Hollow Mast Wood

Discussion in 'Materials' started by abosely, Jul 4, 2015.

  1. abosely
    Joined: Mar 2015
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    abosely Senior Member

    I thought I'd ask about this on a new thread.

    Not specific design details but question about lighter wood vs using thinner, stronger, heavier wood.

    Say a 5.5" x 35' mast speced in Douglas-Fir with 4 sides of 1" wood and corner pieces to start, then rounded. I don't know what the actual wall thickness would be, but a starting point.

    If the mast was made out of a stronger but heavier wood. How much can thinning the walls to optimize the strength to weight ratio work so there isn't much of a weight penalty?

    I realize some specific data would be needed.

    I'm wondering about using Albizia Lebbeck. I can see three advantages. Cost, looks & hardness, so the masts don't get dinged up as much.

    I'm just asking in general, not wanting to know actual dimensions. I don't know how the weight to strength ratio would work and the stiffness of the different woods how that would come into play.

    But just as a rough idea of practical or not.

    D-F Albizia Lebbeck
    Weight
    lb cu/ft 32 40

    Gravity .45-.51 .51-.63

    Hardness 620 1330

    Rupture 12,500 13,730

    Elastic 1,765,000 1,836,000

    Crushing 6,950 7,160

    I'm using D-F in the comparison since we know the material wall thickness to start out with for a rough comparison.

    If I don't use Albizia and need to buy wood for the masts I'll use Sitka Spruce to get the lightest masts possible. But if I understand it correctly I'll probably need to go to a bit thicker on walls when going from D-F to Sitka Spruce. But I'll have all the information and have the masts calculated, engineered. Also if it would work to use Albizia, I'll have it engineered.

    I'm guessing that the best mast material will be Sitka Spruce. But thought I'd ask about the possibility of using Albizia, just in case. I don't want to use Albizia if it isn't a good idea, the masts are kinda important!

    Cheers, Allen
     
  2. alan white
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    alan white Senior Member

    Thicker walls mean slightly greater stiffness if that is what is desired. Spruce would be my choice for that reason. Not that fir isn't a good replacement but when spruce is available I would choose it. Sitka spruce is ideal but other spruces are also close in desirable features such as light weight and resistance to cracking/splitting.
    Adirondack spruce and eastern white spruce are also good choices.
    Look for straight-grained clear grain or pieces you can scarf to work around any knots.
    Nowadays, you don't see the best straight and tall old growth trees being cut (unless you are rich) and a lot more scarfing is required than in the old days.
    Again, fir is fine and usually very available. It is normally sold without knots and is used for flooring, often outdoors. My own boat has a fir mast. I had to lengthen it by a bit to go from a deck to a keel-stepped socket. I spent maybe $8 for the wood (8 ft.), hand-picked at Home Depot. This wasn't a hollow mast, but similar in size to a Herreshoff 12 1/2 mast. Most people can't see the scarf. The new wood looked like it came from the same tree!
    While I don't suggest you limit your mast sections to 8 ft, you could do it and save hugely. You could rip boards from fir dimensional lumber as well (2 x 8, e.g.) but beware pitch pockets and cracks. Cracks are usually due to too-rapid drying. Avoid fir that sticks to your hands that looks like it's sweating little beads of sap.
     
  3. whitepointer23

    whitepointer23 Previous Member

    If the boat is for practical use I would look at alloy spars. Much more robust and light. Usually plenty of used ones around. Saves cutting down alloy trees.
     
  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    For solid spars there is a definite advantage for woods like Sitka spruce, but for hollow spars this advantage goes away somewhat. You still need to select wood that is dimensionally stable, at least once assembled, but the strength to weight in tension and compression becomes more critical, particularly in compression if it is stayed, but the stiffness for weight is still critical to resist buckling. You could certainly use the heavier wood and reduce the wall thickness somewhat, but to know by how much you would really need to re-engineer it. You could also consider increasing the number of stays and such. I wouldn't be afraid to try a wood of different density but I would redesign the mast and rigging to suit it. While doing so you could also optimize it for your specific needs. i.e. crossing oceans versus local sailing.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    In longer lengths, Douglas fir (unless you ask Jammer Six) is a better choice for several reasons. Sitka will still have better compression qualities, per pound, but fastener pull out and stiffness will improve with Douglas fir and on larger mast, where the loads increase exponentially, this is a major concern.

