The problem with the 7.5 mm is finding a ball cutter to cut the track. 9/32 and 5/16 are much more common sizes that I can use without breaking the bank.
thanks for the input though. 7.5 mm definitely gives me an in-between size that I hadn't considered
Background:
Ive finally compiled all the parts necessary to start my first build. Its going to be a 55" rear handle plus, euro style gun with a poured epoxy enclosed track. stock dimensions right now are 1.8" wide and 1.6 thick. The main use for this gun will be reef hunting but I will b attaching a reel and I would love to take this gun offshore for dolphin some time. I plan to power the gun with between 2 and 3 5/8" power bands. 2 for reef fishing and adding a 3rd for offshore or spooky reef fish.
Question:
Im stuck between using a 9/32" and 5/16" shaft for this gun. I currently shoot a riffe 100X with the 7.1 mm (9/32) and have had no problems with it at all. However having a discussion through one of Tinman's threads about shaft diameter, Im torn between the shaft that Im comfortable with (9/32) and that has less recoil and the shaft that has superior range and punch for larger fish but also higher recoil.
Thanks guys
Try Under Armor underwear/compression shorts. They are extremely comfortable they are form fitting and they can be used as regular underwear when you aren't diving. I wear them every day as regular underwear but they are awesome underwater and because they are synthetic they don't stretch out underwater.
The stock is 6 lams of Teak made by WoodGuy and the dimensions are intended to model the Riffe euro series which is the gun I currently shoot. I think what Im going to synthesize all the info you have given me and do what best fits my budget.
I agree with the idea of the wicks and this does seem to be a major oversight on my part. What I think I will do to save money on shipping and other materials will be to do a half pour with the microfibers (shipping and restocking is almost what I payed for them). Then instead of buying the 404 I will just do a graphite and catalyzed epoxy pour to be milled out. Think that will work? I had thought about the UHMW tracks but I don't want to have to deal with the thermal expansion.
I am aware that the 207 provides almost no UV protection and I have been comparing the different protection options and Envirotex is one of my top choices. The reason for the 207 is because I plan to wrap the gun in a CF sleeve and I want it to be very clear. I also have some other friends who have some projects that want to use the 207.
Thanks for the help you have definitely brought up some things that I had not considered.
Thats very helpful. The reason I was directed towards the 207 is because I planned on using it to coat the gun. and The 105/207 combo was only a few dollars more than the 206. The stock dimensions for my gun are 1.75" w by 1.5" high so it is a very slim profile. As for your mix directions are those by volume or by mass? Ive seen a lot of posts that recommend the 403 why is that?
Im looking to start my build on a 55" rear handle plus speargun. I plan on pouring an enclosed track and I have the West Systems 105/207 epoxy and the 403 microfibers. Im hoping some of the more experienced gun builders can comment on the proper ratio for the epoxy:microfiber:graphite. Also I have a fairly large supply of Molybdenum graphite ( molybdenum disulfide) which I have left over from my years in the pinewood derby. I was wondering if it is a suitable replacement for standard graphite? In dry applications it is a superior lubricant to graphite but because it is metal based I don't know how it will mix with the epoxy or if it will eventually corrode from the saltwater exposure. All input is much appreciated.
I dont have any "ethical" problem with using chum. IMO it takes a bit of the sport out of it. In certain situations like Dan and Jeff mention it may be necessary but i much prefer the sport of finding the fish and exploring the reef/wreck/structure to the target practice of chumming. However I will admit on days where I go out to shoot dinner I have used chum when the action is slow.
By making it longer you would be able to add mass but then you would sacrifice speed and pull even again. The comparison of shaft diameter is only effective if that is the only variable. Just like in any science experiment it becomes a lot harder to measure multiple variables at once. By keeping everything the same (bands, gun, shaft length, etc you can calculate the effect of a change in mass.
I guess the real trade off here is mass vs recoil and velocity. In water a much heavier object will continue to move longer but will be much slower. A fast moving spear will get to the fish much faster. A really heavy fast moving spear creates a lot of recoil.
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?
I found .5 as the drag coefficient for a cone (found online) which is the shape of the spear tip (minus the flopper but I doubt you could find that info). And the italic p is rho = the density of the fluid. For water I used one although salt water would be a little denser. The graph would probably look similar to 1/x the lighter shaft starting at a higher velocity but initially decelerating at a faster rate. The heavier shaft having a lower initial velocity and not decelerating as fast. The drag wont stop them completely because as the velocity approaches 0 so does the force of drag. So the graphs of both shafts should approach zero and the farther from the initial point the closer the graphs should get.
Ok for the first part i meant cross sectional area of the spear and your right those numbers arent correct. they should be (.076 for the 5/16) and .062 for the 9/32. Sorry about that I wasnt very careful when I was transferring this from paper.
