pre-lam or prefabricated flat panels-developable

Discussion in 'Boat Design' started by tugboat, Jul 13, 2012.

  1. tugboat

    tugboat Previous Member

    Hi--I have MAL Lows tutorial but im stumped-there is no other info on this method called -pre-lam.

    he suggests this-
    make a formica table, then lay out flat panels about 18 ft long by 60 inches wide--they are built by using various weights of cloth -i.e. 24 -30 and 15 oz cloth. he doesnt specify biaxial or roving etc.

    but they end up being quite thick with about 7-8 layers of average 22 oz layups.

    the process is -

    laminate on table -gel coat - fibeglass -resin cure- repeat many times-
    i,.e glass- resin - cure etc etc. till the thickness is achieved.

    .then use those panels, i assume once cured, like plywood sheets. cut to shape join the panels using taped seams form the hull--in my case its a boxy hull--flat bottom. plumb sides and a fantial. he suggests using a heat gun to help form to the tighter radii.

    my quesiton is--will they be able to develop once cured??..will they be too stiff?? they are quite thick. i want to use them for my tugboat. they are all curved(by means of bending stress) in a single plane only...any ideas as to how this could be done? if possible? any info is helpful

    --it would save a mold cost..
    please any advice here is appreciated..thanks..
  2. bntii
    Joined: Jun 2006
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    bntii Senior Member

    The sheets you describe will be floppy like noddles and will easily take a bend in one plane.
    I question the sanity of using this method for hull building however with all the joining work etc.

    Years ago I saw a clinker built runabout formed from individual FRP planks screwed to frames with riveted seams. I loved it as a historic artifact but the technique seemed to fall short of the beauty of a one part glass fiber hull. I understand that the panels will be fully knit together in your approach, still seems like a bother compared to other one off techniques.
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  3. tugboat

    tugboat Previous Member

    yea its a different method- but an option...i see it as frp plywood in a sense. and could be sticthed and glued using epoxy...there would still need reinforcement...
    thanks for your reply--!!
  4. messabout
    Joined: Jan 2006
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    messabout Senior Member

    I have used that method or miscelaneous parts for years. It works very well for skins for rudders, daggerboards and the like. Those parts being constructed as if an airplane wing, with spars,ribs, etc. The structure becomes a stress skinned part that is light, strong, and has a very smooth surface that needs almost no sanding..

    You can get a beautifully smooth surface on the bottom side of the FRP plank where it is in contact with the Formica or even a plate glass surface. This scheme also has the advantage of being easy to vacuum bag where you can regulate glass/resin ratio to suit your self.

    Doing this for thick panels has occurred to me but I have not tried it for the purpose of making a whole boat. Stiffness of the panels will vary with thickness as you well know. Roughly speaking, the stiffness will increase as a function of the third power of thickness which means that doubling the thickness will increase the stiffness by a factor of eight.

    You could layup a series of narrow strips of various thicknesses to get a hands on feel for the bending potential. I would think it a bit chancey to stress the glass planks more than a small amount. Horsing an acute bend or twist into the material might run the risk of shearing some of the resin to glass bonds.

    It is concieveable that one might lay up some relatively thin sheets and build the boat. After the thin skinned build you could proceed to add as much thickness as you deem necessary, on the inside of the hull. (I have built some RC model boats this way) The drawback is that you would be forced to use epoxy resin for the interior secondary layups. That might get prohibitively expensive on anything but a small dinghy.

    This is worthy of discussion but I think it is a bad idea for a boat of signifigant size, especially a work boat.
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  5. groper
    Joined: Jun 2011
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    groper Senior Member

    This is common practice in Australian 1 off boat building today and is also the method im using. Read thru some of my threads and you can see pics etc.

    In a nutshell, the panel stiffness comes from the thickness and also from the weight and orientation of the fibres. Its difficult to bend at 90deg to the fibre axis, and quite easy at 45deg or less.

    Its a good way of building with glass...
  6. philSweet
    Joined: May 2008
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    philSweet Senior Member

    The technique itself works just fine- but for a tugboat???

    Use it for the superstructure. Make an inside panel and an outside panel and bond to a suitable foam. cut your windows out and go. The cutouts can often be used up for hatches and whatnot. I think it would be an awkward way to build the hull unless somebody else has the table or will construct the panels for you. I made these panels from time to time in the Keys. There is usually a fair bit of waste even with good planning. The price of a gallon of resin is now more than we used to pay for a barrel of the stuff, so I would rather waste lumber than resin. And I can reuse the lumber. My only concern is that the plans may show bend radii too tight for plywood

    I'm pretty sure it will cost more than a top quality okoume ply and epoxy and glass skin hull once waste is factored in.

    Building a decent table isn't as easy as you might think. And not all Formica is created equal. I prefer a single piece plate glass table, or just go ahead and use tooling gel. The Formica would have to be delivered flat in one piece. No $3 rolls of discontinued stuff.

    It will be heavier.

    It will take at least as long to build.

    There's hardly any hull to see on a tug, so what is the value of a mirror finish? It will all get covered with strakes and fenders and such.

