These are my thoughts on the length of the spike http://spearfishing.world/spea…-spike-for-spearguns.html Basically just long enough to be able to detonate a PH.
No matter how long or sharp a spike is you will NEVER be able to kill a free swimming shark with it, these are the sharks we're concerned about. My experience is that shark skin is very tough yet very sensitive. The sharks react to the poke instantly not giving the tip a chance to sink in. If the spike were able to sink in deep it would not reach any vital organs and it would not hurt the shark any more seriously but it would increases the possibility of the shark ripping the speargun out of your hand. I remember reading an old book where divers would make a shark stick from a wooden dowel by nailing an upside down beer cap to the tip of it.
Consequently for it's intended use I don't think the spike has to be long at all, it need not protrude more than 1.5" in front of the speargun muzzle. This is also long enough to accommodate and detonate a slip-on powerhead.