Carbon / Foam keel fin

Discussion in 'Fiberglass and Composite Boat Building' started by Cpalm, Feb 8, 2012.

  1. Cpalm
    Joined: Mar 2011
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    Cpalm Junior Member

    Hello,

    I am planning to build a keel fin for my sailboat project using seven layers of 1/4 and 1/8 " corecell foam laminated together. This laminate I will then carve down to the foil section (an eppler e836) and laminate unidirectional carbon over top to build up a skin thickness of approximately 1/4 ". In between each layer of foam, I am debating adding a layer of carbon and epoxy or a colliodial sillica / epoxy filler.

    My question is which would be the better route? I'm fairly certain the filler would create a stronger bond between foam layer and reduce the chance of delamination, but is there a benefit in putting skin layers in between the main outer structural skin?

    To give some details the fin is a 6' span with an ~800 lb bulb. The bulb's Cg is pretty close to center fore and aft on the fin so im not too worried about twist loads, etc, though i have taken them into consideration. The fin has a 15" chord and is a simple rectangular profile
     
  2. tunnels

    tunnels Previous Member

    You are asking a bit much and carrying all that weigh ?? what sort of thickness is the finn going to have ?? 6 foot long x 15 inchs x ??? how do you antisapate attaching top to the hull and bottom to the bulb ?? lots to thinkabout in design and then how to construct . Uni is ok but some of it has to have angles to hold it's self together . Plus maybe a light double bias first and last!! uni 's strength is all one way the other way its O
    tell me what your thought are and how you intend to go about making it !!What is the bulb being made from ?? lead ?? and hows it going to be fitted together ?? what about the top how do you invisage making it fit the hull ?? pocketed up inside and through bolted what about the hull internall grid to hold the pocket and take all the loading !!Made some race boats a while back and had 1850 kgs of galvanised steelframe and lead bulb with glass skins ( filled with foam to cover the keel strut and the bulb!!
    Or a flang and bolted or a slot and pass throught the hulls keel and attach inside ? the leverage of any of these methods is horrendous and yes there is twist and bend and flex factor and if you hit anything !!!! @#$@! i dont want to think about that bit . Drawings pictures of the boat !,give us something as a starting point !!!!:confused:

    I just had some thoughts and could run them by you !. Uses a small amount of really high density foam to form box shapes then carbon over the outside of each one then attached together and carbon the outside you are needing to gain as much strength as possible and a good percentage can come from the shape of things within the outside skins !!, like aircraft wing design !! outside skins with a internal framing !! the profile needs to be a foil not a rectagular shape , foils are better than rectangular's when moving through the water !!.Just find a good one !!!
     
  3. Cpalm
    Joined: Mar 2011
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    Cpalm Junior Member

    Thank you for the response, the keel fin is ~2" at it's max width. Unidirectional is simply the fabric type i have chosen at the moment. The exact layup is yet to be determined, however a 1/4" skin on each side calls for about 18 layers so at least 10 of those will be running the length of the keel. I would run other layers at +/- 45 and at 90 degrees of course!

    As for fastening to the hull the keel will be housed in a box of similiar layup molded around the fin itself. The keel lifts vertically to deal with the excessive draft. This box or tube has the same section as the fin, and is fixed to the hull (with local reinforcement) and the other end to the top of the cabin house. The keel when fully lowered will keep 18" in the box. The box is also supported by a bulkhead molded to the hull/ deck / cabin house.

    The bulb will be cast in lead, with two halves thru bolted to one end of the keel fin.
     
  4. tunnels

    tunnels Previous Member

    Where the fin entres the lead bulb i would make a tapered fit all 4 sides so it always stays tight and wont come loose
    How do you propose to raise and lower the keel and what sort of stopper have you thought of if the lifting thing breaks and the keel suddenly wants to free fall out through the bottom of the hull to the sea bed ??
    With just 400 mm of fin staying inside the casing and the casing having to take a hell of a loading side ways as the boat heels from one side to the other when tacking The trunk sides will have to be exceptionally strong as would be the fixing to the hull as well , not forgetting the strike an object factor at full tilt and it wanting to tear the guts out of the hull bottom Carbon is not good at handling overloads and shock so just be awear Oh yes its strong to the point and then its gone . theres not tell tale signs of when its going bye bye it just not there any more So consider a mixture of glass amongst your choices of materials and use good quality resins and heat cure and do all that nessasary to get the best from what you are using or going to use !!. All interesting !!:D:p:p
     
  5. michael pierzga
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    michael pierzga Senior Member

    Never looked into composite keel engineering .

