Can any one put into plain English how Vanishing Stability works?
is it just about adding more ballast? What is it that makes the Contessa 32 the yard stick in measurements?
Makes sense to me if you have a deeper keel with weight low down it will be harder to roll , would love to hear from our mathematical gurus here please be gentle.
The angle of vanishing stability is the heel angle when there is no more righting moment.- i.e, she aint going to pop back up.
Modern broad beam boats are more stable initially (at lower angles of heel), but once they lose it, they become more stable in the upside down position. The good point of the old Contessa style is greater angle of vanishing stability, and not very stable upside-down-so it is more likely to pop upright.
Modern boats do have the ballast mass much further down, but it is lighter in order to enable these boats to get on the plane. (lower ballast ratio).
There is a theory which states that modern boats are OK because it takes more energy to get to the angle of vanishing stability. Not so sure I agree, as the energy of big waves is a hell of a lot more than the piddly amounts involved in the stability curve.
One interesting real-life experience is that in the 1998 Hobart, the boats with a high angle of vanishing stability had a HIGHER rate of severe capsizes (ie going past about 140 degrees) than the boats with a low angle. Similarly, the boats that copped most of the criticism in the 1979 Fastnet survived the storm after their crews had abandoned them - arguably if gear had not failed they would have had the same safety record as the Contessa that became the yardstick. Finally, the Contessa 32s had pretty much the same chance of rolling to about 90 degrees as the lightweight half tonners that copped so much flak, and it seems that as many people died in that situation as in boats that rolled (although there were fewer of the latter).
Thanks for replies guys, so what is the answer ? Is the 51% ballasted yacht more sea worthy ? Though more uncomfortable? Or just keep the ends pointed into the waves and reduce the risk of a roll over.
Nice explanation Yara!
I'd like to add it is worth being aware of AVS' partner in crime called Righting Moment. Where AVS is the angle at which the boat has no more resistance to inversion, RM is the amount of force needed to push against the bouyancy to get the boat over to it's AVS.
Example: my fat beamy design has a worse AVS (100-110) than a canoe shape hull which might be 125.
But because of the factors like hull shape (wide beam), deep keel and lightweight rig, I have a lot more force pushing up against the heel , so I have a high RM. A canoe shape hull with a shoal draft keel and a bloody great radar up an alloy mast has minimal resistance pushing back against heel, so it has a low RM.
The two work together and are often difficult to get stellar results for both in a boat design, as improving one usually has a reverse impact on the other. If you look at a GZ curve:
- the height of the curve shows the amount of force needed to heel the boat. A curve with a high vertical peak has more resistance to heel. (Righting moment)
- the point at which the curve crosses the 0 looking from left to right is the angle the boat doesn't pop back up (AVS). A higher AVS means she'll come back even if you're rolled over further.
I'm rated for Cat 0 ocean racing but I only have an AVS of about 110 I think, which is pretty naff. But thankfully the design has lots of Righting Moment, as its the combination of the two that is used to determine a boats stability. A high AVS means a boat can afford to have less RM and be classed as having the same stability.
So in the same given conditions :
new fat beamy designs will:
-- roll less, so stiff. (high RM)
-- are more violent in wave action (high RM)
--resist flipping more, but will flip earlier (high RM and low AVS)
canoe blue water design will:
-- roll more, so tender (low RM)
-- are less violent in wave action (low RM)
-- resist flipping less but will flip much later (low RM and high AVS)
both will have different AVS and RM numbers , but can be deemed to have the same stability rating.
Select to expand quoteshaggybaxter said..
Nice explanation Yara!
I'd like to add it is worth being aware of AVS' partner in crime called Righting Moment. Where AVS is the angle at which the boat has no more resistance to inversion, RM is the amount of force needed for the boat to get there.
Example: my fat beamy design has a worse AVS (100-110) than a canoe shape hull which might be 125.
But because of the factors like hull shape (wide beam), deep keel and lightweight rig, I have a lot more force pushing up against the heel , so I have a high RM. A canoe shape hull with a shoal draft keel and a bloody great radar up an alloy mast has minimal resistance pushing back against heel, so it has a low RM.
