Knowledge Base

• How can I distinguish a planing from a conventional hull?

In order to minimise undue water resistance due to flow separation at the stern and bilges, conventional displacement hulls, such as those of merchant ships, have longitudinal and transverse hull curvature. On the other hand, high-speed craft are designed with abrupt buttock lines at the transom stern so as to induce early flow separation. The geometry of planing craft is characterised by sharp corners at the chines and transom which achieve distinct water flow separation, and hence help minimise hull side flow attachment while maximising dynamic lift. In the absence of these abrupt corners the flow paths would become unpredictable, planing resistance would increase and dynamic lift and stability would deteriorate. Planing hulls belonging to this family of hulls, as shown in Figure 1(a), are known as “hard chine.”  

Hard chine and round bildge planing hulls.

Most popular forms of semi-planing mono-hulls are the “round bilge-hulls” and “deep-V” forms shown in Figure 1(b) and (a), respectively. Hulls with more than approximately 22 degrees deadrise are referred to as “deep- V” forms and are renowned for their good seakeeping qualities in comparison to the “round bilge” type.

Hull features such as bottom steps and rails play a very important role in reducing the boat wetted surface and resistance. Other characteristics of planing hulls include trim tabs, transom notches, keel pads and transverse steps. Lastly, both reverse chines and running strakes can extend longitudinally along the hull periphery and act as spray rails which deflect the spray away from the hull, reducing thus the wetted surface area, and helping keep passengers and cargo as dry as possible.



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