Ultimate Speargun Properties Calculator

[Jworthy]

Hi folks,

I've been working on a spreadsheet based calculator tool that would be useful to any manufacturer/home builder.
As an engineer I am aware that the performance of a speargun can be broken down into quantifiable values. In this way, designs can be built with performance in mind, set up correctly, and compared objectively.
The calculator is built using tried and tested engineering formula and experimental data. The interface features a huge range of inputs, with additional overrides to allow for custom arrangements. See attached pics.

The calculator is comprehensive and extremely functional, and covers most aspects of speargun design such as:

• Load
• Energy
• Momentum
• Initial Velocity
• Buoyancy
• Barrel Flex

There are still a few things to do before it's ready for release, but it would be really helpful to gauge who would be interested in this.

Many thanks,
Jacob

 

[sharkey]

Accuracy & reliability are first for me. I'm not convinced that the power to stretch & hold the rubber is the end of the story. There are so many different rubber choices now & they are not all equal. I believe that there is rubber which is easier to stretch yet shoots the spear faster & further than others. Just putting rubber on a set of scales & measuring the force required to stretch is not the best way to compare rubber IMO. There needs to be a way to measure how snappy or fast it is. Also tunning a gun is very important for accuracy. Not just getting the spine or stiffness of the spear to rubbers correct, but that little bit of bias into the rail of the gun to get the spear to push into the rail rather than out when fired, similar to tunning a bow. This last bit seems to be overlooked in mass production guns.

 

[Jworthy]

You're correct. Load against extension is not an accurate measure of the rubber's properties. Since a rubber with a certain length or diameter would not compare directly to a rubber with different dimensions. Which is why us engineers use Stress and Strain. This effectively non-dimensionalises the material so the properties can be compared directly. This is what has been applied here to 5 predefined types of rubber. See attached graph.

The properties of rubber are incredibly complicated when considering the aging effect and hyteresis. Just take a look at the properties of the Apnea branded rubber, measured 2 years apart. However for the purpose of analysis we just want to know the maximum performance of the rubber to calculate maximum velocities and loads. So it is essential to model the rubber as new. For this purpose the predefined rubber types are more than sufficient.
However, to keep everyone happy, I have also included an option to insert custom rubber data.

Regarding rubbers which are snappy or fast, the rubber will only EVER move as fast as the spear. This is primarily dictated by the energy stored in the bands and the mass of the spear. Marginal gains could be made however if the mass of the bands/wishbone is very light and have minimal resistance. Perhaps this is what you have experienced.

You refer to tuning, and what you are describing is the vertical component of force. This is what pushes the spear into the rail. This value has been calculated and is in the results section under "Actual force in Y direction". You'll notice that for roller guns this value is significantly less. This is due to the load being distributed over both the top and bottom of the barrel (Forces Cancel).