When a spear is fired from a gun it leaves at its maximum velocity, after that hydrodynamic drag and towing the shooting line to the target slow it down while the force of gravity pulls the shaft towards the bottom. Except for judged lobbed or parabolic shots the most useful part of the flight to the target is the linear trajectory travel zone. In addition to the spear tip driving through the water and the suction created behind the spear tail the wetted length of the shaft surface area adds to hydrodynamic drag. If a cavitation sheath can be created early in shaft flight, which takes considerable gun power, then the drag on the wetted shaft surface area is reduced thereby prolonging the flight at higher velocity and the length of the linear trajectory travel zone.
Integral tip shafts have low drag, but have poor durability when striking rocks, but it seems to me an old idea can be revived that will both minimize this problem by using replacement tips and assist in creating a cavitation sheath. The small rear step in tip diameter could serve as a vortex or turbulence generator to produce a cavitation sheath along the shaft length and possibly some rings grooved into the replaceable tips would help create this effect. The sheath will collapse eventually, especially at higher pressure with increased water depth, as the spear velocity falls, but it may buy some extra linear travel distance and decrease the reaction time available to the fish for avoiding the shaft.
What I don't know is the best place for the fine grooves to be located, but if the attachment screw thread makes the threaded wall thickness too thin then they may be better placed forwards on the replaceable tip. This vortex-cavitation idea was canvassed once before, but the series of ring grooves then ran along the shaft which would be like shooting a very long rat tail file from your gun! It could ream the gun’s shaft guide track if the fluid squeeze film collapsed which the grooves may just do being like the sipes on a tyre tread.
Integral tip shafts have low drag, but have poor durability when striking rocks, but it seems to me an old idea can be revived that will both minimize this problem by using replacement tips and assist in creating a cavitation sheath. The small rear step in tip diameter could serve as a vortex or turbulence generator to produce a cavitation sheath along the shaft length and possibly some rings grooved into the replaceable tips would help create this effect. The sheath will collapse eventually, especially at higher pressure with increased water depth, as the spear velocity falls, but it may buy some extra linear travel distance and decrease the reaction time available to the fish for avoiding the shaft.
What I don't know is the best place for the fine grooves to be located, but if the attachment screw thread makes the threaded wall thickness too thin then they may be better placed forwards on the replaceable tip. This vortex-cavitation idea was canvassed once before, but the series of ring grooves then ran along the shaft which would be like shooting a very long rat tail file from your gun! It could ream the gun’s shaft guide track if the fluid squeeze film collapsed which the grooves may just do being like the sipes on a tyre tread.
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