Carbon mast construction in 2 female 1/2's

Discussion in 'Fiberglass and Composite Boat Building' started by bjl_sailor, Apr 21, 2005.

  1. bjl_sailor
    Joined: May 2004
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    Location: MASS

    bjl_sailor Junior Member


    I'm working out the rigging details of my K-19 sport boat and am looking at alternatives to a conventional aluminum extruded mast.
    The plans call for a 26' 2" mast based on the Dwyer DM-3 mast section which is a 'flattened' circle cross section about 2.15" x 2.75".

    I'm thinking I can build a 4" diameter carbon fiber/composite mast for somewhat more than the $500 the DM-3 section costs myself -- and would greatly appreciate a sanity check and any advice from those who ahve spar building experience.

    First, rather than using a male mandible, I would use 4" PVC piping which will cut in half and joined to the 26' plus length by cutting the couplers in half also.
    Here is a CRUDE drawing of what I am talking about below....

    Once the PVC tube is cut in half and a mold release wax applied ( butcher's paste wax) I'll first run a layer of 6" S glass tape -- to make an easier sanded/finished surface.

    My carbon lamination scedule would be as follows:
    1 layer of bidirectional 5.7 OZ carbon fiber tape
    2 layers of 6" wide unidirectional carbon tape
    Next, 3/4 x 3/4 harwood strips will be temporarily screwed and expoxied to the cut lip of the PVC mold on the inside -- providing an internal longitudinal rib and a means of rejoining the cut halves.

    Final internal lamination will be a 6" bidirectional kevlar tape. A wood / epoxy plug would be built for the base, top and spreaded location along with additional reinforcement at the mast hounds....

    Once the molded halves are cured they'd be popped out and the two halves joined with epoxy and s glass tapes. This would then be faired and smoothed. I'd next route a 1/4" deep by 1/4" wide slot along this seam and fill it with carbon strands and epoxy. Final fairing and then I'm not sure what to do about the sail track and tang/spreader attachements.

    I've done the pricing of the material -- my composistes would cost about $500 --plus the cost of resin, mold material and hardwood -- probally about $200 more than the aluminum alternative. My concern is the mast hardware, sail track, spreaders, tang attachment points...

    Is this insane? Is the lamination schedule 'about right'? Any feedback would be appreciated.


    Attached Files:

  2. cristofa
    Joined: Oct 2004
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    Location: Cornwall, UK

    cristofa Junior Member


    When you make a thing like a mast in two halves, and join them together, the lack of fibre continuity will inevitably make the result heavier, or weaker, than it might be.

    I have become a great fan of tubular carbon braid, and would suggest that it could be an ideal material for your mast.

    There are a number of ways you could use it for a one-off - here's one of them:

    You can mould your mast on to the inside of your 4" plastic pipe.

    Get some 4" diam tubular plastic bagging - the kind that is widely used with a heat-sealer for people to bag up small items. Feed on your first layer of braid - you might find this easier if you seal the ends temporarily and partially inflate the bag. I think I would then use a bit of spray glue to attach some 1" uni tape to the front and back, and some extra carbon in the spreaders region. You then need a resin infusion medium - I use, and like, Lantor SORIC - 2" strips on either side should do the trick. Then feed on a second layer of braid and use a string to pull the whole thing up the inside of your 4" pipe.

    All you are doing, this way, is vacuum-bagging on to the inside of the pipe, so you need to bag-tape the bag onto the pipe; evacuate the space between the bag and the pipe from one end, and feed resin in to the other end. Just remember to make sure you have a (SORIC) path from the resin feed in to the laminate, and from the laminate to the vac out.

    If you want the mast profile to be ovoid, you could use clamps and strips of wood to squash the pipe.

    Now all you've got to do is mix up your (low-viscosity infusion) epoxy; infuse the part; allow it to cure, and demould - I think I'd use a router with a cutter set just less than the pipe thickness.

    Having tried recently, with great difficulty, to detach something that had been glued on with SikaFlex, I would happily use that to attach the sail track.

    And you might want to paint the mast for finish and protection.

    Or perhaps it would be better to shape up some (surfboard) foam and then, using just the same layup, bag the outside. This would have the advantage of being able to let wooden blocks in to the foam for spreader attachment - you could also taper the mast, which always looks better. This way you'd have the surface texture of the braid's weave, but that's OK.

    I will be making the ROCAT oars this way in a closed mould, with moulded foam cores.

  3. yokebutt
    Joined: Aug 2004
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    Location: alameda CA

    yokebutt Boatbuilder

    Try this one on for size, instead of cutting the PVC tube in half, cut one half say 1/2" bigger than the other. (this method assumes that you make two parts from one mold) Then you add a few layers of masking-tape to build up a thickness equal to your first lay-up onto the extra 1/2" lip. Proceed to making two parts with a layer or two of carbon fabric from the mold. You now have two half-circle parts that you can bond together into a circle with the extra lip jogged in just enough to bond to the inside of its mate.

    That's your pre-form, now you can laminate on unis and braided socks and whatnot to your hearts content. (don't forget to add local stiffening before bonding the two halves together)

  4. John ilett
    Joined: Nov 2003
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    Location: Perth Australia

    John ilett Senior Member

    This all sounds very dodgy I feel. Just about all carbon tubes and spars are made using pre preg carbon fibre which has much better characteristics than that of wet laid carbon even if it's vac bagged etc.

    Your laminate of 5.7oz cloth and two unis.

    I think 5.7oz is actually 200g and lets say that your unis are a heavier 450g. At 1000g per millimetre your mast of just 1100g would be 1.1mm thick.

    At a rough guess most sports boats carbon spars may have a wall thickness of about 3-4mm which is 3000-4000g of carbon. So the cost of your mast may have just gone up considerably.

    The ply joiner also falls into the dodgy category.

    DIY composites is cool if your re glassing your deck, repair or building a simple hull but to try really structural components like masts could be a very costly mistake unless you have some good guidance and instruction.

    Apologys for being a little negative, I just like to see jobs work out right.

  5. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    I've used prepreg carbon quite a bit for race vehicles, etc. It's not easy to work with- and wetlay's even more difficult. Carbon-fibre of any sort MUST be vacuum-bagged, and ideally should be autoclave-cured.
    I would never think about building a carbon mast myself, especially a hollow one. The stresses on a mast are far higher than they appear, and there are concentrated loads that simple math will miss.
    Your ambition is good, BJL. Still, having seen many homemade carbon-fibre components fail under much lower load conditions, and looking at how light your lamination schedule is, I'm pretty wary of this one. And yes, the split will weaken the mast- not a little, but quite a lot.
    Carbon-fibre's not exactly a DIY thing like fibreglass. To get it right requires a lot of time, training and expensive equipment. You also have to be able to recognize failed laminates (it's not easy to do) as carbon components, unlike metal, do not give you any warning- they simply shatter.
    Sorry to seem pessimistic- like John, I'd rather see this work right the first time.
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