About SpearTool

SpearTool is an excel based calculator designed to perform a variety of functions, from speargun design and set up for the home-builder, to objective speargun comparison with a vast number of inputs. Effective use of this tool enables the user to quantify performance parameters, and modify these to achieve the desired results. 


In order to make this tool as accurate as possible. All formulas represent ideal conditions (no material defects, no energy losses, no resistance forces). Accounting for all these factors would compromise the functionality of the tool by the sheer number of additional variables.

In order to account for real life conditions, included in the spreadsheet is an input to define the mechanical losses as a percentage. Energy loss due to resistance and frictional forces equate to somewhere in the region of 10%. Although the true value depends on how the gun is set up, the efficiency of moving parts, materials and manufacture.

To confirm that SpearTool gives accurate results, rigorous experimental testing has been undertaken including high-speed video analysis and CAD simulations. SpearTool's calculated velocity is accurate to within 6% of measured values.

CAD simulations confirmed that the calculated value for barrel flex is accurate to within 0.5%. Even under extreme loading.

SpearTool is as accurate as practically possible while retaining usibility. The accuracy of SpearTool is dependent on the accuracy of the inputs. An understanding of the mechanics behind how a speargun works is recommended but not essential.


SpearTool has been developed with a focus on its functionality, and has been loaded with features to achieve this. The interface has been designed so that with some basic measurements, any speargun enthusiast can effectively model standard equipment. Getting more technical, materials and section properties can be applied to model irregular designs.

SpearTool Main Features

Effective modelling of three types of power system

Twin Bands

Circular (sling) Bands

Roller Bands

Several predefined rubber types selectable from a drop down menu

Three standard barrel profiles to choose from



Integrated Rails

Live graphs showing relevant results

Outputs that highlight barrel failures due to overloading.

Save/Recall buttons to store previous setups for comparison without needing to open multiple files.

Scale limits allow the user to define his own performance criteria.

Conditional formatting to highlight input errors

Property Overrides

Since certain designs may deviate from the standard inputs, a property overrides section has been added to enable these spearguns to be modelled effectively. This allows you to add your own material properties (barrel and rubber), your own section properties for non-standard barrel profiles (usually taken from CAD), and component force overrides to model the barrel under defined loads.

Comparison Graph

Since the properties of a speargun have different units it can be difficult to compare multiple set ups on a single graph. SpearTool's comparison graph has a great solution to this problem. This graph has upper and lower limits which scales the properties against a defined value. This allows the properties to be displayed in a single chart despite having different units.

The comparison graph displays the key properties of saved and current setups for comparison. The full results for each can be viewed by simply recalling these from the memory bank.

Email:  admin@speartool.com

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