Funny, I'm actually working on a new polespear right now . . .
For simple cylindrical shapes, I think that clear PVC heat shrink is pretty tough to beat for low price and simplicity. The problem arises when you want to do a shape that isn't round or elliptical. The heat shrink tends to pull tight at the corners, but not along the flat sides. I suspect (because I have never tried vacuum bagging) that it is more and more advantageous as the shape gets more complex.
Very cool. Is the excessive foaming with epoxy a problem only at the surface (maybe not a problem if the blank was a bit oversized), or does it affect the integrity of the whole glue joint?
In speaking with a gunbuilder that's already tried vac bagging with epoxy, I was told it created excessive foaming at the glue joints due to all the air being sucked out of the grain of the wood . . .
Nice fish Davie! 
Truly I don't know what someone would do to make it deteriorate. Reminds of the the idea that a poly rope is prone to tangles as compared to the tube float line. I never got a tangle in it so I just don't get it.
Poly frays easier than a pvc tubing. Poly is softer and thus is more prone to tangles and knots. The pvc tubing is too thick to knot.
I made my first homemade float line last winter. It's certainly not the best, and i've had to make some changes to it over the months, but if I had to make a new one today, I would definitely know the best way of doing it. Its a learning process, but i'd def stick with my homemade float line.
I think what Jeff was looking for was simply what are the theoretical thresholds of band speed, and adding more bands vs a heavier shaft. Simply put how many bands do you really need and at what point does that number become ineffective or a waste and would it be more effective to increase shaft diameter or add more bands?
Exactly. Simply put, is there a point where a smaller shaft no longer goes faster, or where adding bands no longer produces more speed.
That would be an interesting graph, and doable with a few simple assumptions:
Let's assume that the full potential energy of the bands is transferred to the shaft for velocity purposes (ignoring track friction, etc.).
In your drag equation:
A = Shaft cross sectional area
C = Some sort of shape / drag coefficient (you used 0.5)?
p = Is this the density of water?
. . . I promise to be on my best behavior.....
Well that won't be any fun . . . <img src= " height="23" srcset="https://spearfishing.world/forum/images/smilies/emojione/1f606@2x.png 2x">
Welome Rich.
For the the cross sectional area for your two bands differs by about .05". so in your drag equation you have .5(v^2)(.5)(.49)=Fdrag(5/16). For the 9/32" shaft you have .5(v^2)(.5)(.44). These simplify to (.12)(v^2) and (.11)(v^2) respectively.
You lost me a little here. You said "area for your two bands", but you used the shaft diameters? Where did the 0.49 and 0.44 numbers come from? They're not the cross sectional areas of the shafts, at least not in inches.
Next comes newtons second law. sum(F)=ma=Fdrag
after the bands have released the shaft the only force on the spear is drag. just to simplify this without knowing the actual mass difference of the shafts: mass of(5/16)= 4. Mass of (9/32)=1. NOTE: this is an exaggeration to explain the difference. As a result the initial velocity of the smaller shaft is twice that of the heavier shaft. and experiences 4 times the drag.
I know you were just trying to illustrate a point, but calling the 5/16" shaft 4x as heavy as the 9/32" shaft threw off everything. Just ratio the squared diameters, and you get 100:81, or the 5/16" shaft is about 1.25 times the mass of the 9/32" shaft.
Now according to Newtons second law sumF=ma the accel for (9/32)= (.11)(4)=.44
Lost again. If you are solving for accelleration (a), then shouldn't a=F/m? Why did you multiply F*m?
So even though this is a significant exaggeration the lighter shaft slows down a huge amount in the first few feet and because momentum = mv and the the shaft is lighter but not enough faster to compensate. Now because the heavier shaft has higher momentum it then hits the fish/target with more punch.
I know that they slow down faster, but it can't be quite so detrimental as this suggests, or no one would use them.
What actually prompted my whole line of thinking was some testing that Wood Guy did with his roller gun in the test tank a while back. If I recall correctly, the 9/32" shaft didn't penetrate the foam target nearly as deep as the 5/16" shaft, even at close range. This was a short shaft, with low mass. So one of the things we talked about was the possibility that lighter and lighter shafts go faster and faster but only to a point. We wondered if in the real world there was a "wall" beyond which latex bands couldn't make a shaft go any faster. Beyond that, a lighter shaft would leave Kinetic Energy "on the table" because lower mass wouldn't be balanced by increased speed.
