Centre of effort and keel position.

Discussion in 'Sailboats' started by AleX`G, Aug 6, 2006.

  1. AleX`G
    Joined: Jun 2006
    Posts: 68
    Likes: 1, Points: 0, Legacy Rep: 22
    Location: Scotland

    AleX`G Junior Member

    I am planning to build a wingsail which rotates fully.
    And is going to tack downwind as well as upwind.
    This would mean the centre of effort moves from fore to aft of the boat.
    I assume the keel goes at the rough centre of effort but if it moves it will have to be a compromise and be in the middle/ were the mast is. Or I could have 2 keels is this reasoning correct.

    Thanks For the Help
    Alex
     

    Attached Files:

    • boat.jpg
      boat.jpg
      File size:
      51.3 KB
      Views:
      932
  2. Doug Lord

    Doug Lord Guest

    Ce / Clr

    Alex, generally the geometric Center of Effort of the rig will be in front of the geometric Center of Lateral Resistance(Pressure) by some amount called "lead".
    Lead is generally expressed as a percentage of the length of the waterline.
    As a practical matter on a very narrow rc monohull(9/1 beam to length at the waterline with very little flair) combined with a high aspect (tall and narrow) rig lead can be as little as 1-2%-less if the hull is narrower than this-- more if the hull is wider or has significant
    flair. I would recommend using a single keel as deep as is legal. It is a proven fact that, even if you wind up with excess wetted surface in the fin, depth trumps fin wetted surface almost every time in a model. Also make sure that your fin section has a thickness /chord ratio around 6 or 7%--same for the rudder.
    The rig position shown in your sketch is too far aft; you have to be very carefull in moving the rig aft as a solution to pitchpoling: the CE /CLR positions still have to meet the criteria of the rule of thumb above and you don't want your hull CB(center of buoyancy) further aft than about .53 of the waterline length aft of the waterline/bow intersection. Go to rc sailing.net and look under " Technology Discussions" for some excellent drawings posted by Claudio in how to set up a rudder t-foil that is tied into your mainsheet winch(hence no extra channel). A rudder t-foil set up as shown in his sketch(similar to Paul Biekers system on the I14) is probably worth looking at. As in everything in sailboat design there is downside: Bieker's system is designed to be effective on a boat that is sailed flat. On a mono such as yours, that will heel, you run the risk of cross inputs from the t-foil as the boat heels upwind. You have to be able to set it a "0"
    reliably to avoid this but even set at zero it is not contributing as much to the boat upwind as does Biekers. I'll be useing a t-foil on my new Bantock designed F100 but I also have a movable ballast system that allows the boat to be sailed relatively flat in most conditions. You said something about the prevailing conditions being light air? If so I would make the t-foil easily removable in case those conditions materialize. Rudder & Winglets - Forums(see post # 5 for automatic system)
    Address:http://www.rcsailing.net/forum1/showthread.php?p=35515
    ---------
    Using a twin foil system as you show in your sketch(w/o a separate rudder) or like CBTF is not practical with just a two channel system and would actually be MORE draggy than a single fin/rudder system. The reason is that for a twin foil system to function properly it must have "collective" control(another servo and channel) to allow both foils to turn the same direction upwind. Not using collective on a twin foil system obviates one of it's most important advantages...
    Good Luck!
    -----------
    You should consider getting the book "Princibles of Yacht Design" by Larsson and Eliasson and others; look under books on this forum.
     
  3. AleX`G
    Joined: Jun 2006
    Posts: 68
    Likes: 1, Points: 0, Legacy Rep: 22
    Location: Scotland

    AleX`G Junior Member

    I doubt the winglet system on rcsailing will work on my boat due to the fact i havnt got a mainsheet ;)
    The wing is free standing and will point into the wind on its own like a weather vane. Then i am going to use one servo to control the flap on the wing to adjust deflection and camber of the wing.

