# Water drag lines - what's it called?

• Iduna
In summary, the conversation revolved around the formation of waves and wakes in fluid mechanics. The original poster was curious about the specific type of wave that forms when a stationary pebble is placed on a flat shore and a wave passes over it. This type of wave is characterized by multiple "V" shapes with increasingly obtuse angles. The conversation also touched on the definition of a streamline and how it differs from a wake. Additionally, there was a discussion about the effects of shallow water and velocity gradients on these types of waves. The conversation ended with the suggestion to upload a picture for further clarification.
Iduna
there's a stationary pebble on the flat shore of the beach. As the last of the wave heads back out to the ocean, it moves over the pebble and creates.. "lines" similar in shape to eg air stream around a jet, etc

Wondering what these 'waves' are called?

sophiecentaur and davenn
Bandersnatch said:
. . . . or bow wave from a moving boat. It's a wave that's continually being formed against the water flow and it falls away on either side, traveling at its wave speed and that forms a vee. Once formed, the wave can travel a long way because there are very few losses (apart from all the other stuff going on).

sophiecentaur said:
. . . . or bow wave from a moving boat. It's a wave that's continually being formed against the water flow and it falls away on either side, traveling at its wave speed and that forms a vee. Once formed, the wave can travel a long way because there are very few losses (apart from all the other stuff going on).
Yes. It is also a supersonic shock wave, not quite the same thing as, say, ordinary flow over a wing.

Tom.G said:
Streamlines
noun
A line along which the flow of a moving fluid is least turbulent.

From: https://en.oxforddictionaries.com/definition/streamline

I don't know who Oxford hired to write the definition for a streamline, but whoever it was clearly wasn't familiar with fluid mechanics. That is not how a streamline is defined.

A streamline is a line that is everywhere tangent to the velocity field.

berkeman, jim mcnamara and Tom.G
sophiecentaur said:
. . . . or bow wave from a moving boat. It's a wave that's continually being formed against the water flow and it falls away on either side, traveling at its wave speed and that forms a vee. Once formed, the wave can travel a long way because there are very few losses (apart from all the other stuff going on).

Hmm... is it wake if the object is stationary and the water is moving around it??

Looking to name it in order to find an online model that shows the angles accurately.. in the 'wake' examples, there is usually a single 'V' with quite an acute angle, whereas with the wave and the pebble there are 2-3 'Vs' with increasingly obtuse angles...

Iduna said:
Hmm... is it wake if the object is stationary and the water is moving around it??
Would you expect a different answer? The motion is relative, just the same.
Multiple wakes would perhaps not be surprising as the water velocity is different at different depths in shallow water. I can't think of an equivalent to the pebble situation for a boat. You could, perhaps observe the Vee formed by a pole in deep water.

sophiecentaur said:
Would you expect a different answer? The motion is relative, just the same.
Multiple wakes would perhaps not be surprising as the water velocity is different at different depths in shallow water. I can't think of an equivalent to the pebble situation for a boat. You could, perhaps observe the Vee formed by a pole in deep water.

It's not that I would expect a different answer, it's that I'm looking to find online a model or illustration of the rippling effect that happens on the water surface in that exact situation - a shallow wave moving over an object on a flat surface...

When I look for 'wake', 'streamline' etc online there's some great examples, but they don't look like what happens with the pebble...

A picture would help here.

Iduna and boneh3ad
Iduna said:
but they don't look like what happens with the pebble...
I have to assume that's because there is a velocity gradient in the water around the pebble and not in a simplified model of a hull moving through deep water.
Also, in shallow water, surface waves are noticeably modified and they can break.
as @mfig says, a picture (with a comparison perhaps) would help. Is it the angle of the V that is different?

mfig said:
A picture would help here.

Can I post a pic from my device? I can only see the URL option...

Iduna said:
Can I post a pic from my device? I can only see the URL option...
Yes, use the Upload button in the lower right corner of the Edit window to Upload a JPEG or PDF copy of your picture.

boneh3ad said:
I don't know who Oxford hired to write the definition for a streamline, but whoever it was clearly wasn't familiar with fluid mechanics.
LOL

Iduna said:
Hmm... is it wake if the object is stationary and the water is moving around it??

I don't see why not. When I'm water skiing the boat that pulls me is at rest (relative to me) and the water is in motion. As long as the speed of the boat relative to the water exceeds the speed of the water waves, I'll get bow waves. Or shock waves, or a wake, or whatever you want to call it.

Looking to name it in order to find an online model that shows the angles accurately.. in the 'wake' examples, there is usually a single 'V' with quite an acute angle, whereas with the wave and the pebble there are 2-3 'Vs' with increasingly obtuse angles...

My guess is that any asymmetry in the rock's shape causes bow waves to be created by more than one spot on the rock. I believe that if you happened to leave the ladder hanging off the stern as you sped away in your motor boat, you'd see more than one vee trailing away in the downstream wake.

I think what the OP is describing might be better described as a stationary wave, which is propagating upstream at the speed of the current, rather than a standing wave, which would be oscillating in place without propagating.

Last edited:

## 1. What are water drag lines and what are they used for?

Water drag lines, also known as water drag systems, are mechanisms used to reduce the resistance or drag of water on a moving object. They are often used in boats, ships, and other watercraft to improve their speed and efficiency.

## 2. How do water drag lines work?

Water drag lines work by creating a barrier between the water and the object, such as a boat hull. This barrier reduces the amount of water that comes into contact with the object, thereby reducing the drag force that slows it down. The lines are typically made of a material with low friction, such as nylon or Teflon.

## 3. What is the purpose of using water drag lines?

The main purpose of using water drag lines is to increase the speed and efficiency of a watercraft. By reducing the drag force, the object can move through the water with less resistance, resulting in faster speeds and lower fuel consumption.

## 4. Are there different types of water drag lines?

Yes, there are different types of water drag lines, each with its own design and purpose. Some common types include drogues, drag chutes, and trim tabs. Each type is designed for specific water conditions and can be used for different purposes, such as slowing down a boat or controlling its direction.

## 5. How do you choose the right water drag line for a specific watercraft?

The type of water drag line that is suitable for a specific watercraft depends on several factors, such as the size and weight of the watercraft, the type of water conditions it will encounter, and the desired speed and efficiency. It is important to consult with a marine engineer or expert to determine the most appropriate water drag line for a particular watercraft.

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