Stopping Spearguns Underwater
Name: Jon G.
What are the physical factors which would slow down and
stop a speargun shaft underwater?
I suspect it would all reduce to the hydrodynamics of the moving spear -- the
size and shape of the point and the overall length of the shaft. These
characteristics would bear upon the frictional (drag) forces experienced by
the spear as it moved through the water.
The major factor slowing down a speargun shaft underwater is the fact that
the shaft must push water molecules out of the way. This factor, sometimes
called resistance or drag, is truly due to shaft molecules crashing into
water molecules. You cannot use the standard equation (Drag force
proportional to v^2) because the shape of the spear has a strong effect. A
narrow pointed object doesn't have to push molecules aside as quickly as a
Another smaller factor is drag at the back of the shaft. As the shaft moves
forward, there is less pressure just behind it: the water molecules haven't
had time to settle in. This also provides a small backward pull, very much
like suction. How the end of the spear is designed can greatly affect the
strength of this force. Again, a blunt end is pulled harder than a narrow
One more factor is how well water bonds with the spear material. Water
molecules stick to many things, including other water molecules. A material
that water runs of of very easily will not disturb molecules alongside the
shaft. A material that "grabs onto" water molecules will be slowed by
binding with the water.
These are the only three significant factors that come to mind.
Dr. Ken Mellendorf
Illinois Central College
Click here to return to the Physics Archives
Update: June 2012