he use the barycenter of these points weighted by respective areas to derive the whole rig center of pressure. He says this rule applies well to wind tunel results. Likely this will give a much better assessment !
    I wonder why designers continues to use old center of area method .
    For example my Tremolino MK4 designed in 2002 with the old rule (a great boat !) , had a very strong lee helm, and I needed to use a huge aft mast rake to get a nearly neutral helm.
    I would like to hear any other's opinions ? Is there another better assessment method (aside aerodynamic flow codes !)

    Patrick
     
  2. gonzo
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    gonzo Senior Member

    Can you name modern designers that use the cardboard system? Tremolino is an ancient design.
     
  3. TANSL
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    TANSL Senior Member

    I guess the carboard method is used to obtain the centroid of the area. The problem that patzefran arises is if it is sufficiently correct to use the centroid as the center of pressure, whether that centroid is calculated with carboard or a CAD program.
    That is, in my opinion, it is not to see what modern designer uses the cardboard but who uses the centroid.
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Whatever method is used, the familiarity of the particular method and percentage of lead used with it what counts IMHO
    And it's center of effort what it's mostly called, not cartboard ;). Remember also sail trim effects among other things..
     
  5. patzefran
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    patzefran patzefran

    I agree the exact name is the centroid of area, cardboard is used by some people I read on this forum . My Tremolino version was a nice recent evolution (2002) by an outstanding old designe Dick Newick. I agree also with this method the percentage of lead is what counts but it relies only on past experience !
    So you have to apply whatever correction you have been learned or build many boats and learn from your errors. It is a little frustating in 2015 no ?
    In Aerodynamic I was teached about center of pressure, the point about which the moment of the aerodynamic force is zero at a particular AoA (which determine the static trim for an airplane), and the aerodynamic center, about which the increment of moment is null when you change the AoA around the previous one (which determine the stability of the equilibrium about the trim). But center of pressure and (mostly called !) center of effort are the same thing, whatever you are used to call it.
    Cheers

    Patrick
     
  6. daiquiri
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    daiquiri Engineering and Design

    Patzefan, it seems to me that you didn't try to perform neither a search of the posts on this forum nor a bibliographic search of papers and books which deal with this matter, before starting this topic. ;)
    The aerodynamic design of sails has made some big steps forward from the times of the method of centroid of sail area. Yes, many designers still use the old method, but that is a matter of their choice. After one has made tens of boats in a certain way, has seen how they behave and has learned from the practical experience, he feels confident in continuing to use the old method. He is not on the cutting edge of the design, but he likely doesn't even care about being on the cutting edge.
    For those who want to use more scientific methods, there are plenty of tools, papers, books and data available around. Just do some more in-depth search on that topic and you will discover a lots of good info.
    Cheers
     
  7. catsketcher
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    catsketcher Senior Member

    Did Dick do wrong?

    Gday Patzefan

    I am intrigued that Dick Newick could design a boat with bad lee helm. Dick was very experienced and was also happy to use computers as well. (I was involved in the building of the shell of a Newick and talked to him a fair bit).
    Maybe your lee helm comes from a non standard new set of amas (Dick did design larger ones for the Tremolino instead of the Hobie 16 hulls) Dick should have designed the Tremolino well either way.

    I haven't come across any papers on helm balance for sailing multis. I can't find much on non ferry sized multis for the average owner. In the book by Larsen (IIRC) he does include a piece on finding the CLR of different hull shapes. The bow entry seems to be very important. he shows the CP of a foil can depend on its AOA but tenmds to stay at 25% chord. This seems to be the right place to start for CP calcs in the rig and the underwater foils.

    To see how this is a bit tricky I designed my little folder with the same basic geometry as my 38 footer. The 38 footer has slight lee helm and the little folder has weather helm yet I used the large one as the basis for the balance calcs. Scale factors do not seem to ever come into the equations I come across.

    In terms of helm I really think there are many other factors involved including float drag calcs (for tris) assymetry issues (increased/reduced drag when depressed/ lifted) and hull profile (bow height, forefoot depth). The great scientist Richard Feynmann said if you can't explain something simply you don't understand it which is why I think there are not simple rules for this that you can easily find in a book.

    cheers

    Phil
     
  8. patzefran
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    patzefran patzefran

    Thanks your reply Phil !
    I agree, Dick Newick was a Great designer, I have sailed my Trem for many years with great pleasure an I succeeded to balance it with a big aft mast rake (like Hobie 16). As you pointed, lee helm may comes from many reason. This particular version of Trem Was bigger WL displacement, Hard chine mainhull with Dory hull all along to the stern, and 19 ft stressed plyood Amas. The Aft cabin was bigger. Wile motoring under bare pole she was very difficult to steer head wind at slow speed , this resulted in I always needed to take moorings going downwind. There is a tendency to streamlined bodies to be unstable around zero AoA and I think this was exagerated by the shape of the abovewater hull and cabin. However, although having much respect to Richard Feynman, and the list of involved phenomenon is long, I suspect main governing parameters are not likely that much complicated (aside some particular case as my Trem !). From what I observed on many tris, including mines, combination of overall beam and hydro drag of the lee outrigger seems (likely !) the main parameter. On my beamy Crowther Twiggy 32 (30' beam for 32' length), Daggerboard was just aft the mast and the balance was good. For very beamy ocean racing tris with high displacement outriggers the design tendency seems daggerboard just under the mast. For moderate beam, lower displacement as my Srike 15, the tendency is further backward.
    Cheers
    Patrick
     
