Hull Strakes

A recent IBEX conference included an informative seminar addressing construction, location, and hydrodynamic aspects of strakes, steps and chines.

Strakes are longitudinal ridges, mostly installed in the forward part of a hull in the vicinity of the waterline.  Their purpose is usually to deflect spray and sometimes to stiffen the hull.  Considering performance on the water, the opinion at IBEX was that strakes should be close to parallel to the keel (see left side of Figure 1) rather than spaced more or less equally between the chine and keel at given sections (see right side of Figure 1).  The reason for this is that alignment parallel to the keel will be closer to the direction of flow and hence incur less resistance than strakes, which cross the flow lines.

Click on figure to enlarge.

Regarding longitudinal location it was also considered inadvisable to extend the strakes forward or aft into areas where they would have no effect or add resistance.  The aftermost extent recommended was the “stagnation” line – which is essentially the waterline at maximum planing speed.

Very convincing evidence was presented for the detrimental effects of stakes (a) being in alignment with the propellers and (b) not having faired endings.  Underwater videos showed clearly that these features either singly or in combination can cause turbulence and cavitation or ventilation to be channeled aft into the prop resulting in loss of performance and, in some cases, engine overheating (see Figures 2 and 3).

Click on figure to enlarge.

Click on figure to enlarge.

For more information contact the NSBA office at 902.423.2378 or email nsbaexec@eastlink.ca.

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