grp boom

Discussion in 'Boatbuilding' started by rob50904, Jul 7, 2010.

  1. rob50904
    Joined: Jan 2009
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    rob50904 New Member

    Hello,
    I'm planing to build a 3m long boom for a 21ft sport boat.
    As i have enough grp left from the hull I was thinking of using grp for the boom. That would save a lot of money because I'd get the boom for free instead of ordering an aluminum or carbon one. Are there any drawbacks (instead of more weight/less stiffness compared to a carbon boom)?
    Grp has a better stiffness/strength than aluminum, so the boom would be lighter or stiffer/stronger than an aluminum one.

    I never heard of grp booms before, so I'm a little bit unconfident.
     
  2. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    What kind of grp you have in mind?? For boom about all unidirectional..
     
  3. rob50904
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    rob50904 New Member

    Yes. Two layers +45/-45 and the rest UD up to an overall thickness of 2mm. in parts of high vertical pointloads (vang, mainsheet, clew) some extra 90° fibres.
     
  4. UNCIVILIZED
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    UNCIVILIZED DIY Junkyard MadScientist

    Tag, as a reference 2 jar my memory later.
     
  5. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    Rob, Welcome to the forum. On such a small boat, you can probably successfully make a workable GRP boom. But you have a wrong assumptions about GRP versus aluminum. GRP is a little more than half the weight of aluminum. Depending on the layup, its strength can be from half that of aluminum to equal or a little greater than aluminum. As for stiffness, GRP is only about 1/5th to 1/10th that of aluminum. So what GRP lacks in stiffness and strength (and for a boom, you need both), you have to use more material to make up for it. The stiffness of a boom (or any spar or structure) comes from both material stiffness (modulus) and geometry (bigger sections are stiffer than smaller sections), so a GRP boom is probably going to be a bit bigger in section than an aluminum boom.

    All that said, a 3M boom on a 21' boat is not a really highly loaded structure. The bigger the boat, the loads go up exponentially, so aluminum makes more sense on larger boats. Aluminum is also cheap compared to carbon fiber construction, and that is why aluminum booms are still so popular. At any rate, I'd say go for it, give it a try. If it breaks or bends too much, build another one that is bigger.

    Good luck,

    Eric
     

  6. UNCIVILIZED
    Joined: Jun 2014
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    UNCIVILIZED DIY Junkyard MadScientist

    I'd venture to say that if stiffness is a concern, then the boom could be built much akin to the carbon ones (for large sailing vessels). That is, that the walls & other areas of the boom are actually built in sandwich construction... like sandwich layup in boat hulls. And that the center of said boom(s) are hollow. Which actually helps their functionality, in that it lets one run reefing lines & hydraulics on the interiors of the booms on larger yachts. Also, that such allows one to put extra reinforcements/and or blocking, in the extremely highly loaded areas. Such as in the neighborhood of the gooseneck, vang & sheet attachments, blocks & cleats for reefing lines, sheave boxes...

    Not that on a boom of that size one needs not go loco about such things. And also, the above info's posted only for the edification of those who aren't familiar with such types of spars.
    Although if a boom were built in the sandwich type of construction for the vessel in question, it'd likely simplify the attachment of hardware to the boom. Making it possible to mostly just epoxy bond the fasteners of the required hardware into place, rather than needing to use semi-complex compression tubes for the more highly loaded fittings.
     
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