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  1. #1
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    Default Birdsmouth flex reality check

    I glued up the birdsmouth mast for the Herreshoff Biscayne Bay 14 I am building over the weekend, and I am a bit surprised at the ease with which it flexes. The mast is 21' long, keel stepped, and stayed. The timber is Sitka Spruce. At this point it is 8 sided.

    My initial impression of the flex came from how much flex the unsupported length flexes, if I clamp the spar to my bench at deck height. I can easily move the head of the mast 3 inches without much force. Maybe that isn't much over the 18 1/2 foot unclamped length.

    Naturally, and more realistically, there is a lot less flex if I put another clamp at the point on the mast where the stays will be. Once I do that there is a quarter to half inch of easy flex in the top 5' of the mast, and a lesser wiggle in the opposite direction in the middle of the mast.

    I realize that Capt. Nat generally designed light masts, and this design calls for a hollow mast of more traditional hollow construction with what seem to me very thin walls. The birdsmouth spar I have built has thicker walls than the design.

    I also realize that a traditional rig often will have a decent amount of flex. I understand this isn't some super rigid, taut modern rig. I'd just like a reality check that the amount of flex here is reasonable, as this is the first hollow wood spar I have built. I figure that if I am going to do something like add material to the mast, it will be a lot easier to do so while it's still 8 sided.

    Thanks.

  2. #2
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    Default Re: Birdsmouth flex reality check

    Stays and shrouds, yes? Sailmakers routinely deal with the amount of movement you report above the stay/shroud location. Some flex is very very helpful in managing the sail and boat.

  3. #3
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    Default Re: Birdsmouth flex reality check

    You need that flex, it helps control draft and twist in the sail in heavy or gusty conditions. Thats why the mast is as lightly built as it is.

    John Welsford
    An expert is but a beginner with experience.

  4. #4
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    Default Re: Birdsmouth flex reality check





    14ft waterline on a 'not relatively fat' 5ft beam with 'relatively not small' 11.3m sail area...gusty bay winds...you need that tip to be floppy. As it bends off the sail flattens at the top and helps depower it in gusts. Desirable especially solo.

    Try to build it exactly to plan. It's a mental challenge as much as building a boat and takes controlled restraint and trust. This is a Nathaniel Herreshoff boat. If your mast staves are thicker, it's already heavier than it should be and your boat now will heel, pitch and heave more etc.

    Was it originally birdsmouth or rectangular hollow section?
    Last edited by Edward Pearson; 09-23-2020 at 04:42 AM.

  5. #5
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    Default Re: Birdsmouth flex reality check

    Quote Originally Posted by Edward Pearson View Post
    Try to build it exactly to plan. It's a mental challenge as much as building a boat and takes controlled restraint and trust. This is a Nathaniel Herreshoff boat. If your mast staves are thicker, it's already heavier than it should be and your boat now will heel, pitch and heave more etc.

    Was it originally birdsmouth or rectangular hollow section?
    As it is already glued up there is little that can be done now. Don't be tempted to plane any off the outside unless you can do the maths to calculate how much so as to match the stiffness and compressive strength of the original design.
    It really is quite difficult to build an ugly wooden boat.

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  6. #6
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    Default Re: Birdsmouth flex reality check

    A modest increase in the weight of the mast may actually reduce the pitching, heaving etc.

    Don't panic. Its a stayed mast, it's supposed to flex.

  7. #7
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    Default Re: Birdsmouth flex reality check

    Thanks all, this is both helpful and reassuring.

    Quote Originally Posted by Edward Pearson View Post

    14ft waterline on a 'not relatively fat' 5ft beam with 'relatively not small' 11.3m sail area...gusty bay winds...you need that tip to be floppy. As it bends off the sail flattens at the top and helps depower it in gusts. Desirable especially solo.

    Try to build it exactly to plan. It's a mental challenge as much as building a boat and takes controlled restraint and trust. This is a Nathaniel Herreshoff boat. If your mast staves are thicker, it's already heavier than it should be and your boat now will heel, pitch and heave more etc.

    Was it originally birdsmouth or rectangular hollow section?
    Good questions; as Nick points out I've already glued up.

    The original was round and hollow, constructed from four square tapered staves, which would then have been hollowed, glued up, and taken down to round. The plans show not only the diameter of the mast but the wall thickness, and near the top of the mast the walls get quite thin indeed. I suppose one would have to be able to accurately measure the inside diameter of the hollowed out mast quarters, and then take the outside down quite precisely to get the design thickness. You'd have to trust you got it right, because the final thickness would only be reached after the hollow staves are glued up.

    While I might aspire to this level of craftsmanship, a man's got to know his limitations, and the birdsmouth method seemed much more accessible to my skillset as well as a more efficient use of stock.

