Intruder: A Device to Improve Hull Performances

The intruder is a device that has been used for a long time now on big fast monohulls like "Acquastrada". It consists of a small vertical thin plate positioned under the transom of a planing or semi-planing hull, jutting out just a few millimetres. It is able to improve hull performances assuring a smaller drag thus a higher speed for the same installed power. For this reason, for its working and installation simplicity, as well as for its low cost, the intruder is becoming increasing popular even with small working and sports crafts. In order to improve the knowledge about the hydrodynamic effects caused by this device, some tests have been performed at INSEAN using three planing and semi-planing models. As the results of tests show, the intruder changes significantly the dynamic trim of the hull and it raises the stern. These effects can reduce the wave pattern and thus the drag. The improvements are larger in the pre-planing phase (1<FnV<2,5) where drag reductions - of more than 20% - have been measured.