Longitudinal Stability - Ships Nostalgia
10:46

Welcome
Welcome!Welcome to Ships Nostalgia, the world's greatest online community for people worldwide with an interest in ships and shipping. Whether you are crew, ex-crew, ship enthusiasts or cruisers, this is the forum for you. And what's more, it's completely FREE.

Click here to go to the forums home page and find out more.
Click here to join.
Log in
User Name Password

Longitudinal Stability

User Tag List

Reply
 
Thread Tools
  #1  
Old 6th April 2012, 17:01
Plane Sailing Plane Sailing is offline  
Senior Member
Organisation: Merchant Navy
Department: Deck
Active: 1979 - 2003
 
Join Date: Sep 2008
Posts: 149
Longitudinal Stability

Esteemed and Learned Gentlemen of SN,

Being on contract at present means the topics of conversation range from the mildly sublime to the utterly ridiculous, however once in a while a reasonably intelligent discussion ensues. This happened a few days ago in which the subject of longitudinal stability came up and specifically trimming moments.

Now it's been a good wee while since I last had to calculate drafts and trim, but the old grey cells haven't totally petrified and I can still remember most of it except for one point, which is where I hope the more erudite members of the forum can help put me out my misery.

A ship trims due to the couple set up between the LCG and the LCB - the ship keeps trimming until the LCG & LCB are again aligned. Also the point about which the ship trims is the LCF. So far so good.

The change of trim can be calculated by (W x (LCB~LCG)) / MCTC ie the trimming moment divided by the MCTC.

Where I'm getting brainlock is what happens if a weight is loaded directly above the LCG. Since the ship trims about the LCF and since the LCF & LCG are rarely in the same place, it would seem reasonable to expect the ship to trim as a result. However if the weight is loaded at the position of the LCG then there should be no change in the position of the LCG and hence, from the formula, no change in the trimming moment (since LCB~LCG = 0) and hence no change of trim. But doesn't that only happen if a weight is loaded at the position of the LCF?

Can someone kindly remind me of what I'm missing here?
Reply With Quote
  #2  
Old 6th April 2012, 17:48
Andrew Craig-Bennett's Avatar
Andrew Craig-Bennett Andrew Craig-Bennett is offline  
Senior Member
Organisation: Merchant Navy
Department: Office / Administration
Active: 1974 - Present
 
Join Date: Mar 2007
Posts: 1,250
If the increase in displacement alters the shape of the waterplane, which it probably will, she will trim until LCB and LCG are aligned again (I think?)
Reply With Quote
  #3  
Old 6th April 2012, 18:01
Super Moderator
Organisation: Merchant Navy
Department: Engineering
Active: 1962 - 2005
 
Join Date: Feb 2005
My location
Posts: 7,128
I gave an answer which was flawed ( so I deleted it )as I was mixing up LCF and LCB ; have to think on this one a bit more . Derek

Last edited by Derek Roger; 6th April 2012 at 18:10..
Reply With Quote
  #4  
Old 6th April 2012, 18:02
Plane Sailing Plane Sailing is offline  
Senior Member
Organisation: Merchant Navy
Department: Deck
Active: 1979 - 2003
 
Join Date: Sep 2008
Posts: 149
Quote:
Originally Posted by Andrew Craig-Bennett View Post
If the increase in displacement alters the shape of the waterplane, which it probably will, she will trim until LCB and LCG are aligned again (I think?)
Yes I thought about that Andrew, but wouldn't the same happen if the weight was loaded at the LCF?
Reply With Quote
  #5  
Old 6th April 2012, 20:08
Chris Isaac's Avatar
Senior Member
Organisation: Merchant Navy
Department: Deck
Active: 1962 - 1980
 
Join Date: Jul 2006
My location
Posts: 3,893
Just when I thought that after 30 years ashore I could finally forget all this stuff you go and drag it all up again.
I expect nowadays they just press a button and its all done for them.
We managed to do all this stuff half pissed. Probably it can only be done when half pissed.
Happy days, not now..... then!
Reply With Quote
  #6  
Old 6th April 2012, 21:08
Andrew Craig-Bennett's Avatar
Andrew Craig-Bennett Andrew Craig-Bennett is offline  
Senior Member
Organisation: Merchant Navy
Department: Office / Administration
Active: 1974 - Present
 
Join Date: Mar 2007
Posts: 1,250
Quote:
Originally Posted by Plane Sailing View Post
Yes I thought about that Andrew, but wouldn't the same happen if the weight was loaded at the LCF?
Ouch! My brain hurts. If anyone mentions Bonjean curves, I'm out of here!

