Re: Birdsmouth spars
Oh those complicated birdsmouth masts!
Consider, for all these numbers being bantied about, all you need do
is pick wall thickness and spar diamter. Everything else falls into
place. Birdmouth builds itself.
If you don't believe me, build a short test spar. Get started now
and in an hour you'll be playing around with a rubber-banded up
birdsmouth 'spar'.
The most difficult part of the entire procedure is setting the depth
of the birdsmouth cut. Set it to 1/2 of the thickness of your stave
(stave thickness is the same as mast wall thickness). Knowing that
will save you 15 minutes over what it took me to build mine.
Do it ye non-believers! BIRDSMOUTH RULES! See the light!
IT'S EASY!!!!
John O'Neill
Consider, for all these numbers being bantied about, all you need do
is pick wall thickness and spar diamter. Everything else falls into
place. Birdmouth builds itself.
If you don't believe me, build a short test spar. Get started now
and in an hour you'll be playing around with a rubber-banded up
birdsmouth 'spar'.
The most difficult part of the entire procedure is setting the depth
of the birdsmouth cut. Set it to 1/2 of the thickness of your stave
(stave thickness is the same as mast wall thickness). Knowing that
will save you 15 minutes over what it took me to build mine.
Do it ye non-believers! BIRDSMOUTH RULES! See the light!
IT'S EASY!!!!
John O'Neill
--- In bolger@y..., wmrpage@a... wrote:
> In a message dated 2/15/01 12:43:27 PM Central Standard Time,
jboatguy@c...
> writes:
>
> least in comparison to elegant, but (apparently) complicated
> "Noble"/birdsmouth technique. The use of shallow rabbets cut with a
dado head
> on two of the four faces might make it as easy, or easier to
assemble than a
> "Noble"/birdsmouth mast, and it would certainly require less
machining of the
> wood, although it would be less adaptable to poor timber. The use
of a square
> section would seem to pose problems with regard to sprits, gaffs or
whatever
> that must bear upon the mast, and he does not address that issue.
Still, an
> intriguing concept.
>
> Bill in MN
>
> *
In a message dated 2/15/01 12:43:27 PM Central Standard Time, jboatguy@...
writes:
writes:
. Per our
friend Michalak, and all else being equal,
In a message dated 2/15/01 12:43:27 PM Central Standard Time, jboatguy@...
writes:. Per our
friend Michalak, and all else being equal,
The 02/01/00 back issue of Michalak's newletter on the subject of hollow
spars is very good, and while he doesn't discuss "Noble"/"birdsmouth"
construction, he does offer some intriguing justifications for a hollow,
square mast constructed of four 3/4" planks. The technique is nothing new or
complicated or non-obvious. He does state that a square spar has "moment of
inertia" (i.e. stiffness and strength, if I understand correctly) 69% greater
than a round spar of equivalent "width" (i.e. 3" diameter of round spar vs.
3" width of the faces of a square spar - while the 3" round spar has the same
diameter however measured, the outside measurement on corners 180 degrees
apart on the square spar with 3" faces would be the square root of 18", which
probably accounts for much of the difference.) He goes on to show an
intriguing aerodynamic concept: stepping the mast rotated 45 degrees from
what might be considered the "normal" configuration and using laced sails
permitting the luff to migrate to the leeward side of the mast. He makes no
great claims for the results, although he did give it a trial and relates
that it seemed to work. It looks very promising on paper, and is what I would
consider a very "Bolgeresque" or "square-boats" piece of engineering, at
least in comparison to elegant, but (apparently) complicated
"Noble"/birdsmouth technique. The use of shallow rabbets cut with a dado head
on two of the four faces might make it as easy, or easier to assemble than a
"Noble"/birdsmouth mast, and it would certainly require less machining of the
wood, although it would be less adaptable to poor timber. The use of a square
section would seem to pose problems with regard to sprits, gaffs or whatever
that must bear upon the mast, and he does not address that issue. Still, an
intriguing concept.
Bill in MN
*
Bill
Hollow masts should be plugged at the bottom, well past the partners,
and TAPERED. Read the Birdsmouth Mast post about how to avoid stress
concentrations.
The physics of hollowing out a mast are incontrovertible. Per our
friend Michalak, and all else being equal, a round, 3" hollow mast
with a 25% of diameter wall thickness (.75") loses only about 6% of
strength to a solid mast the same size. Add a mere 10% to the
diameter, to 3.3", make the wall considerably thinner, at 15% of
diameter (<.5"), and the mast ends up about 11% STRONGER than it's
3", solid counterpart.
And, considering that a solid spar made of less than ideal wood WILL
BE weaker than the same spar made birdsmouth, birdsmouth is the way
to go. Birdsmouth does not tolerate knots and grain irregularities.
By the time it goes together there's nothing but good wood left in
the spar.
All this about taper and plugs and stress concentrations makes it
SEEM difficult, but building a hollow plug is truly a breeze after
building an entire hollow mast, and building the mast is not all that
much more difficult. I personally found it as easy as building solid
spars, easier in many respects.
John
Hollow masts should be plugged at the bottom, well past the partners,
and TAPERED. Read the Birdsmouth Mast post about how to avoid stress
concentrations.
The physics of hollowing out a mast are incontrovertible. Per our
friend Michalak, and all else being equal, a round, 3" hollow mast
with a 25% of diameter wall thickness (.75") loses only about 6% of
strength to a solid mast the same size. Add a mere 10% to the
diameter, to 3.3", make the wall considerably thinner, at 15% of
diameter (<.5"), and the mast ends up about 11% STRONGER than it's
3", solid counterpart.
And, considering that a solid spar made of less than ideal wood WILL
BE weaker than the same spar made birdsmouth, birdsmouth is the way
to go. Birdsmouth does not tolerate knots and grain irregularities.
By the time it goes together there's nothing but good wood left in
the spar.
All this about taper and plugs and stress concentrations makes it
SEEM difficult, but building a hollow plug is truly a breeze after
building an entire hollow mast, and building the mast is not all that
much more difficult. I personally found it as easy as building solid
spars, easier in many respects.
John
--- In bolger@y..., "Samson Family" <Bill.Samson@t...> wrote:
> I don't THINK so. Look at Fraser Howell's cautionary tale in the
archives.
> For newbies, his beautiful birdsmouth Chebacco mast snapped off a
foot or so
> above the partners at a most inconvenient time.
>
> Bill Samson
>PS My two cents: birdsmouth is the only way to go. It is so cheap,I don't THINK so. Look at Fraser Howell's cautionary tale in the archives.
>easy, and accurate, and you get such a strong, light spar as a
>result, with no worries--ever--that a good breeze, aided by some left
>over knots, will turn your cheap wood stick into a stump, that to me
>there is simply no comparison to other methods.
For newbies, his beautiful birdsmouth Chebacco mast snapped off a foot or so
above the partners at a most inconvenient time.
Bill Samson