Aluminium & Composite Construction

Discussion in 'Boat Design' started by Crowie, Jan 19, 2005.

  1. Crowie
    Joined: Jan 2005
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    Crowie Junior Member

    Hi All,
    I have been reading these threads for a while and though I would throw in an idea.....
    I am designing a 6.5m plate alloy centre console boat and was thinking if it was possible to mold the hull sides (port and starboard) seperately out of epoxy/glass sandwich with a 30mm thk core. This way you could have the best of both worlds, a tough customisable plate alloy boat with a bit of style (I was thinking some tumblehome aft and a nice shear line) and you would have built-in bouyancy without sacrificing underfloor storage space.
    All the alloy frames & hullskins would have to be laser cut for an accurate fit with the molded parts at the chine, but you could design a range of hulls to suit your molds, (deep V, bay boat etc).
    So, has anyone had any experience joining the two different materials together in such a critical location I would imagine there would be a "sikaflex" type material that could allow for the differing thermal expansion rates.
    I'm interested in any thoughts on the whole process..........
     
  2. Raggi_Thor
    Joined: Jan 2004
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    Raggi_Thor Nav.arch/Designer/Builder

    I don't know if it has been done, but it's a good idea. I have been thinking about an aluminium bottom structure with sides, deck and interior of plywood.
     
  3. Dutch Peter
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    Dutch Peter Senior Member

    The idea is nice, but mixing these two different materials with their different properties is going to give you headacks for sure!!!
    And what are you really gaining? Weight, space, to my opinion you can make the same tough boat fully out of composite (FRP).
    The notion that you can use the same sides of a hull combined different bottoms will prove very difficult as the boats properties will change and no boat will be at it's optimum.
    Ofcourse a boat is always a compromise, but I feel you will have a worse compromise with this scheme.

    But, if you feel you can pull it of, don't let my 2 cents wear you down.
    I'd love to be proven wrong.

    Good luck.
     
  4. BrettM
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    BrettM Senior Member

    I assume that you are thinking about joining the two at the chine? Could be problematic since this is a critical area that is subjected to plenty of abuse. If you really want some grp parts why not think about items such as the deck and console. There are a few moulds about...

    Getting a good sheer line is possible in al but flare is another thing (without cutting the plate). Sheer is probably more important imo.

    30mm core sounds pretty extreme too for a 6.5m topsides. My first guess would be half that but this ultimately depends upon a few factors.

    Do you have any experience with CNC patterns for laser/plasma/router kit boats? If not, I may be able to help, having done a few in the past few years as part of my job. Where in Oz are you?
    Brett
     
  5. Crowie
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    Crowie Junior Member

    Thanks for that Brett, I'm in Sydney, I agree 30mm of foam is thick but the thought is from a marketing point of view people seem to like the whole unsinkable thing at the moment. It could be seen as having the volume of a RIB tube stretched vertically. As to the join at the chine, I was thinking of maybe an extrusion that the sheet of the bottom slots in one side and moulded side into the other, I've seen manufacturers in the US use something similar, thus eliminating some of the difficulties associated with getting the chine line just right......
     
  6. Are you refering to the, deck to top of the hull, joining seam? That would be much easier and structurally sound. Any thing could be joined there, as it is a accepted meeting edge. Visually, it would look a lot classier than the, " all white jobs ". Weight wise , you would gain a few pounds. A attractive deck beats a few pounds any day.
     
  7. BrettM
    Joined: Apr 2002
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    BrettM Senior Member

    Crowie,

    sorry, I haven't replied. Busy times.

    A typical RIB in the 6-7m range has a tube diameter of 500mm upwards. Putting that into a 30mm panel would make it 6.5m high which isn't practical. Better off making the unsinkable component under the cockpit sole.

    Apart from that, if you could get a guaranteed bond between the topside grp panel and the bottom plate using a chine extrusion (much like the modern tinnies) and sika, it might be ok. I/we designed a 9m runabout a couple of years ago that used sika to attach secondary stiffening structure to the hull plate. Primary structure was welded though.

    Depending on the quantity of boats you intend to build, GRP topsides tooling (if you are looking for fancy curves etc) could be cost prohibitive compared to an alloy topsides.

    Brett
     
  8. Crowie
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    Crowie Junior Member

    I totally agree with the fact that we're talking about only about 15% of the the bouyancy of a similar RIB tube but perception is a funny old thing......

    I guess if we lift our heads out of the technical challenges for a bit, what started me off in the first place was a visit to the Sydney boat show back in winter and seeing what was offerred by the large alluminium boat builders, now the sales people where very nice, but as an mechanical engineer with a large R&D focused multi-national who has now moved into a more sales role (and studying at Westlawn at nights) I was very unimpressed with what was on offer. I don't know whether there just isn't the volume of boats produced to invest in what's required to moved out of the "ribbed tin can" look or if it's more "if it ain't broke don't fix it" mentality.
    So I looked around at what I thought looked great, there was a powerboat in Wooden boat mag called Nectar a few issues back that I thought had beautiful lines, all flare and tumblehome. (It's now being marketed as the 25' Monomoy Angler by
    http://www.peaseboatworks.com/pbw_monomoyangler.html)

    This started me thinking about combining the Australian market's acceptance of plate alloy boats, and bringing into it some style, something in a similar vein to Wally's very cool is new wallydinghy (http://www.wally.com).

    So maybe the question is, can you combine together good classic hull form, a markets perception of a material as being superior (alloy) and modern design in one boat, and sell enough to make whole exercise worth the time and effort..........
     

  9. Gilbert
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    Gilbert Senior Member

    Hi Crowie,
    About 20 years ago a fellow named Ray Wadsworth built a high speed seine boat here in Washington that (assuming I remember correctly) had a composite structure similar to what you are talking about. I don't recall if they used any foam core. I believe the boat was between 50 and 60 feet long and was supposed to go 50 knots or so with a gas turbine engine for "traveling" and a piston engine for fishing. I have no idea if there is any information about this vessel or Ray on the web, but you could try a search. If that proves futile, you could get back to me and I'll see if I can't come up with some more solid information about it.
    Good Luck,
    Gilbert
     
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