Hi folks,

Can anybody explain in very simple English the moment of inertia of a
mast, and how the Ixx and Iyy numbers work - I am a little like Pooh
Bear, I have a poor brain so any help is appreciated.

A million grovelling thank you's in advance

Pete

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.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"pete" wrote in message
...
Hi folks,

Can anybody explain in very simple English the moment of inertia of a
mast, and how the Ixx and Iyy numbers work - I am a little like Pooh
Bear, I have a poor brain so any help is appreciated.

A million grovelling thank you's in advance

Pete

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).

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"
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.

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.

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.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

wrote in message
oups.com...
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).

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.

On Thu, 13 Oct 2005 11:50:11 -0400, "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. 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

On 13 Oct 2005 06:31:49 -0700, 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

Even with all the righting moment calculations spar sections for a given
mast height tend to fall in a fairly narrow range. A competent designer can
calculate the requirements very closely but the spar makers tend to put in
more safety margin and you are practically limited to the available
sections. Here is a "rule of thumb" for aluminum masts from Dave Gerr's
book you might use as a double check.:

For a single spreader rig the transverse mast section should be about 1/90
of the length from cabin top to masthead. The fore and aft section should
be about 1.4 times the transverse section and the thickness of the aluminum
should be about 1/35 of the transverse section.

Double spreaders reduce the required transverse section by 10 to 15% but not
the fore and aft or thickness. Heavy displacement and blue water boats
might go up 5 to 10% in all dimensions.

The only way to drastically reduce the weight of a spar is to go carbon but
brace yourself. After I got a price for composite spars at the Annapolis
show I went directly to Pussers for a #4 painkiller. :-)

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com



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.