This is a General Overview of Physics as it applies to Spearfishing.

The Cavemen were the 1st Physicists. They didn't have formulas, slide rules, calculators and the such. They learned by cause and effect. A couple of Far Side comics to show my point.

All that you do can be tied back to formulas, theories and concepts. These are modern man's way to decribe and work with the "laws of nature".

This section is not a physics course, but just highlights of practical information that the typical spear-o should be interesting in.

More topics will be added in the future once I complete all of these.



Gravity -

The force that attracts a body toward the center of the earth, or toward any other physical body having mass. For most purposes Newton's laws of gravity apply.

When you release a polespear or a spearshaft from a gun, the gravitation force from the earth will effect the pole or the spearshaft. The longer the shot, the more time available for this force to lower the shot.

Short shots, this can be neglected. Longer reach shots you might consider aiming a touch higher on the target.

Buoyancy can also effect the Pole Spear. This will be discussed in detail below.


Buoyancy - Archimedes' Principle -

Archimedes' Principle defines buoyancy. "The Buoyant Force is equal to the weight of the displaced water"

The mass of the water displaced by your polespear will reduce the force of gravity. Below is a diagram to illustrate. If your polespear is neutrally buoyant, this will eliminate the effect of gravity on your pole.


The natural example of this is an iceberg. When sea water freezes it expanded in volume. The frozen ice will displace more water than it actually weighs.


This is the same concept why large steel ships float. The water displacement for these ships is amazing.



Buoyancy is used to navigate a submarine by filling ballast tanks with Air or Water.

Note: 1 foot square volume of Sea water will weigh ~ 64 pounds. Fresh water is a little less ~ 63.5 pounds.

- - - E X A M P L E - - - 1 / 2 " SOLID Aluminum Rod for DIY Polespear - - -

Let's do a calculation for a DIY polespear. You made a 6 foot solid Aluminum Pole 1/2" diameter. Density of Aluminum = .098 lbs/in3. Volume of Pole is 14.14 in3. The weight of the pole is ~ 22 oz.

This pole will displace same volume of water ~ 8 oz.

Out of the water the pole weights 22 oz, In the water it will feel like it's 14 oz.

- - - E X A M P L E - - - 5 / 8 " TUBULAR Thin wall DIY Pole - - -

Let's do a calculation for a DIY polespear. You made a 6 foot Tubular Aluminum Pole 5/8" diameter with a wall thickness of .065". Density Aluminum = .098 lbs/in3. Volume of Pole / tube is 8.23 in3. ( Neglect weight of AIR inside tube) The weight of the pole is ~ 13 oz.

This pole will displace same volume of water ~ 13 oz.

Out of the water the pole weights 13 oz, In the water it will feel like it's 0 oz or Neutral.

Let's look at one more example where the DIYer decided on a tubular carbon fiber design. The mass of this material is much less than alummium and can have similiar volume displacements for bouyancy.

This DIY-er will soon learn his pole with float and the travel of the shot will go up from a positive bouyancy.

Another factor is surface tensions, but this will be neglected here. I might add a section later, but tough to explain without getting to technical.


Center of Gravity -

The Concept is one of the most neglected items by designers. The concepts are really simple to approximate.

Take a typical push broom, balance it and you can find the center of gravity.


I'll show the method for precise numbers, and also how to get a good number without getting too far down a mathematical rat hole.


Potential Energy -

The energy possessed by a body by virtue of its position relative to others, stresses within itself, electric charge, and other factors.


Gravity: If a rock is carried to the top of a cliff and tossed off. As you carried it up the mountain you added potential energy. Tossing it off the cliff you are now converting the stored potential energy to kinetic energy as it falls.

Gravity from the moon causing our ocean tides is another good example.


Rubber bands

Rubber Bands: Our rubber bands are really just a rubber SPRING that can store potential energy. When you stretch the band you are transfering energy from your muscles to the rubber.

Here is a graph of Various Rubber sizes and the amount of energy stored in the Rubber as it it stretched.

Potential energy is stored in the rubber, when you release it you are converting that energy into kinetic energy as the pole moves forward to the fish.

Rubber Tubing is a huge topic of interest. Here is a Link for more details on just RUBBER.



Springs: Compressing or extending a metal spring you are adding ELASTIC potential energy to the spring. Elastic is added to the term to show you are NOT doing permanant deformation of the spring.

Keeping the spring in the elastic region allows you to get the stored energy back out of the spring.

NOTE: The Linear Elastic Region on the Stress/Strain chart Highlighted in RED.

If you go past elastic region, the spring will deform and not be able to return the original state.

Metal springs are not so common for use in the spear-o realm. Some older gun designs used them vs a rubber, The properties of rubber used today has made the use of mechanical springs for power obsolete for the most part.


