Conference unifix::sailing

    Hobby horsing is made worse by weight in the ends and weight aloft.
    Sounds like you can't do a whole lot about it.  The mast is probably
    very heavy and it probably has furniture inside even to the overhangs.
    The bow sprit won't help matters either.
    
    By the way, except in light air with leftover seas, the way a boat
    handles on the mooring is no indication of its sailing characteristics.
    Fat Tuesday used to roll like a bitch on the mooring, but never
    while sailing.
    
    Dave

A mechanical system (eg, a boat) tends to oscillate when acted upon by a 
periodic external force (eg, passing waves). The magnitude of the 
oscillation depends on the amount of damping or friction in the system 
and on how near the frequency of the external force is to the resonant 
frequency of the mechanical system. In the absence of damping or 
friction, the magnitude of the oscillation will become infinitely large 
when the frequency of the external force equals the resonant frequency 
of the system.

For boats the explanation I have heard is roughly this: The pitch
damping of a sailboat hull does not particularly depend on the keel type
or ballast ratio. Rather, it depends on the waterline hull shape at the
bow and stern. When a wave strikes the relatively narrow bow of a boat,
a certain volume of the hull at the bow is immersed in the wave. When
the wave reaches the stern, an equal volume of hull will be immersed.
When the hull is wider aft than forward, the hull needs to rise or fall
less far to immerse the same volume. Since it is harder to immerse a
broad object than a narrow one (the energy from the wave is quickly 
dissipated), repetitive pitching (hobby horsing) doesn't occur. Hulls
with similar waterline shapes fore and aft will be more prone to hobby
horsing. A lot of weight in the bow and stern (ground tackle, fuel
tank(s), etc, tends to increase pitching. 

An example: The shapes of the Valiant 32 and 40 hulls are quite similar 
when viewed from the bow and side. However, the stern of the 32 is much 
fuller (relatively much more volume in the stern) to reduce pitching. 

The effect of resonance can be easily observed. For most boats there is 
a wavelength that, when sailing to weather, will cause the boat to pitch 
badly. A small change in course (which changes the wavelength seen by the 
hull) sees) will often reduce the pitching substantially. 

re .1:

On the other hand, a heavy mast provides significant roll damping and 
reduces the probability of capsize. I recall Judy Lawson complaining 
about the horrible rolling of her boat after the mast came down during 
an OSTAR race.

A lot of pitching at anchor might be an uncomfortable problem if you
spend much time aboard while anchored or moored.

    
    If you're spending the night on board and the pitching is bothersome,
    try using "mexican hats" at the bow and stern.  These are plastic
    disks suspended from the hull and held underwater by sash weights.
    They're supposed to dampen pitching or rolling.  Never having used
    them, I have no idea of their efficacy.  
    
    As Alan says, just about all boats will pitch if the wavelength
    is right.  To pitch or not to pitch is not the problem.  Where you
    pitch can be a problem.  High freeboard, flat hulled, light-in-the-ends
    boats do their pitching on top of the waves.  Low freeboard, sharp
    hulled, heavy ended boats do it IN the waves.  That can make for
    a very wet ride.  If you're in the latter category the only cures
    are a spray dodger and dramamine.
    
    - gene

RE: .2,

I agree with Alan's explanation, with a couple exceptions:

1)  I don't believe similar waterline shapes fore and aft are required to
    produce excessive pitching.  Less volume at either end will make the
    situation worse, as per the following comment on clipper bows. 

    A clipper bow is partly to blame.  They're quite pretty, but require
    fewer pounds-per-inch immersion than other types.  As a result, a given 
    force will tend to push the bow in further (or let a wave ride higher) 
    than on an otherwise similar hull with spoon bow, for instance.

2)  The Valiant sterns, I believe, are designed to be broad not so much to 
    reduce pitching per se, but rather to keep the stern from squatting when 
    approaching (or trying to exceed) hull speed.  Double-ended boats are 
    known for the fact they do take stern seas well, but squat when they 
    approach being overpowered.  (I've watched a 30' dbl-ended pulling 
    boat swamp while being towed in calm seas at 12 kts; the stern was 
    sucked right down into the trough of the wave it was making, and the 
    crest of the stern wave came right over the stern.)  

J.

re .4:

When I mentioned that the stern of the Valiant 32 is fuller to reduce 
pitching compared to the Valiant 40 I was paraphrasing a comment by
Robert Perry, the designer of the Valiants. But you're right, John, the
sterns of double enders do need to be full to have sufficient buoyancy
aft. The aft overhang of conventional transom designs provides
significant additional buoyancy as the stern immerses. 

