Dinghy scantlings

Discussion in 'Boat Design' started by Chuck Losness, Dec 25, 2012.

  1. Chuck Losness
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    Chuck Losness Senior Member

    Thinking about making a new fiberglass dinghy. My current dinghy is plywood stitch and glue and after 5 years of hard, 24/7 use and my mistakes in construction it is getting tired. I want to make the same dinghy but in fiberglass. I have looked for scantlings for a dinghy and haven't found anything. I plan to make a simple female mold. I plan to use biaxl 1708 cloth because I find this cloth easy to work with. I also plan to use epoxy because it gives me a longer working time. I don't know how many layers of the cloth to use or what kind of stiffening, stringers, etc., to use.
    So can anybody point me to a source where I can find scantlings for a fiberglass dinghy.
    Thanks for your help
     
  2. Tad
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    Tad Boat Designer

    Not enough information......

    Size and operating speed, sea state, max load(pounds and/or people), power, positive flotation....a picture of the existing boat might help....
     
  3. tom28571
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    tom28571 Senior Member

    Chuck, I don't mean to be negative but I think this may be a mistake. A solid fiberglass dinghy will be much heavier than a comparable plywood one if it is to be stiff enough to not be floppy. 18-08 is a very strong material though and the scantling thickness necessary for adequate strength in a 8' x4' dinghy will be pretty thin. Therein lies the problem. Its plenty strong but not very stiff so the hull panels will be limber. The scantlings will need to be increased way beyond what is strong enough to get enough stiffness for your little dinghy. That is why such dinghys are usually cored with something and, for home builts, that is usually plywood.

    A single layer of 18-08 filled with epoxy will weigh about 0.46#/sq ft while 6mm occume ply weighs about 0.6#/sq ft. It will take several laminates of the 18-08 to equal the stiffness of the 6mm ply. You see where this is going.

    Maybe we will hear from someone who has actually built a one off solid glass dinghy. Maybe I am being too pessimistic.
     
  4. Chuck Losness
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    Chuck Losness Senior Member

    Sorry for not including the spec's on my dinghy. And I have attached some photo's. My dinghy is a modified Devlin guppy. I talked to Sam before I modified his plans to get his input. The forward half is according to Sam's plans. The aft half is modified. I took all the shape out of the aft half of the dinghy and made it with a constant deadrise. I don't remember what the angle is. It is a fairly deep v and it just slices through 1' to 2' wind chop. The boat is very stable and dry and has exceeded all of my expectations. It also rows well. It is 9'9" LOA and 4' beam. Weight was around 110 lbs without engine or fuel. The most that I have carried is about 450 lbs plus the weight of dinghy. So heavy displ would be in the neighborhood of 650 lbs. I use an old 8 hp Nissan two stroke. I tried a 9.9 hp Nissan and it was way to much motor. I might make the new one slightly bigger. About 11' LOA and 4'6" beam.
    I know the fiberglass would be heavier than a plywood cored boat. But I think that it would be more durable. After I figure out the scantlings I can make a weight estimate to estimate what the difference would be.
    Thanks for your help
     

    Attached Files:

  5. PAR
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    PAR Yacht Designer/Builder

    It will be very difficult to build a solid skin craft under say 15', that can rival taped seam plywood, in both strength and stiffness, plus puncture resistance. As mentioned, it's simply the physics of the materials used. This said, you can likely get close with a cored build and you can get lighter, if you move to exotic materials, but costs will jump up exponentially. This is why you see so many plywood designs in smaller craft, as pound for pound and dollar to dollar, it's tough to beat.
     
  6. messabout
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    messabout Senior Member

    Chuck you have gotten five years of frequent use from the present dink. So how long would you require the FRP dink to last in order to pay you back for the investment in time, itchy epidermis and money.? Build a new one with ply and anticipate another 5 years of good service, You are likely to want something different by then anyway.

    Wooden boats have a soul and are easy to love.. Glass boats are zombies with no soul.
     
