making an 18 foot wooden mast

Discussion in 'Boatbuilding' started by sailmonkey, Sep 29, 2014.

  1. sailmonkey
    Joined: Sep 2014
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    sailmonkey Junior Member

    Greetings. I have seen several plans call for a birds mouth hollow mast design. For an 18 foot mast, I am just wondering if it would be just fine to use two pieces of wood, use a router to hollow out what will be the inside, epoxy them together, and shape outside to suite? Any particular advantage to a birds mouth design? It is much more difficult to build than what I am suggesting. For an 18 foot mast (14 foot dingy), the two piece method may be strong enough?

    Thank you.

    John
     
  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    two peice wood masts are fairly common, the strength and stiffness of the mast however will depend on the cross sectional shape and area of the mast. the birds mouth design (it is actually not that difficult to make) allows you to use more smaller peices, so the chance of a defect in the wood that would weaken it is much lower. But if your crossectional area is the same, and you can good quality clear wood, and the external dimensions are approximately the same, than it will have similar strength. Watch out for your overall mast weight as well, adding weight up high would not be good for sailing a small boat.

    Also, by hollowing out larger pieces of lumber, you will spend more on materials than on a bird's mouth mast, most of the lumber you buy will be routed out and turned to shavings.
     
  3. GTO
    Joined: Jul 2007
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    GTO Senior Member

    I just glued 3 spruce 1x4s together, pre-cut prior to gluing. Made a 19'6" mast. Spruce is pretty light. Birds mouth seems like a lot of work for a small boat but some do it.

    I used two 16 foot pieces for the lower sides, with a 12 foot piece inbetween.
    Then glued two 4 foot sections on top of the 16 footers and an 8 foot on top of the 12. It has survived a storm or two, heavily loaded, all sail up, and really bending.
    Don't spare the glue and make sure it is clamped straight.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There's no reason to not make a hollow mast, regardless of build technique. A box section is the easiest and birdsmouth is also quite easy, plus you can use a lesser quality of stock.

    If you post the spec's of your mast (height, sections at the tapers, length of tapers, amount of sail area it has to hold up, the type of rig, etc. I can tell you what it will weigh, using the various build methods and commonly employed species.

    For example, lets say this 18' mast will be simply stayed and carry a masthead sloop rig of 120 square feet. If built with a 20% wall from Douglas fir, it'll weigh about 11 pounds (not counting glue) and have a 2.5" heel, with a continuous taper to 2" at the head. If made from sitka spruce, it'll weigh about 8 pounds. A solid stayed mast of the same sail area and rig, will need to be 2 3/8" at the heel and 1 7/8" at the head and will weigh 12 pounds if sitka spruce and over 15 pounds if Douglas fir. That's a difference of about 50%, between a solid Douglas fir and a 20% wall, hollow sitka stick. Doesn't seem like much, but it can be if you're trying to hoist it in stiff winds, plus a holloew mast will permit internal wiring, halyards and lifts, which dramatically decrease windage.
     
  5. sailmonkey
    Joined: Sep 2014
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    sailmonkey Junior Member

    PAR, thank you for your offer. Here are the details: the mast in question is from the plans for a SpinDrift 12 foot centerboard sloop, designed by B&B Yacht Design in NC. Stitch and glue plywood boat, more than 1000 plans have been sold in various sizes. Fractional rig. The main is 60 sq ft and the jib is 25 sq ft. The mast is 18 feet, 3 inches tall. The bottom section, with birds mouth 8 piece and 8 sided construction, is 2 1/2 inches across opposite flats. This gradually tappers down to 1 1/2 inches across opposite flats at the hounds, which are 155 3/4 inches up, and that diameter is continued to the top. The wall thickness over the entire 18 foot length is suppose to be 1/2 inch.

    I feel like I could probably make a fairly good one after I made 5 or 6, but the first one might be pretty rough. Just using pieces of wood and hollowing them out like GTO described has some appeal.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The birdsmouth method is easy, surprisingly so and the reason it's so popular too. The SpinDrift has wooden birdsmouth spec's in the plans if I'm not mistaken, though a quick call to Graham would probably get this sorted out easily enough. If not Graham, the gang over a Messing-About.com will have likely tried this before. I happen to have the composite aluminum sections for a SpinDrift here in the shop, looking for a home. It just needs the little top section to complete it, it's tapered, disassemable and LPU painted. Pay for the shipping and it's yours. Trying to carve out a uniform 1/2" wall on the 2 piece method described above sounds a lot more difficult then running some staves through a table saw. There's lots of videos online about birdsmouth mast building, have a look.
     
  7. The Q
    Joined: Feb 2014
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    The Q Senior Member

    I built an 18ft mast for a gaffer, by using 1 piece of 2 X 8 Douglas, cutting it down the middle carving out the insides ( there are solid sections where the gooseneck, gaff jaws and sheeve boxes are). Reversing one piece end to end and then epoxing together before rounding the outsides. Very simple to do and has been successfull.
    I found it easier to use a hand plane on the outside than using a power plane.
     

  8. sailmonkey
    Joined: Sep 2014
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    sailmonkey Junior Member

    Thank you PAR, I sent you email about the mast. And thank you everyone else for your comments. It seems that for this size, using a few pieces of wood and hollowing them out produces a mast which is strong enough and light enough.
     
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