• Welcome to the DeeperBlue.com Forums, the largest online community dedicated to Freediving, Scuba Diving and Spearfishing. To gain full access to the DeeperBlue.com Forums you must register for a free account. As a registered member you will be able to:

    • Join over 44,280+ fellow diving enthusiasts from around the world on this forum
    • Participate in and browse from over 516,210+ posts.
    • Communicate privately with other divers from around the world.
    • Post your own photos or view from 7,441+ user submitted images.
    • All this and much more...

    You can gain access to all this absolutely free when you register for an account, so sign up today!

Measuring speed of the shaft

Thread Status: Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.
I did a first measurement with new speedometer. I measured the speed necessary to separate the shaft from the piston. I did it by hand and the speed was 2.7 m/s. Mass of the shaft was 0.3 kg. So the energy is probably less than 1 J. I said probably, because speed less of 2.7 m/s would be maybe sufficient.
 
Mirage 70 on 19 bar with 770/8 mm spear and 1 mm mono. Vacuum barrel is Tomba800 (higher result). Lower results are with water barrel. I believe this results are satisfactory good and accurate.



I made measurement for Cyrano 850 too (994/6.5 mm spear, 265 gr) that has much larger water exit hols in muzzle, but only for starting speed for vacuum barrel and water barrel. On 20 bar, 995/6,5 mm shaft, 1 mm mono, Tomba650, Cyrano 850 had 30,3 m/s vacuum barrel, and 29 m/s water barrel. Vacuum barrel was 9 % better (energy) than water barrel.
 
Last edited:
Results for speed and energy of the shaft in water.
Mirage 70 on 19 bar, 770/8 mm and 300 g heavy shaft. Spear with the line slide (Tomba800) and mono nylon 0.92 mm.
I combined the results with the results of stretching the rubber hose for a sling gun for a distance of 4.5 m This method with a rubber hose that I have used in the past proved to be very useful for me because it is simple and the result (best-same-worse) can be immediately seen. Basically the vacuum overall energy is about 25% higher than that of the water in the tube. For initial speed of 0 m I did extrapolation since the minimum distance at which I could measure the initial velocity was about 20 cm. I believe that these results are pretty reliable even if not 100% sure.
The highest energy difference vacuum/water is for 0 m distance - 28 % and drops down to 27%, 26%, 25% on 4.5 m distance. Maybe because higher speed shaft has higher losses in water than lower speed shaft.

I had to create a new account (trromic) because I could not attach any new file and could not delete any elder attachment to free the space.
 

Attachments

  • RezultatiMirage8.jpg
    54.6 KB · Views: 396
Last edited:
I made one measurement in power switch position for reduced power (Mirage 70). Speed ​​was about 22 m/s as opposed to full power, when it was about 30 m/s.
With Mirage, speed can be regulated even more. In power switch position for easy loading could be achieved even lower power depending on the number of pumping strokes. If the rifle is pumped 7 times, the shaft could be loaded with two fingers of one hand. For normal charging, loading forces is nearly 30 kgf (8 mm shaft).
 
Reactions: Diving Gecko
I have just put together graphs for 7 mm and 8 mm shaft, measured before. On Y axis is (m/s), on X (m). Distance X is from tip of the spear after reaching max speed.
Better results were for 7 mm. I suppose harpoon with two flopper on 8 mm is not very hydrodynamic. That's why it loose speed more rapidly.



 
I paid subscription for imageshark for one month, so the images are now visible.
 
Last edited:
Reactions: grantl
Hello @tromic ,
I am very late to this game, but just read the whole thread. Such amazing work!
I might actually build a magnet tester like this someday as I have a digital recorder already and with my minilathe and/or the 3D printer, it should be fairly easy to do build the part.

A few quick questions;
- The tube with the magnets doesn't have to be waterproof at all, right?
- Some of you images got lost here. Can you possibly share the images of your latest version of the speed tester if you have them?
- Is there an easy, reliable way to deduct average speeds? Say, we have recorded speed at the initial start of the shot and at 3m, with those two data points can we calculate the average speed over the distance of 3m?
 
Hello @Diving Gecko,
1. There are two tubes with its own windings. Both windings are connected in parallel. Only windings and connection cable to the recorder must be sealed.
2. The latest images are on links:
#99 and #104
3. Average speed is simple:
Va = (Vo + V1)/2
but I think it is more useful to have initial speed (Vo) and final speed (V1 ) separate.
 
