"DSK" wrote in message

I'd say that either displacement or cubic capacity equals comfort.

As a rule, yes, but a hull of a given volume can reach a point beyond which
increasing displacement has a negative effect on comfort, not to mention
safety. An example of this was a Norsea 27 in which a middle age man set
out to circumnavigate some years ago. The boat has an unladen disp/length
ratio of nearly 450, and with stores, extra fuel, water, equipment, etc. the
boat was simply too heavy and performed horribly in beam seas, which rolled
her over several times before he turned tail and headed home. Displacement
seems to offset cubic capacity somewhat, making really voluminous but light
boats uncomfortable and making voluminous and heavy boats more comfortable.

Max

I'd say that either displacement or cubic capacity equals comfort.


Maxprop wrote:
As a rule, yes, but a hull of a given volume can reach a point beyond which
increasing displacement has a negative effect on comfort, not to mention
safety.

Agreed.

... An example of this was a Norsea 27 in which a middle age man set
out to circumnavigate some years ago. The boat has an unladen disp/length
ratio of nearly 450, and with stores, extra fuel, water, equipment, etc. the
boat was simply too heavy and performed horribly in beam seas, which rolled
her over several times before he turned tail and headed home.

THe problem is that he was at the margin... if you graphed motion &
stability against increasing displacement on a given hull volume, you'd
go from extremely bouncy (too bouyant) at the light extreme, to
increasingly comfortable, then back downhill again as the boat lost
responsiveness & stability (which is at least half due to bouyancy,
let's not forget), then back up again as the boat becomes a submarine...
the smoothest ride is 40+ fathoms down!


... Displacement
seems to offset cubic capacity somewhat, making really voluminous but light
boats uncomfortable and making voluminous and heavy boats more comfortable.


Yep. Everything is a trade-off. That's why life is such a good metaphor
for sailing....

Fresh Breezes-
Doug King

"DSK" wrote in message
I'd say that either displacement or cubic capacity equals comfort.

Maxprop wrote:
As a rule, yes, but a hull of a given volume can reach a point beyond
which increasing displacement has a negative effect on comfort, not to
mention safety.

Agreed.

... An example of this was a Norsea 27 in which a middle age man set out
to circumnavigate some years ago. The boat has an unladen disp/length
ratio of nearly 450, and with stores, extra fuel, water, equipment, etc.
the boat was simply too heavy and performed horribly in beam seas, which
rolled her over several times before he turned tail and headed home.

THe problem is that he was at the margin... if you graphed motion &
stability against increasing displacement on a given hull volume, you'd go
from extremely bouncy (too bouyant) at the light extreme, to increasingly
comfortable, then back downhill again as the boat lost responsiveness &
stability (which is at least half due to bouyancy, let's not forget), then
back up again as the boat becomes a submarine... the smoothest ride is 40+
fathoms down!

LOL. Beneteau has, but does not distribute, graphic studies of net
displacement vs. stability for all of its boats built after the mid-90s.
Not sure if the French gov't. requires them or what, but I think they are
all just computer models depicting the safety margins of their boats. I've
seen the ones that apply to boats in the size range that interests me, and
frankly it's a bit scary. Bottom line: don't overload a 39' Beneteau. And
I'd suspect the same admonition might apply to similar boats, such as
Jeanneaus, DuFours, Catalinas, Hunters, etc.= the plastic fantastics. What
was interesting is that there seemed to be an over-center point, beyond
which the stability of the boat fell off precipitously. It was not a smooth
curve, rather it peaked, then plunged. I'm surprised that boat builders
don't publish a *do not exceed* load figure for each boat, even if they
don't want those graphs out and about.

... Displacement seems to offset cubic capacity somewhat, making really
voluminous but light boats uncomfortable and making voluminous and heavy
boats more comfortable.


Yep. Everything is a trade-off. That's why life is such a good metaphor
for sailing....

Aw, heck, I want it all, dammit. g

Max

Maxprop wrote:
.... Beneteau has, but does not distribute, graphic studies of net
displacement vs. stability for all of its boats built after the mid-90s.

I wonder if they assume that the center of gravity stays the same?

Not sure if the French gov't. requires them or what, but I think they are
all just computer models depicting the safety margins of their boats. I've
seen the ones that apply to boats in the size range that interests me, and
frankly it's a bit scary. Bottom line: don't overload a 39' Beneteau. And
I'd suspect the same admonition might apply to similar boats, such as
Jeanneaus, DuFours, Catalinas, Hunters, etc.= the plastic fantastics.

