A funnel is the smokestack on a ship used to expel A boiler is a closed vessel in which water or other fluid is heated under pressure. ...boiler smoke or engine Exhaust gas is gas which occurs as a result of combustion of fuel such as gasoline/petrol or diesel. ...exhaust
First of all, as Michael and Remco pointed out, the ship was moving as she took her final plunge, and so the forward momentum of the ship might have alot to do with why funnels can be found so far from the wreck.
But on the other hand, remember that the funnels were made from very thin metal, and compared with the rest of the ship they were VERY light, and I wouldn't rule out the possibility that they drifted with the current a little bit as they sank.
Regarding, the funnels, there is a very nice image (from the 1995 expedition) obtained from the side-scan sonar, which shows Britannic's hull and the exact location of each of the four funnels on the sea floor.