You will find small springs in gun trigger mechanisms.


Compressed Air: Energy stored in the form of pressure. Scuba Tanks and pneumatic spearguns are most typical uses of this form of energy in Diving.



Other Types of Stored Energy: Chemical, and Electrical are the last two types of common potential energy sources. [ e.g. wooden log, battery, magnets, capacitor, gun powder, gasoline and coal ]


Kinetic Energy - (Motion)

Energy that a body possesses by virtue of being in motion.

Examples are a bullet or anti-tank round in motion, a 100 mph baseball pitched across homeplate for a strike.

Potential Energy stored in a rubber band is transferred to a polespear as it goes into motion. Same for speargun shaft.

Kinetic Energy is from motion of an object.

In our attempts to kill a fish, we are looking to hit the fish with as much Kinetic Energy as we can. To do so we need to decide how to balance 3 things.

-1- Speed ( velocity )

-2- Mass of the polespear or gun shaft( weight )

-3- Potential energy stored in the rubber band.

Designs can go in many directions around these 3 items. This will lead to other factors to consider as well.

A speargun or pole spear is just an device that converts Potential Energy into Kinetic energy.


Just a touch on this topic, but realize the Kinetic energy has two parts the motion towrds the fish and also the rotation of the polespeasr or gun shaft. This is more interesting when you apply it to an actual bullet.

Did you know the bullet out of a AR-15 Rifle can spin at 180,000 rev / minute.


Momentum -

Momentum is really simple it's just "Mass in Motion". Momentum = Mass x Velocity

It is often confused to be the same as speed or velocity. The cue ball transfers it's momentum to the other balls


The momentum of the steel ball in motion will transfer through the other balls. The last Ball will have the momentum from the 1st one. This is an example of conservation of momentum.


Another example, lets take a Killer Bee and NASCAR Semi-truck hauler. Both are headed down the road at the same speed of say 60 mph. The Truck is much heavier than a Bee and it has much more momentum due to the larger mass. Collision, what happens?

Momentum can also be thought of as a characteristic of an object to stay in motion. If the Bee and the Truck both hit a brick wall. The bee will be stopped "splat" and the truck will pass thru the wall and have not lost much speed.

In the Polespear and Speargun world, we need a balance of Mass with Speed. All Mass and little Speed or All speed and little Mass are not the answer. The balance is the trick for performance.


Displacement - Velocity - Acceleration

Let's take a baseball hit by a batter. The Ball goes up and out of the Ball Park.


This is the baseline for general ballistics. This works for many things like firearms, spearguns, polespears, basketballs, throwing rocks and civil war cannons. And many more.....

Below are the typical equation for position, velocity and acceleration.

These equations EXCLUDE many factors to make it easy to understand. Looking at the baseball example. Think of all the different types of pitches from the knuckleball to the slider.

Add in the pine tar and spit or look at the sandpaper in the pitchers glove, plus the weather, bat size, corking etc. and on and on. Go watch some U-tube videos if you want more info. Some good some bad like anything.

Here is a random example that looks at AIR flow on the object in motion.


Ballistics -

Spearshaft Ballistics:

This really is a complicated topic. I'm just going to do a general overview on a few factors for spearo.

Firearms in Water:

Shaft diameter and length:

Open / Enclosed tracks:

Gun recoil, Ballast, Stabilizers:

Band Power:

Spear Shaft Flex and Harmonics:


Penetration of targeted Fish:

Drag, Shooting Lines, Reels and Float Lines:


Firearms in Water

Below are a couple of videos on the ballistics concerning shooting a firearm in water.

Special Thanks to Dustin of - Smarter Every Day - for the amazing common sense presentations.


Shaft Diameter and Length:

The short length on the bullet along with the super fast speed vs low mass of the bullet slug causes the tumble of the slug and limited range in the denser media of water vs air.

The length of a speargun shaft or pole spear is important to flight stability. The range is also much better having the Kinetic Energy shifted from all speed to more mass vs the bullet slug. Digging into the details of cavitation will help explain the design of firearm bullets for use underwater.

Open / Enclosed tracks:

The idea for an enclosed track is to pevent/reduce the spear shaft from flexing when it is loaded and rebounding when it is fired. When you use a thin spear shaft with strong bands, the enclosed track can prevent shaft whip.

The enclosed track will add more surface contact and energy loss from friction. Low friction materials are common choice for the enclosed track.

The Open track spearguns will have less frictional drag on the gun during the shot. Heavier stiffer gunshafts tend to be on an Open track gun.

The enclosed track will also assist loading the gun. Once in the slot it will not fall out. Kind of a minor deal but worth noting.


