Page 1 of 2 12 LastLast
Results 1 to 35 of 38

Thread: Birdsmouth spar calculators

  1. #1
    Join Date
    Feb 2002
    Location
    Uki, NSW, Australia
    Posts
    26,806

    Default Birdsmouth spar calculators

    Posted before but a refresh doesn’t hurt.
    http://www.duckworksmagazine.com/10/howto/birdsmouth/
    The definition of stupid has got to be the belief that more guns will negate the bloodshed done with guns.

  2. #2
    Join Date
    Feb 2007
    Location
    Blue Hill, ME
    Posts
    977

    Default Re: Birdsmouth spar calculators

    Thanks, I've had that bookmarked. I was reading an article in the old Boatbuilders magazine touting the virtues of 12 staves, more strength, less waste. I've done 8 staves before but now thinking of 12. Anyone else tried that?
    Steve B
    Sjogin IIIa
    PAYTON 13' Pea Pod

    RIVUS 16' Melonseed


    "If a man must be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most." E. B. White

  3. #3
    Join Date
    Dec 2001
    Location
    Northern NSW Australia
    Posts
    73,438

    Default Re: Birdsmouth spar calculators

    Only 8.
    '' You ain't gonna learn what you don't want to know. ''
    Grateful Dead

  4. #4
    Join Date
    Feb 2002
    Location
    Uki, NSW, Australia
    Posts
    26,806

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by SBrookman View Post
    Thanks, I've had that bookmarked. I was reading an article in the old Boatbuilders magazine touting the virtues of 12 staves, more strength, less waste. I've done 8 staves before but now thinking of 12. Anyone else tried that?
    The beauty of 8 is the birdsmouth is a 45 degree cut, whereas 12 is one more stage in the set up process...I'm not saying don't do it, I just didn't want to.
    The definition of stupid has got to be the belief that more guns will negate the bloodshed done with guns.

  5. #5
    Join Date
    Jan 2011
    Location
    Norwalk CT
    Posts
    1,001

    Default Re: Birdsmouth spar calculators

    8 staves, 45 degree cuts, almost impossible to muck it up.

  6. #6
    Join Date
    Feb 2007
    Location
    Blue Hill, ME
    Posts
    977

    Default Re: Birdsmouth spar calculators

    30/60 for the 12. I'm sure it would be easy to muck that up. I'll try a mock up before mucking it up. Another issue will be making a 20' mast in a shop not much bigger than that.
    Steve B
    Sjogin IIIa
    PAYTON 13' Pea Pod

    RIVUS 16' Melonseed


    "If a man must be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most." E. B. White

  7. #7
    Join Date
    Jul 2011
    Location
    Lindstrom, MN
    Posts
    2,269

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by willin woodworks View Post
    8 staves, 45 degree cuts, almost impossible to muck it up.
    Betcha I could do it. I'm pretty capable of mucking things up.

    There are several ways to muck this up with table saws and router bits: http://forum.woodenboat.com/showthre...rds-mouth-spar
    The Lee Valley router bits look handy.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  8. #8
    Join Date
    Feb 2007
    Location
    N62 53´52" E27 41´10"
    Posts
    262

    Cool Re: Birdsmouth spar calculators

    Quote Originally Posted by willin woodworks View Post
    8 staves, 45 degree cuts, almost impossible to muck it up.
    Think so? Been there, done that

    stave.jpg

    Dreaming a schooner since 1988:

  9. #9
    Join Date
    Jul 2008
    Location
    Modesto, CA
    Posts
    271

    Default Re: Birdsmouth spar calculators

    Where's the birdsmouth calculator for a tapered spar? Not that I anticipate needing it... The question is more an academic one.

  10. #10
    Join Date
    Feb 2007
    Location
    N62 53´52" E27 41´10"
    Posts
    262

    Default Re: Birdsmouth spar calculators

    I made a spreadsheet for my mast build. That included the calculation to make the wall thickness to decrease together with diameter. So the wall is always the same % of the diameter instead of ending up with a solid stick near the top.

    I can dig it up if someone needs it.

    Dreaming a schooner since 1988:

  11. #11
    Join Date
    Aug 2008
    Location
    Gowen, MI
    Posts
    384

    Default Re: Birdsmouth spar calculators

    Bird's Mouth Spars - Revisited

    http://duckworksmagazine.com/04/s/articles/birdsmouth/

    Scroll down to section re tapering bird's mouth spars.

