Canting keel Model

Discussion in 'Sailboats' started by awhapshott, Apr 15, 2012.

  1. awhapshott
    Joined: Apr 2012
    Posts: 14
    Likes: 1, Points: 0, Legacy Rep: 26
    Location: Southern England

    awhapshott Junior Member

    17 year old keen sailor building a 1.2m Canting winged keel model from Carbon/Balsa.

    I've designed and built the hull, now onto the more complicated bit..

    For the keel, what would be the best foil section to use? :confused: I am planning to build it from HD Foam with Carbon skin with localised kevlar reinforcement.

    And in terms of 'lead' (A big topic I know..) what would be best? I wasn't planning on any, but maybe about 4-5%..? I know there is a lot of judgement involved, so if anyone has experience with Canting keels.. (Expect so :D) The boat has a large fully battened mainsail with a relatively smaller blade-style Jib, and also has twin rudders.. (Very Open 70 like.. similar chines too.)
     
  2. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,002
    Likes: 205, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Awhapshott,

    We cannot really comment too much on what you ask because you did not supply any drawing or model. Many factors from both the hull and rig designs go into the decisions you are trying to make on the keel. But I can offer this:

    On a model, it really doesn't matter too much what foil section you use--just about anything reasonable looking one will work. I have seen people build and race full-size boats very successfully with a keel section that was drawn by hand on the back of an envelope and scaled up to full size with a photocopy machine. I dare say you will get a lot of recommendations from posters that follow me, but for what it's worth, you can still to the simplest sections in the book "Theory of Wing Sections" by Abbott and Von Doenhoff.

    As for lead, for deep, vertical keels, place the center of area of the sail plan directly over the leading edge of the keel blade. It's as simple as that, and you'll have pretty darn good balance. (On sailboats that don't have deep vertical keels, it's a little trickier to get it right.)

    As for keel blade construction, on a boat this size the loads are tiny, so if you are going to use carbon fiber, you really don't need any other material. Kevlar is not necessarily a good choice--it is difficult to cut, to wet out, to repair if damaged, and it has lousy compression strength. Stick with just the carbon, that's all you'll need.

    I hope that helps.

    Eric
     
  3. awhapshott
    Joined: Apr 2012
    Posts: 14
    Likes: 1, Points: 0, Legacy Rep: 26
    Location: Southern England

    awhapshott Junior Member

    Eric,

    Thanks very much, that helps a lot! I had a rough idea, but I want to be 100% sure before I cut anything.. (I haven't got enough materials to make a mistake, and managed to source everything for free so far..) I've been helping to build skiff's from Carbon/foam so happy with how to build, its just the designing that is relatively new to me.

    Sorry I can't post a drawing.. I haven't got a scanner and using free software that only allows 3 surfaces...

    I've experienced design issues with foils on a skiff where the wrong shape = Ventilation which always results in a capsize.. I suppose with a slower speed, and tiny area it won't be so much of a problem so long as its symmetric and roughly the right shape.

    I will take your advice about using just Carbon. I know that Kevlar is difficult to work with, but I hoped that I wouldn't need to sand it, although I've just realised I probably will so I would be shooting myself in the foot!

    Thanks for your help,

    Andrew.


    Ps. That pretty much answered all my questions, but if anyone spots a fatal error in my plan please post below!
     
  4. Doug Lord
    Joined: May 2009
    Posts: 16,496
    Likes: 289, Points: 93, Legacy Rep: 1362
    Location: Melbourne, Florida

