A sharp edge enables better flow separation.  That means water is less inclined to stay attached to the surface and get lifted.  If the flow stays attached at the bow it creates spray and there is a small cost in drag by accelerating that water.  For a long slender hull with a canoe stern there may be a slight difference in the hump behind the hull due to flow staying attached but there could be benefit in pressure equalisation at lower speed with smooth flow around the chine.  A wet transom will lift water causing added drag if the edge is not sharp but not applicable to a HP..

The other benefit of a hard chine is the reduction in pitching.  I can feel this on a pedal boat where there is a slight slapping as the bow with a flat bottom enters a wave.  You can hear the wave slapping (popping sound) of the very lightly loaded stabiliser as it hits waves in this link:

This is something I model and can give numbers for different section shapes in terms of wave induced drag in given wave conditions but I have no way to measure the reduction in drag to confirm my calculations.   The difference between 2 and 30mm radius is too small to make much difference in added drag due to changes in pitch damping. 

The linked paper has some interesting images and results from adding spray rails to a soft chined “slender" planing hull:

It discusses both calm water drag and wave induced drag.  It is not directly applicable to a HP hull but it gives insight into attached flow and pitch damping.  The bottom photos in Fig 4 of the hull without the rails really shows what attached flow looks like although in this case it is off the aft sections rather than the bow that I have seen on the 18m proa and other soft chined narrow hulls.

Rick

 

• Privacy • Unsubscribe • Terms of Use

SPONSORED LINKS