Choice of timber ...

Discussion in 'Sailboats' started by Claus Riepe, Sep 15, 2006.

  1. Claus Riepe
    Joined: Sep 2006
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    Claus Riepe Junior Member

    You know how it is, often you cannot get the 'right' timber in the 'right' length or dimension, so you must make do with what you CAN get at the time.

    My first question would be what timber(s) must be excluded straightaway from being used on boats.

    Second, for unstayed spars that will be subjected to considerable mechanical stress, what would be preferable
    a) thinner profiled tropical heavyweight hardwood
    or
    b) thicker profiled ash or softwood?

    Very specifically, for a seriously mechanically stressed 8 ft. jib club boom, any advice NOT to use stiff tropical hardwood like Bangkiray?

    Also,
    some new softwood stuff has come to the market, they call it 'gluewood' or similar, it consists of bundles of thinner and shorter lengths glued together, axially interconnected with combs and grooves to make long lengths, primarily made from fir and pine softwood. It is graded as construction timber for spanning roofs and floors.
    The huge advantage is it can be had in very long lengths and serious profile dimensions, it is fairly inexpensive, and it remains straight as an arrow.
    I built two new -longer- masts from it, copying the very same diameter dimensions of the previous quality timber masts, and later used them in several blows at sea with full confidence and without any probs, over two years now.
    Has anyone here any experience with respect to these new 'gluewoods' for use in boats, good or bad?

    Claus Riepe
     
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Claus;
    The finger jointed timber that you mention is a bit scary for use in spars. Although I have seen no failures in those items, the strength and durability of the timber is dependant on the quality of the joint. You as the customer, have no influence on the integrity of the joint that was made by someone other than yourself. Use of that lumber should be regarded with some degree of caution.

    You may use vertical grain fir with confidence. Dependable scarf joints can be done by yourself. A word about wood as a material in general. You ask about the suitability of using "thinner" sections of tropical hardwoods as opposed to thicker sections of lighter woods. It is true that some wood is stronger than others but the difference in relative strength is not sufficient to cause compromise in dimensions. The real strength of a beam like structure (an unsupported mast) is a function of the depth of the beam. In this case the diameter of the mast. The function is an exponential one wherin the stiffness is affected in proportion to the cube of the depth. That reality makes the strength of the particular wood less signifigant than even small increases in spar diameter. Bottom line: do not decrease the diameter of the mast in deference to stronger woods.

    You can make remarkably good and thoroughly reliable hollow spars by glueing up eight pieces of wood into an octagonal shape. Each of the eight pieces will have a birdmouth groove in one edge only. The birdmouth is to be a 90 degree profile. A vee groove if you will. the square edge of one piece fits into the birdmouth groove of the adjacent piece. Continue around the hexagon by repeating the process with the remainder of the pieces. When assembling the spar, clamp the eight pieces together with the birdmouths facing upward. Pour epoxy into the birdmouths and brush it out along the surfaces of the birdmouth. Now you can assemble the parts. Hold them in place by using a series of ordinary hose clamps with light pressure applied. This is an easy process as the parts almost assemble themselves. Calculating the dimensions of the planks is easy. Select a diameter that you wish to use. Say 100mm. The width of the eight planks will be 0.4 times the diameter or 40mm. The thickness of the planks can be from 0.15 to 0.2 times the diameter. 15 to 20 mmm. If you wish to have a tapered mast then clamp all pieces together and plane down the square (not the birdmouth) sides.

    A hollow mast is very nearly as strong as a solid one. Lighter weight aloft will be very pleasing to both you and the boat.

    Smooth sailing
     
  3. Claus Riepe
    Joined: Sep 2006
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    Claus Riepe Junior Member

    Thank you, Messabout.
    I can follow your conclusion, that the use of harder and heavier tropical hardwood timbers could not automatically lead to significantly thinner spars.
    On the other side, if hardwood beams could not be dimensioned significantly thinner, their usability for small boat beams would really be very limited, because that would result in the boat weight going up vs the softwood alternative.

    Building hollow timber masts is a skill I will not make an attempt at myself. I think I am quite happy with the solid profile ones made from prefab glue wood.
    Encouraging that you too had not heard about actually failing finger joints.

    For lighter unsupported spars like topmasts I had already started to use carbon fibre windsurfer masts as substrate, they work brilliant, and -being a commodity item- for very little money. But for jib booms/bowsprits that practically have to hold the mast up, I was looking for something more substantial.
    I suppose it's back to Douglas fir timber for that, and away with the Bangkiray.

    Thanks again,

    Claus
     

  4. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    Claus,

    The strongest and stiffest woods for the lightest weight, as used in spars, are Sitka Spruce and Douglas Fir. No other woods come close except for Port Orford Cedar, but that is extremely rare.

    In general, I have found that wood is not strong enough or stiff enough for a free-standing mast on larger boats, say over 25' LOA or so. From there to about 35' or so, aluminum tubes are acceptable. By 35' to 40', you have to go to carbon fiber construction to get enough strength and stiffness for a reasonable weight.

    Eric
     
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