http://upload.wikimedia.org/wikipedia/commons/e/e0/T-foil.PNG
The T-foil is used in combination with two transom mounted trim tabs providing ride control forces equivalent to a pair of keel mounted T-foils ensuring a smooth ride in heavy seas. The retractable T-foil is used only when it is needed hence there is less drag to slow the ship, with resultant savings in fuel and less chance of the foil striking a submerged object.
http://www.incat.com.au/domino/incat/incatweb.nsf/v-title/Passenger%20Comfort?OpenDocument
