How to tackle this impulse ?

[Externet]

Hi all.

My speargun takes 80 lbs-force (355 Newton) to pull its elastic rubber band that will impulse a 0.4Kg spearshaft when pulling the trigger.

Assuming no friction nor drag by water, when 'firing', the elastic band applies its force to the spearshaft while contracting during -say 0.02 seconds-
The push force decreases linearly? to zero by the moment the spearshaft 'takes off'; I believe as a compressed spring pushing a streamlined object.

Is it rational to say that a lightweight spearshaft will have little penetration into a target; and if it is too heavy, lacking initial and final velocity will also have little penetration into target ?

How to determine the optimal mass (or weight) of a spearshaft for most penetration at target for a given initial force ?

Just a picture as aid ----> http://www.thespecialistsltd.com/files/Speargun.jpg

[swansont]

What is the best measure of penetration? Kinetic energy or momentum?

[Externet]

Unsure if in English is also called impetus.

[studiot]

+1, good question swans.

 

If your target is hard ( and brittle) then the speartip will be supplying energy to newly created fracture surface in order to penetrate.

No large fracture force is required just a continuous supply of energy, so maximum impact kinetic energy is required.

 

If your target is soft, as I suspect, then the target material reflows around the speartip and no fracture surface is created.

In this case driving the spear in is more like piledriving or nail driving and the maximum driving force is required.

This, of course, means maximum impact momentum.

[Danijel Gorupec]

But did you suppose an immovable soft target? I have a gut feeling that a small fish my be pushed away by a very slow projectile thus the penetration depth might be reduced - so I guess that projectile speed might be important too.

[Externet]

Still suspect the magnitude of interest here is impetus.

Impetus is defined as being very similar to the suggested momentum. What I do not see clear is the difference between impetus and momentum, being the latter, mass times velocity. Implies a gravity constant. Can someone explain ?

 

----> https://en.wikipedia.org/wiki/Theory_of_impetus Where one says is weight times velocity.

 

Hitting a plain golf ball with the club; and hitting another containing a lead core should behave different in reach, speed and size of the splash into the pond.

 

I have fallen into this same dilemma repeatedly; as example the size and stiffness on diver fins matching the wearer strenght for optimal propulsion. Also, size of oars. Similar to electronics impedance matching, which I do understand.

But never found the same with dynamics. How is it called ?

[swansont]

For collisions, the most efficient transfer is when the two objects have equal mass. The incoming object stops and the outgoing object moves off with all the momentum and all the energy, if it's an elastic collision. But we don't have that situation here. And you may not want the most efficient collision.

 

For an elastic band, you have to figure out how much energy is left in the band when the projectile leaves it, at the point where it's no longer stretched; that's where everything is moving as fast as it can. The faster the band moves the more energy it has that's not transferred.

 

Then you can see how much energy and momentum the projectile has, and see if that has a maximum. The most efficient transfer uses up (essentially) all the KE, but that means a very massive projectile. A very light projectile only uses a part of the energy, but moves a lot faster. Maybe impact speed is also a criterion, if you need some minimum speed to penetrate a surface.

[imatfaal]

Empiricism Rocks. Buy some cheap fish you don't mind repeatedly nailing to a target and experiment. The only time I used a speargun the shaft was hollow with a heavy screw-on tip (changeable for trident, barbed, or plain) - if yours is of similar design maybe try weighting with fishing split shot and testing the underwater penetration on your deadfish.

 

Post photos, videos and results.

 

I will tell you that the spear gun shafts and heads were significantly heavier than the arrows I was more accustomed to using - which makes me think it is a medium thing rather than a penetration thing. In archery you change the head to small narrow and pointed to pierce armour; to wide, barbed and bladed for hunting; and rounded and smooth for targets.

 

The force on a shaft due to fluid friction / resistance is not going to change if you use a heavier material but the same shape. Force equals momentum change over time - which is to say mass times velocity change over time. If the force remains the same but the mass increases then the change in velocity is decreased.

 

Perhaps it is also worth thinking about how the fluid would behave around a very fast moving object - would you start to get cavitation? - would small inconsistencies in the symmetry of the projectile change the flight in a non-linear way?