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Thread: Hollow vs solid wood mast

  1. #1
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    Default Hollow vs solid wood mast

    Now, I know I could go aluminum to make it even lighter, but I am pondering trying to make the mast on my upcoming 8 foot dinghy as light as I can and still make it wood. The builder (Dave Gentry) recommends building it solid out of two pieces of 1 3/4" by 1 1/2" spruce or pine, and gluing them together before shaping. This is an unstayed 8 foot long mast. The 22 foot long mast on my GP14 is built similarly, but has a hollow routed out of it for halyards and lightness.

    Do you think I would gain anything (or lose strength) by putting a similar round hollow up the centre of my Dinghy's mast? I know in metal, the strength is all in the surface and by being hollow you can come close to doubling that strength, does wood work the same?
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    Default Re: Hollow vs solid wood mast

    The spruce masts we have for our woody El Toros (8') are pretty damn light. The mast for an 8' boat is of such small dimension that there may not bee a lot of weight to be saved by taking some wood out of the middle.

    Of course, the carbon mast my son has for his newer racing El Toro is a good deal lighter!

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    Default Re: Hollow vs solid wood mast

    The difference will be negligible but building a hollow birdsmouth mast will be fun; at least more fun then gluing two pieces of wood together....

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    Default Re: Hollow vs solid wood mast

    A hollow mast can be lighter and stiffer, but must have a bigger outer diameter.
    I think the convention dimensions are available online.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Hollow vs solid wood mast

    Hollow would be lot more work for very little gain in such a short spar. I've made quite a few shortish solid spars from big box standard dimensional lumber. They are light weight and plenty strong enough.
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    Default Re: Hollow vs solid wood mast

    Pine?
    geebus find nice spruce, build as designed, bobzyeruncle
    bruce

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    Default Re: Hollow vs solid wood mast

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    Default Re: Hollow vs solid wood mast

    The mast is only eight feet? Even an el toro is at least eleven. Is the spar for a lug or sprit or what not?

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by JimD View Post
    Hollow would be lot more work for very little gain in such a short spar. I've made quite a few shortish solid spars from big box standard dimensional lumber. They are light weight and plenty strong enough.
    This. Unless you just want the experience of building a birdsmouth stick - with future projects in mind.
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    Default Re: Hollow vs solid wood mast

    A hollow mast that size will not save much weight, less than say a can of beer.
    But a solid mast has an advantage of being more robust in the event of dings and damage sustained while stepping, transporting and etc.
    One could always do the math... two days work vs. a can of beer?

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by Canoeyawl View Post
    two days work vs. a can of beer?
    but that scales with time and use...... 1 can of beer every single time out adds up over time.......

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by JimD View Post
    Hollow would be lot more work for very little gain in such a short spar. I've made quite a few shortish solid spars from big box standard dimensional lumber. They are light weight and plenty strong enough.

    I disagree. I built a 10í birdsmouth spar side by side with a 10í sprit. The sprit was slightly smaller diameter but both spars were tapered at both ends. The milling time on the birdsmouth spar took very little time - rip the eight staves to size, tilt the blade on the table saw and two passes makes the birdsmouth. Tapering the staves is easy on the saw - a quick pass with a tapering jig on each end. Then glue up with some hose clamps and you have an eight sided tapered spar.
    For the solid spar first glue up the two pieces. Then plane or saw to biggest dimension. Then carefully mark out the tapers on one face. Plane or saw to the lines. We have four taper lines you are cutting to. Then tip over and mark the taper lines on the other faces and plane to the lines. This is harder to do with a saw because you already have a taper on the other face. After all this tapering it is time to make it eight sided with your spar gauge and plane. After that you are at the same point you are at after gluing up the birdsmouth.
    Itís hard to know how long all these steps take in the real world but when I made the two spars it was clear the solid stick was more work.

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    Default Re: Hollow vs solid wood mast

    I'm in the leave it solid camp on this one. By the time you plug the ends and (probably) fill in appropriately where the mast rests against the partners I just don't think you are saving an appreciable amount of weight.

    On the other hand a small birdsmouth spar like this could be quite a fun, elegant little woodworking project, especially if you keep to the original design spec and maintain the 1 1/2" x 1 3/4" dimensions. Off the top of my head you need to increase the diameter by 5% (please verify this) going from solid to hollow so it will wind up a bit larger but not by much.