The two work together and are often difficult to get stellar results for both in a boat design, as improving one usually has a reverse impact on the other. If you look at a GZ curve:
- the height of the curve shows the amount of force needed to heel the boat. A curve with a high vertical peak has more resistance to heel. (Righting moment)
- the point at which the curve crosses the 0 looking from left to right is the angle the boat doesn't pop back up (AVS). A higher AVS means she'll come back even if you're rolled over further.
I'm rated for Cat 0 ocean racing but I only have an AVS of about 110 I think, which is pretty naff. But thankfully the design has lots of Righting Moment, as its the combination of the two that is used to determine a boats stability. A high AVS means a boat can afford to have less RM and be classed as having the same stability.
So in the same given conditions :
new fat beamy designs will:
-- heel less, (stiff)
-- are more violent in wave action
--resist flipping more, but will flip earlier
canoe blue water design will:
-- roll more (tender)
-- are less violent in wave action
-- resist flipping less but will flip much later
both will have different AVS and RM numbers , but can be deemed to have the same stability rating.
Shaggy what a great description and thanks for such in-depth reply, I always thought the beamy boats where more comfortable in a seaway and the narrow higher ballast more uncomfortable.
Primrose of Primrose and Illingworth fame went down the extreme beam road and lost his life to the yacht not coming back up .
The seawoethiness of a yacht has a lot to do with the skipper and preparation of the boat IMO.
Select to expand quoteZzzzzz said..shaggybaxter said..
Nice explanation Yara!
I'd like to add it is worth being aware of AVS' partner in crime called Righting Moment. Where AVS is the angle at which the boat has no more resistance to inversion, RM is the amount of force needed for the boat to get there.
Example: my fat beamy design has a worse AVS (100-110) than a canoe shape hull which might be 125.
But because of the factors like hull shape (wide beam), deep keel and lightweight rig, I have a lot more force pushing up against the heel , so I have a high RM. A canoe shape hull with a shoal draft keel and a bloody great radar up an alloy mast has minimal resistance pushing back against heel, so it has a low RM.
The two work together and are often difficult to get stellar results for both in a boat design, as improving one usually has a reverse impact on the other. If you look at a GZ curve:
- the height of the curve shows the amount of force needed to heel the boat. A curve with a high vertical peak has more resistance to heel. (Righting moment)
- the point at which the curve crosses the 0 looking from left to right is the angle the boat doesn't pop back up (AVS). A higher AVS means she'll come back even if you're rolled over further.
I'm rated for Cat 0 ocean racing but I only have an AVS of about 110 I think, which is pretty naff. But thankfully the design has lots of Righting Moment, as its the combination of the two that is used to determine a boats stability. A high AVS means a boat can afford to have less RM and be classed as having the same stability.
So in the same given conditions :
new fat beamy designs will:
-- heel less, (stiff)
-- are more violent in wave action
--resist flipping more, but will flip earlier
canoe blue water design will:
-- roll more (tender)
-- are less violent in wave action
-- resist flipping less but will flip much later
both will have different AVS and RM numbers , but can be deemed to have the same stability rating.
Shaggy what a great description and thanks for such in-depth reply, I always thought the beamy boats where more comfortable in a seaway and the narrow higher ballast more uncomfortable.
Primrose of Primrose and Illingworth fame went down the extreme beam road and lost his life to the yacht not coming back up .
Happy it makes sense Zzzz,
I notice the difference most when I'm slow.
As a low RM boat heels, the mast head decelerates till it pauses at that point of max heel, before swinging back accelerating slowly as it does so. All nice and gentle. Bigger arc, faster movement through the middle, but more gentlemanly.
A high RM boats doesn't waste time being nice, the masthead decelerates quickly, pauses for a brief second and snaps back accelerating hard, much more aggressive in its motion. Lesser arc, slower through the middle, but more sudden on the edges.
It's only at higher speeds when you're travelling faster than the waves where it gets really smooth, then a stiff boat can feel like a limo.
