Re: [bolger] hollow mast progress
>The material was clear white spruce, glued with epoxy. The glue didn'tI think the design of the hollow mast doesn't allow for concentrated loads,
>fail, the wood did. It might be that the stiffness of the hollow mast
>concentrated the stress, progressively crushing the material in the way
>of the partner.
such as you will find in an unsupported mast, but rather distributed loads,
like you will find in a mast supported with shrouds (yes, I know this is an
oversimplification). The concentration of load must be allowed for in the
design. It's likely that it was the unsupported hollow shape, where
stresses were concentrated (at the mast partner), that led to the
failure. If so, I think you might want to fill that spot in the mast with
solid wood. The design of the mast as described in the WB article has
solid plugs at each end, where the loads were expected to be concentrated.
To fit a plug in the mast where it passes through the partner, make a dummy
of the staves to the dimension of that part of the mast (I'm assuming some
sort of taper, so it won't be the same dimension throughout). Assemble the
dummy, and measure the dimension of the inside of each stave. Make an
8-sided plug of some soft wood just slightly undersized. If the section of
mast that needs the plug is tapered, of course you'll need to taper the
plug. It will need to be inserted as the staves are assembled, probably
with thickened epoxy on all sides to glue it to the inside of the mast.
If a structural engineer wants to chime in, I'll listen, but I feel the
plug ought to be 2x the diameter of the mast in length, centered on the
stress point.
If you still want to pass the shrouds up inside the mast, you will need to
drill some holes in the plug, probably twice the size of the shroud, and
chamfer them to make it easier to insert the shrouds.
>I still like the concept, and I'll build another.I very much like the concept myself, and was quite disappointed in your
minor disaster.
Regards,
Ron Butterfield
Perhaps if you were to make the diameter of the hollow mast greater
than that of the solid one with the weight the same or maybe a bit
less.
--- Inbolger@egroups.com, "Peter Vanderwaart" <pvanderw@o...>
wrote:
than that of the solid one with the weight the same or maybe a bit
less.
--- Inbolger@egroups.com, "Peter Vanderwaart" <pvanderw@o...>
wrote:
>splinters.
> I'm sure it was painful to see all that work come down in
> Condolences. Under the circumstances, I admire the plucky tone ofmast
> your posting.
>
> You didn't say in your report of the disaster whether the hollow
> was plugged at the partners. I think it certainly should be pluggedthe
> with a tapered plug as others have said. My guess is that cutting
> plug to exactly the right size is the most difficult part of thewhy
> construction. Some would put plugs in way of the boom and gaff jaws
> as well, though that would complicate using the hollow for halyards.
>
> I am interested that the hollow mast seemed stiffer. I don't see
> it would be stiffer than a solid mast of the same material. Anyone
> have any ideas?
>
> Peter
I'm sure it was painful to see all that work come down in splinters.
Condolences. Under the circumstances, I admire the plucky tone of
your posting.
You didn't say in your report of the disaster whether the hollow mast
was plugged at the partners. I think it certainly should be plugged
with a tapered plug as others have said. My guess is that cutting the
plug to exactly the right size is the most difficult part of the
construction. Some would put plugs in way of the boom and gaff jaws
as well, though that would complicate using the hollow for halyards.
I am interested that the hollow mast seemed stiffer. I don't see why
it would be stiffer than a solid mast of the same material. Anyone
have any ideas?
Peter
Condolences. Under the circumstances, I admire the plucky tone of
your posting.
You didn't say in your report of the disaster whether the hollow mast
was plugged at the partners. I think it certainly should be plugged
with a tapered plug as others have said. My guess is that cutting the
plug to exactly the right size is the most difficult part of the
construction. Some would put plugs in way of the boom and gaff jaws
as well, though that would complicate using the hollow for halyards.
I am interested that the hollow mast seemed stiffer. I don't see why
it would be stiffer than a solid mast of the same material. Anyone
have any ideas?
Peter
Hi Fraser,
Sorry to hear that the hollow mast let go. I imagine the forces as
those on a cantilever beam, with significant stresses being imposed
where the cantilever hangs its hat: the partner area. I'm wondering
if you might consider thickening the wall of the mast more than the
WB mag article suggests -- especially the lower area from the sole on
up perhaps to where the boom jaws rest -- if that is practical.
Plugging that lower part sounds good too.
