Underwater Gun Velocity

I am a Speargun Manufacturer and would like to like to pick your brains about a practical way of measuring the velocity of a spearshaft underwater.

My best success has come from marking a swimming pool with distace markers and filming underwater using a video camera, which takes 30 frames/sec, we measured the amount a shaft travels in one frame. Because of the cavitation of the shaft, the images are blurry and results inaccurate.

A speargun would shoot only about 40 feet underwater, regardless of how much power is applied, and the initial velocity would be about 100~150 fps.

I am looking for a low cost practical solution and would prefer not to build something only a rocket scientist would understand.

Thank you for any ideas.

Max

Since the drag is so high in water, and the velocity loss is probably higher at higher speeds, the location of the measuring device will greatly affect the results. If you are only concerned about the initial velocity, I would measure it in air and not worry about it too much. If you have a big budget for this, I would expect Oehler Research can address your needs.

I have tried shooting over a chronograph in air to check the velocity, but it kept giving me false readings. I can't explain this since my archery arrows give consistant readings and I think the velocity should be about the same. If it had worked, I would have enclosed the chronograph in a U/W housing.

I was told that radar won't work underwater, since the cops can't use their radar guns in the rain.

Max

If the spear velocity is actually in the 100-150 ft/sec range, most chronographs will probably consider the result an error since it is absurdly low for bullets. The way I would try is to set the screen spacing switches to 10 feet but set the screens 1 or 2 feet apart so the chronograph thinks the velocity is 10 or 5 times higher than it is. Then calculate the correct number by hand. If you don't have a chronograph that you can do that with, I do and we could probably get together without too much difficulty.

I have 2 chrony's. One that folds out and has a cord that has a display and one that just folds out. Unfortunately, in one testing session, I shot one of the sensors off one of the machines.

I see what you mean about the shaft being too slow for the chrony to read accurately. The chrony is designed to measure bullets in the 1000 fps+ range.

I think I'll give it some thought and come up with ideas while I stay indoors to stay out of the rain. I will try unmounting the sensors off the chrony and put them closer together to give it a false sense of speed and see if that works.

Thanks for the imput.

Max