Static Stability

IAINT
27th February 2010, 17:37
Hi All,

Can anyone advise the meaning of ''static stability'' it's in relation to the inquest
into the loss of the Peterhead registered fishing vessel ''TRIDENT'' IN 1974.

Regards
Iain T

John Cassels
27th February 2010, 19:46
Simply put ; the ability of a vessel to return to the initial position after being
inclined by force.

fred henderson
27th February 2010, 20:03
Iain, if you go to one of my old threads you will find my effort to try and explain stability in simple terms: -

http://www.shipsnostalgia.com/showthread.php?t=6372

The thread was written about cruise ships but the forces involved apply to all vessels. To shorten my piece I used the term Intact Stability, in an explanation of the effect that the ship's inbuilt Static Stability has in correcting a roll (and to point out that the vessel is undamaged).
In stability terms, the word Static is generally used to describe the way that forces are counteracted instantaneously, as opposed to the word Dynamic were forces are absorbed gradually. If a ship starts loading a heavy object from a jetty, it will roll towards the jetty as its crane takes the weight. This is a static movement, because once the object is off the ground the ship's angle of list remains constant until the weight is moved. There is a precise and predictable relationship between the position of the weight and the angle of heel. If the sling then breaks, dropping the weight onto the quay, the ship will roll violently the other way, then roll back again several times before it settles. This is dynamic movement.

IAINT
27th February 2010, 21:02
Thanks John/Fred.

Therefore an inclination test, to check the static stability of the boat when she was built should have been carried out.

Regards
Iain T

chadburn
28th February 2010, 12:14
The statical stability curve is an important factor affecting stability. This curve is generally assessed by the values of the metacentric height, the maximum ordinate of the curve, and the range of the curve. For small angular displacements of the ship from the upright the effort to heel the ship is directly proportional to the metacentric height.
At greater angles other factors affect the effort required to heel a ship. These factors are governed by the form of the ship. The form curve is influenced by freeboard, the shape of the vessel above and below the waterline and by the nature and extent of the erections above the weather deck, the curve is very much influenced by the draught.
The wind must have a greater effect the larger the amount of surface exposed by the ship, Squall effect's are of course the worst.

fred henderson
28th February 2010, 20:47
Thanks John/Fred.

Therefore an inclination test, to check the static stability of the boat when she was built should have been carried out.

Regards
Iain T

An inclining experiment is normally only used to establish a vessels displacement. You start out with the vessel level. If you place a known weight, a measured distance from the centre-line and then measure the angle of inclination, you can mathmatically arrive at the weight of the ship. It is usually only carried out on the first-of-class vessel.

Supergoods
1st March 2010, 02:58
An inclining experiment is normally only used to establish a vessels displacement. You start out with the vessel level. If you place a known weight, a measured distance from the centre-line and then measure the angle of inclination, you can mathmatically arrive at the weight of the ship. It is usually only carried out on the first-of-class vessel.

Not so Fred,

The inclining experiment is used to calculate the centre of gravity of the lightship using observed angles of heel and the tabulated metacentre to provide GM and hence the VCG.

A lot of pre-calculation is required to inventory the weights that are not included in the lightship.

When the effect of these weights are deducted from the original calculation the result, theoretically, is the VCG of the vessel as well as the LCG and TCG.

I have done several of these on oil rigs and they can be required whenever the classification society demands it, usually when records of "additions to lightship" are inaccurate or non existant.

One I recall had a definate error caused by grounding the port quarter on the side of the hole.

Displacement can be calculated by observing drafts and the density of the water without a full inclining test.

fred henderson
1st March 2010, 15:15
I have no direct knowledge of the Peterhead fishing vessel Trident, but I would suggest that as she was was one of a series of similar, modestly sized vessels, the lack of an inclining test would not be unusual.

Having read the internet reports on the current inquiry, I see that it is thought that she shipped a large amount of seawater, which was unable to drain before she capsized. It sounds like another sad case of inadequate scupper capacity, which should have been prevented during the design approval stage and would not normally be covered, even if in an inclining test was carried out when she was completed.

gadfly
18th March 2010, 02:55
Having read the internet reports on the current inquiry, I see that it is thought that she shipped a large amount of seawater, which was unable to drain before she capsized. It sounds like another sad case of inadequate scupper capacity, which should have been prevented during the design approval stage and would not normally be covered, even if in an inclining test was carried out when she was completed.

Fred, it may be that you are not far wrong with your post as the Trident was provided with less than 50% of the freeing port area (scupper capacity) that was recommended by IMCO

If you combine water on deck with debatable stability reserves then this is a recipe for capsize.

The lack of an inclining test is one of the reasons why the actual stability reserves of the Trident are unknown. An inclining test that was carried out on a sister to Trident suggested that she did not meet IMCO's minimum stability standard

regards

Gadfly

fred henderson
18th March 2010, 15:53
Unless Trident was a new design (and from the photos on the internet she looked to be a bog-standard side trawler) it would be very unusual for an inclining experiment to be carried out. If I am correct in assuming that she was a side trawler, then hauling in a full net would be likely to tell the owners all they needed to know about her stability.

