## rig effect on pitching

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• The moment of inertia of a mast, plus the damping effect of the sail moving through the air, makes the motion of a sailboat gentler than that of a motorboat in
Message 1 of 6 , Jun 1, 2003
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The moment of inertia of a mast, plus the damping effect of the sail
moving through the air, makes the motion of a sailboat gentler than
that of a motorboat in the same conditions.

characteristics of a boat to fit its home water conditions, and it
occured to me that you could change the pitch (and roll, for that
matter) frequency of a gaff-rigged boat by raising or lowering the
gaff, in other words, by reefing.

The effect would work with any rig that has a spar aloft, for
instance, a lug, and the heavier the spar, the greater the effect.

In addition, if you have a boat that has an unpleasant motion, it
should be possible to slow its pitch rate (and possibly decouple a
harmonic pair, two parts of the boat with the same frequency) by
mounting a longer mast, or switching to a spar-aloft sail type.

One way to experiment before taking a big step would be to raise a
weight up the mast while the boat is pitching upleasantly, to see
what effect that has on the motion. If raising the weight has a good
effect, then changes would be indicated.
• What of the relationship of the ocilations of the mast in relation to the rondness of the bottom and rolling and heaving, and the resultant harmonic rhythm,
Message 2 of 6 , Jun 1, 2003
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What of the relationship of the ocilations of the mast in relation to
the rondness of the bottom and rolling and heaving, and the
resultant harmonic rhythm, leading to the disipation of energy ceated
by the interaction of the two forces? Jake
• ... to ... ceated ... Mmmm...the rounder the bottom, the less it will damp a roll...the mast is best considered as one of a set of pendulums making up the
Message 3 of 6 , Jun 1, 2003
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--- In boatdesign@yahoogroups.com, "jakeman19652002"
<jakeman19652002@y...> wrote:
> What of the relationship of the ocilations of the mast in relation
to
> the rondness of the bottom and rolling and heaving, and the
> resultant harmonic rhythm, leading to the disipation of energy
ceated
> by the interaction of the two forces? Jake

Mmmm...the rounder the bottom, the less it will damp a roll...the
mast is best considered as one of a set of pendulums making up the
whole boat. It doesn't actually create energy, it will simply
reinforce a motion (such as that from a swell) that matches its
period. If two or more of the various parts of the boat have the
same harmonic period, when you hit a swell with that magic period,
you are in for a rough ride.

Part of the design process is making sure you don't end up with those
inconvenient harmonic matches. They can make any sort of structure
shake itself to pieces.
• Yes, but what about the masts gyration in an ocean of motion. J
Message 4 of 6 , Jun 2, 2003
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Yes, but what about the masts gyration in an ocean of motion. J
• Jake wrote: What of the relationship of the ocilations of the mast in relation to the rondness of the bottom and rolling and heaving, and the resultant
Message 5 of 6 , Jun 2, 2003
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Jake wrote:

What of the relationship of the ocilations of the mast in relation to
the rondness of the bottom and rolling and heaving, and the
resultant harmonic rhythm, leading to the disipation of energy ceated
by the interaction of the two forces? Jake

Rolling and heaving and oscillations, I think I am starting to feel a little squeamish.
The United States Yacht Racing Union did a report called Safety From Capsizing. I have an interim report from June 1984. In the report they talk about all the aspects of roll moment of inertia. It is far too long to detail here.They also provide a computer programme for making calculations titled gyradius program for MHS Data, and a programme for IOR data. I think you need to be a DOS type programmer to be able to enter this data. But that was 1984, maybe they have something newer. A quote from one paragraph says The rig, in a typical boat, makes up some 60 to 70 percent of the total moment of inertia, and deserves the closest of attention, firstly because small changes in weight up in the rig make large changes in inertia; etcetera
BC Mike C

[Non-text portions of this message have been removed]
• ... Makes sense because (if I recall correctly) R^2 M is the general formula for moment of inertia, and the length of the rig means it has the greatest R on
Message 6 of 6 , Jun 3, 2003
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--- In boatdesign@yahoogroups.com, "Michael Casling" <caslingm@s...>
wrote:
>A quote from one paragraph says The rig, in a typical boat, makes up
>some 60 to 70 percent of the total moment of inertia, and deserves
>the closest of attention, firstly because small changes in weight up
>in the rig make large changes in inertia; etcetera

Makes sense because (if I recall correctly) R^2 M is the general
formula for moment of inertia, and the length of the rig means it has
the greatest R on the boat.
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