A Contessa 32 is not the benchmark.
Try this.
Standard Mumm/Farr 30 is close to 140 degrees
Usually the clear winner save for a 12m which has an AVS of about 180 degrees.
Don't start me about AVS.
Had the most inclined boat in the country for a while there.
Inclining for AVS if you have never done it.
So use the acceleration taken from moving 2 degrees one way to 2 degrees the other and extrapolate that to 180 degrees.
makes us safer.
Of course does not measure the effect/flotation of the coach house or having on open transom and stuff like that.
interesting IMOCA 60s have a minimum cabin volume to stop the boat squatting by the stern when inverted and becoming more stable upside down.
Of course inclination started as way to handicap boats as it measured righting moment which is reflected in speed.
less stability equals lower rating so you always looked for less measured stability.
Somehow after 1998 if became a benchmark of a safety index as it was easy to measure and could be justified on some grounds.
Select to expand quoteshaggybaxter said..Zzzzzz said..shaggybaxter said..
Nice explanation Yara!
I'd like to add it is worth being aware of AVS' partner in crime called Righting Moment. Where AVS is the angle at which the boat has no more resistance to inversion, RM is the amount of force needed for the boat to get there.
Example: my fat beamy design has a worse AVS (100-110) than a canoe shape hull which might be 125.
But because of the factors like hull shape (wide beam), deep keel and lightweight rig, I have a lot more force pushing up against the heel , so I have a high RM. A canoe shape hull with a shoal draft keel and a bloody great radar up an alloy mast has minimal resistance pushing back against heel, so it has a low RM.
The two work together and are often difficult to get stellar results for both in a boat design, as improving one usually has a reverse impact on the other. If you look at a GZ curve:
- the height of the curve shows the amount of force needed to heel the boat. A curve with a high vertical peak has more resistance to heel. (Righting moment)
- the point at which the curve crosses the 0 looking from left to right is the angle the boat doesn't pop back up (AVS). A higher AVS means she'll come back even if you're rolled over further.
I'm rated for Cat 0 ocean racing but I only have an AVS of about 110 I think, which is pretty naff. But thankfully the design has lots of Righting Moment, as its the combination of the two that is used to determine a boats stability. A high AVS means a boat can afford to have less RM and be classed as having the same stability.
So in the same given conditions :
new fat beamy designs will:
-- heel less, (stiff)
-- are more violent in wave action
--resist flipping more, but will flip earlier
canoe blue water design will:
-- roll more (tender)
-- are less violent in wave action
-- resist flipping less but will flip much later
both will have different AVS and RM numbers , but can be deemed to have the same stability rating.
Shaggy what a great description and thanks for such in-depth reply, I always thought the beamy boats where more comfortable in a seaway and the narrow higher ballast more uncomfortable.
Primrose of Primrose and Illingworth fame went down the extreme beam road and lost his life to the yacht not coming back up .
Happy it makes sense Zzzz,
I notice the difference most when I'm slow.
As a low RM boat heels, the mast head decelerates till it pauses at that point of max heel, before swinging back accelerating slowly as it does so. All nice and gentle. Bigger arc, but more gentlemanly.
A high RM boats doesn't waste time being nice, the masthead decelerates quickly, pauses for a brief second and snaps back accelerating hard, much more aggressive in its motion. Lesser arc, but more sudden on the edges.
It's only at higher speeds when you're travelling faster than the waves where it gets really smooth, then a stiff boat can feel like a limo.
Shaggy I remember reading Voss and him saying on his first trip on a sailing ship the motion was horrendous and his skipper saying to him he had loaded the cargo to heavy down low so he learnt to put some load higher to slow it down
In the late 1970s we went through days of IOR when especially little boats on choppy venues had all internal ballast and wooden keels.
1977 1/2 worlds in Sydney.
Less pitch equals more speed
Select to expand quotelydia said..
Inclining for AVS.
So use the acceleration taken from moving 2 degrees one way to 2 degrees the other and extrapolate that to 180 degrees.
makes us safer.
