Being mathematically challenged could any one explain how to work out limit of stability for my yacht in plain English please.
My eyes glaze over with x times y divided by z 
It is normally determined by Naval Architect using a computer model of the yacht, supported by inclination tests.
If you create a free account on the orc web site you can view certificates of selected boats. If you are lucky there will be a certificate of a sister boat in the data base:
www.orc.org/index.asp
Good discussion on the ORCV site :
www.orcv.org.au/safety/stability-requirements
A boat that comes with stability and build certification paper work is worth thousands if you intend to race offshore at some time if the future. It is a big job to measure, test and model from scratch.
A
I'm right into this topic at the mo, as I failed a heel test that was completely at odds with my ISO cert.
It is a mind numbingly complex subject in parts, but quite fascinating even understanding the fundamentals.
It is voddoo physics but that is what gets used.
Soon we will have a time when you will not get insurance to sail coastal passages without AVS data.
As Shaggy says AVS data is worth thousands when buying a boat because it costs thousands to get with an older boat.
It has been topical just lately in cruising circles because of one boat.
The Omni 40 has an AVS of 89 degrees so does not right from horizontal yet is considered a capable ocean cruising boat.
Also many French productions boats with Builders certificates and Iso certificates can't replicate the numbers when actually measured for ORCi for instance by inclination.
what type of boat and might be able to help?
Select to expand quoteshaggybaxter said..
I'm right into this topic at the mo, as I failed a heel test that was completely at odds with my ISO cert.
It is a mind numbingly complex subject in parts, but quite fascinating even understanding the fundamentals.
Out of interest why did you need the test ? Doesn't the ISO cert cover you ?
A
Thanks for the in depth response.
Will look through that site tonight.
Do you think the ballast is a good indicator to how good the righting moment with out needing a computer, or is there much more to it e.g. how wide the yacht is etc
A look at www.orc.org/index.asp?id=15 and also the link at the bottom of that article to the "Stability & Hydrostatics Datasheet Explanation" might also help the understanding.
Select to expand quoteBROWNSMTRACE said..
Thanks for the in depth response.
Will look through that site tonight.
Do you think the ballast is a good indicator to how good the righting moment with out needing a computer, or is there much more to it e.g. how wide the yacht is etc
Have a play with this to calculate RM
www.seldenmast.com/en/services/calculators/rm_calculator.html
Select to expand quoteAndrew68 said..shaggybaxter said..
I'm right into this topic at the mo, as I failed a heel test that was completely at odds with my ISO cert.
It is a mind numbingly complex subject in parts, but quite fascinating even understanding the fundamentals.
Out of interest why did you need the test ? Doesn't the ISO cert cover you ?
A
HI Andrew,
Not in Aus, which makes us a bit unique in this regard. Sailing Australia only accept an ORC stability test rating for Cat 2 and 1 races. Everywhere else in the world accepts the ISO cert. No bloody logic as to why, it just is.
Individual clubs will overrule it for a race. QCYC did for the Gladstone, and for next year Prince Alfred club got me clearance for PIttwater to Southport, bless 'em..
CYCA have not for the Hobart.
In part it because the blood sucking incompetents at sailing Australia get to force you buy two rating certificates. IRC for results and orci to demonstrate stability so send another thousand.
Select to expand quoteBROWNSMTRACE said..
Thanks for the in depth response.
Will look through that site tonight.
Do you think the ballast is a good indicator to how good the righting moment with out needing a computer, or is there much more to it e.g. how wide the yacht is etc
For similar designs, ballast is very a good indicator of righting moment. Very generally a Ballast to Displacement ratio of over 40% or a stability index over 115% is considered "stiff", however lots of other factors come into play that affect ability to resist capsize. In fact some argue that it makes very little difference at all when a wave picks up a boat and spins it upside down. Practically relates more to how much sail a boat to hold and how comfortable it rides.
A
Join this forum and ask your question there. Supply the details of your boat or find the same boat there. One of the real naval architects will help you.
www.boatdesign.net/forums/stability/
Thanks for the replies .
Andrew in regards to a wave getting hold of you I would think a more modern boat may handle it like a surf board same as a cat would and run out of danger ??
which has a higher AVS, a Mum 30 or an S&S 34?
Your call
Don't confuse stiffness with stability.
Shaggy's boat has incredible stiffness ( mainly through form stability) but according to the AVS measurement very little stability.
For a while I had one of the most inclined boats in Australia so have dealt with the issue a great deal.
B, what type of boat.
Currawongs are about 118 to 123
S&S 34s are about 123
Most other 30 foot production boats will be 106 to 110
IOR 1/2 and 1 tonner anything from 93 to 110
From memory a Mumm 30 (Farr 30) is off the dial at 136
A 12 meter about 180
Also there is an easy 5 degrees in how you up for the inclination.
There would be a lot of cruising boats that have serious compromised their AVS through things like Davits and roller furlers which weigh heaps and are above the centre of gravity and lowering the AVS
So Lydia I assume the 12m is so high because it is stiff but also narrow making it so much easier to come back?
But of course I would assume some cabins would resist capsize and also help righting
Why does the Contessa 32 hold the yard stick for Limit of Positive Stability for other yachts to be compared to .
I have an S&S34. Many have had their AVS measured and I understand they range from 115 to high 130s.
A big deck stepped mast, solid boom with a furler all reduce the AVS with added top weight. A high mounted radar like wise.
The engine low above the keel is better for AVS than one mounted higher up under the cockpit.
So even knowing a design AVS a particular boat may be vastly different.
Righting moment is also affected by hull form and beam. A beamy roomy boat will be harder to right I.e. need a bigger wave to tip it back up.