I understand that a rubber band isn't the same as a speargun band. Look at Mori's test data from years ago and it's obvious that there is enormous variation even within band material.
But so far, I haven't found any high speed close up camera footage of speargun bands. I figured the rubber band would at least give an "order of magnitude" estimation.
Below is the relevant part from your PM. Your point about whether the shaft reaches max speed in the short length of the band stretch was my point all along. Whether it does, or not, depends on a lot of things, and one of them is how fast the bands are capable of contracting.
If you really got into it, you would probably find that the spring rate of rubber isn't linear, and likely varies with the percent elongation. One problem at a time . . .
Without doing any math or research that sounds perfectly accurate. And there is definately a maximum rate of band contraction. The only thing that you seem to forget is wheter that shaft can reach that max speed in the length of the track. If it would take 100 in for 4 bands to accelerate the shaft to the max speed and 6 bands can do it in 60in it would make sense to add the bands and recoil. However if neither are capable of reachin max velocity within a feasable length of track or if the gain is minimal then like you said it wouldnt make sense to add that much power. Thats why most manufacturers show approximate ranges for each gun depending on the length of the gun not by which band set up. Longer gun equals longer accel for a longer time.
So to answer the question yes there will be a point where adding more bands no longer results in more velocity. But that could be a lot of bands in order to reach that velocity in the length of a gun. Somewhere there becomes a trade off between the number of bands that can be comfortably controlled, the length of the gun that is manuverable and how far you really need to shoot.
I never really meant to suggest that the contraction speed of the bands is ACTUALLY a limiting factor on any typical guns. I was just curious how quickly they snap.
Here's a high speed video of a rubber band snapping back. It looks like his clock starts about 0.4360 seconds, and the band is fully contracted at about 0.4400 seconds. If I estimate the distance as 3", then the speed is 3" per 0.004 seconds, which is about 62 ft/sec.
Under the section titled "Dynamics"
I agree that it's probably academic for spearguns of any practical size. Just curious how fast bands could really contract.
I completely understand and agree with your point about light, fast shafts losing energy quicker AFTER the shot, but that's not really what I'm focused on.
But just to clarify, it's not the energy stored in the bands that counts. What matters it the ability to transmit that energy, in the form of work, to the shaft. And that ability is very much dependant on the difference betwen the potential contraction speed of the band, vs. the speed of the shaft. Right?
In the real world, bands may contract a zillion miles an hour, such that they always have the ability to pull on the shaft. Just wondering.
This is one of those things that may have little practical application, but it still rattles around in my brain sometimes.
How fast can a rubber speargun band contract, and what (if any) limitations would that place on gun design?
My thinking goes like this. Suppose I am pushing my kid in a wagon. No matter how big or strong I am, (or how big or small my kid) that wagon will never go faster than I can run. I can get three friends to help me, and we might get the wagon up to to speed sooner, but it will still never go faster than we can run. All the extra strength of my three friends is wasted, because the limit is how fast we can run.
So I was wondering. There must be some maximum speed at which a speargun band can contract. So at some point, adding more bands to a gun might not increase the shaft speed at all. Or, moving in the opposite direction, using a lighter shaft might not result in any additional speed.
Anyone ever seen info on how fast a rubber band can contract?
Very clean looking! I never thought of doing a line anchor that way.
I am a freediver. I go diving very frequently and when I dive two days back to back, I find that my ears cannot equalize the second day. I literally cannot clear my ears. It feels that my eardrum doesn't not move to equalize. Its like the muscle that moves the eardrum is frozen in place. I do a hands free equalization and it works very well for me.
Today I went out and my ears locked up half way through the day while I was lobstering in 30ft. I pushed it even though it hurt like hell. Now it feels like I have lost some hearing in my ears.
Does anyone have any suggestions as to how to help my ears equalize more easily when diving back to back?
thats exactly right... its colder on top and the warmer water was down below.
went out "south of Port Everglades" today. I think I saw you Putmeincoach. did you have a red yak on the first reef?
We got two legal grouper and 20 lobster today between 4 ppl. Viz was ok.. with that layer of brown water on top of the clean water which kinda messed with the viz. I was actually getting really cold wearing my 1.5mm mako wetsuit today. Guess its time to get that 3mm out!
3.. why?
any idea when they'll have their 2011 schedule up? I coulnd't make any of the courses in 2010 and am dying to take the course!
finally a recipie that I can use with the "only asian ingredient" kitchen supplies that I have lol!
