a ballast is a weight that is positioned low in a boat to inrease ballance and stability, it is possitioned as low as bossible to give it a bigger effect. it is made from a heavy material such as lead concret or ion depending on the amount of room available water can allso be puped in to create an adgustable ballast on larger ships. smaller vessels ballast in the form of a keel that runs bellow the boat allong the ghost line, and yachts often have a bulb on the und of there keel to act as a ballast with more leverage due to the extra distance from the pivot point.

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information: http://www.glen-l.com/resources/glossary.html
images:http://news.bbc.co.uk/2/hi/7392072.stm
http://massbay.mit.edu/exoticspecies/ballast/