Havung said that, if stability is a requirement best to start with a design that has heaps of it.
The contessa does not really just relative to older boats and even then it is only 132 from memory
Select to expand quoteBROWNSMTRACE said..
So Lydia I assume the 12m is so high because it is stiff but also narrow making it so much easier to come back?
But of course I would assume some cabins would resist capsize and also help righting
Why does the Contessa 32 hold the yard stick for Limit of Positive Stability for other yachts to be compared to .
This is also where it is bull****, AVS for ORCi ignores cabin volume while the French IMOCA boats (open 60s)have a minimum cabin volume requirement to help with righting from an inverted position which makes much more sense.
The 12 meter has so much weight in the keel relative the weight of the boat.
Beam is not the determining factor you would think because AVS is asking a different question which is how the vessel rights not how it gets inverted.
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Select to expand quotelydia said..BROWNSMTRACE said..
So Lydia I assume the 12m is so high because it is stiff but also narrow making it so much easier to come back?
But of course I would assume some cabins would resist capsize and also help righting
Why does the Contessa 32 hold the yard stick for Limit of Positive Stability for other yachts to be compared to .
This is also where it is bull****, AVS for ORCi ignores cabin volume while the French IMOCA boats (open 60s)have a minimum cabin volume requirement to help with righting from an inverted position which makes much more sense.
The 12 meter has so much weight in the keel relative the weight of the boat.
Beam is not the determining factor you would think because AVS is asking a different question which is how the vessel rights not how it gets inverted.
Agree the BS in how this issue is managed.
To clarify my understanding:
AVS is the point where the boat will lose positive stability and continue to invert in smooth water.
As the boat starts to heel form stability, from hull shape, is the major factor in righting the boat until the keel weight takes over at higher heel angles. Form stability is zero at 90 deg heel and starts to increase again as the boat inverts. Once the boat is Completely inverted form stability will keep it inverted until a wave knocks it back past its AVS when keel weight takes over.
To right an inverted boat it needs to overcome the form stability first, so the keel weight can do its job.
A wide beam boat has more form stability and is more stable inverted therefore harder to move back beyond its AVS so the keel can right it.
Select to expand quoteshaggybaxter said..
I'm right into this topic at the mo, as I failed a heel test that was completely at odds with my ISO cert.
It is a mind numbingly complex subject in parts, but quite fascinating even understanding the fundamentals.
It is really not that complicated in basic principle, the difficulty comes in calculating the buoyancy from the immersed hull and superstructure for each angle of heel. Would be very interesting to see details of the failed heel test and the ISO data. Do you have any pics of the test?
Yep but cant post them right now.
It is all very unexciting as the boat is only inclined 2 degrees each way
So the old narrow 51% ballast ratio wooden boats, if they didn't leak where more seaworthy??
Select to expand quoteBROWNSMTRACE said..
So the old narrow 51% ballast ratio wooden boats, if they didn't leak where more seaworthy??
More stable maybe, but seaworthiness covers much more than stability.
I do believe from a story of one that ended on Middleton Reef that they were very tender, the righting moment causing a very sharp motion.
Select to expand quoteYara said..shaggybaxter said..
I'm right into this topic at the mo, as I failed a heel test that was completely at odds with my ISO cert.
It is a mind numbingly complex subject in parts, but quite fascinating even understanding the fundamentals.
It is really not that complicated in basic principle, the difficulty comes in calculating the buoyancy from the immersed hull and superstructure for each angle of heel. Would be very interesting to see details of the failed heel test and the ISO data. Do you have any pics of the test?
G'day Yara,
Here ya go, this is my stability test...only gives me an AVS of 112.
The test was 2 x 5 mtr pole with 100kgs hanging off the end, it didn't heel the boat more than two degrees. What they are actually determining is the centre of gravity, not actually the amount of righting moment, so hence the low heel angles are not a factor.
I'd perosnally be happier heeling to a bigger angle to get a more accurate result.
Or do it on a non windy day.
Aaannd......this is my ISO cert numbers, an AVS of 118.
I need an AVS of 115 for Hobart, or any Cat 1 race. So the Pogo is illegal in any Cat 1 or Cat 2 race in Aus under ORCI. The French are still wetting themselves laughing about it, as we are the only country in the world that the 12.50 does not qualify for Cat A ocean racing, or a Cat 1 race.
The issue appears to be a formula tweak in the ORCI calcs for modern beamy designs. There's an awareness at ORCI of there maybe a problem, but no action yet.
Select to expand quoteMorningBird said..BROWNSMTRACE said..
So the old narrow 51% ballast ratio wooden boats, if they didn't leak where more seaworthy??
More stable maybe, but seaworthiness covers much more than stability.
I do believe from a story of one that ended on Middleton Reef that they were very tender, the righting moment causing a very sharp motion.
That's interesting MB. Fusion is very stiff, so when beating to windward (non planing) in short sharp seas the motion is very sharp, violent almost. This is where the wide beam and a deep heavy keel work against me.
If I lift the keel, the motion is obviously a lot more rolly polly, but much more gentle in the oscillation.
Once the wind is abeam, or you're running, and you start planing, the motion is completely different. Even in big seas it's very smooth and stable.
Sidenote: Its interesting, I have had only one guy (complete newbie) get seasick to date. Most stomachs seem to find a stiff boat more palatable even though the action is more violent.
Hey Lydia,
According to the stab test results, I have a sink rate of 29.2 kgs per mm of freeboard.
What is say a Sydney 38, do you know offhand?
Interested in comparison of the two hull shapes, I dunno if that looks right.