If I understand your description correctly, the wood split along the
grain -- horizontal shear -- is that right? Or was it compressed to
crushing? Does your mast rotate when you sail or is it fixed at the
base? Were there twisting -- torsional -- forces at work? What
thickness did the pieces finish out at?
Dick
Sorry to hear that the hollow mast let go. I imagine the forces as
those on a cantilever beam, with significant stresses being imposed
where the cantilever hangs its hat: the partner area. I'm wondering
if you might consider thickening the wall of the mast more than the
WB mag article suggests -- especially the lower area from the sole on
up perhaps to where the boom jaws rest -- if that is practical.
Plugging that lower part sounds good too.
If I understand your description correctly, the wood split along the
grain -- horizontal shear -- is that right? Or was it compressed to
crushing? Does your mast rotate when you sail or is it fixed at the
base? Were there twisting -- torsional -- forces at work? What
thickness did the pieces finish out at?
Dick
Hi Fraser,
Sorry to hear that the hollow mast let go. I imagine the forces as
those on a cantilever beam, with significant stresses being imposed
where the cantilever hangs its hat: the partner area. I'm wondering
if you might consider thickening the wall of the mast more than the
WB mag article suggests -- especially the lower area from the sole on
up perhaps to where the boom jaws rest -- if that is practical.
Plugging that lower part sounds good too.
If I understand your description correctly, the wood split along the
grain -- horizontal shear -- is that right? Or was it compressed to
crushing? Does your mast rotate when you sail or is it fixed at the
base? Were there twisting -- torsional -- forces at work? What
thickness did the pieces finish out at?
Dick
Sorry to hear that the hollow mast let go. I imagine the forces as
those on a cantilever beam, with significant stresses being imposed
where the cantilever hangs its hat: the partner area. I'm wondering
if you might consider thickening the wall of the mast more than the
WB mag article suggests -- especially the lower area from the sole on
up perhaps to where the boom jaws rest -- if that is practical.
Plugging that lower part sounds good too.
If I understand your description correctly, the wood split along the
grain -- horizontal shear -- is that right? Or was it compressed to
crushing? Does your mast rotate when you sail or is it fixed at the
base? Were there twisting -- torsional -- forces at work? What
thickness did the pieces finish out at?
Dick
A couple of months ago I built a hollow (unstayed) mast for my Chebacco.
I liked the description of the birdsmouth technique that appeared in
Woodenboat, and thought it would be neat to run the halyards inside. I
was quite happy with the thing until it broke. No warning, sudden,
compound fracture. Just like Prada.
I think that it had been weakened during stronger winds, as it failed
under moderate conditions, with a reef in the main. It certainly had
stood up to far greater stress. The break started at the partner, with a
vertical failure across 2 strakes, then a split that went up to the boom
jaws, where the remainder let go.
The material was clear white spruce, glued with epoxy. The glue didnt
fail, the wood did. It might be that the stiffness of the hollow mast
concentrated the stress, progressively crushing the material in the way
of the partner.
I still like the concept, and Ill build another. Ill try protecting
the area between the partner and boom by wrapping it in epoxy and cloth,
and I might try carbon-fibre. Im open to suggestions.
It seems like a lot of work, but the mast was that much lighter and
stiffer that the boat went much better to windward, and I still have the
exit blocks, which were the most expensive part.
I kept the old solid mast, so am still in business.
Cheers
Fraser Howell
I liked the description of the birdsmouth technique that appeared in
Woodenboat, and thought it would be neat to run the halyards inside. I
was quite happy with the thing until it broke. No warning, sudden,
compound fracture. Just like Prada.
I think that it had been weakened during stronger winds, as it failed
under moderate conditions, with a reef in the main. It certainly had
stood up to far greater stress. The break started at the partner, with a
vertical failure across 2 strakes, then a split that went up to the boom
jaws, where the remainder let go.
The material was clear white spruce, glued with epoxy. The glue didnt
fail, the wood did. It might be that the stiffness of the hollow mast
concentrated the stress, progressively crushing the material in the way
of the partner.
I still like the concept, and Ill build another. Ill try protecting
the area between the partner and boom by wrapping it in epoxy and cloth,
and I might try carbon-fibre. Im open to suggestions.
It seems like a lot of work, but the mast was that much lighter and
stiffer that the boat went much better to windward, and I still have the
exit blocks, which were the most expensive part.
I kept the old solid mast, so am still in business.
Cheers
Fraser Howell