Larger people pushed faster in current?

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

billt4sf

Contributor
Messages
2,561
Reaction score
1,151
Location
Fayetteville GA, Wash DC, NY, Toronto, SF
# of dives
500 - 999
I am 6'6" (about 198 cm) and 250 lbs (about 110kg). I swear I get pushed faster in current than other divers, who are of course usually smaller people. I frequently find myself being pushed ahead of the group and the DM for example, even when I am at the same distance from the reef and the same height in the water column.

Does anyone else agree?

Why would this be so?

From a theoretical perspective, it seems to me that if water is moving at X speed, unless we swim against it, we are all going to go at X speed once we are accelerated to that speed. However I do feel as if I get pushed faster than my fellow divers.

If I think about acceleration to get to X speed, then yes I have a greater surface area in contact with the water (surface area is proportional to the square of length), but I have a far larger mass to be pushed (mass is proportional to the cube of length) therefore I should accelerate more slowly.

I can't get this to make sense but it is my observation.

- Bill
 
Last edited:
Much more important where you are in the water column. Generally if you hang higher you go faster.
 
I also suspect it is true. My theory is smaller people are more streamlined (eg pointy) so a lot of the current that hits them is pushed over, under and around them. A larger person will have more cross-section to catch the current. I also note that in stronger currents, people with lots of gear (eg twin tanks, deco tanks etc) will have a harder job swimming into the current than a more streamlined person.
 
but I have a far larger mass to be pushed (mass is proportional to the cube of length) therefore I should accelerate more slowly.

Is that far larger mass really going to matter when you consider the immense power behind huge walls of water?
 
Hydrodynamic drag/force is proportional to cross sectional area times a slipperiness constant times the velocity-difference between you and the water squared.

Assuming we are talking same trim and same water speed, the bigger person (with bigger cross sectional area) will feel more force.

This does not mean they are accelerated faster than a smaller person.

Acceleration equals force divided by mass.

The bigger person has more mass...and human mass increases faster than cross section does.

So, bigger people will, all other things equal, be accelerated less. (This is also true in bicycle racing where the big guys do better on windy days because weight and power output tend to go up together in fit people...this also applies to swimming into a current).

That said, in water the drag force is so strong you're pretty quickly accelerated until your speed matches the water if you're not swimming. Notice in the description of the force that the force on you goes to zero when you're going same the velocity as the water. So, pretty quickly, everyone will be going the same speed. The small people likely will be a bit farther downstream than the bigger people, but going the same speed.

Also note the "all things being equal." If all things are not equal, the person with poorer trim or poorer streamlining will feel more force. But still will have same final velocity in water moving the same velocity.

So, if you're finding yourself going a different velocity than someone else, they might be better trimmed/streamlined, might be swimming harder or likely are in water moving differently than the water you're in.

Note: this is from a diving noob who only knows the physics of hydrodynamics....there usually are other little gotchas out there, but this should be mostly true.
 
I
Hydrodynamic drag/force is proportional to cross sectional area times a slipperiness constant times the velocity-difference between you and the water squared.

Assuming we are talking same trim and same water speed, the bigger person (with bigger cross sectional area) will feel more force.

This does not mean they are accelerated faster than a smaller person.

Acceleration equals force divided by mass.

The bigger person has more mass...and human mass increases faster than cross section does.

So, bigger people will, all other things equal, be accelerated less. (This is also true in bicycle racing where the big guys do better on windy days because weight and power output tend to go up together in fit people...this also applies to swimming into a current).

That said, in water the drag force is so strong you're pretty quickly accelerated until your speed matches the water if you're not swimming. Notice in the description of the force that the force on you goes to zero when you're going same the velocity as the water. So, pretty quickly, everyone will be going the same speed. The small people likely will be a bit farther downstream than the bigger people, but going the same speed.

Also note the "all things being equal." If all things are not equal, the person with poorer trim or poorer streamlining will feel more force. But still will have same final velocity in water moving the same velocity.

So, if you're finding yourself going a different velocity than someone else, they might be better trimmed/streamlined, might be swimming harder or likely are in water moving differently than the water you're in.

Note: this is from a diving noob who only knows the physics of hydrodynamics....there usually are other little gotchas out there, but this should be mostly true.
'
I agree with the physics as you have presented it -- and thank you for your clear explanation -- but it is not in line with my experience, hence the post.

- Bill
 
No. You do present more drag/area when the group turns into the current and your power output may not be proportional to the larger flat plate area. But just drifting along, static, in the water column, no, everyone would move at the same rate.

N
 
Last edited:
https://www.shearwater.com/products/perdix-ai/

Back
Top Bottom