An inadequate freeing port area is an entirely different matter, because an accumulation of water on deck can quickly overcome an otherwise perfectly stable ship. This major design error should have been corrected at the drawing approval stage of her construction.

Andrew Craig-Bennett
18th March 2010, 16:04
I wonder if I might draw attention to an interesting thread on the static stability of sail training vessels, with comments from crew members who survived losses and from naval architects specialising in the type, here:

http://www.woodenboat.com/forum/showthread.php?t=110145

and more particularly here:

http://www.woodenboat.com/forum/showthread.php?t=110414

chadburn
18th March 2010, 16:38
Interesting comment by R.Long in regards to my#5 and squall's

Andrew Craig-Bennett
18th March 2010, 20:13
Yes, very much so; those graphs are an eye opener!

wbeedie
18th March 2010, 20:31
With regards to there not being enough scuppers ,I dont think that holds due to sister vessels of the Trident being given extra ballast to the tune of between 7.5 and 10 tonnes ,the wave tank in Denmark also suggested that had the Trident carried even 7.5 tonnes extra in ballast she would have survived and as she was steaming at the time of the foundering then wouldnt be hauling a net over her side and she was a stern trawler not a side trawler

fred henderson
22nd March 2010, 14:35
I have already stated that I know nothing about the boat itself. There is a photo of her on:

http://news.stv.tv/scotland/north/131062-new-inquiry-into-trident-fishing-tragedy-to-begin/

From this she does not look like a stern trawler, but you guys may know more.

I have not suggested that she was fishing when she foundered, in fact I have read that she was on her way to have a survey when she was lost 36 years ago.

wbeedie
22nd March 2010, 15:53
The vessel was designed as a pelagic herring trawler,the gantry forrard was for the herring net being tightened ,there were thre vessels of this class designed Trident, Persevere and Stanhope all fitted out in Bute
The difference to a stern trawler is they took the net aboard aft and side trawler took it over the side like the Aberdeen,Hull and Grimsby near and distant water trawlers ,you might be thinking of vessels with forrard wheel house as being the only stern trawlers a better pic here http://www.trawlerphotos.co.uk/gallery/showphoto.php?photo=31921&cpage=3#poststart and here http://www.trawlerphotos.co.uk/gallery/showphoto.php?photo=20095

Satanic Mechanic
22nd March 2010, 16:44
Hmm Seems to be a lot of different teminology about. Just for the record this is my understanding - feel free to disagree.

Static stability - never heard this term used in reference to ships
Dynamic Stability- or this one

Stability - the ability of a vessel to return to its upright position split into
Intact stability and Damaged stability.

Inclining Experiment - used to get the coordinates of the centre of gravity and therefore its stability characteristics.

Lightship test - used to find out the weight of the vessel - using a lot of very fun devices for measuring the draft (Rube Goldberg and Heath Robinson would have loved doing this!!(Jester))

There is of course an angle that a vessel won't recover from and if you have a large ampount of free liquid for the breadth of the vessel the free surface effect of this can heel the vessel over to this angle ala Herald of Free Enterprise. It would therefore, on vessels where water is shipped onto an open deck , be wise to make sure that the stability and the drainage are sufficient to allow for this.

Lancastrian
22nd March 2010, 16:59
Not a bad attempt for an Engineer!
However Chapter IX of the fabled Kemp & Young has a section entitled "Dynamical stability and the moment of statical stability", so they do have maritime relevance.
Dynamical stability = area under statical stability curve x W.
Moment of statical stability = W x GZ.

Satanic Mechanic
22nd March 2010, 17:31
Not a bad attempt for an Engineer!
However Chapter IX of the fabled Kemp & Young has a section entitled "Dynamical stability and the moment of statical stability", so they do have maritime relevance.
Dynamical stability = area under statical stability curve x W.
Moment of statical stability = W x GZ.

which means what exactly......................

Just need a definition thats all.

Lancastrian
22nd March 2010, 18:47
Try this - http://www.johnsboatstuff.com/Articles/dynamic.htm (http://www.johnsboatstuff.com/Articles/dynamic.htm)
or this - http://cmst.curtin.edu.au/local/docs/pubs/gourlay_dynamic_stability_of_ships_in_waves.pdf

Basically its still water or lumpy water.

John Cassels
22nd March 2010, 20:21
SM ; angle of vanishing stability.

Derek Roger
22nd March 2010, 20:28
A number of herring boats have been lost in Canada and a lot of work done with regard to the carrige of herring .

Herring being very "fluid " act like water when a vessel rolls .

The requried trim and satbility book has to include a number of loaded conditions and special requirements when the cargo is herring .

Typically the holds have to be longtitudinaly subdivided to reduce free surface effect .

A single long bulkhead reduces the free surface effect by 1/2 squared ie 1/4

3 long bulkheads provide 4 compartments which reduce the free surface effect by1/4 squared ie 1/16.