  12. #12
    Join Date
    Feb 2007
    Location
    N62 53´52" E27 41´10"
    Posts
    262

    Default Re: Birdsmouth spar calculators

    That method keeps the wall thickness the same. You should leave the top O.D. larger originally and then plane to final O.D. By reducing the thickness you end up with the same wall/diameter ratio on the whole length of the spar.

    Dreaming a schooner since 1988:

  13. #13
    Join Date
    Dec 2001
    Location
    Northern NSW Australia
    Posts
    73,438

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by SBrookman View Post
    30/60 for the 12. I'm sure it would be easy to muck that up. I'll try a mock up before mucking it up. Another issue will be making a 20' mast in a shop not much bigger than that.
    Don't forget that the assembly has to be perfectly straight as you glue up. Not just supported at the ends .....or the middle sags and you end up with a curved mast. I don't want to explain how I know this but it's easy to do in the rush of glue going off.
    '' You ain't gonna learn what you don't want to know. ''
    Grateful Dead

  14. #14
    Join Date
    Feb 2007
    Location
    Blue Hill, ME
    Posts
    977

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by PeterSibley View Post
    Don't forget that the assembly has to be perfectly straight as you glue up. Not just supported at the ends .....or the middle sags and you end up with a curved mast. I don't want to explain how I know this but it's easy to do in the rush of glue going off.
    Or an unintentional raked mast that worked out pretty well, just like I hadn't planned.
    Steve B
    Sjogin IIIa
    PAYTON 13' Pea Pod

    RIVUS 16' Melonseed


    "If a man must be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most." E. B. White

  15. #15
    Join Date
    Nov 2002
    Location
    St. Petersburg FL
    Posts
    149

    Default Re: Birdsmouth spar calculators

    The 30/60 cut is no more difficult than the 45. Cutting on the table saw the blade is set at 30, first cut the stave is vertical against the fence, and the second cut it is horizontal on the table top. adjusting the height as necessary.

    The cut is the same for six sided spars- useful for sprits and other small diameter spars.

    Set up and try the cuts with an 8 foot board and the cut into 8" lengths to experiment, as a six and 12 sided spar.

  16. #16
    Join Date
    Aug 2008
    Location
    Gowen, MI
    Posts
    384

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by Timo_N62.9_E27.7 View Post
    You should leave the top O.D. larger originally and then plane to final O.D. By reducing the thickness you end up with the same wall/diameter ratio on the whole length of the spar.
    Not sure I understand your explanation. If you build a bird's mouth mast with the top O.D. the same as the bottom O.D. the I.D. will be the same throughout the length of the mast. If you then plane to taper to the final top O.D. the I.D. will not be affected, and the wall/diameter ration will not be the same at the top as at the bottom. And wouldn't the outside of the V-notch be planed away? Not sure that's a real issue because it would be planed after the mast is glued.

    On the other hand... if the thickness of the staves is tapered before assembly, how would that be accomplished with an acceptable degree of precision?

    My experience is with smaller (18' long, 4" O.D.) masts so I may be missing something that could (?) work on a larger mast. What size masts/spars are you working with?

    Thanks.

  17. #17
    Join Date
    Feb 2007
    Location
    N62 53´52" E27 41´10"
    Posts
    262

    Default Re: Birdsmouth spar calculators

    The O.D. at the top is not the same as at the bottom, it is a certain calculated amount smaller so that when the rest of the taper is done by planing the outside we end up with a spar that has the desired taper on the O.D. and the wall thickness decreases proportionally towards the top being the same % of the O.D. all the way.

    If yoy don't do this your 4" mast that tapers to 2" at the top will have wall thickness say 25% = 1" at the bottom. At the top it would be 50% meaning you have a solid stick. The example is extreme but demonstrates what I mean. If you taper the staves a little less aiming at the correct ratio between bottom and top I.D. and plane the rest from the outside you acheieve a spar with the desired taper and the wall tapers as well keeping the diameter/wall ratio the same.

    I realized this when I first made my hollow gaffs that have a serious taper. They ended up being almost solid at the peak. So for the masts I developed the calculation to avoid that. Saving weight higher aloft is usually a good idea.

    P.S. my masts aren't any bigger, the main is ~6 meters and the fore 5.6.

