[hpv-boats] Wetted Surface Area

[Rick Willoughby]

Ian
Somewhere between the 250mm and 300mm will give the lowest drag  
hull.  The wider you go the more wavemaking so you are better off  
staying on the narrow side of the lowest wetted surface.

You can do a little better with a rounded section but then the beam  
goes up unless you go longer.  For speed under 12kph the round  
section will be 1 to 2% better then the rectangular in total drag if  
the same weight but I find it possible to build the boat lighter with  
flat sections.  You also get a slight advantage from the flat bottom  
with planing forces above 14kph.

There is some fine tuning in the ends to get the best combination of  
friction resistance (wetted surface) and wave drag but what you have  
is not far off it.

Rick

Rick
         Have been thinking about what you said about
wetted surface area. I did some sums based on a
canoe shaped hull wth flat bottom 5m long and with
a load of 100kg.
I worked out the wetted surface areas for different beams
from 200mm up to 400mm.
Beam  200mm WSA  2.14m2  Draft 145mm
Beam  250mm WSA  2.02m2  Draft 116mm
Beam  300mm  WSA  2.00m2  Draft 97mm
Beam  350mm  WSA  2.00m2  Draft 80mm
Beam  400mm  WSA  2.10m2  Draft 72mm
 >From this the 350mm hull might be the best as it
has the lowest draft to WSA. Shows that the narrower
you go does not always give you the fastest hull.

Ian