Newbie question that needs some clarification

Discussion in 'Boat Design' started by k5blazerboy, Aug 23, 2011.

  1. k5blazerboy
    Joined: Aug 2011
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    Location: Central WI

    k5blazerboy New Member

    OK, after a lot of digging and not being able to find an answer, I have finally decided to sign up and just ask my question. I am currently building a a mold for a boat, and will actually be making a boat this weekend. It is a hunting boat propelled by oar only, no transom or anything, no wood, pure glass, 10' long, just over 4' wide. My questions are: Using 1.5oz mat, I need to make up 1/8" thick. My calculations put me at 2-3 layers thick. Does this sound correct? I plan on also hand laying cloth on another hull. I have 10 oz and 6 oz. How many layers of each would it take to match or slightly exceed the structural integrity of the mat? And which cloth would I be better off using? Thanks a lot, hoping you guys can help me out
     
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Describe the boat with a little more detail. If it is to have a flat bottom or other wide unsupported areas, then you will need a lot of glass. If the boat is divided into small panels or has a round section, then you might use less glass. In either case the boat is pretty wide for the given length.

    Why do you want to build using no wood at all? You can make the boat much lighter and probably stronger if you use a wood core. The WEST system has long proven the durability of glass/wood composites.
     
  3. k5blazerboy
    Joined: Aug 2011
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    k5blazerboy New Member

    The boat will look like this, this one I built out of wood and glass and it weighed in at 215 pounds. The bottom is flat, but has a cockpit for the person to lay in. I have since changed a lot on the design, but it is the same general idea. An all glass boat will weigh about 120lbs. There will be honeycomb board on the floor, but that is the only other thing besides glass.
     

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  4. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    That is a pretty interesting looking boat. It must be a special purpose design, but I cannot imagine why it is shaped like a bath tub. Hey! it doesn't matter if it is something that pleases you or fits the specific application.

    I and a bunch of other forum folk can build that boat of wood and glass in the 65 to70 pound range. You could too.

    Maybe some of the others will jump in here with more advice about the FRP layup thicknesses. Matt will suck up a lot of resin so, if weight is a consideration, be conservative with the matt content.
     
  5. BKay
    Joined: Jan 2011
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    Location: Reedville, VA

    BKay Junior Member

    k5 -- looks like you plan on slaying some ducks! Interesting.
     
  6. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    define 'wood' - plywood, planks, what kind if wood ? How much fibreglass ?

    215lbs is waaaaay overweight

    Solid glass will weigh more than say plywood and epoxy, while fibreglass with foam core should come in less.
     
  7. erik818
    Joined: Feb 2007
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    Location: Sweden

    erik818 Senior Member

    K5,
    I understand that you want to get a laminate thickness of 1/8" and wonder if 2-3 layers of 1.5 oz matt sounds right. You will not get a straight answer from me either; I lack data. When I had a similar problem I calculated the thickness by using the total weight and the density (specific gravity) of the applied materiel. This is how you calculate:
    Find the density of the polyester/epoxy/vinyl or whatever resin you are using. I expect you are using polyester since you use matt, and the density to be approximately 1.2 g/cm^3, but you should be able to find the correct figure in data from the manufacturer.
    Find the density of the glass in the matt. If you don't succeed, assume the density of glass to be approximately 2 g/cm^3.
    Find out how much polyester the matt is likely to need before being saturated. If I recall it correctly, I used approximately the same weight epoxi and glass cloth per area for my latest boat. I don't know what the ratio will be with polyester and matt. The experienced boat builders on this forum might be able to come up with a figure.

    Since you know the weight per area of matt you will be able to calculate the weight per area of polyester. You will now be able to calculate the volume of the (solid) glass and the volume of the polyester between the fibers. From there it's just to divide the total volume with the area to get the thickness.

    Example:
    You use 3 layers of 300 g/m^2 matt = 900 g/m^2. If the ratio of polyester to glass is 1:1 (in weight) you will also use 900 g polyester per square meter. 900 g glass with density 2 g/cm^3 results in 450 cm^3 per square meter. 900 g polyester with density 1.2 g/cm^3 results in 750 cm^3 per square meter. In total you will have 450 + 750 = 1200 cm^3 per square meter. 1200 cm^3 = 1200 *10^-6 m^3= 1.2 * 10^-3 m^3. Divide this with one square meter and you find that you have a 1.2 mm layer.

    You state that you have 1.5 oz matt. If that is per square foot you have 300 g per square meter. If my assumption of 1:1 ratio for resin to matt is correct, you will need 8 layers to reach 3 mm (1/8").

    I have no opinion if 1/8" is the proper thickness to use.

    You can also do the calculations above in imperial units to get a clue why only three major countries in the world (USA, Liberia, Burma) still refuse the metric system.

    It would also be more helpful to be able to write exponents than to write smilies.:p

    Erik
     
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  8. erik818
    Joined: Feb 2007
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    erik818 Senior Member

    K5,
    Some corrections to my previous post. The calculation principles are correct, but my assumed data are very wrong for polyester and mat.
    1.5 oz per square feet corresponds to 457 g / square meter.
    I happend to come by Dave Gerrs figures on the subject. Typical glass content by weight when using polyester and mat is 28%, so the glass to polyester ratio is 1:2.57 and not 1:1. If you use roving you will get a higher glas content, 1:1.6. According to DG you will need 1.36 kg per square meter and mm (mat only), so 1/8" thickness equals 4.35 kg per square meter. 28% is glass: 1.22 kg per square meter or a little less than 3 layers 1.5 oz mat. The rest of the weight is polyester.

    The figures are available in Dave Gerrs book "The elements of boat strength" which I recommend.

    Good luck,
    Erik
     
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