    Doug Lord Flight Ready

    Andrew, are you using the wings on the keel for lateral resistance when the keel is canted? For racing Graham Bantock of "Sails etc" in the UK recommends thin sections (6-8%) T/C ratio-but as Eric says I don't think it makes much difference on a model like this. If you are using the "wings" for lateral resistance you need to be aware of how the hull changes pitch with heel so that the wings aren't put in too high an angle of attack or in a negative angle of attack with heel.(unlikely) You can change the axis of rotation of the canting keel slightly if need be.
    I built several canting keel models years ago and the best winch for the purpose is the RMG by Rob Guyatt. They're a bit expensive but you'll probably need the power. Shoot for fast movement- 2 sec side to side- and a max cant angle of at least 55-60 degrees.
    You might consider the "KFOIL" which I came up with to allow canting keel wings to retract when you don't need them-see below. There is another, simpler way to do the KFOIL: put the "wing" with an asymetrical section up against the bottom of a semicircular lead bulb. Put the flatish side of the asymetrical section up against the bulb. Simple to make and allows a slightly longer wing. Don't really think you need retractable wings-just an idea for consideration.
    PS-Check in with Graham Bantock! http://www.sailsetc.com/ Graham designed a one meter canting keel boat for me many years ago-never got to production.

    pictures: KFOIL retracted and extended-
     

    Attached Files:

  5. awhapshott
    Joined: Apr 2012
    Posts: 14
    Likes: 1, Points: 0, Legacy Rep: 26
    Location: Southern England

    awhapshott Junior Member

    Hi Doug,

    Thanks, I'll drop Graham an email.

    I was thinking of making the wings adjustable via a rod down the keel.. Unfortunately I won't have enough time to do this now (Its an A-level project and the deadline nears.)
    Yes, my plan is that they will act as lateral resistance when canted, and to account for the slight change in pitch I planned to angle them at 1' negative angle of attack, maybe not enough but I don't want it to 'dig' downwind.
    The folding wings look quite cool, I might play around with that once I've got this one built.. Its my first attempt so not expecting perfection (But close would be nice!)

    RMG seems to be the one to have, if only it was a bit cheaper.. :rolleyes:

    Did you use a purchase for it? I am looking at a 2:1..
     
  6. Doug Lord
    Joined: May 2009
    Posts: 16,496
    Likes: 289, Points: 93, Legacy Rep: 1362
    Location: Melbourne, Florida

    Doug Lord Flight Ready

    --------------
    Andrew, not possible for me to say whether 2/1 is ok or not without knowing:
    1) weight of bulb and keel, 2) distance from the pivot of the keel/bulb CG, 3)
    Cant angle,4) Winch specs including torque and speed. I know I used some purchase-it was different on every boat. It is essential that you use first class ball bearing blocks like pekabe*(if they are still around) and thin line(but not too thin or it will get caught between the sheave and sides of the block)-and this is pretty much regardless of the weight-anything less than ball bearing is too much drag on the system. Also you need to design in a "slack eater" system so that slack is removed from the unloaded line to prevent snags. See below for alternative to canting keel**.
    * Harken just introduced a very small new block for full size applications that might work.

    **A few years ago I spent many hours studying and testing this stuff. One of my final conclusions was that a sliding rack PBS(Power Ballast System) was probably superior to a canting keel. You have a relatively light weight fixed keel(just to aid in self righting) coupled with a carbon tube rack that slides across the boat and fore and aft. You put the boats battery+ lead as required in the box that slides on the rack. Both the box and the rack slide. The ends of the rack are supported by trapeze wires and the rack pivots to keep the windward side low and lee side clear of the water. More like a skiff and allows variable ballast. Simpler to make than a canting keel.The battery is much more efficient as ballast this way -and you can use (and will need) a larger battery. Just food for thought......
     

    Attached Files:


  7. awhapshott
    Joined: Apr 2012
    Posts: 14
    Likes: 1, Points: 0, Legacy Rep: 26
    Location: Southern England

    awhapshott Junior Member

    Thanks doug. Your Power Ballast system is cool! -Very skiff like :)

    I've cut the slot for the keel now so no going back!

    Sourced some tiny ball bearing blocks.. had some left over from my dinghy refit that are the smallest harken do and look about right. Using a thin dyneema line, on a 2:1 with the RMG winch as suggested.

    Off to fit it out and get afloat, thanks for all your help!

    Andrew
     
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.