Dose anybody know if the the answer to this question is right because I'm finding conflicting answers and i tried T.C web site for clarification? To me it dose not make sense to mandate oil tankers to have them by 2015 in Canada, and then let them carry fuel oil ( Diesel) in them, do oil tankers have different restrictions then general cargo and passenger ships ?

Q- What is the double bottom tank of a ship used for?

A-The double bottom tanks of a ships are primarily used for ballast when the ship is traveling light or for trimming the vessel loaded. Although they can be used for carrying fuel oil and fresh water, fuel oil storage tanks must only be used for fuel oil.
In cruise ships the double bottoms may be used for holding grey water (washing and laundry water) until it can be discharged overboard when in open waters.

Thanks too anybody that has something to add

That is an excellent question and I have moved it into it's own thread so it will not be missed.

(And IMO, the TC website is horrible for clarification of any issue.)

I wondered about this same question a few weeks ago when I was studying for the 3rd class ekg.

From what I understand, MARPOL states that tankers of double hull construction are to use the wing and double bottom tanks surrounding cargo tanks and pump rooms as ballast water tanks, meaning that they can't carry any kind of oil product in the double bottom or wing tanks that protect the cargo tanks or cargo machinery.

I hope that helps.

My experience on tankers has been that double-bottom tanks forward of the cargo pump room are not to carry anything other than ballast water, whereas double-bottom tanks in the cargo pump room and the engine room can carry all hydrocarbons necessary for the operation of the ship: diesel fuel, lubrication oil, hydraulic oil, etc. Wing tanks can only carry ballast water, no matter where they are located. Deep tanks forward of the cargo pump room can be filled with liquid hydrocarbons as long as there is a wing tank separating the deep tank from the ships' side. For updated rules you can look up the latest version of ISGOTT, which is the tankerman's ''bible''.

One important function of the double bottom tank is structural strength. The first ship to be fitted with double bottom tanks was Brunel's S.S. Great Britain and she went aground on the Irish Coast on her maiden voyage and survived intact for a year before she was salvaged, and she is still with us in drydock at Bristol.
The longitudinal strength of a ship can be viewed as a Box girder, the ships sides, deck and bottom make one box girder, the double bottoms make a 2nd Box girder and the cargo /Machinery spaces make a 3rd. If wing tanks are fitted these make an additional 2 box girders, and greatly improve longitudinal strength.
The double bottom and wing tanks can be stiffened with transverse floors and longitudinals in a way that would be impractical in machinery or dry cargo spaces and impossible to clean in cargo tanks.
One of the advantages of a double hulled tanker design is that the frames can be put in the wing and double bottom tanks, making draining/ stripping of cargo quicker and easier and also facilitating tank washing.
Some tanker designs even have the frames for the deck plating on deck rather than inside the tank.

I wouldn't be worried too much about pollution from oil in double bottom tanks, oil floats on top of sea water, and unless the tanks are completely full or the ship eventuall breaks up, little oil will be lost.
There was a very cogent arguement put forward for designing tankers on this principal rather than adopting double hulls, but it required a basic level of technical comprehension to understand, so it was far beyond any Politician's comprehension.

Good luck.