    More important is appropriate diameter, coupled with stave thickness in hollow spars. Also rounded square mast should be reinforced in the internal corners (see attached). This way, when the spar is rounded, the wall has uniform thickness. This particular type of mast construction has only one advantage over other types and that is everything is a square cut. You still have to make long stave tapers, but the edges are simple rips. I make them in halves, so I can install internal reinforcements, like blocking at the gooseneck, heel and head for example.

    Jamie has it, the stick will need to be redone, in terms of diameter and wall thickness to take advantage of the better physical attributes. The mast shown below is done exactly this way. The mast is Douglas fir, the boom SYP and the gaff spruce with a SYP center spacer.
     

    Attached Files:

  6. whitepointer23

    whitepointer23 Previous Member

    I have to ask why wood spars and not alloy. I just cannot see why you would make them out of wood when ally is so strong ans easy to obtain. I guess you have your reasons.
     
  7. Jamie Kennedy
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    Jamie Kennedy Senior Member

    For a small wooden boat wooden spars that match the gunnel would not be much trouble. Another thing that can be done for round spars is aluminum tube with wood plugs on the ends so you still have that woodsy feel. You could use a very tough wood for those plugs and they can incorporate whatever traditional fittings a wooden spar might have there. I think I have seen the lower mast of a mirror dinghy done this way. At some point once a boat is over a certain size I would opt for aluminum spars and put the extra attention into some place else, but I think wood is always worth considering. Wooden spars and spar making certainly is an art in itself worth keeping, but some art is best be left for others. ;-)
     
  8. PAR
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    PAR Yacht Designer/Builder

    In smaller sizes it's tough to beat wooden spars, if designed properly. Around 20' long, aluminum becomes a better choice. I've done a number of composite (aluminum lower section with a birdsmouth upper) sticks now and this can offer the look you want, without much of a weight burden. Sometimes, wooden spars are simply the only choice, like the boat above which is all wood, bright sheer strake and bright rig. I guess you could use a faux paint job on an aluminum stick, but as soon as a halyard slaps it, you'll know.
     
  9. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I suppose wooden boats and wooden spars are another good reason to try and keep things smaller. It really does give you a special feeling. But if you do need to go up in size for whatever reason it is good to have these other materials to switch in if appropriate for your needs. I love the lamp post mast on Mingming II.
     
  10. Charlyipad
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    Charlyipad Senior Member

    They look beautiful
    question: why bother with the rabbets when they all get cut away with the chamfer cuts?
     
  11. abosely
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    abosely Senior Member

    Well a couple reasons for going wood spars. Aluminum is more expensive over here & is only available in 20' lengths, I need about 35' & don't want to have to splice them.

    Plus she's going to be a traditional Islander looking boat.
    With some bright work.
    So Aluminum wouldn't look right or sound right.

    Will build birds mouth spars in two halves, glassed inside then joined. The glass & epoxy isn't for strength, just 4oz to seal the inside good & not have a place for checks to form.
    Outside of mast will get layer of 4oz glass to seal it up good & bright finished.

    Most of the cleats, deadeyes etc... will be wood. Wharram designed her rigging so it can be rope & wood. Using IPE for these things. Strong, won't rot & oil it to keep it looking nice, but no varnish.

    Cheers, Allen
     
  12. whitepointer23

    whitepointer23 Previous Member

    Thanks . Now I understand.
     
  13. Jamie Kennedy
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    Jamie Kennedy Senior Member

  14. abosely
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    abosely Senior Member

    There's Ohia, it's heavy, a little over twice the weight of D-F and amazingly hard. But not good rot resistance at all unfortunately.
    But IPE is available here & isn't expensive and is very hard, strong & just about doesn't ever rot, even in water.

    Cheers, Allen
     

  15. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Good choice. It's going to be a beautiful collection of craftwork when you are done.
     
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