    Dreaming a schooner since 1988:

  18. #18
    Join Date
    Sep 2010
    Location
    Bainbridge Island WA
    Posts
    2,613

    Default Re: Birdsmouth spar calculators

    I'm curious about how you worked the taper into the smaller sections of the spar. I thought about it when I built the roughly 6 meter main mast for Marianita but in the end left the wall thickness alone because there are two mast bands up there and whittling away to create a shoulder for them to sit on already reduced the wall too.
    Steve

    Boats, like whiskey, are all good.
    R.D Culler

  19. #19
    Join Date
    Jul 2011
    Location
    Lindstrom, MN
    Posts
    2,269

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by MW Jones View Post
    The 30/60 cut is no more difficult than the 45. Cutting on the table saw the blade is set at 30, first cut the stave is vertical against the fence, and the second cut it is horizontal on the table top. adjusting the height as necessary.

    The cut is the same for six sided spars- useful for sprits and other small diameter spars.

    Set up and try the cuts with an 8 foot board and the cut into 8" lengths to experiment, as a six and 12 sided spar.
    Now that you mention it, it seems obvious. All of the angle pairs for any number of staves add up to 90°, so this works for any number of staves. Set the blade for one angle and rotate the stock 90° against the fence and table.

    One suggestion for using a plane to taper a stack of staves: Use straight strips as plane guides and shims to angle the staves, then clamp and plane down to the guides:
    Capture.jpg
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  20. #20
    Join Date
    Sep 2010
    Location
    Bainbridge Island WA
    Posts
    2,613

    Default Re: Birdsmouth spar calculators



    That is what I'm doing here, planing down the "back" of the staves, the "V" is down. I supposed you could do more or less the same thing to taper the wall thickness, you would want to work both sides evenly. At some point though you have to do a cost-benefit-analysis for these things, as an old friend from North Carolina was fond of saying "That's a lot of squeezin' for not much juice".
    Steve

    Boats, like whiskey, are all good.
    R.D Culler

  21. #21
    Join Date
    Feb 2007
    Location
    N62 53´52" E27 41´10"
    Posts
    262

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by stromborg View Post
    I'm curious about how you worked the taper into the smaller sections of the spar. I thought about it when I built the roughly 6 meter main mast for Marianita but in the end left the wall thickness alone because there are two mast bands up there and whittling away to create a shoulder for them to sit on already reduced the wall too.
    It sounds (and feels) complicated when explained verbally but really isn't. First you calculate the ratio between O.D. top and O.D. bottom - or partners if you taper also to the shoe. Using that ratio and wall thickness you get the I.D. top and bottom. Wall thickness and top I.D. gives you the RAW top O.D., which you use to calculate the stave tapering. This is slightly more than the FINAL top O.D. which is acheived by planing some material away after glueing it together. Now you have a mast that has all three - I.D., wall and O.D. - reduced with the same ratio from bottom to top.

    Dreaming a schooner since 1988:

  22. #22
    Join Date
    Feb 2007
    Location
    N62 53´52" E27 41´10"
    Posts
    262

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by stromborg View Post

    I supposed you could do more or less the same thing to taper the wall thickness, you would want to work both sides evenly.
    It is easier to taper the wall after glue up while rounding the mast

    Dreaming a schooner since 1988:

  23. #23
    Join Date
    May 2006
    Location
    Ballard
    Posts
    8,082

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by stromborg View Post

    That is what I'm doing here, planing down the "back" of the staves, the "V" is down. I supposed you could do more or less the same thing to taper the wall thickness, you would want to work both sides evenly. At some point though you have to do a cost-benefit-analysis for these things, as an old friend from North Carolina was fond of saying "That's a lot of squeezin' for not much juice".
    I lash the staves together with blue painters tape before running them through the thickness planer. (I don't run the tape through planer. I locate the tape outside the area I want to taper.) I crank down the planer handle ever so slowly as the staves feed through, removing thickness from the backs of the staves. It takes a pass or two to get it more or less how I want it. I do this for both tapers at the foot and the head of the spar.

    I'm not sure how many birdsmouth spars I've made at this point. Perhaps eight or nine of various lengths and scantlings. I did the first couple by hand until I saw Hvalsoe knock out a taper by eye with his thickness planer. It was a moment of clarity for me.
    Last edited by Yeadon; 11-20-2018 at 12:01 PM.
    Quote Originally Posted by James McMullen View Post
    Yeadon is right, of course.

