SUPER CAVITATION TIP SHAFTS

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.

The actual sharpened tip could be a pencil point, a tri-cut or a quad-cut termination. The cone built into the rear of the tip provides a flow shadow for the folded flopper located immediately behind it. Super cavitating ammo has been developed by the US Navy, but involves a propellant that is carried on board that provides a nose jet for the cavitation sheath generation and a rear jet for continued propulsion after exiting the gun barrel.

http://www.military.com/video/…-ammunition/5389871935001

http://www.defensereview.com/r…s-debut-photos-and-video/

The idea here is to keep it simple, standard spear shaft with an integral flopper and a replaceable ring grooved tip which can have different case hardened points. If you smash the tip into a rock then you replace it with another tip, the benefit expected is improved flight time in the critical first section of spear travel. As shots rarely go beyond 30 feet underwater, and are often around 15 feet off the tip, then any gain would be worthwhile and you don't whittle down your integral tip shafts through repeated re-pointing on the workshop bench grinder.