[PCW] A question of weight

[Grahame Shannon]

Weight is important, in that hull resistance is directly proportional to
weight. In other words, a heavier boat takes more power to push it.
 
If you consider an automotive equivalent, these days car makers often
publish a cd (coefficient of drag) which indicates how streamlined the
vehicle is. These range from about 0.2(very smooth sports car) to 1.00 (flat
plate).  But this doesn't tell you the total drag. That calculation involves
cd multiplied by FRONTAL AREA. Thus a Hummer will have more drag than a
Mini, even if the cd is the same.
 
Boat hulls are similar, but the interaction with water is more complex.
Still, FRONTAL AREA, or if you like maximum cross sectional area, is a key
factor in total resistance. Frontal area is related to displacement.  A
heavier boat will need fatter hulls to keep it floating right, and these
will have greater frontal area.  The alternative is to keep the area the
same and increase the length. Long skinny hulls have less frontal area than
short, fat hulls of the same displacement.
 
Weight matters because as the boat moves, it has to constantly push an
amount of water equal to its own weight out of the way. It takes power to
push that water.
 
If this was not true, you could use the same size engine on a 100' boat as
you would use in a runabout.
 
Heavier boats have a place, and keeping a boat light is tough. It is human
nature to keep adding gear and equipment to fill the available space. A
heavy boat can carry more fuel, water, fuel, and the comforts that make life
aboard more self sufficient.  So, if it goes slower, why worry.
 
Kind Regards,
HYPERLINK "mailto:designer@aviadesign.com"Grahame Shannon
HYPERLINK "http://www.aviadesign.com/"Avia Design Group Inc.
4884 Skyline Drive
North Vancouver, BC
V7R 3J3 Canada



 


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