Doug Levin is the Associate Director for the Center for
Environment and Society at Washington College in Chestertown, Maryland,
and is an expert in underwater exploration technology, as well as
designing fun programs that teach complex engineering concepts.
It’s Saturday morning, the sun is up, and I’m sitting here with a cup of hot tea looking out onto my tidal creek. The wind is blowing out of the west, but the tide is going out, emptying to the east. I know this through observation. The wind is pushing waves up the creek. At the same time, I see what appears to be just the top of a basketball floating out with the tide (it belongs to Chip, the son of my neighbor). Hold on a bit, I’ve got to go retrieve it for him…
…Okay, I’m back. The basketball was pretty deflated, which is why it floated so low in the water. I used my bike pump to inflate it and then I threw it back in the creek. It floated with more of the ball visible. It’ll be easier for Chip to find now. (Just kidding, I didn’t throw it back.) Watching the ball in the creek, the waves show me the direction that the wind is blowing and the basketball the direction of water movement.
The basketball reminds me of a drogue, so now I’m thinking, “What do I have in my kitchen that I could use as a drogue?” What’s a drogue, you ask? Well, a buoy floats and is anchored to the bottom. A drogue is an unanchored device that allows us to measure the direction and speed of water movement.
Would the metal bowl float? Would the potato?
Photo by Kiritin Beyer, My Shot
So, I’m in the kitchen and I need something that has a specific level of buoyancy. If it’s positively buoyant it will float; if it’s negatively buoyant it will sink; and if it’s neutrally buoyant, it will stay in the water column, neither sinking nor floating. So, I’m searching my kitchen for something that is slightly positively buoyant. It should float just enough for me to see it and watch its movement. Like any good scientist, I want to test my materials. Here’s what I test and how it behaves when I put it into my kitchen sink filled with warm water:
• A dry tea bag floats–positively buoyant–when wet, it doesn’t float as well
• Brasso metal cleaner container–sinks; negatively buoyant
• A full dog food can–sinks (metal can and heavy food); negatively buoyant
• A full can of soda–just the top of the can is seen; slightly positively buoyant
• A new plastic jar of peanut butter–sinks; negatively buoyant (who knew?)
• A new container of dish soap–slightly positively buoyant
• A half-full container of dish soap–positively buoyant
• An orange–just the top of the orange is seen; slightly positively buoyant
The orange is a perfect drogue: It sits low in the water; it will not be
affected by wind; and it will be easily seen. I go to the creek and
throw it in. With the wind blowing from the west, the orange starts
moving to the east, in the opposite direction. My orange “drogue”
confirms that the tide is going out. I retrieve the orange for my
You can conduct this simple drogue experiment with students in the
classroom or with your own children at home. Just fill a basin with
water (1 foot deep is good) and grab 5-10 common items to test for
buoyancy. Have students predict which items they think will be (a)
positively buoyant, which will be (b) negatively buoyant, and which will
be (c) neutrally buoyant (with younger students you might want to call
these conditions (a) float (b) sink and (c) dip). Ask students to
identify specific characteristics of the objects they think will make
them sink or float (e.g. weight, shape, material, absorbency, etc).
Thought question: How are submarines similar to drogues? (Hint, think about the 3 types of drogues.)
I’ll tell you the answer in my next post tomorrow!
This is a mini-drogue that I built from parts found at Lowes. I can track it on the Internet using a GPS tracker. With the GPS this cost me about $150. I’m trying to get its cost down further.
By Doug Levin