Current Assessment Question

[Cacia]

Just when I think I can gauge a current, somebody I respect comes up with a speed I

am surprised by. Usually, somebody is saying a current is 2 or 3 knots and perhaps

they are overestimating considerably. So, I was wondering if you can make a rough

guess based on the degree the bubbles are drifting?

It seems the physics would be pretty constant, unless I am forgeting something.

Anybody have any comments? I'll take a stab and say it was... .25-.5 knot, I could still swim against it with the camera, but didn't really want to very far. When the boubles are twice this angle, I have trouble pulling down the line. Does this look about 20-30 degrees?

 

[nolatom]

Excellent question. I have absolutely no idea.

But I wonder if there's a "one angle fits all depths" answer. Wouldn't the increasing density, or pressure, of the water when they're deeper, affect the amount from vertical that the rising bubbles are offset, as opposed to the shallower depths?

Any hydrologists out there?

 

[Walter]

As bubbles ascend, they expand. As they expand, they increase their rate of ascent. They also break up into smaller bubbles, so the angle would be difficult to use to measure current speed. It can certainly be used to judge a strong from a weak current, but to determine the actual current speed, I wouldn't use this method.

A knot is 1 nautical mile per hour. A nautical mile is 6076 feet. A 1 knot current will travel approximately 8½ feet in 5 seconds. So to determine approximate current speed, count 5 seconds while measuring the distance a floating object moves.

.25 knot = 2 ft
.5 knot = 4 ft
.75 knot = 6 ft
1 knot = 8 ft
1.5 knot = 12 ft
2 knots = 17 ft
2.5 knots = 21 ft
3 knots = 25 ft

You're not likely to encounter a current faster than 3 knots and you don't want to get in the water in that.

 

[do it easy]

Excellent question. I have absolutely no idea.

But I wonder if there's a "one angle fits all depths" answer. Wouldn't the increasing density, or pressure, of the water when they're deeper, affect the amount from vertical that the rising bubbles are offset, as opposed to the shallower depths?

Any hydrologists out there?

I think it's like catching baseballs- after a while you stop doing the math in your head and just put your hand where the ball is going to be.

 

[Charlie99]

My method is similar to Walter's, but I count the time that it takes to move 10'. The key fact I remember is that 1 kt is 10' in 6 seconds. So I just time either my drift past a stationary point 10' away, or watch junk in the water move past me while I'm stationary.

The alternative method which is pretty close is to listen to every one else's estimate and divide it by 2.

I'll have to pay more attention to the bubble angle. If the smallest bubbles are going up about 60fpm, then that is 1fps or 10sec to do 10'. Since 6 sec per 10' is 1kt, that's about 0.6kt. So 0.6kt should move the smallest bubbles at a 45 degree angle. Obviously, since bigger bubbles move faster, then their angle would be less.

 

[captndale]

I learned a long time ago that estimates of current speed by divers are much like estimates of the size of the fish that got away by fishermen.

 

[captndale]

I learned a long time ago that estimates of current speed by divers are much like estimates of the size of the fish that got away by fishermen.

 

[Cacia]

1 kt is 10' in 6 seconds. So I just time either my drift past a stationary point 10' away, or watch

Oh thanks.

As bubbles ascend, they expand. As they expand, they increase their rate of ascent. They also break up into smaller bubbles, so the angle would be difficult to use to measure current speed

I knew I was missing "something"

you can see the pattern, the andle is decreasing, becoming more vertical as the depth is less

I have seen bubbles blowing pretty horizontal from other divers on an upstream bouy, got on the line and could not pull myself down.(with the camera, then without)

 

[Walter]

The alternative method which is pretty close is to listen to every one else's estimate and divide it by 2.

That usually results in overestimating the speed of the current.

I'll have to pay more attention to the bubble angle. If the smallest bubbles are going up about 60fpm, then that is 1fps or 10sec to do 10'. Since 6 sec per 10' is 1kt, that's about 0.6kt. So 0.6kt should move the smallest bubbles at a 45 degree angle. Obviously, since bigger bubbles move faster, then their angle would be less.

Keep in mind, bubbles that ascend at 60 ft/min are foam, very, very small.