Without subdivision and a partial cargo of herring the free surface effect is severe and in the event of heavy rolling can have a very adverse effect on the righting capability of a vessel .

I dont know what the requirements were for Trident ; however can tell you all Canadian boats fishing herring have these conditions and precautions clearly laid out for the Captain and crew to follow .

Insufficient freeing ports would of course compound the problem .

Satanic Mechanic
22nd March 2010, 20:40
SM ; angle of vanishing stability.

thats da bunny aka the angle of dangle

John Cassels
23rd March 2010, 10:16
thats da bunny aka the angle of dangle

Sorry , you've lost me again.

gadfly
23rd March 2010, 12:35
With regards to there not being enough scuppers ,I dont think that holds due to sister vessels of the Trident being given extra ballast to the tune of between 7.5 and 10 tonnes ,the wave tank in Denmark also suggested that had the Trident carried even 7.5 tonnes extra in ballast she would have survived and as she was steaming at the time of the foundering then wouldnt be hauling a net over her side and she was a stern trawler not a side trawler

Inadequate scupper area in itself would not have been sufficient to trigger Trident's capsize - many small fishing vessels of that era were similarly equipped.

However when this is combined with the poor stability reserves and characteristics that the Trident undoubtedly possessed then capsize would be much more likely.

I would agree that additional ballast would have prevented capsize in the sea conditions that prevailed at the time of her loss.

Regards

Ronda

Satanic Mechanic
23rd March 2010, 16:08
Sorry , you've lost me again.


yeesh get with the groove daddyo ;)

It was just something I use to remember the term "angle of vanishing stability". Because a vessel will stay at that angle until something changes the situation either way I remembered it as the ......."Angle of Dangle"




it works for me ok:p

gadfly
23rd March 2010, 16:16
yeesh get with the groove daddyo ;)

It was just something I use to remember the term "angle of vanishing stability". Because a vessel will stay at that angle until something changes the situation either way I remembered it as the ......."Angle of Dangle"



SM maybe you're thinking of angle of loll

Regards

Gadfly

wbeedie
23rd March 2010, 16:21
no more like the angle of Oh S**t come on ya bass get back over

Satanic Mechanic
23rd March 2010, 17:13
no more like the angle of Oh S**t come on ya bass get back over


that one works as well

Satanic Mechanic
23rd March 2010, 17:14
SM maybe you're thinking of angle of loll

Regards

Gadfly


Nah I just remeber that one by its name

Lancastrian
23rd March 2010, 17:56
Some homework for you SM. http://www.mcaorals.co.uk/Stability%20Definations.htm

John Cassels
23rd March 2010, 20:52
Some homework for you SM. http://www.mcaorals.co.uk/Stability%20Definations.htm

Think we had all beter miss this this link. Wouldn't even teach this rubbish
to a first trip apprentice and the English is even worse than SM's.

Lancastrian
23rd March 2010, 21:00
Is that possible? Agreed it is not well written, but it might suffice for the education of Engineers and it does point out the difference between AOL and AOVS. (Wave)

gadfly
16th May 2010, 22:07
A video explaining some basic stability concepts in very simple terms (and with relevance to the capsize of Trident) is available on the following site:

http://the-trawler-gaul.blogspot.com/

Guaranteed GM free

regards

Gadfly

gadfly
17th July 2010, 13:31
The court hearings into the loss of the Fishing vessel Trident have now concluded in Aberdeen and from recent reports in the press it would appear that the investigating Authorities, represented by Scotland’s Advocate General, are pressing the Sheriff to conclude that:

- Trident capsized and sank due to heavy weather only
- Trident’s deficient stability did not contribute to her capsize in any way

The taxpayer has been charged 6.2m for this nonsense

A critical assessment of Trident’s stability is given on:

http://the-trawler-gaul.blogspot.com/2010/07/stability-of-fishing-vessel-trident.html

regards

Gadfly

Billieboy
17th July 2010, 16:30
Another cock-up, it would have been better to give the 6million to the families of those lost!

gadfly
18th October 2010, 00:30
Seems like the DfT has been busy shredding the evidence:

http://the-trawler-gaul.blogspot.com/

Regards

Gadfly

jimthehat
18th October 2010, 11:16
Not a bad attempt for an Engineer!
However Chapter IX of the fabled Kemp & Young has a section entitled "Dynamical stability and the moment of statical stability", so they do have maritime relevance.
Dynamical stability = area under statical stability curve x W.
Moment of statical stability = W x GZ.

Yoy beat me to it was just going to quote from my dog eared copy of Ship stability by kemp and Young and statical and dynamical stability was very much a intregral part of stability for masters.

jim

gadfly
18th October 2010, 20:23
Yoy beat me to it was just going to quote from my dog eared copy of Ship stability by kemp and Young and statical and dynamical stability was very much a intregral part of stability for masters.

jim

It's a shame they didn't have a copy of this book in the Aberdeen Sheriff's Court

Regards

Gadfly