Help with mast calculation

Glenn,
That is an interesting way to express it.
It sure would be is easier for a ordinary person to understand.
I will have to remember that. It may save hundreds of bar napkins.
Matt Colie

Glenn Ashmore wrote:
> OK, you asked for it. Prepare to be confused:
>
> Mast design starts with the transverse righting moment. That is the force
> in foot pounds required to overcome the initial stability and heel the boat.
> It is usually measured at 30 degrees. 30 degrees is where the sail area
> perpendicular to the wind starts to reduce faster than the wind speed
> increases so it is about the point of maximum stress. The calculations
> require finding the center of gravity and the center of buoyancy and the
> metacenter at the heel angle. The Metacenter is the point at which a
> vertical line through the COB crosses the centerline of the hull
that is inclined (when upright the two are coliniar)
.. The
> horizontal distance between the COG and the COB is the lever arm. The
> righting moment is the mass * the acceleration of gravity (32'/s/s)* the
> lever arm.
>
> The actual mast section depends on the righting moment at 1 degree and 30
> degrees, the number of spreaders, baby stays, runners and several other
> factors. I am not even going to try to get into that.
>
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Many thanks for your response Glenn. I do have a set of figures from
the architect, but all the mast manufacturers want to give me a mast
that I feel is far heavier and stiffer than I would prefer for my type
of sailing.
Obviously I am not going to argue with the architect or the mast
suppliers, but I just wanted to have an idea of the whole concept
(about which I have no knowledge at all) so I can maybe get the mast I
want without compromising any of the architects intentions - well not
too much anyway!

To make it a bit tougher for me I'm trying doing this in French, so
I'm struggling a bit to make sure I am completely understood and I
completely understand them. "Knowledge is everything" is never more
true then when your a stranger in a strange land. I have a seven page
questionnaire from the manufacturers full of technical terms that
would make my head ache in English let alone French!

Thanks, Pete

On Thu, 13 Oct 2005 07:57:32 -0400, "Glenn Ashmore"
<gashmore.DeleteThis@cox.net> wrote:

>There is no simple explanation. Brion Toss's "Rigger's Apprentice" and Dave
>Gerr's "Nature of Boats" have a fairly straight forward explanation. Both
>have charts to help you estimate the RM. Larsson & Eliasson "Principles of
>Yacht Design" has a much more complete explanation but requires more math.
>Read all three in that order and you will have a pretty good grasp of the
>subject.
>
>Best way to get accurate numbers is to send your hull dimensions to a spar
>maker and ask them to specify a mast. They all have computer programs that
>can crank out a report in seconds.
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On Thu, 13 Oct 2005 11:50:11 -0400, "Glenn Ashmore" <gashmore DeleteThis @cox.net>
wrote:

>OK, you asked for it. Prepare to be confused:
>
>Mast design starts with the transverse righting moment. That is the force
>in foot pounds required to overcome the initial stability and heel the boat.
>It is usually measured at 30 degrees. 30 degrees is where the sail area
>perpendicular to the wind starts to reduce faster than the wind speed
>increases so it is about the point of maximum stress. The calculations
>require finding the center of gravity and the center of buoyancy and the
>metacenter at the heel angle. The Metacenter is the point at which a
>vertical line through the COB crosses the centerline of the hull. The
>horizontal distance between the COG and the COB is the lever arm. The
>righting moment is the mass * the acceleration of gravity (32'/s/s)* the
>lever arm.
>

That's pretty much exactly what I was after, thanks. I don't need to
work out the maths myself, I just wanted to understand the why and
wherefore.

>The actual mast section depends on the righting moment at 1 degree and 30
>degrees, the number of spreaders, baby stays, runners and several other
>factors. I am not even going to try to get into that.


That I understand perfectly,

Gosh this newsgroup is useful isn't it?

Many thanks,

Pete
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On 13 Oct 2005 06:31:49 -0700, dbohara RemoveThis @mindspring.com wrote:

>Is this simply the moment of inertia of the mast and objects along it
>or is it some kind of definition specific to yachts. Does it include
>the contribution due to sails and rigging? If not, moment of inertia
>is a fairly simple concept. If it is the simple physics definition,
>let me know and I'll show you how to estimate it (or even calculate for
>simple cases).

Thanks, I don't really want to calculate it myself, I'm trying to
understand the correlation between figures given to me by the
architecht, and the tree trunk mast sections that the mast suppliers
want to give to me.

Pete
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