For the sumF=ma the 9/32 spear was assigned the mass of 1. so the .44 number is the force divided by 1. 4 is the the velocity (2) squared not the mass.
This Is pretty exaggerated and in practical application the difference in mass is not nearly what I described. I simply used those numbers to simplify the calculations. In hind sight probably a bad idea.
But throughout the entire shot the 9/32 spear looses power. If you were to graph the velocity vs time for both shafts in water you would see that the velocity of the two shafts quickly become very similar and at this point which isnt very long after the shot the lighter shaft is now at a disadvantage because is has less momentum. This is further compounded when striking the target. Impulse (J) = change in momentum/ time it takes. And because the target exerts about the same force on both both spears the lighter shaft slows down faster and penetrates less.
So while none of these factors seem that large individually they add up and result in penetration.
As for why would anyone use them if it seems like they have no advantage my thinking on this would be that even though they have less penetration it is usually enough to penetrate most fish that would be shot and there is less recoil making them a little more accurate for smaller guns. But when lots of penetration is really needed and recoil can be offset like in large tuna guns I would be very surprised to see a 9/32 in shaft used by anyone. Ideally you could make a 9/32 shaft have the same mass as a larger spear and then you would see then used more.
This is actually related to your other question of the rate of band contraction. More bands doesnt always equal more velocity and more velocity isnt always better. Shooting a heavier spear at the same velocity gives you a better punch than a really fast light spear. And even for objects that are very hydrodynamic water does have a significant effect and the only way to really "combat" this is with added mass.
This is so much more interesting than the projectile motion we do in physics.
Energy is only conserved in the absence of friction. In this scenario there is both friction from the spear on the track and the drag force of moving through the water. Because of this the energy stored in the band will never equal the kinetic energy of the spear.
Jeff the spring rate of the band is definitely not linear but that is a completely different problem.
I think some of the confusion here might be with the use of KE. In this scenario the conservation of mechanical energy doesnt apply because of the force of friction. Instead what is most applicable are the equations: sum(F)=ma. and p=mv. These are Newtons second law of motion and the equation for momentum (p). Also Force(drag)=1/2(p*(v^2)(C)(A) is critical. What is most important in this formula is the fact that the drag force is proportional to the velocity squared. In your test for penetration, using the same bands shooting different mass spears your acceleration will different. the lighter spear will, like you said have a higher velocity.
From the instant the bands no longer apply a force on the spear the only force acting on them will be the the force of drag. 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.
To summarize, the (9/32) shaft has a smaller mass, a higher drag force and a higher initial velocity.
the (5/16) shaft has more mass, less drag and a lower initial velocity.
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.
Now according to Newtons second law sumF=ma the accel for (9/32)= (.11)(4)=.44 and for the (5/16) = (.12)(1)/4 =.03 creating a ratio of about 15:1. Because of this in very short distances the lighter shaft slows down significantly. This will change at longer ranges because the drag force will decrease as the velocity decrease. There will be a point some distance from the barrel that both will penetrate the same distance.
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 really hope this makes sense and that I didnt loose anyone with the simplifications or the notation. Jeff this should explain your experiment. This also explains why heavier shafts can travel farther. Now I need to go do my actual physics homework.
Hey Jeff can you post what I PMd you the other day. And although that video shows us that a rubber band contracts at one speed the chemical composition of each band will change its properties so that number cant be used for spearfishing bands.
That knot that most shops use to attach line to the reel is a uni-knot.
Any of those programs available for a Mac? Mac has the iMovie but I dont know how well it works with HD Mp4
I'd Love to participate in this so count me in. I also have a really nice dSLR camera if you wanted some stills from above the water. When is this project projected to take place?
Hey Wood Guy, I dont have the ability to pm yet. Can I email you? My email is Pergakis88@gatech.edu
There is a housing that is made for the go pro called the eyeofmine and it has a flat lens and great pressure rating. Ive seen some great footage taken underwater at depths over 100 ft.
Ive seen some pretty good footage from those cameras. A tip I heard about using those cameras is to use the eyeofmine housing. It has better depth capabilities and is very clear. Check out some vids on youtube by a guy with the sn blessumm. He shoots his bids with a gopro hd and they are great. I was wondering what kind of skill level are you looking for to film this video. I would love to participate but Ive only been spearfishing for a little over a year and havent done any videography. I am big into photography though and i just havent had a chance to get into filming my dives.
Thanks for the warm welcome guys. I have already found a huge bank of knowledge here and cant wait to start on my first gun. LunkerBuster ill definitely let you know next time I'm home. Ive started to out dive most of my hs friends and Im looking for a dive partner. Go Jackets!!!
Hey Wood Guy, looks like a great build. Is there any chance your selling those wood blanks?