    This has been my go-to for working out the math on birdsmouth spars: https://www.duckworksmagazine.com/04...es/birdsmouth/
    Steve

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    Default Re: Hollow vs solid wood mast

    Per J. Welsford.
    "Masts and spars, I favour the solid wood sections hollowed with a router, but if that seems like hard work, or clear lumber is not available birdsmouth..."
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    Default Re: Hollow vs solid wood mast

    The mast on my 8' Nutshell is solid spruce, about 10' or so. It's darn light and I wouldn't ever feel compelled to make it lighter. That said, I've made hollow spars about that length but they were used as yards for lugsails. Reducing weight aloft was the goal. In any case, if I were undertaking the same musings as you are, I'd probably end up making it solid. If you had to glue up 2 pieces of wood, though, it would be fairly easy to do some selective hollowing.

    Good luck on your project.
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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by willin woodworks View Post
    The difference will be negligible but building a hollow birdsmouth mast will be fun; at least more fun then gluing two pieces of wood together....
    I found this handy illustration in The Sailing Yacht by Juan Baader.
    Attached Images Attached Images

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by Autonomous View Post
    Per J. Welsford.
    "Masts and spars, I favour the solid wood sections hollowed with a router, but if that seems like hard work, or clear lumber is not available birdsmouth..."

    As I had to glue up two pieces to make it, that was how I was planning on hollowing out the middle, run it over my router table and then glue it all up. I will increase the dimensions slightly to make up for it. Gentry's Piankatank Pram is skin on frame, so very light to begin with, any weight aloft (this includes the spars) would probably help a lot in keeping it stable.
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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by John Kohnen View Post
    I found this handy illustration in The Sailing Yacht by Juan Baader.
    That's a really interesting chart!

    I built a 13.5' mast for my First Mate and debated solid or hollow. I modeled a 20% wall hollow mast in CAD and found with the required blocking it'd weigh 82% of a solid mast. I did build it birdsmouth just to do it, but I only saved 2.83 pounds.

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    Default Re: Hollow vs solid wood mast

    Thanks to John for the chart - it's terrific.
    Art, it sounds like you've got your plan set. I might be arriving late and spoiling the fun, but: having built a Gentry whitehall (but no sailing rig for it) I'll say that I would probalby start with something simple and add complexity as needed. For an 8 foot dinghy used in casual conditions, I wonder about trying something like a closet rod dowel.
    But that wouldn't be nearly as fun as what you're planning! Keep us posted on the results.
    Also: I just finished hollowing a 16 foot mast by kerfing out with a saw, then using a chisel and non-powered router. Some pictures here: http://forum.woodenboat.com/showthre...lifornia/page4


    James

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by John Kohnen View Post
    I found this handy illustration in The Sailing Yacht by Juan Baader.
    Copied and saved to http://forum.woodenboat.com/forumdis...erence-Threads
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Hollow vs solid wood mast

    Since spar and mast bend is always a critical issue to deal with and allow for when designing and building sails for small boats, it would be interesting to also get some idea of the flexibility differences with those various constructions, though I suspect trying to generate reliable data would be a tremendous amount of work.

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by Art Haberland View Post
    As I had to glue up two pieces to make it, that was how I was planning on hollowing out the middle, run it over my router table and then glue it all up. I will increase the dimensions slightly to make up for it. Gentry's Piankatank Pram is skin on frame, so very light to begin with, any weight aloft (this includes the spars) would probably help a lot in keeping it stable.
    Yes. The thing about a solid piece of wood as a spar is unless it's perfect, and you're to some extent lucky, it'll warp. But it's much less likely to warp if you glue 2 or 3 pieces together. And if you're doing that, you can use a router to remove inner material.

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    Default Re: Hollow vs solid wood mast

    Have any of those of you who have built a wooden mast sheathed them in Fibre Glass?

    Is there a strength to weight advantage to be gained in doing so which, together with protecting the wood from the elements, makes it worthwhile?

    Steve

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by stevegooda View Post
    Have any of those of you who have built a wooden mast sheathed them in Fibre Glass?

    Is there a strength to weight advantage to be gained in doing so which, together with protecting the wood from the elements, makes it worthwhile?

    Steve
    eye've dun the opposite, peeled glass off rotted masts

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by stevegooda View Post
    Have any of those of you who have built a wooden mast sheathed them in Fibre Glass?

    Is there a strength to weight advantage to be gained in doing so which, together with protecting the wood from the elements, makes it worthwhile?

