# Help me calculate drag on objects in water

In summary, the conversation discusses the calculation of drag for different missile/torpedo shapes in water. The question asks if there is a formula for calculating the drag coefficient and if there is a rule of thumb for the amount of frontal drag versus laminar/surface friction. The formula for drag is mentioned, but the exact values for the drag coefficients of the shapes are unknown. The conversation also mentions finding drag coefficients for the different torpedo noses and the need to calculate the drag on the body of the torpedo as well.

## Homework Statement

How can I calculate the drag of different missile/torpedo shapes in water? I have five different shapes, similar cylinders with different "noses" One is a long cylinder with a flat nose, another is a cylinder of the same diameter with a hemisphere nose. Third and fourth are short and long cone-shaped noses, and the fifth is a large hemisphere (twice the diameter of the cylinder, sort of a mushroom shape). They all have identical surface area (were designed that way) and are powered by identical electric motors. But they move through water at different speeds, presumably because of their different shapes or "streamlining."

We're talking about low speeds here, these are models of approx. 14-18 inches long propelled by a small 3 volt electric motor.

Is there a formula for calculating the drag or drag coefficient of these different shapes? Is there a rule of thumb (I have heard 90 %) for the amount of drag caused by the torpedos' frontal area/cross section vs. the amount of drag caused by laminar/surface friction?

Thanks, any help is appreciated!

## The Attempt at a Solution

You can use the formula here, however I am unsure as to the exact values for the drag coefficients of your shapes.

The link you pasted is not active, but I bet you're suggesting the formula for drag, i.e. the force of drag = one-half the density of the medium, times the velocity squared, times the cross-sectional area of the object, times the drag coefficient for that shape. If correct, I've done some searching and found drag coefficients for all my "shapes" of torpedo. Actually, it's the torpedo "noses"...flat cylinder, hemisphere, long cone, short cone.

So I can basically calculate that, and as I understand it the "frontal" drag is the main drag effect.

However each torpedo has a body too, a fuselage if you will...so how do I calculate the drag on that, the "laminar" resistance or whatever, and add it to the frontal drag for a total?

I've found lots of examples for calculating the frontal drag of different-shaped objects, but the length and shape (pure cylinder, gentle taper, rounded, etc.) of the fuselage/body must matter too...how to measure drag there?

## 1. How is drag force calculated on objects in water?

Drag force on objects in water can be calculated using the formula: FD = 1/2ρv2CDA, where ρ is the density of water, v is the velocity of the object, CD is the drag coefficient, and A is the cross-sectional area of the object.

## 2. What factors affect drag force on objects in water?

The main factors that affect drag force on objects in water include the shape and size of the object, the velocity of the object, and the properties of the water such as density and viscosity. The angle of the object's movement in relation to the water's flow can also impact the drag force.

## 3. How does the drag coefficient affect the calculation of drag force?

The drag coefficient is a dimensionless quantity that represents the amount of drag produced by an object in a fluid. It is dependent on the shape and size of the object, as well as the properties of the fluid. A higher drag coefficient will result in a higher drag force, while a lower drag coefficient will result in a lower drag force.

## 4. Can drag force on objects in water be reduced?

Yes, there are various ways to reduce drag force on objects in water. One way is to change the shape of the object to make it more streamlined and reduce its cross-sectional area. Another way is to decrease the velocity of the object, which will result in a lower drag force. Additionally, using materials with lower drag coefficients can also help reduce drag force.

## 5. How is drag force on objects in water important in real-life applications?

Understanding and calculating drag force on objects in water is crucial in many real-life applications, such as designing boats and ships, optimizing swimmer and diver performance, and predicting the behavior of objects in ocean currents. It is also important in fields such as marine engineering, naval architecture, and sports science.