Reducing weight by using Kevlar or carbon

Discussion in 'Boat Design' started by Flying Flivver, Oct 3, 2011.

  1. Flying Flivver
    Joined: Oct 2011
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    Location: Ottawa ON canada

    Flying Flivver Junior Member

    I’m planning on building a new flivver, by using the existing one a male mould. The dimensions are 8.5’ long 4’ beam 1’ deep. I used the scantling rule for my basic hull thickness of 7.6 mm +20 %for speed and got 8.64 mm which consists of roving, mat and polyester resin and is consistent with the original boat.

    The boat is a tunnel hull design with a open top and a 25 hp outboard.

    My question is, if I use a Kevlar or carbon inner skin and an e-glass exterior how much can I reduce the weight?

    Attached Files:

  2. tunnels

    tunnels Previous Member

    Mixing of materials is not advisable glass stretchs carbon dosent so its pointless . Stay with one or the other !! why you want to build light ??

    What sort of resin are you going to use or think about using ?? :D
  3. Flying Flivver
    Joined: Oct 2011
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    Location: Ottawa ON canada

    Flying Flivver Junior Member

    I have no trailer for the boat which means a lot of lifting for me.

    west system 105/206

    Did I supply enough information?
  4. tunnels

    tunnels Previous Member

    I made Boats about the same size as rescue boats two man 30 hp inflateables . The ridged bottom as 4 layers of unidirectionall glass and used only polyester resin . you can build light boats from just poly and straight glass !! Forget the exotics . Other ways to gain strength is from Design and shape Panels of cars get thinner and lighter as the years go by but look at the designs !! Look how they gain strength its from Shape!!, each and every panel has ribs ,creases, and indents all that gets added increases the ridged strength . theyre not adding weight just increasing ridged strength .
    Recently i replaced the transom in my boat , was the usual glass 50 mm of wet ply and more glass . Its now all glass has lots a shape and a little core in 3 places , the whole weight is less than half what the original transom was and yet its probably twice as strong using less materials . something to think about also !,the hp of the outboard went from tired old 65 hp omc 20 inch to 115 hp vee 4 yamaha 25 inch and yet the transome is way way stronger than it was !! Shape gave me the one thing i was looking for and needed most .
    Think about your design ,how can you increase ridgeity and make it stronger without increasing weight .its worth thinking seriously about believe me . Go look at your car and how is it made !! how is it constructed ! how do they make doors and bodies so strong ? yet each year cars are getting lighter !! . Boat designers are caught in this thing of using stronger materials to gain strength and are completely hooked by it when really its design that can be the greatest step forward !!. :eek:
    Look also at aircraft design how is it that planes are so strong ?? Boat designs need to change and the thinking behind designs has to change as well .
  5. Flying Flivver
    Joined: Oct 2011
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    Location: Ottawa ON canada

    Flying Flivver Junior Member

    Obviously the 5’ by 2’ perfectly flat panel where I sit has little structural integrity. Ridges in the center panel would increase stiffness, like the car hood.

    i would like to stay relatively true to the original

    Attached Files:

  6. tunnels

    tunnels Previous Member

    you missing the obvious !!
    Look at whats behind those thin strong panels !! Look at wings and the fuslage of a aircraft how is it so strong and so roomy inside

    You want to make you boat strong yet light weight ! use good quality glass and vinylester resin !!infuse it !!6 oz cloth on the outsdie with 3 layers of unu glass and 1 double bias on the inside with a monococ internal frame also using double bias !! you can work out the glass weigths . With a racing 25 merc it will fly !! The oriantation of the 3 uni's is you most important decission . Uni is the most fantastic glass and it will bend a mile and never break even carbon fishing rods use glass on the ends so the wont break !!!
    The same weight of carbon and epoxy will snape and shatter ! which one would you use ??
    Kevlar for the weight you need is thicker so uses more Epoxy so is heavyer .
    Glass !!
    Carbon !!
    Kevlar !!
    Pick one !!
  7. Flying Flivver
    Joined: Oct 2011
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    Location: Ottawa ON canada