    Steve
    Bad idea, pretty much any way you look at it.
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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by stevegooda View Post
    Have any of those of you who have built a wooden mast sheathed them in Fibre Glass?

    Is there a strength to weight advantage to be gained in doing so which, together with protecting the wood from the elements, makes it worthwhile?

    Steve
    couple coats of varnish will protect it. Its a dinghy mast, glassing it would add weight and strength it wont likely require.

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    Default Re: Hollow vs solid wood mast

    It was mainly the question of additional strength I was thinking of.

    Strip built boats are sheathed in Fibre Glass and a Birdsmouth mast is strip built. Why should it be a No, No for the mast but standard for the hull made of strips?

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by stevegooda View Post
    Have any of those of you who have built a wooden mast sheathed them in Fibre Glass?

    Is there a strength to weight advantage to be gained in doing so which, together with protecting the wood from the elements, makes it worthwhile?

    Steve
    I would think a layer of composite between the half’s before gluing up could add some stiffness depending on orientation.
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    Default Re: Hollow vs solid wood mast

    Forum member WX posted an account of building a Birdsmouth mast in 2015 which was well received, despite the fact that he sheathed it in Fibre glass, see

    http://forum.woodenboat.com/showthre...ght=Birdsmouth

    if it was acceptable then, why not now. Presumably masts that have been painted or varnished also succumb to rot from time to time. If the fibre glass stiffens the mast at an acceptable increase in weight I don't understand why it should not be done.

    I'm thinking of building one myself and I hoped that there may be good reasons for or against to guide my decision. Thanks anyway.

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    Default Re: Hollow vs solid wood mast

    Forum member WX posted an account of building a Birdsmouth mast in 2015 which was well received, despite the fact that he sheathed it in Fibre glass, see

    http://forum.woodenboat.com/showthre...ght=Birdsmouth

    if it was acceptable then, why not now. Presumably masts that have been painted or varnished also succumb to rot from time to time. If the fibre glass stiffens the mast at an acceptable increase in weight I don't understand why it should not be done.

    I'm thinking of building one myself and I hoped that there may be good reasons for or against to guide my decision. Thanks anyway.

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    Default Re: Hollow vs solid wood mast

    What is the point of the extra work and weight? The reason for varnishing spars is to see damage or conditions that may cause rot early. Well maintained varnish is quite protective of the wood and is a system that has withstood the rest of time. Epoxy under varnish is much more difficult to repair.

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by CK 17 View Post
    I would think a layer of composite between the half’s before gluing up could add some stiffness depending on orientation.

    It would also not hold the moisture in. When I reglued the wooden mast on my GP 14 (22 feet worth of spruce) I used slightly thickened epoxy, but did not worry about putting any composite materials in.
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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by CK 17 View Post
    I would think a layer of composite between the half’s before gluing up could add some stiffness depending on orientation.

    Wouldn't that put the layer exactly on the neutral axis in one plane? My inclination would be to join the ranks of those who would hollow out two halves and then glue them together-then varnish.

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by Todd Bradshaw View Post
    Since spar and mast bend is always a critical issue to deal with and allow for when designing and building sails for small boats, it would be interesting to also get some idea of the flexibility differences with those various constructions, though I suspect trying to generate reliable data would be a tremendous amount of work.
    It can be calculated without too much difficulty as the stiffness is directly proportional to and the deflection is inversely proportional to the Area Moment of Inertia (MOI). Aside from the teardrop shape, the rest can be looked up vs. having to be derived from the definition.

    For instance, D has the MOI formula Pi/64*(A*B^3-a*b^3) for sideways bending, where A and a are the outer and inner length along the major axis and B and b are the outer and inner widths along the minor axis and if you make it a circle so the major and minor axis are the same length and thus are the inner and outer diameters, the formula is Pi/64(D^4-d^4) where D is the outer diamer and d is the inner diameter. There is even a calculator so you can plug in your values at: https://structx.com/geometric_properties.html

    The plywood will be more difficult because the plywood is a different material and will behave different compared to solid wood due to the alternating grain pattern so you can't use a simple formula.

    A little calculation will allow you to find the deflection of one of the other shapes compared to a known amount for a solid spar as the ratio of the deflections will just be the inverse of the ratio of the MOI values.

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    Default Re: Hollow vs solid wood mast

    Quote Originally Posted by stevegooda View Post
    It was mainly the question of additional strength I was thinking of.
    If you want to go that route, you could just build a carbon fiber mast https://duckworksmagazine.com/16/how.../#.YfiSxabMLIU

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