    I would have thought that the CE (tilting force) would move towards the mast when the boom is perpendicular. When reaching(i think its reaching) surely there would be almost no healing force therefore a keel would not be required.
    When sailing into the wind the CE would move aft of the mast .
    And when sailing out of the wind the wing would rotate around the mast and be facing backwards so to speak therefore bringing the CE fore of the mast. This is unlike a normal sail configuration because the CE canot move forwards of a set point using standard sails.

    Or so i think thats probably all a load of rubbish but I cant see how it wouldnt work like that.
     

    Attached Files:

  4. Doug Lord

    Doug Lord Guest

    wing rigs

    I think you're asking for trouble using a wing rig with a controllable flap because in practical rc sailing it's very hard to keep a wingsail properly trimmed. You might consider a "wing tip rig" ( www.microsail.com ) look at the America One and F3 rigs. These rigs have some of the advantages of a wing rig and square topped rig as used on ACC boats but because a carbon "gaff" supports the rectangular planform sail full battens are not necessary and the sails are quite effective in light air.
    On a wing rig or high aspect rig of almost any type you'll have far less CE movement than on a low aspect sail. But I caution you: the rule of thumb I gave you is PROVEN over many years in models and full size sailboat design-don't deviate much from it or you'll have excess weather or lee helm(excess weather helm if you place the rig relative to the keel as shown in your sketch above).
    ----------------
    F3 Wing Tip Rig"--- "Gaff" fits in bearing at top of mast; boom is fixed to rotating mast; Gaff rotates independently of mast. Twist controlled by vang tension and UPPER OUTHAUL.
    mfoilerf3_14a.jpg
    Address:http://www.microsail.com/images/mfoilerf3_14a.jpg Changed:9:24 AM on Sunday, August 6, 2006
     
  5. AleX`G
    Joined: Jun 2006
    Posts: 68
    Likes: 1, Points: 0, Legacy Rep: 22
    Location: Scotland

    AleX`G Junior Member

    It really is a shame as I am sure a wing sail would work well if under control. Which with electronic systems on board could easily be done. BUT the guy who laid down the race rules says its illegal so. I guess the next best thing is the most efficient flexi rig you could possibly make. I like the wing tip idea. And the way it controls twist.
    One question is what does the jib do on a bermuda type rig.
    Is it like the leading edge control surfaces on a wing which control the maximum camber.
    To be honest i dont think it would be feasable to build a solid wing in the time i had availiable as well as a flexi wing.

    What is vang tension and upper outhaul. Sorry for being a noob.

    It would also mean i could use the adjustable winglets on the rudder to help prevent nosediving.
     
  6. Doug Lord

    Doug Lord Guest

    Vang & upper outhaul

    You asked "what does jib do?"- thats a big subject; you might want to read the "Sail Aerodynamics" thread. The primary reason I use a jib on my rc boats is to facillitate reefing. All my mains can be "roller reefed" to prevent having multiple rigs like most models require. To make the system work well(on non spinnaker equipped boats) the SECOND reef in my system is to remove the jib and move the mast forward to maintain excellent balance.
    The Wing Tip Rig utilizes a manually adjustable upper outhaul that allows the upper back end(peak) of the sail to move fore and aft. This allows the camber at the top of the sail to be fully adjustable as is the camber in the foot with a "normal" outhaul.
    Vang tension refers to a mechanically adjusted system(usually mounted diagonally between part of the mast closest to the deck and the boom) that allows an increase or decrease in mainsail leach tension. On the Wing Tip Rig the vang tension also controls twist in the sail while still allowing the upper part of the rectangular planform sail to have automatic gust response.
     

  7. AleX`G
    Joined: Jun 2006
    Posts: 68
    Likes: 1, Points: 0, Legacy Rep: 22
    Location: Scotland

    AleX`G Junior Member

    NOOOOOOOOOOOOOOOOOO not sail aerodynamics.
    lol
    its such a long thread I have read a few pages but it is very heavy on the mind.
    I will read some later today. Thank you for your help.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.