Of course does not measure the effect/flotation of the coach house or having on open transom and stuff like that.
interesting IMOCA 60s have a minimum cabin volume to stop the boat squatting by the stern when inverted and becoming more stable upside down.
Hi Lydia is there more information on doing this ? And is this the norm for racing vessels?
would be interesting to see how they work out the stresses put on those long thin keels
So you create two levers with kite poles, then move a known weights (say 25kg increments) from one extremity to the other while measuring the inclination usually with a manometer set up across the stern of the boat.
then freeboards are measured at the bow and stern.
The hull plan is in the computer so input the data and press enter.
The answer was 116.3 that time btw so Hobart legal.
Select to expand quoteZzzzzz said..lydia said..
Inclining for AVS.
So use the acceleration taken from moving 2 degrees one way to 2 degrees the other and extrapolate that to 180 degrees.
makes us safer.
Of course does not measure the effect/flotation of the coach house or having on open transom and stuff like that.
interesting IMOCA 60s have a minimum cabin volume to stop the boat squatting by the stern when inverted and becoming more stable upside down.
Hi Lydia is there more information on doing this ? And is this the norm for racing vessels?
would be interesting to see how they work out the stresses put on those long thin keel
Norm is you want to go to Hobart generally.
as to keel stuff I will answer that later
Select to expand quoteZzzzzz said..
Thanks for replies guys, so what is the answer ? Is the 51% ballasted yacht more sea worthy ? Though more uncomfortable? Or just keep the ends pointed into the waves and reduce the risk of a roll over.
I would suggest joining this forum and read especially the section on Stability.
www.boatdesign.net/forums/
Plenty of naval architects there to answer specific questions.
Select to expand quoteRamona said..Zzzzzz said..
Thanks for replies guys, so what is the answer ? Is the 51% ballasted yacht more sea worthy ? Though more uncomfortable? Or just keep the ends pointed into the waves and reduce the risk of a roll over.
I would suggest joining this forum and read especially the section on Stability.
www.boatdesign.net/forums/
Plenty of naval architects there to answer specific questions.
Thanks Ramona
In multihulls at least, there is also a concept called roll moment of inertia - or rotational inertia. If you get hit with a wave on the beam the boat will roll to the impulse or torque created by the wave. If the boat has a higher rotational inertia it will be harder to start turning and more resistant to capsize.
You get higher rotational inertia from moving weight away from the centre of rotation. In multihulls, this is one reason why cats are much better at handling beam seas than tris even though they have less beam - tris have a much lower rotational inertia.
Rotational inertia increases with mass and beam so one way to increase it is to get a big heavy Roberts Spray. A small centreboarder with internal ballast will have a lower rotational inertia.
Select to expand quoteshaggybaxter said..
So in the same given conditions :
new fat beamy designs will:
-- roll less, so stiff. (high RM)
-- are more violent in wave action (high RM)
--resist flipping more, but will flip earlier (high RM and low AVS)
canoe blue water design will:
-- roll more, so tender (low RM)
-- are less violent in wave action (low RM)
-- resist flipping less but will flip much later (low RM and high AVS)
Also need to keep in mind that tipping forces and righting moments on flat water may differ from those in conditions of rough seas.
A flat bottomed craft is tipped more strongly than a round bottom in a side-on swell/wave situation.
Think about a flat board lying in the water, vs a round log. A wave rolling in side-on will tend to grip and tip the board more easily than the log.
You can experience it first hand by paddling a flat bottom canoe vs a sleek narrow kayak, with waves coming in side-on.
If you do an incline test on a catamaran inside the harbour, the righting moment is fantastically high. But if you place that cat side-on on a steep wave, wave face at 45deg, there is no force attempting to right it at that point.
Select to expand quoteLazzz said..
Looks like everyone fell into a hole.
would not a flat bottomed boat slide forward with the wave like a boogie board ??
Select to expand quoteZzzzzz said..Lazzz said..
Looks like everyone fell into a hole.
would not a flat bottomed boat slide forward with the wave like a boogie board ??
Until it catches an edge.
