What causes some sailboats to hobby horse? Long overhangs? Bow
design? Stern design?
G

Gordon wrote:
What causes some sailboats to hobby horse? Long overhangs? Bow
design? Stern design?
G

Waves.

--
Roger Long

"Roger Long" wrote in news:EWBgh.38775$zB4.32537
@twister.nyroc.rr.com:

Waves.

--
Roger Long



Don'tcha just love it when a marine architect goes off on this long
technical explanation with all these drawings and huge mathematical models
to depict why a hull behaves the way it does?.....(c;


Larry

For this we needed 8 years of university education??...hee hee

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.

--
Roger Long

"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

Gordon wrote:
What causes some sailboats to hobby horse? Long overhangs? Bow design?
Stern design?
G

I'll start ...

Its a complicated question. For instance, long overhangs reduce
hobby-horsing because they add reserve buoyancy, but they increase it
because of added mass in the extremities.

One design feature that hurts is too much symmetry fore and aft. In
this case there is little change in buoyancy as the boat pitches.

If you have a boat that pitches too much, you can try reducing the
weight in the bow. This is why catamaran owners will avoid heavy
ground tackle, etc.

Mostly caused by overloading the bow and stern with excess weight.
Better to to store heavy weight near the center of rotation (in the
middle of the boat). Its a phenomenon of rotational inertia.


In article , Gordon
wrote:

What causes some sailboats to hobby horse? Long overhangs? Bow
design? Stern design?
G

Rich Hampel wrote in news:151220061439253714%RhmpL33
@nospam.net:

Better to to store heavy weight near the center of rotation (in the
middle of the boat). Its a phenomenon of rotational inertia.



Yeah, but who can stand all that wife bitching every time she has to crawl
over those big tool boxes on the main salon deck to get to the head?

Larry

Isn't she the reason they were stowed in the ends in the first place?...(c;

Here is something I want to try sometime, maybe. Maybe someone has
tried it already or knows why it will not work.

Drop a big ol Fortress anchor over the stern with about 30 feet of line.

It would be heavy enough to hang pretty near straight down, maybe? When
the stern comes up the flukes act as a break slowing the rise. When
stern goes down the flukes just sink.

I think it would change the harmonic frequency, but it may just make
things worse.

Any thoughts?

Howard

Larry wrote:
Rich Hampel wrote in news:151220061439253714%RhmpL33
@nospam.net:

Better to to store heavy weight near the center of rotation (in the
middle of the boat). Its a phenomenon of rotational inertia.



Yeah, but who can stand all that wife bitching every time she has to crawl
over those big tool boxes on the main salon deck to get to the head?

Larry

Isn't she the reason they were stowed in the ends in the first place?...(c;