  24. #24
    Join Date
    Jul 2013
    Location
    Aquitaine
    Posts
    878

    Default Re: Birdsmouth spar calculators

    While we are at it..
    I built birdsmouth spars for my Oughtred; 8 staves. Easy and quite fun.
    Now, I have a 23ft one to make with a taper from 3.5" down to 1"

    Any comments about suitability for BM? or is the little end going to be a bit tricky? As designed, it is nearly solid, with a 1/2" square hole up the middle, laminated.
    A2

  25. #25
    Join Date
    Aug 2008
    Location
    Gowen, MI
    Posts
    384

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by Timo_N62.9_E27.7 View Post
    The O.D. at the top is not the same as at the bottom, it is a certain calculated amount smaller so that when the rest of the taper is done by planing the outside we end up with a spar that has the desired taper on the O.D. and the wall thickness decreases proportionally towards the top being the same % of the O.D. all the way.

    If yoy don't do this your 4" mast that tapers to 2" at the top will have wall thickness say 25% = 1" at the bottom. At the top it would be 50% meaning you have a solid stick. The example is extreme but demonstrates what I mean. If you taper the staves a little less aiming at the correct ratio between bottom and top I.D. and plane the rest from the outside you acheieve a spar with the desired taper and the wall tapers as well keeping the diameter/wall ratio the same.

    I realized this when I first made my hollow gaffs that have a serious taper. They ended up being almost solid at the peak. So for the masts I developed the calculation to avoid that. Saving weight higher aloft is usually a good idea.

    P.S. my masts aren't any bigger, the main is ~6 meters and the fore 5.6.
    Thanks. Your second paragraph describes what I tried to describe above: without tapering/partially tapering the staves before assembly, the O.D. and I.D. will be the same at the top and bottom of the mast.

    Just so I understand, you partially taper the staves on the inside surface so that when assembling the mast, the top is a smaller (but not final) O.D. (which results in a smaller I.D.. Then, once assembled, you plane the outside of the mast (the length of the taper) to get to the desired top O.D. Is your sequence: cut staves, cut the V-notch, taper 'inside' surface of staves a certain calculated amount, assemble mast, and plane to round, with the result being a consistent wall thickness percentage-wise?

    What is the disadvantage of having a constant wall thickness throughout the mast? More weight aloft seems to be the obvious one. Are there others?

  26. #26
    Join Date
    May 2006
    Location
    Ballard
    Posts
    8,082

    Default Re: Birdsmouth spar calculators

    Taper only the side of the stave that does not have the joint cut into it. Once you click everything together the taper will be apparent.
    Quote Originally Posted by James McMullen View Post
    Yeadon is right, of course.

  27. #27
    Join Date
    Mar 2014
    Location
    Hasslö, Blekinge, Sweden
    Posts
    643

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by BobW View Post
    What is the disadvantage of having a constant wall thickness throughout the mast? More weight aloft seems to be the obvious one. Are there others?
    If you run electric wires and halyards in the hollow section it may become too cramped.
    I can on the other hand see where you'd want a relatively speaking stiffer mast top, when the top of the mast is unstayed and too weak to carry a topsail.

    /Mats
    Yes the avatar depicts me; yes I drew the comic boat pic, it's a joke on the pop song I'm not a robot by Marina and the diamonds

  28. #28
    Join Date
    Feb 2007
    Location
    N62 53´52" E27 41´10"
    Posts
    262

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by BobW View Post
    Thanks.
    Just so I understand, you partially taper the staves on the inside surface so that when assembling the mast, the top is a smaller (but not final) O.D. (which results in a smaller I.D.. Then, once assembled, you plane the outside of the mast (the length of the taper) to get to the desired top O.D.
    The original taper is not on the inside but on the opposite side to the v-notch, exactly as described on the duckswork pages. This reduces the circumference and creates the taper. This is used to get the desired I.D.

    The sequence is:
    1. plane to thickness both ways
    2. make the notch
    3. make the taper to each stave
    4. assemble
    5. plane to final size

    The disadvantage of constant wall thickness is to have a more top heavy mast than needed. And saving weight is the main reason why we bother with the bird's mouth construction.

