"IanM" wrote

You *are* missed.

Well, thank you. OK, I'll throw something into the pot. Here's a first
draft of something I may come back to later. A discussion along the same
lines got started here quite a while ago and disolved into the usual flame
war. I brought it up over at the high class bar where I hang out now and
another RBC refugee complained that I never finished my explanation here and
was I going to leave them hanging over there as well? It's winter and there
are only so many hours a day I can work on my boat so I whipped this up:

http://www.rogerlongboats.com/Stability.htm

Sorry, Virginia, buoyancy is imaginary.

--
Roger Long

On Sat, 21 Feb 2009 17:15:04 -0500, "Roger Long"
wrote:

"IanM" wrote

You *are* missed.

Well, thank you. OK, I'll throw something into the pot. Here's a first
draft of something I may come back to later. A discussion along the same
lines got started here quite a while ago and disolved into the usual flame
war. I brought it up over at the high class bar where I hang out now and
another RBC refugee complained that I never finished my explanation here and
was I going to leave them hanging over there as well? It's winter and there
are only so many hours a day I can work on my boat so I whipped this up:

http://www.rogerlongboats.com/Stability.htm

Sorry, Virginia, buoyancy is imaginary.

Roger, welcome back, your expertise is missed.

We once had a discussion about anti-roll tanks and how to design them.
Some where in that discussion or in a stability discussion, the
subject of roll period came up, ie, the time for a complete roll, over
and back. My recollection is that I estimated the roll period for
our GB49 trawler to be about 3 seconds, and that some discussion
ensued about whether or not that was impossibly fast. I have since
then taken more recent measurements and come up with a more accurate
figure by averaging a number of rolls. It came out to about 3.2
seconds, quite close to my original estimate. Perhaps you could
comment on the implications of that as it relates to stability.

The GB49 is a ballasted, full length keel, semi-displacement trawler
with hard chines, a displacement of about 70,000 lbs and a draft of 5
1/2 ft.

"Wayne.B" wrote
I have since then taken more recent measurements and come up with a more
accurate
figure by averaging a number of rolls. It came out to about 3.2
seconds, quite close to my original estimate.

Is that a complete roll, all the way from one side, over to the other, and
then back again to the starting point?

--
Roger Long

On Sat, 21 Feb 2009 21:05:59 -0500, "Roger Long"
wrote:

"Wayne.B" wrote
I have since then taken more recent measurements and come up with a more
accurate
figure by averaging a number of rolls. It came out to about 3.2
seconds, quite close to my original estimate.

Is that a complete roll, all the way from one side, over to the other, and
then back again to the starting point?

Yes, one complete roll.

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.

--
Roger Long

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.

On Sat, 21 Feb 2009 17:15:04 -0500, "Roger Long"
wrote:

"IanM" wrote

You *are* missed.

Well, thank you. OK, I'll throw something into the pot. Here's a first
draft of something I may come back to later. A discussion along the same
lines got started here quite a while ago and disolved into the usual flame
war. I brought it up over at the high class bar where I hang out now and
another RBC refugee complained that I never finished my explanation here and
was I going to leave them hanging over there as well? It's winter and there
are only so many hours a day I can work on my boat so I whipped this up:

http://www.rogerlongboats.com/Stability.htm

Sorry, Virginia, buoyancy is imaginary.

Rather a long drawn out exercise in semantics, isn't it? All to prove
that, a beautiful, buoyant ball bounding over the wine-dark sea,
isn't.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)

On Sun, 22 Feb 2009 08:19:02 +0700, Bruce in Bangkok
wrote:

On Sat, 21 Feb 2009 17:15:04 -0500, "Roger Long"
wrote:

"IanM" wrote

You *are* missed.

Well, thank you. OK, I'll throw something into the pot. Here's a first
draft of something I may come back to later. A discussion along the same
lines got started here quite a while ago and disolved into the usual flame
war. I brought it up over at the high class bar where I hang out now and
another RBC refugee complained that I never finished my explanation here and
was I going to leave them hanging over there as well? It's winter and there
are only so many hours a day I can work on my boat so I whipped this up:

http://www.rogerlongboats.com/Stability.htm

Sorry, Virginia, buoyancy is imaginary.

Rather a long drawn out exercise in semantics, isn't it? All to prove
that, a beautiful, buoyant ball bounding over the wine-dark sea,
isn't.

For me it's more than that. The best explanation of why a boat floats
that I've seen, and a straightforward explanation of what affects boat
stability. Of course I'm not an engineer, and do get a bit befuddled
when the going gets a bit heavy.
I did glean that Wayne's boat rolls quickly due to a lengthy GM, and
that it's not necessarily a bad thing.
Have to go back and reread to figure out what the hell GM is.

--Vic

On Sat, 21 Feb 2009 20:23:27 -0600, Vic Smith
wrote:

On Sun, 22 Feb 2009 08:19:02 +0700, Bruce in Bangkok
wrote:

On Sat, 21 Feb 2009 17:15:04 -0500, "Roger Long"
wrote:

"IanM" wrote

You *are* missed.

Well, thank you. OK, I'll throw something into the pot. Here's a first
draft of something I may come back to later. A discussion along the same
lines got started here quite a while ago and disolved into the usual flame
war. I brought it up over at the high class bar where I hang out now and
another RBC refugee complained that I never finished my explanation here and
was I going to leave them hanging over there as well? It's winter and there
are only so many hours a day I can work on my boat so I whipped this up:

http://www.rogerlongboats.com/Stability.htm

Sorry, Virginia, buoyancy is imaginary.

Rather a long drawn out exercise in semantics, isn't it? All to prove
that, a beautiful, buoyant ball bounding over the wine-dark sea,
isn't.

For me it's more than that. The best explanation of why a boat floats
that I've seen, and a straightforward explanation of what affects boat
stability. Of course I'm not an engineer, and do get a bit befuddled
when the going gets a bit heavy.
I did glean that Wayne's boat rolls quickly due to a lengthy GM, and
that it's not necessarily a bad thing.
Have to go back and reread to figure out what the hell GM is.

--Vic

Given that the dictionary gives one definition of buoyancy as:

2. (Physics) The upward pressure exerted upon a floating body
by a fluid, which is equal to the weight of the body;
hence, also, the weight of a floating body, as measured by
the volume of fluid displaced.
[1913 Webster]

It seems that the writer has gone the long way round to prove an
already accepted meaning for the word.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)

Roger Long wrote:
.... Here's a first
draft of something I may come back to later. A discussion along the same
lines got started here quite a while ago and dissolved into the usual flame
war. I brought it up over at the high class bar where I hang out now and
another RBC refugee complained that I never finished my explanation here and
was I going to leave them hanging over there as well? It's winter and there
are only so many hours a day I can work on my boat so I whipped this up:

http://www.rogerlongboats.com/Stability.htm

Sorry, Virginia, buoyancy is imaginary.

--
Roger Long



I enjoyed that piece. You probably make too much of your
"buoyancy is imaginary" thesis, but you use the same criteria for
buoyancy as everyone else, so what harm is that?
I enjoyed the railing against the Coastguard.
That's what it is to be American, after all.
A healthy disrespect for the folks who try to keep us safe(?)

:-)

Regards

BrianW