  7. TANSL
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    TANSL Senior Member

    Watch Chuck, wooden boats are lovely and all that. All what others tell you is true but surely you want to do the boat you want to do and not what we would do. So, if you give me details of the boat, its structure and mats / rovings you want to use, I will make an estimate of the layers you should apply and the required thickness. My email: 657677483@orange.es
     
  8. tom28571
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    tom28571 Senior Member

    Some really tough boats have been built with plywood core and biax or triax skins. Lighter weight cloth could be used on the inside and teh result should satisfy some rugged use. If you use the same basic (monohedron aft) design stretched to 11', it should be a harbor terror with the 9.9 engine.

    If you stick to the solid FG scheme, I think you are looking at well over 200# and I, for one, don't want to have to hoist such a beast around on a sailboat.
     
  9. PAR
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    PAR Yacht Designer/Builder

    I have a dinghy design a few inches longer than that, taped seam plywood of course, where the full up hull is under 70 pounds if okoume (85 if meranti)). This would be hard to rival in a solid laminate, though you could get fancy and knock it down to 50 at considerable expense. Tom's point of hoisting the puppy, is the major focus for a tender or dinghy.

    You might want to consider a SOF approach, but use Spectra and hollow fabric tube like stringers. This would be an all inert materials build and very light.
     
  10. FAST FRED
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    FAST FRED Senior Member

    The best longest lasting dinks are aluminum.

    Light , cheap, used 40 years old is fine , they are easy to row and power.

    Everything else is usually heavier and can not take 1/10 the beating , bashing a dink will get.

    YRMV
     
  11. michael pierzga
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    michael pierzga Senior Member

    A ten foot dyer dink weighs 125lbs

    http://www.newmanandgrayboatyard.com/ngbsaledd10.html

    The PT 11 nesting dingy weighs 85lbs.

    http://www.ptwatercraft.com/pt11_brochure.pdf

    In the end its up to the customer.

    Plastic is heavy, but it can take more abuse. Wood, glass is light, sporty and you have to take care of it.

    As for layup...Im not a glass guy. I would think a dingy would need two layers of structural cloth with a bulker fabric as a core and a layer of mat on the exterior to achiece an acceptable finish.
     
  12. tom28571
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    tom28571 Senior Member

    While I am a bit skeptical about 125# for the Dyer Dink (a typical 8' plywood Optimist pram weighs 80#) , it does have a major factor in its favor. All hull surfaces are compound form. This allows a thinner laminate for a given stiffness than flat or two dimensional hull shapes. It also means that the round Dyer hull is not the most stable dinghy at the dock. Also, vacuum forming makes for lighter weight because of a higher glass/resin content.

    Let us know how it all comes out.
     
  13. Chuck Losness
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    Chuck Losness Senior Member

    Thanks for all the suggestions. Don't want an aluminium dinghy. It would have to be welded. The riveted ones in my limited experience don't hold up well in saltwater. And bottom paint is an issue.
    And I know I can always make another stitch and glue dinghy like I have now. It's a great dinghy. What I am trying to figure out is what the scantlings would be to make my current dinghy out of fiberglass. If I had to guess I would make it with 3 layers of the biaxl 1708 cloth with some kind of stiffeners glassed in between the 2nd and 3rd layers. Also by the time you add the gunnels and all the supports for the interior seats, etc. you are going to add a lot of stiffness to the fiberglass.
    Thanks again for all your suggestions.
    Chuck
     
  14. michael pierzga
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    michael pierzga Senior Member


  15. PAR
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    PAR Yacht Designer/Builder

    If you use combo fabrics (like 1708) with an epoxy layup, you'll just make the boat heavier then it needs to be, without any additional strength. For best strength to weight ratios, use straight biax.

    As to the actual scantlings, especially if weight is an issue, full up specs on the boat will be necessary. It'll still be heavier than a taped seam build, but it'll be inert.
     
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