    Dreaming a schooner since 1988:

  29. #29
    Join Date
    Feb 2007
    Location
    Blue Hill, ME
    Posts
    977

    Default Re: Birdsmouth spar calculators

    Another reason that I'm thinking going 12 is that it is almost round making it possible to make a BM plug to fit inside, designing it such that it would leave room for wiring. Here's a pic I found from someone that read the same Boatbuilder's article.
    Last edited by SBrookman; 11-20-2018 at 05:56 PM. Reason: added link
    Steve B
    Sjogin IIIa
    PAYTON 13' Pea Pod

    RIVUS 16' Melonseed


    "If a man must be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most." E. B. White

  30. #30
    Join Date
    Aug 2015
    Location
    Santa Fe NM
    Posts
    298

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by Yeadon View Post
    I lash the staves together with blue painters tape before running them through the thickness planer. (I don't run the tape through planer. I locate the tape outside the area I want to taper.) I crank down the planer handle ever so slowly as the staves feed through, removing thickness from the backs of the staves. It takes a pass or two to get it more or less how I want it. I do this for both tapers at the foot and the head of the spar.

    I'm not sure how many birdsmouth spars I've made at this point. Perhaps eight or nine of various lengths and scantlings. I did the first couple by hand until I saw Hvalsoe knock out a taper by eye with his thickness planer. It was a moment of clarity for me.
    Yeadon,
    Thank you for this tip. I was just about to ask for input on how to taper the staves. I like the sound of this planner method. Anyone else have input on how to taper the staves?
    "Yeah, well, that's just, like your opinion man"
    -The Dude-

  31. #31
    Join Date
    Aug 2008
    Location
    Gowen, MI
    Posts
    384

    Default Re: Birdsmouth spar calculators

    Okay, I am definitely missing something.

    It seems to me your responses in post 12 and post 28 are - or seem to be - contradictory. Tapering the mast as described in the link in post 11 does not affect wall thickness. It does taper the mast and the desired I.D. can be calculated - but that taper does not affect wall thickness.

    After playing with the calculators, I can spiral in on the solution, so I think I understand what you are doing. I don't have a high level of confidence that I can get a consistent taper over the length of 10 - 12' (referring to the mast I built) in order to maintain that wall thickness/O.D. ratio.

  32. #32
    Join Date
    Sep 2010
    Location
    Bainbridge Island WA
    Posts
    2,613

    Default Re: Birdsmouth spar calculators

    My solution (and it only tapers the width of the stave, not the wall thickness) is to calculate the widths needed to achieve my desired O.D.. I then mill the staves to the largest width and cut the notch, I take one stave and mark along it's length what the final width wants to be at each point, rough cut and hand plane to the finished width then use it as a pattern for a second stave. Next step is to clamp all the staves together with the two patterns on the outside (see the picture I posted above) now I can go in with my power plane and mow the middle six to size, finishing by hand with a block plane.
    Steve

    Boats, like whiskey, are all good.
    R.D Culler

  33. #33
    Join Date
    Nov 2010
    Location
    Vancouver BC Canada
    Posts
    357

    Default Re: Birdsmouth spar calculators

    Very cool thanks. Can one use any decent wood adhesive probably preferably waterproof ?

  34. #34
    Join Date
    Feb 2002
    Location
    Uki, NSW, Australia
    Posts
    26,806

    Default Re: Birdsmouth spar calculators

    Ill be fitting new rubrails in the next couple of days and I’m going to use a polyurethane called Purbond, it’s waterproof. The reasoning is it stays flexible so if the rubrails cop a good bump the glue line shouldn’t crack.
    The definition of stupid has got to be the belief that more guns will negate the bloodshed done with guns.

  35. #35
    Join Date
    Aug 2008
    Location
    Gowen, MI
    Posts
    384

    Default Re: Birdsmouth spar calculators

    Quote Originally Posted by BobW View Post
    Okay, I am definitely missing something.

    It seems to me your responses in post 12 and post 28 are - or seem to be - contradictory. Tapering the mast as described in the link in post 11 does not affect wall thickness. It does taper the mast and the desired I.D. can be calculated - but that taper does not affect wall thickness.

    After playing with the calculators, I can spiral in on the solution, so I think I understand what you are doing. I don't have a high level of confidence that I can get a consistent taper over the length of 10 - 12' (referring to the mast I built) in order to maintain that wall thickness/O.D. ratio.
    Timo: My apologies for not re-reading the tapering method described in post 11, and for not remembering that method specifically does not taper wall thickness. I misunderstood your response (post 12) and thought you somehow tapered the wall thickness before assembly and finally planing to final size. This misunderstanding was entirely of my own making.

    Working with paper, pencil and the calculators last night, I worked out dimensions for a tapered mast maintaining a 20% wall thickness/O.D. ratio after planing to final size. I still have concerns with my ability to create that consistent taper - but that has nothing to do with calculating the dimensions.

    Thanks for making me think and take a harder look at the discussion.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •