BTW, Bilge Keels are great additions .... funny part is, that their best
reduction in rolling ( note I said "rolling" ) is a mere 10%.

The way I understand it, bilge keels -increase- the rolling period (the time
from heeling on one side to the other), and I'd guess that 10% is about
right. I've never worked (or sailed for that matter) on a cruise ship
(well... not entirely true... I was a cadet aboard a retired US Lines ship),
but I understand their "active stabilizers" do a better job at keeping the
vessel flat.

Incidently (and we're way off topic), the most comfortable riding ship I
ever worked on was the RV Kilo Moana, a SWATH (Submerged Waterplane Attached
Twin Hull). You could literally leave your cup of coffee on a table in 20
foot seas and it wouldn't budge. SWATH's are actually -more- comfortable
with seas on the beam, we frequently lied abeam to do scientific work.

As for blowing out windows/portholes.... heading is immaterial, vessel
size is immaterial.... the right sea at the right moment, now, THAT'S
important.

Good point. Mother Nature is rarely nice enough to give us seas from
only -one- direction. Depending on your vessel's heading, your wake can
reflect or refract off the side of your hull, interact with a sea and send
it in nearly any direction at all.

On the ships I've served on, all portholes up to the main deck had
"deadlights", heavy, solid metal covers for when the weather gets nasty.
When it got nasty, we just dogged them down. Never had one fail (not to say
it isn't possible though).

But things are probably different on the modern cruise ships. I've berthed
near a number of cruise ships, and one I remember in particular had "sliding
glass doors" to cabins above the main deck! You wonder what the designer
was thinking...


All the best,

--
Paul

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renewontime dot com wrote:

a SWATH (Submerged Waterplane Attached
Twin Hull).

SWATH: SMALL Waterplane AREA Twin Hull

Rick

renewontime dot com wrote:
BTW, Bilge Keels are great additions .... funny part is, that their best
reduction in rolling ( note I said "rolling" ) is a mere 10%.


The way I understand it, bilge keels -increase- the rolling period (the time
from heeling on one side to the other), and I'd guess that 10% is about
right. I've never worked (or sailed for that matter) on a cruise ship
(well... not entirely true... I was a cadet aboard a retired US Lines ship),
but I understand their "active stabilizers" do a better job at keeping the
vessel flat.

Bilge keels have no effect on roll period, only rolling. Roll period is
determined by GM. The higher the GM, the shorter the roll period.
Active (fin) and passive (Flume) have a far greater ability to reduce
rolling. G probably the best system includes all three.


Incidently (and we're way off topic), the most comfortable riding ship I
ever worked on was the RV Kilo Moana, a SWATH (Submerged Waterplane Attached
Twin Hull). You could literally leave your cup of coffee on a table in 20
foot seas and it wouldn't budge. SWATH's are actually -more- comfortable
with seas on the beam, we frequently lied abeam to do scientific work.

For most conditions, these ARE considered about the most comfortable
surface types, though reports I've heard say the can be pretty wet in
beam seas and quite noisy in head seas.



As for blowing out windows/portholes.... heading is immaterial, vessel
size is immaterial.... the right sea at the right moment, now, THAT'S
important.


Good point. Mother Nature is rarely nice enough to give us seas from
only -one- direction. Depending on your vessel's heading, your wake can
reflect or refract off the side of your hull, interact with a sea and send
it in nearly any direction at all.

On the ships I've served on, all portholes up to the main deck had
"deadlights", heavy, solid metal covers for when the weather gets nasty.
When it got nasty, we just dogged them down. Never had one fail (not to say
it isn't possible though).

On the widely viewed picture of that Sea River (x-ATTRANSCO) tanker in
rough seas, the porthole which was taken out, was on the Boat Deck.
Having taken that ship through similar conditions, it was either bad
luck or age, G which caused that blow out. I always tell people to
look at the foremast on the foc'sle and the deck lights at the top of
that mast. On at least one of those ships (class) you'll find the
brackets which hold those lights, bent up, from seas coming aboard the bow.


But things are probably different on the modern cruise ships. I've berthed
near a number of cruise ships, and one I remember in particular had "sliding
glass doors" to cabins above the main deck! You wonder what the designer
was thinking...

G Aside from the fact of the type of glass used and
construction/location above the water, they probably considered that a
cruise ship will normally do everything it can to stay clear of most
serious weather.


otn

You fella's are both quite right in your corrections. My appologies.
Sometimes the connection between what my brain is thinking and what my hands
are typing is something less than 100% at 0230 in the morning. Nice to know
we have such a knowledgeable group here to keep things straight... ;-)

As for how a SWATH handles seas on the beam, bow and quarter: the only
SWATH I've sailed on was the KM, so my opinions are based on her. I'm
guessing she was designed for seas up to about 20 - 25 feet, as that's about
how high the bottom of the main superstructure is above the WL. One might
expect beam seas to be a problem, but in reality in that range they weren't
a problem, and when they were on the bow there was some pitching, but no
much. The only time I could feel any real motion (still nothing compared to
a conventional mono hulled ship) was underway with the seas on a quarter,
which gave the motion a wierd "cork screw" kind of feeling too it. Still
nothing compared to any other ship I've been on...

When the seas got bigger than that, it was a bit spooky, as the seas would
pound the unprotected superstructure (in our case, the mess area), and
continue pounding under the superstructure until they passed underneath.

Overall though, SWATH's make an ideal research platform as they are very
stable and maneuverable while lying a hull or station keeping. They do have
their weaknesses (most troublesome was her extreme sensitivity to any shift
or change in our carried load) but otherwise a very nice ride.

--
Paul

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