    I like stitch and glue boatbuilding, but I don't find the thought of working on diamond cut FRP edges for hours on end very attractive. Can you say hamburger hands?
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  7. Deering
    Joined: Feb 2005
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    Deering Senior Member

    Seems like an interesting solution for a flat panel surface such as a barge, maybe cabin walls or a roof. But I'd be leery about bending it very much as well. That would generally place the gelcoat in tension, which I anticipate would lead to premature cracking.

    A tugboat's a working boat. It doesn't need a yacht finish. And weight isn't much of an issue. I'd go with the lowest-cost marine-grade fir plywood, and epoxy/glass it heavily on both sides and slap some paint on it. Put some pigment in the outer layers of epoxy to almost match your paint. That way any scuffs and scratches won't show from a distance, but you'll be able to spot them so you can touch them up before UV does damage.
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  8. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    A long but narrow test piece before committing would be the best way to check.

    I would expect Epoxy covered ply would be cheaper, stronger and easier as Deering suggested, as non cored Glass is very heavy for given strength, and a lot more expensive.
  9. tugboat

    tugboat Previous Member

    I have to agree on the price- and wastege factored in etc..the fir and glass-the-crap-out-of-it-approach,Other than steel) seems to be the best way. It is interesting about how the designer specifies it in the tutorials...I DO like fiberglass for a workboat..then it doesnt need a lot in the way of appearances...its not a yacht which will be chartered or whatnot-- great posts ...Pure frp vs wood epoxy?..think the upkeep would be more on frp??
  10. viking north
    Joined: Dec 2010
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    viking north VINLAND

    Is this to be a working tug. If so it has a limited life anyway, why not use steel, especially so since you are working in fresh water.. If it is to be a working tug, i would not use ply as a core, I.E. Epoxy "FRP" skin on the outside and epoxy coat on the inside. As a working tug it is subject to impact and punctures of the FRP skin and once that occurs you will have moisture getting into the ply. Unless immediately discovered and repaired this requires a considerable period of buisness down time to dry out. This is especially critical in our climate of seasonal weather cycling. If the core is not completely free of moisture before repair, the freeze expansion will lift the skin from the core over a much wider area than the origional damage. I would think one could build for less cost and much faster in steel than FRP or ply cored at todays resin prices. The faster built the faster it can be put to work. A damaged steel section would be childs play to repair with very little down time. Steel is a win win in this case.

    A yacht is not defined by the vessel but bu the care and love of her owner---
  11. tugboat

    tugboat Previous Member

    Hi Viking north--its going to be a cross-over vessel--meaning light commercial duty. occasional tows etc.. not a full working tug

    thanks for the make some good points..
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Pre made, flat GRP panels make for a louse panel in terms of physical properties. They tend to be heavy and still too flexible, if comparable to other building materials and methods.

    I toyed with the concept years ago and could never get them stiff enough for their strength and still have a competitive weight per square foot. I tried several methods and the one that looked most promising was foam stringers on the inside, applied after the panels where hung, the edges joined and the hull shell rolled over. It was a lot of extra work and you still ended up with a compromise multi chine shape. I also tried diagonal planking with flat panels, but again required considerable reinforcement on the inside, just to get the stiffness necessary, though not nearly as much as a flat panel hull form.

    Unless looking to produce a bunch of boats, over a well designed jig, I see no real advantage to this technique. Not that making flat panels doesn't have some merit in some situations, as has been listed above, but the biggest advantage of GRP construction is the ability to permit the hull shape, to add stiffness and decrease flexibility, resulting in less 'glass, thus a lighter structure with nearly uncompromising shape. In short there's no advantage if required to build it heavier then other methods, even if you do get a good finish, right off the bench, the seams still need treating and the flat expanses of panel, reinforcement. This is the same issue with metal building, shape and weight limitations for the strength and flexibility requirements of the design cause building and designing issues to make you think about other materials (all things being equal).

  13. tugboat

    tugboat Previous Member

    thanks for the great post PAR, I still have always liked steel over anything else--but frp seem like the best choice if you dont want to retool your whole workshop. I do have frames for the tug built in steel. I would have loved to see those experiments- do you have any pics??
    in the end I figured out that steel wa snot cheaper because initially the steel was cheap--but add in new tools-sandblasting and coatings and it might as well be a frp/grp hull...I think in the end im going to continue in steel...i liked the low maintenance of grp/frp but there were some sound posts on here that got me thinking about this method and why not to use it.
    yours being one of them. I realized whether its steel-frp-wood/epoxy-they come out to pretty close for the same steel makes the most sense..

    Im tired of spinning my wheels and tomorrow im ordering more steel. I realized if you think it over too much you keep spinning the ill try to salvage the rest of the summer and fall and see how far i can get
    using steel- i can build well into fall...

    It sure would have been cool to have a heavy tug in frp- i have seen them. but as you mentioned steel is a compromise too in shape, since its conically/cylinderically developed. not too many designers sell tug plans i would ever build...

    Par--please design a classic radiused chine stock plan for a 25 ft tug and id love to own a set(if the price was right and i could afford them) but regardless someone needs to do that!...think about it--there is a market for A well designed frp tug for tug lovers out there...practically no one has any "real tugs" other than Mals flat bottomed designs (tug rules: wide bow, counterstern is a must-if it doesnt have a fantail-it aint a tug and i dont look twice at it, low freeboard aft for line handling and towing, and small wheelhouse to maximize deck area, nice smooth shear sweep and high gear ratio. for slow turning props,)...

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