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

... A diagram could explain this much
much better.


Maxprop wrote:
Not really--that's quite clear.


Allow me to compliment your grasp of trig relationships. My boss doesn't
get it even after studying a diagram.



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


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


That's a bad sign by itself IMHO. Consider the opposite case, a
catamaran... static righting moment hits max as soon as one hull clears
the water, maybe 15 degrees or less. The curve is almost vertical for
the first little bit, then drops off and becomes negative well before 90.

As a monohull becomes boxy & lightly ballasted, it's stability curve
trends more towards that catamaran type curve... good for sailing
performance under ideal conditions, but poor for "real life" sailing
IMHO. It can still have a good Limit of Positive Stability, though,
which the real meat of the matter.

Fresh Breezes- Doug King

"DSK" wrote in message

... A diagram could explain this much much better.


Maxprop wrote:
Not really--that's quite clear.


Allow me to compliment your grasp of trig relationships. My boss doesn't
get it even after studying a diagram.

The study of optics is almost pure and applied trig. I enjoy trig, so it is
relatively easy and intuitive for me.

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


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


That's a bad sign by itself IMHO. Consider the opposite case, a
catamaran... static righting moment hits max as soon as one hull clears
the water, maybe 15 degrees or less. The curve is almost vertical for the
first little bit, then drops off and becomes negative well before 90.

As a monohull becomes boxy & lightly ballasted, it's stability curve
trends more towards that catamaran type curve... good for sailing
performance under ideal conditions, but poor for "real life" sailing IMHO.
It can still have a good Limit of Positive Stability, though, which the
real meat of the matter.

I had a rather lengthy discussion with a Beneteau factory rep at a boat show
a few years back. I was inquiring whether, in the company's opinion, their
boats were truly offshore passage capable. He, predictably, answered to the
affirmative. I asked him why, and he gave me all sorts of reasons, such as
the method by which they laminate all bulkheads to the hull and deck
structures, their rigid gridwork under the sole, etc.--things of a
structural nature. When I asked about stability and seaworthiness, he
seemed at a loss. I commented that many of their boats had less than a 1:3
ballast/displacement ratio, and he said, "Oh that's really not terribly
important." Sure wish I'd had the company's graphs during that discussion.
It seems to me that the reps are spoon-fed the company standard line with no
particular enlightenment.

Max

Maxprop wrote:
I had a rather lengthy discussion with a Beneteau factory rep at a boat show
a few years back.

Why?


Sometimes when I find a boat salesman of amusing mendacity, I'll start
playing along just to see how far he'll push his act. It's really
amazing what these guys will say some times.


.... I was inquiring whether, in the company's opinion, their
boats were truly offshore passage capable. He, predictably, answered to the
affirmative. I asked him why, and he gave me all sorts of reasons, such as
the method by which they laminate all bulkheads to the hull and deck
structures, their rigid gridwork under the sole, etc.--things of a
structural nature. When I asked about stability and seaworthiness, he
seemed at a loss. I commented that many of their boats had less than a 1:3
ballast/displacement ratio, and he said, "Oh that's really not terribly
important."

Well, it depends on what your sailing goals are.

.... Sure wish I'd had the company's graphs during that discussion.
It seems to me that the reps are spoon-fed the company standard line with no
particular enlightenment.


Some of them are good & knowledgeable sailors who will say anything to
sell a boat, others are simply not well educated in naval architecture.
The most mendacious salesmen I've met, as a group, are the ones pushing
the Seaward line. Don't know why that brand seems to attract them, but
one really outrageous character was trying to sell us a Schock and
making all sorts of ludicrous statements, including "heel angle has
nothing to do with sailing speed or helm balance." At this point I was
semi-serious about that particular boat and challenged him to explain
why, at which point he got impatient and insisted that "when you have
much experience as me, you'll understand."

Anyway, the ballast certainly contributes to reserve stability & a high
LPOS but it's not the whole ball game. Given a choice for hard offshore
passagemaking between two boats, one with a 10% lower B/D ratio but a
much higher LPOS, the other with more ballast but also some negative
factors like a wide flat deck, big unprotected hatches, or the like, I'd
pick the one with higher LPOS.

And remember that some extremely under rated factors in seaworthiness
are things like... do the cabin sole access panels lock in place, ditto
the galley cabinet doors, does the bilge pump have a good suction
screen, how good is the non-skid in the shower...

But I digress... sorry...

A high ballast ratio is a very good thing for hard sailing. Gives sail
carrying power and makes the boat more manageable, at the very least.
But it takes away from all the amenities that help sell boats!

Fresh Breezes- Doug King

On Mon, 28 Mar 2005 09:00:29 -0500, DSK wrote
this crap:

Sometimes when I find a boat salesman of amusing mendacity, I'll start
playing along just to see how far he'll push his act. It's really
amazing what these guys will say some times.


So you admit to being a liar and a pussy?





This post is 100% free of steroids

"DSK" wrote in message

one really outrageous character was trying to sell us a Schock and making
all sorts of ludicrous statements, including "heel angle has nothing to do
with sailing speed or helm balance." At this point I was semi-serious
about that particular boat and challenged him to explain why, at which
point he got impatient and insisted that "when you have much experience as
me, you'll understand."

You're a relative youngster. They don't try to pull that crap with me any
longer. :-(

Anyway, the ballast certainly contributes to reserve stability & a high
LPOS but it's not the whole ball game. Given a choice for hard offshore
passagemaking between two boats, one with a 10% lower B/D ratio but a much
higher LPOS, the other with more ballast but also some negative factors
like a wide flat deck, big unprotected hatches, or the like, I'd pick the
one with higher LPOS.

As would I. Some designs simply have so many inherent flaws as to be
disqualified for anything beyond coastal daysailing. Small boats,
especially, often have no bridgedeck to speak of, or have companionways so
offset as to promote filling the hull on a knockdown on that side.

And remember that some extremely under rated factors in seaworthiness are
things like... do the cabin sole access panels lock in place, ditto the
galley cabinet doors, does the bilge pump have a good suction screen, how
good is the non-skid in the shower...

But I digress... sorry...

A high ballast ratio is a very good thing for hard sailing. Gives sail
carrying power and makes the boat more manageable, at the very least. But
it takes away from all the amenities that help sell boats!

Apparently, or should I say obviously. Short keels, tall topsides (high
freeboard), and megabeam carried well aft seems to be the rule in plastic
boats these days. Volume at all costs. Personally I think it is better for
a boat to look good than to sail good. :-) (apologies to Billy Crystal)

Max

... I'd pick the
one with higher LPOS.

Maxprop wrote:
As would I. Some designs simply have so many inherent flaws as to be
disqualified for anything beyond coastal daysailing. Small boats,
especially, often have no bridgedeck to speak of, or have companionways so
offset as to promote filling the hull on a knockdown on that side.


Agreed. Some boats are clearly designed for sitting on while drinking
beer at the dock, and aren't really practical for that either.

Off center companionways bug me. Sure it can make sense to give more
room to the galley, etc etc, but I just don't like it. Some therwise
wonderful boats have that feature, though.



.... Personally I think it is better for
a boat to look good than to sail good. :-) (apologies to Billy Crystal)


L. Francis Herreshoff once said, "If it looks right to your eye, chances
are it'll look right to the sea." Of course his mastery was in producing
boats that looked hypnotically right, and yet didn't have much in the
way of racing successes. Personally, while I think any boat that wins is
automatically beautiful, I also put a high value on looks and have only
owned one boat that was not at least pretty... some called the Hunter 19
cute, but it always looked pudgy to me.

Fresh Breezes- Doug King