    The Wooden Boat articles documenting the build report a final mast weight by the Herreshoff method of 16 1/2 pounds. Mine is currently a little under 23 pounds, and it will lose some weight in getting from 8 sides to round. Some of the extra weight is in the lower end, as I extended the plug in the birdsmouth a bit farther up the mast than the design has solid. I'm interested to see what the final weight is.

  8. #8
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    Default Re: Birdsmouth flex reality check

    ^ A couple of extra pounds acting 8 or 10 feet up is not going to make much difference in the grand scheme of things. If it bothers you, eat some big dinners.
    It really is quite difficult to build an ugly wooden boat.

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  9. #9
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    Default Re: Birdsmouth flex reality check

    Quote Originally Posted by Peerie Maa View Post
    ^ A couple of extra pounds acting 8 or 10 feet up is not going to make much difference in the grand scheme of things. If it bothers you, eat some big dinners.
    Cool. Or maybe I can put a few more pounds of lead in the ballast.

  10. #10
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    Default Re: Birdsmouth flex reality check

    Quote Originally Posted by nrs5000 View Post
    Cool. Or maybe I can put a few more pounds of lead in the ballast.
    The road to doom. Little bit more weight in the ballast, to off set the extra mast weight, then you start mucking around with other important things, yer gonna drown! Or not..
    Birdsmouth is a very succesfull construction method and masts need to flex a bit. Just go with it.
    A2

  11. #11
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    Default Re: Birdsmouth flex reality check

    Quote Originally Posted by Andrew2 View Post
    The road to doom. Little bit more weight in the ballast, to off set the extra mast weight, then you start mucking around with other important things, yer gonna drown! Or not..
    Birdsmouth is a very succesfull construction method and masts need to flex a bit. Just go with it.
    A2
    Right, good point!

  12. #12
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    Default Re: Birdsmouth flex reality check

    If it breaks, you're in luck as you already know how to make a mast.
    Quote Originally Posted by James McMullen View Post
    Yeadon is right, of course.

  13. #13
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    Default Re: Birdsmouth flex reality check

    Sounds like you are going to have a very good mast in your boat! Incidentally, what kind of glue did you use! Herreshoff was stuck with hide glue back then but it worked just fine as long as spars were painted or varnished. Fortunately tremendous
    advances have been made in adhesives since Captain Nat's time. G/flex is the new kid on the block for spars as it will flex as the spar is loaded. Even the old urea formaldihide glues we used sixty years ago were better than hide glue was.
    Wishing you great adventures under sail!
    Jay

  14. #14
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    Default Re: Birdsmouth flex reality check

    Quote Originally Posted by Yeadon View Post
    If it breaks, you're in luck as you already know how to make a mast.
    Hah, good point. And the next one should go together a lot faster.

    Quote Originally Posted by Jay Greer View Post
    Sounds like you are going to have a very good mast in your boat! Incidentally, what kind of glue did you use! Herreshoff was stuck with hide glue back then but it worked just fine as long as spars were painted or varnished. Fortunately tremendous
    advances have been made in adhesives since Captain Nat's time. G/flex is the new kid on the block for spars as it will flex as the spar is loaded. Even the old urea formaldihide glues we used sixty years ago were better than hide glue was.
    Wishing you great adventures under sail!
    Jay
    Thanks Jay!

    After some research, I used regular West 105 and 206 slow hardener, moderately thickened. Many people have built successful birdsmouth spars with this product and it's what I am most familiar with. I get the added flexibility of G Flex and I have used it in other applications but it costs more and is quite viscous, so I think it would be a bit of a pain to get it all mixed and spread out. I found that putting the mixed, thickened 105/206 epoxy into a gallon freezer bag and cutting off a tiny bit of corner made a nice dispenser kind of like a pastry piping bag to get the glue into the v shaped notches on the birdsmouth staves in an efficient manner.

  15. #15
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    Default Re: Birdsmouth flex reality check

    Today as I walked past the shed, I checked out four masts. Three of the four were bending 1" or so under their own weight, over a span of an average of 11' or so. The fourth mast was a Laser top section that did not bend at all. The others were the top section of a Tasar dinghy wingmast, an International Canoe carbon-topped mast, and a Windsurfer mast.

    As another vague check, my 28'er mast, which is a J/24 and Etchells section, probably sags roughly 6-8" under its own weight when horizontal, and the boat bounces around in offshore races under a #4 headsail and reefed main with no problems.

    As Edward says, dinghy masts are generally supposed to bend. Certainly Francis H wrote that spars should bend. Bermudan dinghy masts without bend are like cars with no springs...sort of.

  16. #16
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    Default Re: Birdsmouth flex reality check

    Thanks Chris!

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