Last edited:
Reactions: Diving Gecko
This is such a nice and simple setup. Big congrats on thinking it up and making it work. High frequency/accurate digital recorders are cheap and plentiful these days and because of my work I already have a few. I might give this thing a go[emoji4].
As for average speed it’s just so it would be easier to compare with other people who measure with a GoPro.

Actually, an audio recording app on a smartphone can very likely be used instead of a dedicated digital field recorder. E.g. I think the iPhone's sample rate is 44.1kHz, so about 44 thousands samples per second. Huge resolution to see spikes on the recording.
As such, I think your testing gadget is even more user friendly these days than when you first made it.

Sent from my iPhone using Tapatalk
 
Last edited:

I am confused, I can't really figure out what is going on in that image. Is this the latest setup? The latest version is no longer the longer tube? But now, the two magnets are on the line instead?
We don't even need any electrical components, right? That was only when you wanted to use the multimeter a live, direct read-out of the speed.
 
Yes, that's right.
 
Reactions: Diving Gecko
tromic said:
Yes, that's right.
Click to expand...

Ah, I think I finally got it. The two tubes are much shorter than the single longer one in the earlier version. They only have to be long enough to hold a coil at the front and the “magnet bullet” at the rear in place until the shot. These two tubes are placed next to each other and have one coil each. Inside the first tube is one magnet tied to the shooting line. In the other tube is another magnet with a 20cm piece of wire cable which makes it pull out of its tube 20cm "later" than when the first magnet is pulled out.

I can see that this system is more flexible as it is easy to change the length between the two magnets but why did you favor this system over the longer, single tube with just one magnet?
 
Last edited:
Davide,
All your observation were right!
Regarding the last question, I think this last system would be more accurate because both magnets would accelerate from 0 m/s to some speed exactly the same way so I can measure the time difference between two pulses more precisely. In a single tubing version it might happened that the speed of magnet between two winding be different than the actual speed of the shaft. This is due to some elasticity and elongation-shrinking of the line connecting magnets to the shooting line and the shooting line itself. The weight of the magnets should be as low as possible because they are under huge acceleration. They have to accelerate from 0 m/s to say (30 -35) m/s instantly. Force to the magnet is mass (m) * acceleration (a), F = m * a
 
Thanks so much for elaborating.
I think this system is very intriguing as it is very, very simple to build and quite easy to use.

I researched a bit more and it seems the same idea has been used by a group of archers (bow shooters) to make a cheap chronograph called the Coil Chrono, though they scale up the coils, space them out more and shoot the arrow itself through the coils.



Someone even programmed a small piece of software, so that when you plug the Coil Chrono into the mic port of a laptop and define the distance between the coils, the software will give your the speed of the shots and log them for you. Not a big deal, as it is still easy to do in Audacity, but pretty neat nonetheless. (And we can probably use it as well - though I don't want my laptop next to a pool.)

Perhaps, with a magnetized spear tip (like the archers use when testing with this Coil Chrone) or a magnet inside the spear tail, we could make the same thing work underwater? Maybe some circuitry would be needed to amplify the signal - though I have no skills at all in that area.

But your system is smaller and you don't risk shooting the chrono to pieces. I may very well also have less reading errors than a much bigger tube you would have to shoot through. As for using small magnets, I think your system could possibly be downsized even more. Neodymium magnets are very strong for their size and the tube will guide the magnet very close to the coils. I wouldn't be surprised if a ø3-4mm cylinder magnet could still give a good signal in your system.
 
Last edited:
I was using such a small 3 mm neodymium magnets in mine design.

That "speedometer" and the previous versions I made I was using to answer some questions:
1. What is the initial speed of the shaft in water and is it different from initial speed in air?
2. What is the speed of the shaft on various distances from the speargun if using different setups: slider, shooting line, different shaft diameter...

But even most useful testing setup for comparing different setups one to another was using the rubber tubing:

#733

 
Reactions: Diving Gecko

Sure, for "internal" use and comparing one setup to another, your rubber tester is smart and easy. But speed is perhaps better if people want to compare different guns shot at different locations.
 
You must log in or register to reply here.
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…