Probably true of any boat that is shaped like a normal sailboat. The
only way to be able to keep stability constant is to load all the weight
increasingly lower down so as to maintain the relationship between the
centers of bouyancy & gravity. Actually, as the beam/disp ratio drops,
the lever arm between CG and CB needs to increase... very unlikely!

... What
was interesting is that there seemed to be an over-center point, beyond
which the stability of the boat fell off precipitously. It was not a smooth
curve, rather it peaked, then plunged.

Maybe like a sine wave, with the peak for the point at which the
gunwhale immerses?

... I'm surprised that boat builders
don't publish a *do not exceed* load figure for each boat, even if they
don't want those graphs out and about.


Ever hear of the Plimsoll marks? For commercial ships, these figures are
published and are supposed to be kept by the captain. For naval vessels,
they keep a whole book of figures on stability.


Yep. Everything is a trade-off. That's why life is such a good metaphor
for sailing....


Aw, heck, I want it all, dammit. g


If you figure out how, let me know.

Fresh Breezes- Doug king

"DSK" wrote in message

Maxprop wrote:
.... Beneteau has, but does not distribute, graphic studies of net
displacement vs. stability for all of its boats built after the mid-90s.

I wonder if they assume that the center of gravity stays the same?

No indication of CoG movement on the graphs I have, but I'd assume that it
would have to change, unless one puts all the additional weight in the
bilge.

Not sure if the French gov't. requires them or what, but I think they are
all just computer models depicting the safety margins of their boats.
I've seen the ones that apply to boats in the size range that interests
me, and frankly it's a bit scary. Bottom line: don't overload a 39'
Beneteau. And I'd suspect the same admonition might apply to similar
boats, such as Jeanneaus, DuFours, Catalinas, Hunters, etc.= the plastic
fantastics.


Probably true of any boat that is shaped like a normal sailboat. The only
way to be able to keep stability constant is to load all the weight
increasingly lower down so as to maintain the relationship between the
centers of bouyancy & gravity. Actually, as the beam/disp ratio drops, the
lever arm between CG and CB needs to increase... very unlikely!

Not sure I comprehend that concept, but I'll assume you know what you're
talking about here.

... What was interesting is that there seemed to be an over-center
point, beyond which the stability of the boat fell off precipitously. It
was not a smooth curve, rather it peaked, then plunged.

Maybe like a sine wave, with the peak for the point at which the gunwhale
immerses?

Sounds a bit extreme, or tongue-in-cheek.


... I'm surprised that boat builders don't publish a *do not exceed*
load figure for each boat, even if they don't want those graphs out and
about.

Ever hear of the Plimsoll marks? For commercial ships, these figures are
published and are supposed to be kept by the captain. For naval vessels,
they keep a whole book of figures on stability.

No, never heard of them, but it might be a good idea for recreational boats
to keep such figures handy as well. Then again, perhaps it's difficult to
exceed those critical figures in terms of load.

Yep. Everything is a trade-off. That's why life is such a good metaphor
for sailing....


Aw, heck, I want it all, dammit. g


If you figure out how, let me know.

Still working on it. No solution in sight.

Max

Maxprop wrote:
No indication of CoG movement on the graphs I have, but I'd assume that it
would have to change, unless one puts all the additional weight in the
bilge.


Well, it makes the calculations simpler if you assume the Center of
Gravity doesn't change, even though that would be almost impossible in
real life.



.... Bottom line: don't overload a 39'
Beneteau. And I'd suspect the same admonition might apply to similar
boats, such as Jeanneaus, DuFours, Catalinas, Hunters, etc.= the plastic
fantastics.



Probably true of any boat that is shaped like a normal sailboat. The only
way to be able to keep stability constant is to load all the weight
increasingly lower down so as to maintain the relationship between the
centers of bouyancy & gravity. Actually, as the beam/disp ratio drops, the
lever arm between CG and CB needs to increase... very unlikely!


Not sure I comprehend that concept, but I'll assume you know what you're
talking about here.


