T&T: Catamaran vs monohull (long rant)

[Faure, Marin]

>I was out again in my Mainship this afternoon, and even in relatively
small waves, the Mainship is rocking and rolling (no problem heading
into the waves) with a following sea being particularly bad, and a beam
sea not very comfortable at all....Now maybe the question should be, "is
the Mainship (34' 1979) just not particularly stable/seaworthy?" or "is
the cat way more stable?"

I think you will find that your boat's behavior is typical for most
monohulls with a semi-planing (aka semi-displacement) hull.  Any boat of
this type with a wide, relatively flat transom will be pushed around by
following seas and particularly if the speeds are low, you will be
pretty busy at the wheel (or your autopilot will be if you have one).

As to rolling, I don't know what you consider excessive rocking and
rolling, but our 26,000 pound GB36 will most certainly roll in a beam
sea, even if the waves are only a couple of feet.  The main difference
between a semi-planing hull (GB, etc.) and a full-displacement hull
(Willard, etc.) is that the rounded chines and bilge of the
full-displacement boat will result in a gentler rolling motion.  The
boat will likely roll farther, however.  The hard-chine GB (and
Mainship, etc.) tends to have a shorter roll, but at the end of the roll
there is a more sudden "snap back" to the other direction because of the
hard chine and flatter aft hull sections offering more resistance to the
water.  Some people prefer the gentler but farther rolling motion of a
full-displacement hull, others prefer the shorter roll but more abrupt
"snap-back" of the semi-planing hull.  Both can be reduced considerably
by the use of active or passive stabilizing systems.

Where the rolling of any boat can get unpleasant and even dangerous is
if the waves are close together.  This is typical of windy days in the
PNW where the inside waters are not subjected to swells, but the waves
can get up to five or six feet in high winds and be very steep and very
close together.  In a beam sea, the wave frequency can get awfully close
to a boat's rolling frequency, and if the seas and the boat's center of
gravity are high enough, the boat could go over (it's happened often
enough out here to be something to be mindful of).

I have heard catamaran proponents say that in beam seas, particularly
the very closely spaced waves we get in this area, a catamaran tends to
"step over" the waves.  So one hull goes up and over but the boat is
stabilized by the other hull, which goes up and over while being
stabilized by the first hull.  So while the boat rolls a bit, it's
apparently not nearly the same degree of movement one gets with a
monohull.   I would also think that in a following sea, since the cat
has less exposed stern surface to be shoved around by the waves,
maintaining directional control should require less effort.

But I would be very surprised if your own boat is doing anything
different than what other monohulls with similar hull configurations as
your boat do under the same conditions.

______________________________
C. Marin Faure
GB36-403 "La Perouse"
Bellingham, Washington