On Sun, 22 Feb 2009 07:41:11 -0500, "Roger Long"
wrote:
3.2 seconds is a remarkably short roll. Now that I've seen your boat on the
RBCprofiles page, I can believe it though. Back when we were talking about
this before I was thinking traditional trawler, deep and fairly round. A
roll period under 5 - 6 seconds on a boat like that would be unlivable.
The bootstrap for the research vessel design phase of my career was
designing this 47 foot boat which has a roll period of about 4 seconds:
http://www.sml.cornell.edu/sml_welcomekingsbury.html
Her comfort and seakeeping became legendary. On the delivery trip, we went
across Buzzards Bay in a typical southwester with the seas just about on the
beam and I was walking around the deck with my hands in my pockets. They
take large deck loads of people out to the lab in 8 foot seas and she
navigates the mouth of the Portsmouth NH harbor which can be a pretty nasty
area when the tide is running against the seas.
Roll period varies with GM. The angle of the roll changes with damping
which is stuff like bilge keels and chines that resist the transverse flow
of water around the hull. The acceleration you experince in rolling is a
function of the roll period and the angle of roll. You can live with faster
rolling if you can reduce the angle and, thus, the resulting G forces.
Your boat must have a fairly high GM and a very healthy amount of stability
for the roll to be so short. Since she isn't ballasted, the only way to get
that GM is with a fairly wide shallow hull. She may be a "trawler" above
the waterline but she isn't below. The chines and fairly flat hull sections
that would produce that GM have a lot of inherrent roll damping. Add the
large keel area typical of trawler types and you have a hull that can be
comfortable with a short roll period.
The key to short roll periods is tuning. A boat rolls most heavily when its
natural roll period matches the period of the waves and there is a
relationship between wavelength and period. You may have noticed that your
boat is quite lively in very small waves. I've heard people say on boats
I've designed, when seeing them bouncing around in tiny harbor waves, "Wow.
This boat must roll her guts out in big waves." However, when the waves get
bigger, the rolling period and wave period no longer match, the hull damping
takes over, and the boat is quite comfortable.
The hull geometry that produces high GM and large damping also raises the
center about which the boat rolls. With the modest freeboard and reasonable
superstructure your boat has, this puts the rolling center closer to your
feet. The result is the deck changing angle beneath you more and less side
to side motion.
I usually try for a 4 - 4.5 second rolling period but am also designing
metal boats with large deckloads of equipment. All in all, it sounds like
the Grand Banks is a very intelligent design although I wouldn't think of
her as a "trawler" from the hull designer's perspective.
Thanks for that analysis, interesting. Supposedly the boat does
have some ballast in the keel, not sure how much, and all machinery
with the exception of ground tackle is located low down. It also has
Naiad dynamic stabilizers which make a huge difference underway.
Rolling is mostly an issue at anchor where there is exposure to an
incoming swell, like Bar Harbor at high tide. We once spent a month
there on the town moorings. Between the incoming swell, the lobster
boats coming and going, and the Big Cat high speed ferry, we got
rolled a lot and started thinking about ways to slow it down, hence
the interest in an anti-roll tank which we would fill only when
needed. It would be fairly easy to put one on the flybridge which is
about 12 feet wide and it could easily hold an additional 1200 lbs of
weight since we sometimes have as many as 10 people up there.