Terminal velocity is a notion that has crept into speargun discussions, but there is no such thing. What they are really talking about is the maximum stable launch velocity out of a band gun. An unsupported spear can be free to buckle under acceleration and bounce off the gun deck sending the shaft anywhere, whereas a similar sized shaft can be blasted at much higher velocities out of a hydropneumatic gun as the spear is enclosed in a barrel. The attraction of rollerguns is that inherent losses in the propulsion system take the edge off the acceleration which technically is the “jerk”, jerk being the change of acceleration with time, just as acceleration is the change of velocity with time.
If you look through the diagrams on this thread for equivalent sized bands the rollerguns are not necessarily more powerful, in fact they are often less. But a standard gun piled up with bands has such a massive jerk when you pull the trigger that unless the gun is a ballasted monster it is not really that useful to shoot with. Such large guns can also be a pain to swim with, although often banded up and ready to go the diver often drops in beside a teased up whopper with one and blasts the fish from relatively close range.
The more elaborate rollerguns allow much longer bands to be installed on a gun body, yet bending bands around roller and hauling moving pulleys on cables all contribute to extra losses which make the gun easier to shoot at the expense of taking longer to reload. This is all shown in the diagrams by those green energy triangles, you just have to study them carefully.
The inverted rollerguns still recoil as is shown by these photos. Here the gun is tilting up with the shot and moving rearwards, lifting the spear up slighty as it departs.
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I think we have to talk about both terminal velocity and stable one and separate well the discussion on the scientific level hence why I think the tests should be done on a fully stable platform.
Unfortunately in my eyes A LOT discussion is on the feeling not backup by any research.
The russian guy tested various shaft sizes going from small diam to big and high pressures and he was not able to surpass some speeds regardless of power setting. So there is sth to it if lets say scenarios that were tested was sth like this (unfortunately cant find the research anymore):
7mm - 27m/s PSI X (chamber pressure - call it power factor)
6.5mm - 27m/s PSI X or PSI+1X (going down in diameter and/or extra pressure added 0 benefit to muzzle exit and or Joules in
penetration.
So if we operate under this concept that majdaq8 is also mentioning and can be seen in his video that often addding power does not mean energy. This can be both atributed to terminal velocity and lost energy due to unstability where the whip takes the energy off the shaft.
Proper tests should be done couse literally all the discussion here and there IS purely theoretical. We might understand how thing work more or less yet engineers are often surprised in areas where there isnt enough testing done and all things are hypothetical.
Ive tried to look at band stretch equations and most of them are done dry, water as medium changes the results becouse of both density properties, cavitation and water springyness(albait the letter one we can almost nulify as it should be within marginal error as at 100PSI its just ~1%
I mean where else to start from if not from basics and go further into theorethical applications?
Regarding the muzzle lift on release - I feel this topic is so complicated that videos like this might not really reflect that real behavior. There is SO MANY FACTORS involved that I would not look into it (simple video) so much yet unless we can do a very good aplication of band strentch/power ratios in first step.
Not saying it is not a thing (becouse it is, I did use an omer cayman 100 with HF head with double 380% 14mm setup and 6.25mm shafts at some point xD), but we should take it step by step instead of fighting with a behavior that can be probably minimalized if we have the proper tool to judge the power to weight/diameter ratios.
I am thinking about simple tool we could use/make to actually measure the shaft muzzle exit velocity on a daily basis to judge band wear or just simple have a way to adjust band lenght properly. Imagine we had a simple and cheap tool to measure this things, a lot good would come out of it and so many myths could be trashed.
On totally side note, talking about proper science, CARBON guns for deep spearfishing in theory are a BAD material. Why?
Carbon has great tensile strengh but its crap at compression, I have seen at least 2 tubes brake at 30m+
.
