"Roger Long" wrote in message
...
What I really meant was: Insufficient damping combined with
first or second order correspondence of excitation cycle with
natural pitching period. The significance of Radius of Gyration
and rotational inertia may be greater in terms of resonant carry
over of motion than gross value and counter intuitive results
may be experienced when adjusting weight distribution.

Translation?

The natural bobbing frequency of the boat is the same as (or a
multiple of) the frequency with which waves strike.

Answers?

1. Damp the boat's bobbing - make the forward pitch a different
stiffness from the rearward pitch. Then it won't bob so much.
Since that means changing the boat's lines, is a bit difficult to
do under way . . .

2. Change the bobbing frequency - shift weight from the centre to
the ends to slow it, shift weight from the ends to the centre to
speed up the bobbing. Leads to a lot of moaning from the boat's
crew, who specially don't like being dunked at the bow. And
hauling a couple of them up the mast, though more effective, isn't
popular either if you're going to windward - it's that lost
stiffness. And sometimes this doesn't work anyway (Roger's
'counterintuitive' thesis) if you try to slow the bobbing
frequency.

3. Change the wave frequency - alter course. Mind you, it might
take longer to get where you want to go, but just occasionally the
extra speed gained my cancel the penalty of shifting off course.
Do the sums . . .
--
JimB
http://www.jimbaerselman.f2s.com/
Comparing cruise areas within Greece and N Spain

JimB wrote:

1. Damp the boat's bobbing - make the forward pitch a different
stiffness from the rearward pitch.

I used to be a great believer in asymmetry until sailing my present boat
which is fairly symmetrical and has a surprising full entrance for a 1970's
design. She has a cruising weight mast, a large anchor hung out over the
bow, water tank under the V berth, batteries and lots of gear aft,
everything that urban legend says should promote wicked pitching. She has
about the nicest motion I've encountered in a sailboat.

It used to be thought that spreading weight out into the ends made for a
more comfortable boat. This was back when boats tended to be more
symmetrical. Now, everyone "knows" that moving weight from the ends to the
middle reduces pitching. The shape of the typical boat has changed but so
may have the conventional wisdom to some extent. Does the boat really become
more comfortable after you have spent a long, hot, afternoon moving all that
heavy stuff from the ends to under the midship berths? Send three people up
to the bow sometime when beating and compare the results immediately.

The physics of pitching are pretty much the same as rolling, a subject I've
pondered enough to have produced some powerboats that are considered
remarkable for their comfort. Like props, it's a complex subject and the
common rules of thumb, 3 blades better, move weight to the middle, are often
correct for a narrow range of typical boats but wrong as general principles.


--
Roger Long

On Sat, 16 Dec 2006 14:00:23 GMT, "Roger Long"
wrote:

It used to be thought that spreading weight out into the ends made for a
more comfortable boat. This was back when boats tended to be more
symmetrical. Now, everyone "knows" that moving weight from the ends to the
middle reduces pitching.

I think there are really two issues related to pitching; comfort and
speed. Putting weight in the ends will slow the pitching moment, and
that tends to be more comfortable. Removing weight from the ends will
allow the boat to pitch at a faster rate, and that tends to be faster
because it keeps the bow from digging into the waves and increasing
drag. Light weight ends also dampen pitching motions more quickly
because there is less angular momentum to deal with.

I can tell you from personal experience that moving fast through big
waves on a lightweight boat with a fast pitching motion is not very
comfortable even though it gives you considerably more speed.

"Roger Long" wrote in message
...
JimB wrote:

1. Damp the boat's bobbing - make the forward pitch a different
stiffness from the rearward pitch.

I used to be a great believer in asymmetry until sailing my present boat
which is fairly symmetrical and has a surprising full entrance for a
1970's design. She has a cruising weight mast, a large anchor hung out
over the bow, water tank under the V berth, batteries and lots of gear
aft, everything that urban legend says should promote wicked pitching. She
has about the nicest motion I've encountered in a sailboat.

It used to be thought that spreading weight out into the ends made for a
more comfortable boat. This was back when boats tended to be more
symmetrical. Now, everyone "knows" that moving weight from the ends to the
middle reduces pitching. The shape of the typical boat has changed but so
may have the conventional wisdom to some extent. Does the boat really
become more comfortable after you have spent a long, hot, afternoon moving
all that heavy stuff from the ends to under the midship berths? Send three
people up to the bow sometime when beating and compare the results
immediately.

The physics of pitching are pretty much the same as rolling, a subject
I've pondered enough to have produced some powerboats that are considered
remarkable for their comfort. Like props, it's a complex subject and the
common rules of thumb, 3 blades better, move weight to the middle, are
often correct for a narrow range of typical boats but wrong as general
principles.


--
Roger Long


I've sailed a Yamaha 30 for a number of years. One of the things that's
interesting about this boat is the old one-lung diesel and fuel tank are
under the v-berth. I don't find it to hobby much at all compared with
similar size boats.

--
"j" ganz @@
www.sailnow.com

"Roger Long" wrote in message
...

I used to be a great believer in asymmetry until sailing my
present boat which is fairly symmetrical and has a surprising
full entrance for a 1970's design. She has a cruising weight
mast, a large anchor hung out over the bow, water tank under the
V berth, batteries and lots of gear aft, everything that urban
legend says should promote wicked pitching. She has about the
nicest motion I've encountered in a sailboat.

Sounds like a lovely old 8m I used to sail. Great long overhangs,
lots of weight - not only in the ends, but all over. Heavy long
keel, tall mast, very slim hull. Went like a demon to windward,
but modern racing handicaps put the end to that type of design.

Hobbyhorsing? That boat hadn't heard of it. For all her overhangs,
her pitching inertia was so great that she just creamed along dead
level (well, relatively) through all seaways. Very comfortable.
Except for one thing.

She had little freeboard, and once the waves got above 1.5m or so,
they just rolled along the deck. With 2m waves, she made like a
submarine half the time. So, there's comfort, and comfort. Motion
versus soaking.
--
JimB
http://www.jimbaerselman.f2s.com/
Comparing cruise areas within Greece and N Spain