    Flying Flivver Junior Member


    i know it as unibody and i will have to do more research on it.

    concidering that most of the stress is on the bow section from the waves hitting the rounded front and from the motor pushing from the back . if there are three layers there should be two uni layers along the lenght and one across the beam

    hopefully i'm right on this one:D

    thank you for your help and advice:)
  8. tunnels

    tunnels Previous Member

    You have to understand the properties of uni glass and the use of overlaps of layers in the right places . :)
    Glass is very robust and forgiving . Look at the glass oriantation of the glass i said !! draw them on a piece of paper !! what do you see ??:confused:
  9. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    If it were me, I would have an engineer do a structural analysis considering the forces that are involved to tell where the strength has to be. Then shave off weight where it isn't needed to the point of being paper thin. You can then introduce exotic materials(carbon/kevlar) with the proper characteristics IF it saves weight. Embeded titanium structural members where needed might make sense if it is compatible with the resins you choose. When human powered planes were to be built as light as possible, they experimented by reducing the thickness of every component until they failed structurally, then dialed it back up slightly to the point is was just strong enough. The new Boeing airliner uses especially designed composites for the skin over metal structure to save a lot of weight...


  10. u4ea32
    Joined: Nov 2005
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    u4ea32 Senior Member

    On such a small structure, you probably want to just do it the easy way: glass and foam. You can skip beer for a weekend and save more weight than using exotics on such little things.

    Just be sure to really use the squeegee: when it looks too dry, its about right, when it looks beautiful and smooth like glass, its far too heavy and weak. The strength and toughness comes from the fibers, the weight and brittleness comes from the resin.

    Kevlar starts out sounding really cool, but it absorbs water. Its hard to get the resin to flow, so in practice the fiber content usually ends up lower, so it can be heavier even when first constructed. It will end up heavier when water vapor migrates through the resin.

    Carbon can be used effectively with glass by using the glass for panels and carbon uni tape for strapping (double diagonals, like a bridge truss) and along the load paths. All structural loads tends to transfer along extremities. So put the carbon tape along pretty much all the lines in your drawing: the gunnels, the keels, etc.
  11. cyclops2
    Joined: Sep 2010
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    cyclops2 Senior Member


    Racing boats get VERY HIGH instantanious bow loads. Going around a course with other boats sets up some VERY steep choppy water.
    Add in to that, the UNSEEN wake from a round bottom cruiser 1/2 mile away as it goes by unnoticed. The high tech carbons ...Shatter completly .....Often with no gradual warning.

    Glass cloth gets cracks. Then starts the taking in water. Then a whole panel starts vibrating. ..............How much more warning signs does a driver need ?

    Seen it happen like that many times. Fibreglass & thin plywood for racing is far safer than any cabon stuff.

    Your life & limbs.

    We are repairing a sponson crack for the 3rd time due to rough water. Driver was not aware. Shop check found it.

    We only use cloths . Why ? Because everything else adds lots of bulk in comparison. Also known as excess money & weight.
  12. sabahcat
    Joined: Dec 2008
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    Location: australia

    sabahcat Senior Member

    Foam core for panel stiffness
    Kevlar in and out with glass biax on the outer for abrasion resistance is about the lightest method used

    7.6mm of solid glass seems insanely heavy
  13. tunnels

    tunnels Previous Member

    glass biax on the outer for abrasion resistance
    Sorry cobber but glass on the outside is not abrassive resistant !! Kevlar is way more abrasive resistant than glass !!!:confused:
  14. Flying Flivver
    Joined: Oct 2011
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    Location: Ottawa ON canada

    Flying Flivver Junior Member

    on the original boat the transom was quite square in section and had started crack along the edges, If I alter the shape of the transom so that it is rounder on the top to relive the stress points will it last longer?

  15. tunnels

    tunnels Previous Member

    then the stress will go somewhere else !! think round from the hull sides to where the motor is mounted :?:
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