Sea Sniper



Gun recoil, Ballast, Stabilizers:



Band Power:



Spear Shaft Flex and Harmonics:

Deeper Perspective's video on shaft flight and shaft flex upon impact to the target. Worth the watch.

Think about a big hammer hitting a thin nail vs a thick nail.


Note the degrees of motion on an arrow. Not a spearshaft, but similiar properties.





Penetration of targeted Fish:



Drag, Shooting Lines, Reels and Float Lines:




Accuracy and Precision -

Precision and accuracy is commonly referenced in firearms and scopes. It is also common in statistics and Quality Assurance in industrial settings.

The easiest way to explain the difference is this bullseye diagram. Do some practice shots with your speargun or pole spear. Determine which group you are in to correct the problem. Pay close attention to details and try to do each shot the same.

LOW Accuracy - LOW Precision : Not a good place to be. Clean and check equipment and repeat testing.

HIGH Accuracy - LOW Precision : Your aiming is good, you might have a minor flaw or just need practice

LOW Accuracy - HIGH Precision : Your shot is great along with your setup. you just need to tweak your aim.

HIGH Accuracy - HIGH Precision : Where you want to be, be sure to practice and maintain your SKILL.



Gas Laws - Pressure Volume Temperature -

The Ideal Gas Law:

For some Spear-o's applications, we can use this formula and hold some of the values constant and remove them from the equation. This will produce these 3 laws, [ Boyle's , Charles's , Avogadro's ] .

Let's look at the change in a divers lungs (volume) as he dives to depth breath holding.

Temperature is constant (assumption) so we will use Boyles's Law.

So your lungs volume will be 1/2 there size at 33 feet. if you go down to 66 feet or 2 atms of pressure your lungs size will be 1/3 of the size on the surface. NOTE: The closer you are to the surface the larger changes in volume.


Let's look at the diver mask. you don't want to change the volume of the mask, so the diver will add air to the mask as he dives to depth. Low volume masks are the choice so you don't have to add too much air.

If you don't do this expect to look like a vampire, the blood vessels will hemorrhage in your eyes.


PSIA vs PSIG: A note on this pressure measurements. This is commonly confused.

PSIA = pounds per square inch absolute | PSIG = pounds per square inch Measured / Gauge.

Absolute pressure is ZERO in outer space. At sea level it is 14.7 psia and measured gauge pressure is ZERO. In order to actually measure pressure the instrument will reference atmospheric pressure. [ psia = psig + 14.7 ] .


Temperature -

Temperature changes not really a huge factor for a spear-o, except in picking out a wet suit to maintain body temperature.

Celsius vs Fahrenheit scales


Visible Light -

Light Spectrum: Think of a Rainbow,the water in the air will refract the sunlight so you can see all of the visible colors.


Adsorption: Water will block the white light from the sun at different wavelengths. First the REDS, then Oranges, and so on. The Blue light will penetrate the deepest.

NOTE: The IR light is blocked 1st and UV light does get somme depth before it is stopped too. Many fishing lures will have UV reflective properties for this reason.


Refracton ( Snells Law ): As light passes through a Water/Air boundry the light will bend. Looking at a fish in the water from the surface is best way to illustraate this. If you move to look straight down, the Apparent and Actual position will be the same.

Aiming Concerns : As long as the fish in directly in front of your mask you are good to go. If you are looking to the side, you will have to compensate for the refraction.



Speed of Light vs Speed of Sound -

Speed of Light: 2.99 x 10*8 meters/sec in Vacuum / Outer Space. light will travel a touch slower in other medias like water and air. Using the Refractive index of the Material.

In water Light will travel at 2.2431 x 10*8 meters/sec which is basicly the same.


Speed of Sound in Water vs Air:

This is more interesting as it changes with temperature as well.

Speed of sound in Air [ 30 F to 100 F ] { 1085 feet/sec to 1159 feet/sec }

Speed of sound in Water [ 30 F to 100 F ] { 4600 feet/sec to 4995 feet/sec }

The speed of sound in Water is ~ 4 times faster than in Air. This is why you can't tell direction of a noise underwater.

Looking at the speed of a speargun shaft at say 150 ft/sec, the light and sound are both at the fish long before the spearshaft. The reaction time for a fish is next to consider. If the fish is 30 feet away, it has ~ 200 msec to react and get out the way.

General chart for different materials


The electromagnetic spectrum.


Material Science -

Going to make a new page off of 101 for all of this, I will have a link here when It's all done.


Stress-Strain Curve:

Strength - Hardness - Toughness - Stiffness:

Stainless Steel:



Rubbers and Plastics:


Tubing vs Pipe: ( Here is a quick look I thought you might like for reference )

Tube and Pipe - The Differences Explained


Friction - Surface to Surface contact