# How can one know where to throw one's spear when spearfishing?

Spearfishing has been around ages and everyone has heard of how great the Native Indians were at spearfishing.

As we all know, light is refracted (bent) when it enters water. The key to spearing fish is to know how much below the fish to aim the spear from the edge of a river or stream.

What is the science principle of understanding where to throw a spear into the water in order to catch your fish?

Spearfishing at Sugar Bowl

• It's just practice. It's not fundamentally different than how you have to throw a ball in a parabolic arc for it to land at your target, you get a feel for it. Feb 8, 2017 at 4:17
• The practice teaches you the physics same as shooting pool. If you know the physics before you start the training curve is going to be much shorter. The less time it takes you to learn, the more likely you will get good at it before you starve to death. Feb 8, 2017 at 14:05
• @whatsisname: You make it sound like you have a choice. In the absence of other forces (eg, wind) all trajectories of flung objects are parabolic. Feb 9, 2017 at 21:14
• Trajectories of flung objects are parabolic only in vacuum. Consider a golf ball hit in still air: to hit the ball far, the golfer uses the club with the lowest takeoff angle, the driver. (Driver loft angles are from 8° to 11°.) The trajectory of a golf ball hit by a driver looks nothing like a parabola. In a vacuum, the optimal takeoff angle for the most distance is 45°, and the trajectory is a perfect parabola. May 28, 2020 at 19:30

## 3 Answers

This is a great explanation of the concept:

Investigating refraction and spearfishing

Refer to the linked Word document inside. It is copyrighted, so I hesitate to include its contents in entirety here.

No matter the angle, no matter the position, you always aim for below the apparent position of the fish.

The apparent position of the fish actually becomes closer to the surface for when further away from the fish. When a person is viewing from a position more directly above the fish, its apparent position is three-quarters of the actual depth of the water. To estimate the actual depth of the fish, estimate how deep it appears to be and add and extra one-third of this distance.

The easy technique is to simply hold the spear in your hand with the tip under the water and stab the fish. Once your spear enters the water, refraction has identical effects on your view of the spear point and your view of the fish, so you can easily guide it to where you want.

Throwing a spear to hit a fish takes a great deal of practice. The physics of refraction is sufficiently non-linear that the only practical way is trial and lots of error.

• Not sure "easily guide" it is possible at stabbing speeds. Do you have any specifics? Feb 9, 2017 at 8:07
• Never tried it with a spear, but if it works when grabbing fish with your bare hands, it should work with a spear.
– Mark
Feb 9, 2017 at 8:56
• I think it would be clearer if you said "...your hand, with the tip under the water surface, and stab..." (emphasis on extra words only for clarity). Feb 9, 2017 at 21:21

The math is trigonometry but I think you would need to get a feel. It is not like you could run the math on the fly anyway.

The ratio of sin above and the sin below is 1.33.

More angle is more bend. At perpendicular there is zero bend. At 45 about 13 degree of bend.

At 45 degrees if the fish looked like a depth of 2' you would need to aim like 7" below by my calculation. You still have to guess how deep the fish looks. Multiply how deep it looks by 1.33.

The best thing would be to set up a string and a target and practice. The spear is also going to push off when it enters the water so need to account for that.

• This answer is essentially the same as the answer by Wigwam but it includes more detail in the body, but does not have references. I don't see any reason for either to be getting down voted. Feb 9, 2017 at 19:09
• @JamesJenkins Mine was posted first. I get down voted a bit. People get to use their down votes as they chose. Feb 9, 2017 at 19:15
• I agree with @JamesJenkins. One of these upvotes is mine. Feb 11, 2017 at 0:45