Carbon versus Ali main beams

Discussion in 'Multihulls' started by waynemarlow, Mar 23, 2015.

  1. Larry Forgy
    Joined: Dec 2011
    Posts: 13
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Maryland

    Larry Forgy Junior Member

    Wow. I haven't priced finished carbon tube, but I will happily supply all you want for $559 per foot. Here is what I will do:

    Buy a length of 4" heavy gauge biaxial carbon sleeve for $5.17 per foot from fiber sleeves.html
    and a length of 9ft/lb unidirectional carbon sleeve for about $7.99 per foot. fiber uni-fabric.html#UNISleeve
    A sandwich construction of biaxial-uni-uni-biaxial will be about 2 ft/lb and a wall thickness of about 0.12". Epoxy (about $8/lb) will double the weight to about 1 lb/ft and increase the wall thickness to about 0.15"
    So far, I am up to about $26 per foot of carbon and $4/foot of epoxy for $30/foot of materials cost. Add in another $30/ft for expendibles (mandrel, vacuum bag, etc) and I will happily supply my labor for $500 per foot.

    I have done a lot of glass and carbon work on various boats, and these days I generally count on carbon being about $30/pound if you do a careful search on the Internet. In this case, the carbon is costing about $60 per pound.
    Larry Forgy
  2. Jetboy
    Joined: Feb 2012
    Posts: 278
    Likes: 6, Points: 0, Legacy Rep: 65
    Location: USA

    Jetboy Senior Member

    I didn't choose that website as a source...

    Lets say we can buy it for $80/foot. The 60 feet of tube I bought for $400 turns into $4,800. That's a very significant increase in cost.

    Unfortunately carbon tube is also very hard to find in telescoping sizes around 3-5" OD. In fact I've never seen an advertised matched pair of telescoping carbon tube in that size. Have you? It can be done - but it would likely involve having a female mold and that gets pretty damned difficult to manufacture 20 foot sticks with .010 or less tolerances. Aluminum is much easier to do the same.

    I'm fully in agreement that Carbon tubes are better in an absolute sense. Carbon FRP is a superior material. They simply are not cost competitive for the small weight reduction for anything short of very high performance boats. We need to remember we're talking 50-75lbs weight difference for $4,000+. Titanium 4" tube is around $40-55/ft. So that's an intermediate option as well. Unfortunately Titanium is pretty hard to weld properly. Although it has roughly the same tensile strength as carbon FRP, it's young's modulus is roughly double that of biaxial carbon FRP, but half that of unidirectional carbon FRP. So it sits somewhere in the middle in terms of elasticity - probably would be very similar in 99% of applications.

    Finally, as I said before, if we're building in carbon - why are we even considering round tubes? The reason we use aluminum tubes is because they are generic and I can drive to affiliated metals and buy 4" 6061 tube off the shelf today and drive home with it. It's not because it's an ideal material. If we're looking for ideal, there are better shapes than round tube.

    In a dream world we would just put in an order with Lexus and have them filament wind us custom shapes on this guy: Someday boats will be entirely built this way.
  3. redreuben
    Joined: Jan 2009
    Posts: 2,010
    Likes: 226, Points: 63, Legacy Rep: 349
    Location: South Lake Western Australia

    redreuben redreuben

    Considering a Tri like the Bucc 24 the attraction of using carbon is not just the light weight but the possibility of losing the water stays as well although from the lack of comments on my previous post few seem to rate this as worthwhile. What I don't like about any off the "off the shelf" tube beams, carbon or aluminium is the drag when pressed, a custom fabricated beam like on Capricorn can use carbon economically, lose the water stays and use curves to get the beam out of the water when pressed.

  4. Skyak
    Joined: Jul 2012
    Posts: 1,461
    Likes: 146, Points: 63, Legacy Rep: 152
    Location: United States

    Skyak Senior Member

    The unavoidable high cost of carbon says to me that it is a bad deal to buy it in off the shelf constant sections when the intended use (AKAs) is tapered strength and deflection. If you want the improved performance, the least you can do is to match the strain to the section. This is an advantage extruded aluminum can not follow.
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.