Now, the LCB is the longitudinal centre of bouyancy, which will lie vertically under the LCG, the longitudinal centre of gravity, or if it does not it soon will do because the ship will trim until it does.

The LCF on the other hand will always be found at the waterline because it is the geometric centre of the waterplane.

Typically, ships being fatter aft than forward, the LCF will lie slightly aft of the LCB.

If we add weight at the LCF, the ship will draw more water until she displaces her original displacement plus weight that we have added.

The LCG will move towards the LCF and the LCB will follow the LCG, moving aft, until equilibrium is again established, with the ship having trimmed down slightly aft.

er...I think?
Reply With Quote
  #7  
Old 6th April 2012, 21:09
EBenarty EBenarty is offline  
Senior Member
 
Join Date: Oct 2005
Posts: 297
Half of them cannnot use a sextant in real and rely on GPS, I have known from experience. I hate to see the day GPS fails and see what happens. Like I say a lot would have forgoton haversines, Logarithms ,Lighthhouses appearing over the horison and do a running fix and vertical sextant angles.
Reply With Quote
  #8  
Old 6th April 2012, 22:16
vectiscol vectiscol is offline  
Senior Member
 
Join Date: Oct 2006
Posts: 673
If a weight is added at the LCG, then one has to re-calculate the trim using the relevant parameters at the new displacement in the hydrostatic tables. Passing from salt to fresh water has the same effect; the displacement will change due to differing density of water, but the centre of gravity of the ship remains the same.
Reply With Quote
  #9  
Old 7th April 2012, 06:28
Keltic Star's Avatar
Keltic Star Keltic Star is offline  
Senior Member
Organisation: Merchant Navy
Department: Deck
Active: 1958 - Present
 
Join Date: Jan 2006
My location
Posts: 3,235
Quote:
Originally Posted by Chris Isaac View Post
Just when I thought that after 30 years ashore I could finally forget all this stuff you go and drag it all up again.
I expect nowadays they just press a button and its all done for them.
We managed to do all this stuff half pissed. Probably it can only be done when half pissed.
Happy days, not now..... then!
Here's one of the programs in use today.

http://www.delftship.net/delftship/index.php/delftload

And for design hydrostatics we use "Delftship"
__________________
No to Canada-EU Trade Agreement
Look what happened to Britain
Reply With Quote
  #10  
Old 7th April 2012, 09:00
Chris Isaac's Avatar
Senior Member
Organisation: Merchant Navy
Department: Deck
Active: 1962 - 1980
 
Join Date: Jul 2006
My location
Posts: 3,893
Thanks for that Keltic Star.
I am sure it is progress it just doesn't seem as much fun as filling the after peak and see what happens.
Reply With Quote
  #11  
Old 13th February 2016, 10:03
Rob Wilkinson Rob Wilkinson is offline  
Member
 
Join Date: Feb 2016
Posts: 1
Longtitudinal Stability

On the topic of Longtitudinal Stability I have a question from my fluid mechanics book that I'm trying arrive at the answer, but not quite sure how to get it.
The question is as follows:

A ship displaces 10,000 metric tons and the area of its plane of flotation is 1,480 m^2. The centre of mass is 49m and the centre of area of plane of flotation is 55mfrom the stern. The metacentric height for pitching motion about the transverse principal axis is 91.5m. The ship is loaded in sea water with 300 metric tons of extra cargo. Find the minimum allowable distance of the mass centre of this extra load from the stern if, when the ship passes from sea water into a freshwater canal, the stern draught must not increase by more than 0.3m. Assume that the metacentric height and area of plane of flotation are not altered by the change in draught and that the density of sea water is 1025 kg/m^3.
Answer 46.2m

I've looked at LONGTITUDINAL WEIGHT ADDITIONS AND REMOVALS from this website http://fas.org/man/dod-101/navy/docs...a/stg4-04.html .

