I used to worry that the notches in my slim Omer 6.3mm shafts might weaken the stainless steel spears - the notches seem significantly deeper than the polished notches on my old 6.6mm RA spring steel spear. I also smoothed and polished the sharp edged Omer notches to work with the Dyneema wishbones that I fitted without cutting them. But for the fish I deal with, they are plenty strong enough. If you are going for 500lb Tuna or Marlin, your spears will be thicker to start with, so even then...
I bought 2 new Apnea spearguns near the end of last year, one smaller than my main Omer XXV spearguns and one longer. I haven't had a chance to try either yet and, with Corona virus not yet peaking here in the UK, unlikely to for some time. I wanted to try sharkfins and both new Apnea speargun have sharkfins, I wanted to try them. I figured they would be stronger. But the first thing that struck (esp. as I has just read a thread on spears and spear velocity on the DeeperBlue.com forum) is that a raised fin or pin will inevitably slow the spear somewhat due to increased drag and probably a significant amount. So the extra power of the double 14mm bands of the smaller speargun and the roller of the larger speargun will be somewhat offset by the raised sharkfins and heavier 6.6mm spears (the fins themselves will also add weight to the thicker spear). Some say the weight and thickness of the spearline can also affect the speed of the spear significantly, that it affects it is no surprise but significantly? No so obvious to me.
Like so many of these things, you need to balanced several factors and make the best choice for your personal circumstances.
In practice, for most fish, I think it makes little difference, provided other factors (spear diameter, power of the speargun in relation to the spear, etc.). For big game fish, perhaps fins are better being stronger but I have a feeling that a lot of world records were probably caught on Rob Allen or Rabitech railguns with notched spears. But if huge fish are your target, perhaps fins and, perhaps, a slip-tip or Rob Allen's fancy new drop barb would be the way to go? It's extra expense, mass, complication and loading effort/time if you don't really need it though.
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