    I would think that they would be constructed the same as a rudder.

    A massive Box section rudder stock with webs and only a fairing skin.

    The Box section buried into a keel trunk and the bulb
     

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  6. tunnels

    tunnels Previous Member

    Some picture of the project from korea . 8 x 36 foot boats racing yachts
    we were very fortunate to have a designer on hand and a young brilliant enginieer . because of time constrates and a no mistakes or else policy things were well planed and done ontime of before . the boat is also a criusing boat so everything is the same racing or cruising all the keel parts were cnc cut and interlocking to make assmbly easy ad quick . galvenised and then placed into the keel lead mould and when all the lead was pored became one complete unit . The glass shells foam filled were then fitted and glassed in place and faired and bogged and painted . All were the same and completely interchangeable with any and all the boats The boata had cradles for with or without keels so just lift place in the cradle and a forklift required to lift the keel and slide the 3 bolts into there holes and tightened . the rudders were fitted at the same time .
    the box for the keel within the hull had a grid frame fitted and glued into place and the framing was all sat and fitted and glassed on to and around the grid there was no way any of it could come loose of come out .
     

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  7. tunnels

    tunnels Previous Member

    All up the complete keel unit weight was 2024 kgs
     
  8. michael pierzga
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    michael pierzga Senior Member

    Yah..steel fin is what Ive seen in the past
     
  9. dougfrolich
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    dougfrolich Senior Member

    Cpalm-
    I've used the approach that micheal pierzga, and tunnels have described above.
    I'm designing one now with similar dimensions to yours. But I am making a central spar from carbon fiber laid over a douglas fir core. The rest of the fin will be shaped in foam and covered in a lighter carbon fiber skin
     

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  10. Cpalm
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    Cpalm Junior Member

    The box section is a very interesting idea... Basically making an I beam like structure from carbon. But am I understanding correctly that steel is perhaps the safer / more widely used method? And that the skin of the keel should not be the load bearing part bit rather an internal structure?
     
  11. michael pierzga
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    michael pierzga Senior Member

    Sure. Ive built many keels this way. Super strong..super stiff. But these keels were for one meter class RC sailboats !!!!

    Better investigate and put your thinking cap on before you proceed.
     
  12. Phil Locker
    Joined: Oct 2004
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    Phil Locker Junior Member

    I'll chime in on the side of those recommending some central structure to the strut. You need it for both a shear web, and for compressive strength at the hull exit.

    800 lbs isn't huge, but you're hanging it on a pretty good moment arm.

    Keel struts of that size we typically mill a solid wood core for (Douglas Fir) - because we can. But its also valid to just use a douglas fir beam as Doug suggests, with fairing pieces of foam on the leading and trailing edges. For additional security the beam can be wrapped with laminate (biax eglass works well there).

    Cheers
    Phil

    Follow our tweets from the workshop @CCIComposites
     
  13. michael pierzga
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    michael pierzga Senior Member

    Sounds logical, sounds cheap and sounds doable. Perhaps special attention to the end grain.
     
  14. tunnels

    tunnels Previous Member

    In a structure like that every part has to be working no mater what it is box for ridgidity and across the structure also the outside fairing skins they to need to carry and be able to work and do there part with twist and the fore an aft forces nothing should be just there for the ride so to speak!! and everything need to all work together!!and be interlocking and tied together from top to bottom sode to side end to end all !!! and there is no room for error any where !!!:eek::D;):p
    I still think you should be looking at a foil rather than a simple rectangular shape, a foil will have deepth and possible to deal with the bending problem you could encounter !
     

  15. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Even on a race boat I would be concerned about a carbon keel. Every boat I have sailed on, eventually runs aground somewhere (does this just say something about me?), and I have doubts that a CF keel would take this load well. It might be ok in California where deep draft is pretty much assumed, but on the gulf coast, and eastern seaboard soft mud groundings are so common we call it "leaving the harbor".
     
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