strip plank

Discussion in 'Boatbuilding' started by TOALL, Jun 19, 2016.

  1. TOALL
    Joined: Jun 2016
    Posts: 23
    Likes: 1, Points: 3, Legacy Rep: 36
    Location: uk

    TOALL Junior Member

    I don't see any reason why I shouldn't mix timber while construction the hull.
    I have a large quantity of Cedar and Douglas Fir.
    Am I asking for trouble?
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    It depends on the cedar, but generally you stick with species of similar densities. If it's Spanish or Alaskan cedar, they'll be close enough to work, but if it's western red, the Douglas fir will be considerably denser, so the cedar will be the weak link in the panels, which isn't desirable. It also depends on the sprik planking type (I can think of at least a dozen distinctly different types). If it's a straight strip plank build, uniform hull shell density is important, but if the strip type uses a significant sheathing or has molded veneers over it, this is less important.
     
  3. TANSL
    Joined: Sep 2011
    Posts: 5,499
    Likes: 152, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    More important than the density are the mechanical properties, flexural strength or compressive strength, of each material. If the difference is large, sure there will be problems. You also should check the stresses on each layer and if the material itself supports it properly. The ratio actual stress/allowable stress should be similar for all layers.
    In my opinion, you should not mix different materials without having made a study of what the real situation of each panel.
     
  4. rwatson
    Joined: Aug 2007
    Posts: 5,756
    Likes: 257, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Thinking on it, mixing species on separate planks might cause problems, but not if you use one species on the bottom, and other on the sides.

    Don't forget, even in the same species, strips can exhibit varying degrees of softness, strength etc depending on the part of the log and the sawing technique.

    Most strip planked hulls are glassed and epoxied anyway, so the differences would be insignificant.

    On a smallish boat, it should not be a show-stopper.
     
    1 person likes this.
  5. SukiSolo
    Joined: Dec 2012
    Posts: 1,270
    Likes: 25, Points: 0, Legacy Rep: 271
    Location: Hampshire UK

    SukiSolo Senior Member

    As PAR says it largely depends on the type of cedar and also what you are trying to achieve. Alaskan yellow cedar is quite a bit denser than W R Cedar and could be used with Doug Fir. Equally you could mix W R Cedar and Sitka Spruce which are close enough but can increase strength locally where desired.
    I've built quite a few foils mixing these timbers to get reasonable stiffness weight and flex. You can use lighter core timbers when cold moulding and usually use a slightly denser outer (and inner) veneer.

    It does depend what you are building. How big is this boat? and what kind of loads are you expecting her to take?.
     
    1 person likes this.
  6. TOALL
    Joined: Jun 2016
    Posts: 23
    Likes: 1, Points: 3, Legacy Rep: 36
    Location: uk

    TOALL Junior Member

    50 ft long, wide beam mono hull.

    Specification for hull construction is
    Externally 27x40 planks. 2x600g biaxial +-45 and 1x160g at 0deg.
    Internally 2x600g biaxial +- 45 deg
    Vacuum bagged

    I have Western Red Cedar
    Douglas Fir.

    I could build the entire boat with the timber I have but would need to mix it. Whether that's alternative strips , bands or half and half.

    It will all be machined at the same time, to a speed strip profile. I intend to comb joint the ends.
     
    1 person likes this.
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    What design is this for and what species does the plans suggest? If you have to mix them, the cedar should be used on the topsides, if only to to save weight, though I do have concerns about using WRC on this hull.

    Western red cedar:
    Weight (dry): 23 lbs/ft3 (370 kg/m3)
    Specific Gravity (12% MC): .31, .37
    Janka Hardness: 350 lbf (1,560 N)
    Modulus of Rupture: 7,500 lbf/in2 (51.7 MPa)
    Elastic Modulus: 1,110,000 lbf/in2 (7.66 GPa)
    Crushing Strength: 4,560 lbf/in2 (31.4 MPa)

    Douglas fir:
    Weight (dry): 32 lbs/ft3 (510 kg/m3) or about 40% heavier
    Specific Gravity (12% MC): .45, .51 or about 40% denser
    Janka Hardness: 620 lbf (2,760 N) or about twice as hard
    Modulus of Rupture: 12,500 lbf/in2 (86.2 MPa) or about 60% stronger
    Elastic Modulus: 1,765,000 lbf/in2 (12.17 GPa) also about a 60% difference
    Crushing Strength: 6,950 lbf/in2 (47.9 MPa) or about a 50% difference

    As you can see there are some pretty huge differences in the physical properties between the two species, so this is a considerable risk, without knowing more about the design. Can you contact the designer and ask him what he thinks?
     
    1 person likes this.
  8. gonzo
    Joined: Aug 2002
    Posts: 13,293
    Likes: 323, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    There are plenty of canoes and kayaks built of cedar strips with mahogany or other dense woods for accent. I haven't seen any particular problems with it.
     
  9. DCockey
    Joined: Oct 2009
    Posts: 4,216
    Likes: 182, Points: 63, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Western red cedar is very popular for strip building canoes, kayaks and small boats. These boats are sheathed on both sides with the assumption the core wood strips will stay dry. I have also contrasting strips of pine, douglas fir. mahogany and other hardwoods used for visual effect.

    My recollection, perhaps incorrect, is that western red cedar has significant longitudinal shrinkage with changes in humidity. For most species longitudinal shrinkage is usually considered insignificant. If the moisture content of strips in a hull changes then an appreciable difference in longitudinal shrinkage between adjoining strips could result in shear stresses large enough to cause failure of the glue bond between the strips and/or failure of the sheathing.
     
  10. gonzo
    Joined: Aug 2002
    Posts: 13,293
    Likes: 323, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    However, if you have that much of a moisture change, there is a major problem with the lamination. On the other hand, the OP has not indicated if this will be sheathed or not. It could be a traditional strip plank hull with nails holding the planks together.
     

  11. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    There's not a lot of loading on a 14' canoe or 16' kayak, in spite of running rapids and all. This wouldn't be the case on a 50' mono sailboat. I hate to see chainplate loads open up the cedar, because this portion of the hull happened to be 50% weaker than the other areas.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.