Am I on the right track looking at this to work it out.
Reply With Quote
  #12  
Old 13th February 2016, 12:29
NoR's Avatar
NoR NoR is offline  
Senior Member
Department: Deck
Active: 1963 - 1979
 
Join Date: Mar 2008
Posts: 2,214
I think that if LCG and LCB are vertically aligned (which they will be if the vessel is at rest ) then placing a weight at LCG will just cause the vessel to sink bodily with no(?) change of trim except that the water-plane shape will be different at the new draft thus causing a change of trim (minimal except for large weights and extreme hull shape)

.........I think ??
Reply With Quote
  #13  
Old 13th February 2016, 13:30
Gulpers's Avatar
Gulpers Gulpers is offline   SN Supporter
ex-Denholm Moderator
Organisation: Merchant Navy
Department: Navigation
Active: 1972 - 1981
 
Join Date: Sep 2005
My location
Posts: 11,591
Smile Welcome

Quote:
Originally Posted by Rob Wilkinson View Post
On the topic of Longtitudinal Stability I have a question from my fluid mechanics book that I'm trying arrive at the answer, but not quite sure how to get it.
The question is as follows:

A ship displaces 10,000 metric tons and the area of its plane of flotation is 1,480 m^2. The centre of mass is 49m and the centre of area of plane of flotation is 55mfrom the stern. The metacentric height for pitching motion about the transverse principal axis is 91.5m. The ship is loaded in sea water with 300 metric tons of extra cargo. Find the minimum allowable distance of the mass centre of this extra load from the stern if, when the ship passes from sea water into a freshwater canal, the stern draught must not increase by more than 0.3m. Assume that the metacentric height and area of plane of flotation are not altered by the change in draught and that the density of sea water is 1025 kg/m^3.
Answer 46.2m

I've looked at LONGTITUDINAL WEIGHT ADDITIONS AND REMOVALS from this website http://fas.org/man/dod-101/navy/docs...a/stg4-04.html .

Am I on the right track looking at this to work it out.
Rob,

On behalf of the SN Moderators, a warm welcome aboard from the Isle of Anglesey.
You will thoroughly enjoy the SN experience and hopefully some more members will be able to help with your query in the meantime have a good look around.
__________________
Ray
. . . . A closed mouth gathers no feet!
Reply With Quote
  #14  
Old 13th February 2016, 20:21
John Cassels John Cassels is offline  
Senior Member
 
Join Date: Sep 2005
My location
Posts: 2,957
Quote:
Originally Posted by NoR View Post
I think that if LCG and LCB are vertically aligned (which they will be if the vessel is at rest ) then placing a weight at LCG will just cause the vessel to sink bodily with no(?) change of trim except that the water-plane shape will be different at the new draft thus causing a change of trim (minimal except for large weights and extreme hull shape)

.........I think ??
No , she will trim round the LCF.
__________________
JC ; same initials-but the other guy did the miracles.
Reply With Quote
  #15  
Old 14th February 2016, 01:13
NoR's Avatar
NoR NoR is offline  
Senior Member
Department: Deck
Active: 1963 - 1979
 
Join Date: Mar 2008
Posts: 2,214
Quote:
Originally Posted by John Cassels View Post
No , she will trim round the LCF.
I think that's in effect what I said.
Reply With Quote
  #16  
Old 17th February 2016, 23:16
RayL RayL is offline  
Senior Member
 
Join Date: Apr 2008
Posts: 2,105
Has anyone else noticed how often one hears people mispronouncing the word longitude? Lots of people--including clever ones--add a 't' and say "longtitude". It is presumably done to match the word "latitude".
Reply With Quote
  #17  
Old 18th February 2016, 00:07
Varley's Avatar
Varley Varley is offline   SN Supporter
Senior Member
Organisation: Merchant Navy
Active: 1971 - 2011
 
Join Date: Oct 2006
Posts: 9,671
I usually pronounce the geographical version Longditude more like the naval architectural term, certainly no 't' but I take the point (especially since learning more properly of Harrison and down-done Maskelyne).