As I understand pitching (an imperfect understanding at best), unequal 
hull volumes fore and aft are required to provide pitch damping. To be 
sure, certain hull shapes, regardless of anything else, do pitch more 
than others. But the damping of pitching is not everything. The 
magnitude of the pitch amplitude is also important. A flat hull (eg, a 
barge) might oscillate (pitch) for a long time after the external force 
(eg, a wake) stops but the amplitude (amount of up and down) might be
small. On the other hand, a boat with a narrow bow and an broad stern
might pitch only once with large amplitude (ie, very good damping). From 
the viewpoint of the base note, I think that the question is why is the 
amplitude of the pitch so great. But amplitude and damping are related. 
The greater the damping, the less the amplitude under all conditions and 
especially near resonance. It sounds like the boat described in the base 
note has insufficient damping and a resonant frequency near that of the 
usual waves in the harbor (not a good combination). 

It is interesting to note that generalizations here, as elsewhere, are
not always correct in specific cases. Our Valiant 32 has a heavy anchor
windlass on the foredeck and 90 gallons of water and some 250' of anchor
chain under the V-berths. So you might suspect that the boat would pitch
a lot. Actually, it doesn't (except in a short, steep sea with light
winds). I suspect that the full stern and flare at the bow are
responsible for this nice behavior. 

re .1 again:

One of the reasons Fat Tuesday rolled when moored and not when sailing 
is that sails are extremely effective roll dampers -- its hard to 
push a sail sideways -- when sailing close hauled. Going downwind there 
is a mechanism where the forces on the sails can actually increase 
rolling to the point of a knockdown within one or two rolls (see 
Marchaj's book Seaworthiness for the explanation) -- the classic 
knockdown to weather under spinnaker. 

    Fine.I feel a little better now.After all that input though I think
    that I should add some other details and observations that could make
    some sense now.Could this pitching be just a pivoting?
    Let's see, amidships or near it we have the 5500 balast 100 gl.fresh
    water and another 100 gl. fuel.(all 4 tanks were full)most of the
    furniture is amidships and the engine is located 8' from the transom.
    The bow has only a couple of anchors and 300-400 1/2" rode and nothing
    under the V berth.The fat a** transom has a lot of volume but hardly
    much weight.From the companionway all the way to the end is a walk-in
    (stoop-in) engine room with enough waisted room that you can easily
    smuggle 6-8 illegals.
    re.4:
    Looking from ashore, when the hobby horsing is in effect,out of abouut
    100 boats mine hobbies the most,second comes a Cabo Rico 38 ...clipper
    bow et.al

    
    My Etchells has a stilleto bow and a broad stern.  The word that
    best describes its pitching motion in a heavy sea is BRUTAL.  Think
    of a 20' x 7' x 3' box with a 10' dunce cap attached to one end.
    That's a rough approximation of the hull shape.  The box part of
    the hull floats on top of the waves.  The dunce cap part (the bow)
    just gets whipped around.  When a wave hits, the bow slices right
    into it.  After the crest has moved along the hull about 10 feet
    it encounters enough hull volume to start lifting the boat up. 
    At this point the bow is whipped up towards the sky as the stern
    sinks into the trough preceeding the wave.  Once the crest reaches
    the stern, that end goes up whipping the bow down into the following
    trough with a resounding BOOM.  As the cycle start again some of
    the water displaced by the bow rolls down the deck and into the
    faces of the crew, usually accompanied by comments suited to the
    occassion.  
    
    If asymmetric ends affect pitching at all, they'll accentuate it.
    When Touche's stern is pushed up by a wave crest, the bow wants
    to go down and there not enough floatation there to stop it.  On
    a boat of comparable length with two full ends, the bouyancy of
    the bow and stern will allow the hull to bridge the wave trough
    to a degree with a reduction in the amplitude of the motion.  
    
    To promote the E-22 world championships in 1981 the fleet officers
    made up posters showing an Etchells riding up the crest of a big
    wave.  It looked so spectacular I thought it was a fake photo. 
    After I bought my boat I found out how easy it is to do wheelies
    in it.
    
    - gene

RE: .6,

Hmmm...I can imagine how you may feel somewhat frustrated.  Were I in your 
shoes, I'd certainly not contemplate changing the hull shape at this point 
;-), but if Alan's observation that your boat's resonance seems to be 
matched by the prevailing wavelengths in your anchorage at your mooring, a 
couple strategies suggest themselves:

1)  Look closely at the wave patterns, heights and intervals in your 
    anchorage -- is there a possibility that you could move to another area 
    where the waves are different enough to not match your boat's resonant 
    pitching frequency so closely?

2)  Since your weight seems to be quite concentrated amidships, I'd
    definitely try placing as much of it as I could out in the end of the boat 
    for an experiment.  Once the easy stuff like anchors and stores are out 
    there (temporarily at first, of course), you could also try putting a few 
    hundred gallons of water in jugs in the ends, balancing them to keep the 
    waterline about level, of course.  That might just change your pitching 
    frequency to one not so sympathetic with the harbor's.  If so, you could 
    then stategize on how to make more permanent re-placement of these items.

J.