The cruise ship Astoria (aka the Stockholm) has a very strange looking stern. I think it has something to do with her stability.
How do you get a substantial free surface effect if you fill up one set of water ballast tank at a time until it overflows through a side or deck discharge port?It is usually futile, and sometimes dangerous, to attempt to cure a rolling problem by working ballast. Think free surface and virtual rise in the position of "G".
Georges, Free surface effect has nothing to do with the content of a compartment. I have no idea what you are writing about. In fact, the Herald of Free Enterprise event is a classic example of what happens when a well deck or card deck is flooded due to taking on a volume of water. This was 30 years ago..... almost to the day.How do you get a substantial free surface effect if you fill up one set of water ballast tank at a time until it overflows through a side or deck discharge port?
Shadowraven posted a message saying simply that "The ship probably would have experienced difficulty in harsh weather." I was asking him/her what the basis of that opinion is.«What's the basis of your opinion, Sahdowraven?»
What do you mean by Sahdowraven, Mark?
I tough you were asking me what was the source of the diagrams I posted …
Jim, I must also admit that I have no idea what you are writing about.Georges, Free surface effect has nothing to do with the content of a compartment. I have no idea what you are writing about. In fact, the Herald of Free Enterprise event is a classic example of what happens when a well deck or card deck is flooded due to taking on a volume of water. This was 30 years ago..... almost to the day.
Free surface effect does not depend on water volume or depth within a compartment. The minute you start filling a ballast tank, or a deck becomes flooded, free surface action begins. It continues until the tank is pressed-up. Don't take my word for it here is the technical position from my Ship Stability & Naval Architecture text book:
Yep! That's what I was trying to tell you. Are you questioning the wisdom of the Naval Architects who very clearly, as I illustrated in my post of No.30, emphasised the futility of what the Bowdich advice you quoted earlier? That's why it is called "Free surface".Erratum… It depends as well on the water «surface area» or the dimensions and shape of the slack tank (length x breadth)!
Georges, show me where it states that these measures should be taken while the vessel is actually engaged in synchronous rolling. In fact, your example agrees with me. The following is an extract from the very publication you use as an example:Jim You and your Naval Architects should modernize, study and amend your stability incantations by reading not only Nathaniel Bowditch American Practical Navigator but also through Captain D.R. Derrett Ship Stability for Masters and Mates written with grateful help and comments afforded by the following personnel in the Maritime Industry. Taking note that those experts agreed and signed their names to «Use water ballast changes to alter the KG of the vessel. This should alter GMt and hence the natural rolling period to a no-synchronous value.»
«1. Use water ballast (not another kind of liquid)to alter the KG of the vessel»
- Captain Sergio Battera, Vice-Chief (Retired) Pilot, Co-operation of Venice Port and Estuary Authority.
- Julian Parker, Secretariat, The Nautical Institute, London.
- Tim Knaggs, Editor of the Naval Architect, Royal Institute of Naval Architects, London.
- Gary Quinn, Head of Testing Services, Scottish Qualification Authority (SQA) Glasgow.
- Roger Towner, Chief Examiner, Department for Transport/Maritime and Coastguard Agency (DfT/MCA), Southampton.
- Captain G.C. Leggett, Area Operations Manager (Surveys and Inspections), Maritime and Coastguard Agency, Liverpool.
- Captain Neil McQuaid, Chief Executive, Marcon Associates Ltd, Southport.
- Malcolm Dann, Partner, Brookes Bell Jarrett Kirmann Ltd, Liverpool.
- Captain I.C. Clark, Maritime Author for The Nautical Institute, London.
- Darren Dodd, Managing Director, Saab Tank Control (UK), Wokingham.
- Colin Jones, Stock Control Manager, DPM Ltd, Liverpool.
I agree that you will have no other choice but to use salt water ballast in the middle of the ocean. Nothing new here. Since the FSE formula integrates liquid density, you will also have no other alternative but to apply sea water density. On other occasions, ballast could’ve been taken in a fresh water harbor prior sailing to expected rough seas. An oil tanker continually manipulates liquid cargo and segregated ballast of various densities. So that’s why I posted a working example using oil; to point out that the liquid density is of prime importance and cannot be ignored.
«The original question was whether Titanic or Olympic had any artificial means of reducing motion in a sea way. They did not.»
Declaring that an Olympic class vessel had no means of reducing motion in at sea is, borrowing your own expression, pure hogwash! She could have easily altered course and speed but to the evidence, it was not in Capt Smith’s priorities! It does not seem yours either. She could as well have taken ballast to increase her drafts, reduce her KG thus rising her GMt and hence the natural rolling period to a no-synchronous value. The cellular wings and double bottom water ballast tanks of an Olympic Class, made taking ballast easily and safely in rough seas. But again, it would have to be done in sequence, one set at a time to reduce the Free Surface Effect to insignificant value and by a mariner who understood the phenomenon.
Jim, like it or not, erratum understood or not…
Rise of GM = GG1 = wd / W₂.
Loss of GM = FSE = lbᶾ / 12 x 1/W₂ x δ x 1/n²
Captain D.R. Derrett...
- where w equals the mass of the water ballast pumped in
- where W₂ equals the initial displacement added by the mass of the water pumped in