Not sure if I can explain, but here's a try:
When you look at stability, two things help. Weight down low and beam.
As the boat heels from 0 to 90 degrees, beam loses it's effectiveness &
weight down low gains. The lever arm between the center of bouyancy &
center of gravity is determined by these, so if you increase weight you
increase the leverage of the boat's beam but sink the hull deeper in the
water which decreases the lateral shift in bouyancy. Unless that weight
is down real low, you're reducing both modes of static stability.

Another way to look at it is by the Capsize Screening Ratio, a number
wich tells how likely the boat is to roll upside down and stay that
way... heavy + narrow = good... but if you're rightside up and want to
stay that way, the same relationship holds true which makes heavy +
narrow = not so good!




... What was interesting is that there seemed to be an over-center
point, beyond which the stability of the boat fell off precipitously. It
was not a smooth curve, rather it peaked, then plunged.

Maybe like a sine wave, with the peak for the point at which the gunwhale
immerses?


Sounds a bit extreme, or tongue-in-cheek.


Not at all. Consider a hull shaped like a cylinder, with a weight at the
bottom. This will have zero stability due to beam; called initial
stability or form stability. It will have very very little righting
moment at low angles of heel, then as it approaches 45 the righting
moment starts increasing steeply, then nears max somewhat short of 90
and increases slowly to the max at 90... a sine wave. A diagram could
explain this much much better.

Most monohulls have elements of a sine curve in the stability.



Ever hear of the Plimsoll marks? For commercial ships, these figures are
published and are supposed to be kept by the captain. For naval vessels,
they keep a whole book of figures on stability.


No, never heard of them, but it might be a good idea for recreational boats
to keep such figures handy as well. Then again, perhaps it's difficult to
exceed those critical figures in terms of load.


Depends on what you're carrying. In the old days, sailors used to love
to carry lumber because it's bouyant and when stacked up & strapped down
on deck, the boat was very safe (although a PITA to work around the deck
load).

About Plimsoll marks-

http://amchouston.home.att.net/plimsoll.htm

The story I heard was that the basic idea behind Plimsoll marks came
from a lowly insurance clerk who tabulated ship losses & their loading,
but could not get anybody to pay attention... ship captains being rather
set in their ways...



Aw, heck, I want it all, dammit. g

If you figure out how, let me know.


Still working on it. No solution in sight.


I don't want it all. I just want a time travel machine.

Fresh Breezes- Doug King

"DSK" wrote in message

Maxprop wrote:

Not sure I comprehend that concept, but I'll assume you know what you're
talking about here.


Not sure if I can explain, but here's a try:
When you look at stability, two things help. Weight down low and beam. As
the boat heels from 0 to 90 degrees, beam loses it's effectiveness &
weight down low gains. The lever arm between the center of bouyancy &
center of gravity is determined by these, so if you increase weight you
increase the leverage of the boat's beam but sink the hull deeper in the
water which decreases the lateral shift in bouyancy. Unless that weight is
down real low, you're reducing both modes of static stability.

Okay. Makes sense.



Sounds a bit extreme, or tongue-in-cheek.

Not at all. Consider a hull shaped like a cylinder, with a weight at the
bottom. This will have zero stability due to beam; called initial
stability or form stability. It will have very very little righting moment
at low angles of heel, then as it approaches 45 the righting moment starts
increasing steeply, then nears max somewhat short of 90 and increases
slowly to the max at 90... a sine wave. A diagram could explain this much
much better.

Not really--that's quite clear.


Most monohulls have elements of a sine curve in the stability.

Hmmm. The Beneteau graphs didn't appear sinusoidal at all, rather
asymmetrical.

No, never heard of them, but it might be a good idea for recreational
boats to keep such figures handy as well. Then again, perhaps it's
difficult to exceed those critical figures in terms of load.

Depends on what you're carrying. In the old days, sailors used to love to
carry lumber because it's bouyant and when stacked up & strapped down on
deck, the boat was very safe (although a PITA to work around the deck
load).

About Plimsoll marks-

http://amchouston.home.att.net/plimsoll.htm

The story I heard was that the basic idea behind Plimsoll marks came from
a lowly insurance clerk who tabulated ship losses & their loading, but
could not get anybody to pay attention... ship captains being rather set
in their ways...

I did a search after posting my reply. Didn't see that website, but several
others. Interesting stuff, actually.

Still working on it. No solution in sight.


I don't want it all. I just want a time travel machine.

Hey, I gotta good used flux capacitor I could let ya have real cheap . . .


Max