  9. patzefran
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    patzefran patzefran

    Daiquiri,
    Thanks, I did not perform a resarch on the topic, sorry, I should have. I know aerodynamics made big progress, I have used many CFD codes and written some. It was not my intention to critic designers. I have much respect for them, an altough being a (now retired !) engineer, I always build my sailboats (multihulls) from good designer plans (instead of making my own errors !). I just looked to understand and to improve quick assessment methods. Looking at the related topics in the forums, I found many interesting discussion dealing to monohulls, none for multihulls !
    It is why I posted in the multihulls forum.
    Cheers
    Patrick
     
  10. catsketcher
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    catsketcher Senior Member

    Gday Patrick

    You have some serious trimaran issues if you have three of them - I also used to have a Twiggy which many on the forum know about and have lots of time for any boat that Richard Woods designs. With all your boats you have about 80 years of design experience behind their designs.

    As to low speed maneuvring I feel that almost all multis have foils that are too small. After sailing dinghies and monohulls that are crisp to tack and meander through moorings the low speed helm of almost every multi is not what I feel we should be aiming for (my own Chamberlin cat included and she has bigger than usual rudders). With foils better suited to normal sailing speeds a typical set of cat foils stall (or don't work as well for other reasons) at slow speeds. Couple this with high aspect foils which can break flow and lose grip easily and many cats need their twin motors to be able to put anywhere whereas the equivalent mono with deep rudder and large keel can wander easily into a marina and mooring area with ease. I experimented a little with my little folder and found it is great to maneuvre with its big rudders and board but with one rudder it becomes a normal tetchy cat at low speeds. I am pretty sure you could solve the Tremolino's mooring issues with a bigger board and rudder. Think crop duster not Concorde.

    As Dick always signed off

    Cheers

    Phil
     
    Last edited: Mar 26, 2015
  11. patzefran
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    patzefran patzefran

    Good day Phil
    Thanks your reply !
    I sold my Twigy 25 years ago, I remember we had to go very fast for harbour manoeuvring, a little hairy, but it worked an has large rudder. To my knowledge, the boat is still afloat. After I had no problem for 15 years with my Formula 40 cat with small area rudders but large daggerboards (I used one outboard motor in the center pod, coupled with the rudders), a litlle hairy also to cruise with my wife. My Trem had good sized daggerboard and rudder, I think the problem was more the windage of the mainhull, I sold her last year after buliding and using it for 6 years. It was OK to take a mooring, but a little tricky in a Harbour ! Now my Strike 15 circle like a charm, with a Bim 20 cat daggerboard and an A class cat rudder. I am now buiding a Strike 20 with Richard's Wood help, as I regret my Trem !
    Cheers
     
  12. Richard Woods
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    Richard Woods Woods Designs

    Internet is sketchy here in the boatyard. Hence the delay in replying

    I have never used cardboard cutouts. But have talked to many people about the "real" CofE, including Tony Marchaj, top sail makers and also America Cup consultants. All agree the CofE is further forward than the geometric area. And you need to factor in the rotating mast if fitted.

    Obviously the sail shape and trim makes a huge difference. The modern fast boats tend to have flatter mainsails and fuller jibs than in the past. That will also affect CofE

    I haven't sailed a Tremolino, only a Val, but I would think much of the problem is the lee outrigger drag. I went on the original Twiggy a couple of times, but didn't sail it. The rig was a long way aft, as was the d'board. I always thought it must make it "twitchy" to sail.

    I have sailed a French built trimaran that went from "lee" helm to weather helm depending on whether the outrigger was immersed or not. It was not a good boat, we sailed no faster than a Laser to windward, very wet to sail, yet the builders sold 400+

    I thought Lock Crowthers d'boards were too small for low speeds. Whether that was in light winds, in a chop, or when it was really windy and the boat was slowed by waves/fear. High aspect foils work on beach cats which generally sail in flat water but are prone to stall when coming out of tacks and when thrown around in big waves. Also of course a high AR implies a thin chord, which can cause structural problems at high speeds.

    Lots of cruising monohulls are impossible to turn one way in reverse. Much harder to manouver than a catamaran

    Having re read this, I think all I am doing is agreeing with Phil, except for the monohull manouverability bit. My cousin's Mike Pocock 38 won't go to std at all in reverse.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

  13. patzefran
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    patzefran patzefran

    Thanks Richard.
    I agree, It is more a matter of, overall beam / outrigger drag when sailing to windward. Daggerboard of my Trem could have been somewhat more forward. In my original post I only said the crude rule proposed by Marchaj for 1st order assessment of CoE (1/4 chord at midheight) was (perhaps!) a progress versus center of area (at least the correction goes in the right direction, although the height appears questionable) . It is even more simple to use than center of area. As for the size of daggerboards, catamarans (hull flying!) seems more suited for the whole range of velocity :
    2 daggerboard at low velocity, 1 at flying hull speed !
    Progress are resuming on Strike 20 ! I will keep you informed.
    Best wishes

    Patrick
     
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