Select to expand quoteshaggybaxter said..MorningBird 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.
i agree about the sea sickness thing pagan has very high stability but being narrow and heavy rolls like a pig though never bangs off a wave gives a beautiful soft motion but does seem to encourage seasickness
when sailing on boss high performance cat extreme beam light weight more stable upside down ridiculous jerky motion at sea nearly impossible to drink out of a cup tea no seasickness
Select to expand quoteAndrew68 said..
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
My Currawong is 120.5 in case your interested Andrew. I presume that calculation was done with the headsail furler drum off.
Select to expand quoteshaggybaxter said..Yara 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.
I cannot see how a simple test with a couple of degrees of heel can give you the angle of vanishing stability. In the STIX data, it looks like it was actually measured at 115.6 and they are saying it exceeds the minimum requirements.
I agree Yarra what happens if initial stability is tender as in old style yachts then picks up incredibly after that ?
Select to expand quoteYara said..shaggybaxter said..Yara 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.
I cannot see how a simple test with a couple of degrees of heel can give you the angle of vanishing stability. In the STIX data, it looks like it was actually measured at 115.6 and they are saying it exceeds the minimum requirements.
Yeah, and to &*+$ with your head more, the following is how ISO measure for STIX. It appears to me exactly the same as how ORCI measure.
It hurts.
---------------------
1. Waterline Beam, BWL
By inspection, establish an approximate BMAX station and hang a plumb bob over each side. Measure BMAX and insets from the plumb lines to the local waterline beam. By subtraction from BMAX, BWL can be found.
2. Freeboard at BMAX station (IOR FMD)
Measure freeboards both sides.
3. Canoe Body Depth (IOR CMD, Centre Mid Depth)
This will normally be found from the designer supplied line plans or other data.
4. Waterline Length, LWP (IOR L)
Measure as for IRC, subtracting forward and aft overhangs from LOA.
The inclining is carried out exactly as for an IRM/IMS inclination, with the exception that the only freeboards required are those at the BMAX station (2. above). The boat may be in either empty condition be in 'light' sailing trim with all gear and equipment necessary to race the boat. If in the light condition, water tanks must be empty, but small quantities of diesel are acceptable. Any in-mast or headstay furling sails should be hoisted and furled. Otherwise, the mainsail should be on the boom and other sails stowed below. The condition must be declared on the input sheet. Environmental conditions are critical. There must be no current and an absolute maximum of 10 knots of wind, preferably less.
--------------------
Select to expand quoteBROWNSMTRACE said..
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 
Browny,
good thread. I can't explain the whole calc sorry. This is what ISO say about calculating the STIX number;
STIX is the product of a number of different safety/stability related features. These are: Length, Dynamic Stability, Inversion Recovery, Knockdown Recovery, Displacement Length, Beam Displacement, Wind Moment, Downflooding, and Reserve Buoyancy.
Who needs sleep anyway?
Select to expand quoteshaggybaxter said..
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.
S
For a sistership which slightly heaver than mine
sink rate 21.16 kg with a wetted area of 27.94m2
My feeling is that there is a problem with the ORCi stability calculation for very wide shallow boats as many are not getting consistent numbers.
Select to expand quotelydia said..
My feeling is that there is a problem with the ORCi stability calculation for very wide shallow boats as many are not getting consistent numbers.
There is not very many Class 40 entries in the ORCi certificate library. I wonder how the Melbourne Osaka boats Class 40's got in ? They don't seem to have certificates in the system.
A
Select to expand quoteAndrew68 said..lydia said..
My feeling is that there is a problem with the ORCi stability calculation for very wide shallow boats as many are not getting consistent numbers.
There is not very many Class 40 entries in the ORCi certificate library. I wonder how the Melbourne Osaka boats Class 40's got in ? They don't seem to have certificates in the system.
A
If you look under the vendor name Andrew you find some. Pogo 40 is a class 40 for example.
Select to expand quoteSandS said..
so do all these righting calcs take sails in the water into consideration ?
That probably throws a spanner in the works.
Like putting the float on the top of a Hobie cat mast.
or a radome that floats put high up on the mast
Select to expand quoteAndrew68 said..lydia said..
My feeling is that there is a problem with the ORCi stability calculation for very wide shallow boats as many are not getting consistent numbers.
There is not very many Class 40 entries in the ORCi certificate library. I wonder how the Melbourne Osaka boats Class 40's got in ? They don't seem to have certificates in the system.
A
Not racing with orc I. Certificates