What about Arctic and Antarctic?
Reply With Quote
  #18  
Old 18th February 2016, 03:35
slick slick is offline  
Senior Member
Organisation: Royal Fleet Auxilary
Department: Deck
Active: 1958 - 2002
 
Join Date: Mar 2006
Posts: 1,516
All,
Reference Longitude pronunciation, as I have alluded to elsewhere Paxman used Longtitude on University Challenge, more recently Michael Portillo's journeys in America at the US Naval College I think I heard a learned person do the same.
My wife thinks I am being picky....

Yours aye,


slick
Reply With Quote
  #19  
Old 21st February 2016, 14:57
Varley's Avatar
Varley Varley is offline   SN Supporter
Senior Member
Organisation: Merchant Navy
Active: 1971 - 2011
 
Join Date: Oct 2006
Posts: 9,671
Quote:
Originally Posted by Varley View Post
I usually pronounce the geographical version Longditude more like the naval architectural term, certainly no 't' but I take the point (especially since learning more properly of Harrison and down-done Maskelyne).

What about Arctic and Antarctic?
How terrible Eric Lousada might have resorted to the cane. No Longditudinals either only longitudinals. Very embarrassing.
Reply With Quote
  #20  
Old 22nd February 2016, 04:36
allanc's Avatar
allanc allanc is offline  
Senior Member
 
Join Date: Aug 2005
My location
Posts: 698
My old seafarer sailing mate refers to me as an Artitect. That really grates, I wonder if it might be Artbishop, Artangel etc?
Reply With Quote
  #21  
Old 22nd February 2016, 08:49
Chris Isaac's Avatar
Senior Member
Organisation: Merchant Navy
Department: Deck
Active: 1962 - 1980
 
Join Date: Jul 2006
My location
Posts: 3,893
Enough !
There is no such word as Longtitudinal and certainly no such word as Longditudinal.
It is Longitudinal...... simples !
Reply With Quote
  #22  
Old 22nd February 2016, 10:21
Barrie Youde Barrie Youde is offline  
Senior Member
 
Join Date: May 2006
Posts: 6,923
And, while we are at it, we are in February - so don't forget the middle "r"!
Reply With Quote
  #23  
Old 22nd February 2016, 11:12
Chris Isaac's Avatar
Senior Member
Organisation: Merchant Navy
Department: Deck
Active: 1962 - 1980
 
Join Date: Jul 2006
My location
Posts: 3,893
And..... if you want some specific it is "specific" not"pacific".
And another thing. On Countdown you ask for a consonant not a continent.

Now I am going back to bed.
Reply With Quote
  #24  
Old 22nd February 2016, 11:39
Varley's Avatar
Varley Varley is offline   SN Supporter
Senior Member
Organisation: Merchant Navy
Active: 1971 - 2011
 
Join Date: Oct 2006
Posts: 9,671
Alright class swots. Some might say understanding the meaning is more important that being speckie-four-eyed with the spolling and grandma, my wrongitude at those can often be calculated.
Reply With Quote
  #25  
Old 22nd February 2016, 11:56
Chris Isaac's Avatar
Senior Member
Organisation: Merchant Navy
Department: Deck
Active: 1962 - 1980
 
Join Date: Jul 2006
My location
Posts: 3,893
Quote:
Originally Posted by Varley View Post
Alright class swots. Some might say understanding the meaning is more important that being speckie-four-eyed with the spolling and grandma, my wrongitude at those can often be calculated.
Yer wot ?
Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Cruise Ship Stability fred henderson Modern Cruise Ships 37 16th September 2012 22:03
Stability Problem dundalkie News and Views from the Shipping World 5 9th August 2011 19:12
Static Stability IAINT News and Views from the Shipping World 38 18th October 2010 20:23
Stability kepowee Container Vessels 21 4th January 2007 10:57



Support SN


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
User Alert System provided by Advanced User Tagging v3.1.0 (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.