What Floats Our Boats? Exploring Density and Buoyancy

The Balclutha is a 3-masted square rigger; 301 feet long and clocking in at 1,689 tons – how in the world do such ships not simply sink to the bottom of the ocean? Much less carry cargo?


Just how does a massive ship, made of steel and wood, manage to stay afloat? The Orange Band began their explorations into the ideas of buoyancy and vessels at the Hyde Street San Francisco Maritime National Park Association. The kiddos had an opportunity to assist in the build of a Bevin’s skiff (a small rowboat) and take a spin in a completed skiff out on the San Francisco Bay!

Boat construction begins with drawn scaled, iterations from multiple angles – this reminded the Orange Band of their processes during project time.

The Maritime Park crew work with high school students from Downtown High to build Bevin’s skiffs. This skiff, being built over a skeleton to support and maintain the shape, is about halfway done.

Progress on the boat must be slow and methodical – here, Lucy, Charlotte, and Phoebe apply adhesive – liquid cement – to the newly attached plank.

Lucy is just the right size to fill in holes with the cement adhesive from the inside of the support frame.

Meanwhile, Justin, Roman, Amiya, and Jeevan pushed off for a trip around Aquatic Park in the Bevin’s skiff, learning the commands for rowing a small boat.

Maneuvering in the water took coordination (with the rower in front or behind you AND at your side) and good listening skills. Glenn, our captain in the skiff, called out directions to keep the rowers and swimmers in the water safe.

Lucy holds her oar, waiting for Captain Glenn’s next direction.

It was a gorgeous day on the Bay – we had spectacular views of the Golden Gate Bridge as the fog began to roll in over the city.

AND got to experience the thrill of sea-life in the wold: sea lions playing in the Bay!

It only takes a bit of imagination to see the SF Bay at the height of its shipping era; just a little squint and the right light and it is teeming with boats of all sizes, once again!

*Meanwhile, Back at the (Brightworks) Ranch*

The Orange Band was raring to get out on the water again – or at least begin boat construction of their own designs. But we had to take a giant step back before setting sail. Before we could jump into building boats in our shop, we needed to become familiar with the science behind what allows any substance float on water.

So, the kiddos were presented with a series of items: ceramic, wood, steel, and plastic. Their task was to measure the volume and mass of each set of items, graph the data, and then compare that to water.

Finding the volume of irregularly shaped (or non-rectangular) objects is tricky – unless you use the water displacement method, credited to the Greek mathematician, Archimedes.

Jeevan works to carefully measure the volume of wood blocks – first in displaced milliliters – and NOT include the tip of his pencil in his data collection.

Justin uses the scale to calculate the mass of the steel in his bag. A big takeaway from this activity was clarifying the difference between mass (how much matter an object is made up of – a constant) and weight (a measurement of the force of gravity on an object)

Phoebe measures the mass of the ceramic tile pieces multiple times to ensure an accurate reading. Good practices for data collection!

Lucy and Charlotte divide the work in finding the volume of multiple items.

Once our measurements had been double and triple checked, kiddos graphed the data and observed four lines with very different slopes. Then the Orange Band measured and graphed the volume and mass of various amounts of water. With little deviation, the data collection revealed that water’s volume and mass are equal in value!

This information, graphed, gave a clear picture of which items would float (wood, with a line slope smaller than water’s) and which would not (any item with a line slope steeper than water’s). The work gave the students an opportunity to see WHY we graph – and brought to light the formula for density (density = mass/volume) and its relationship to buoyancy. Next up? (Small) Boat Building!

Next, kiddos were given a challenge: build a boat out of  a 12″ x 12″ piece of aluminum foil that holds the MOST mass. While the constrictions of the challenge were met with some resistance (Couldn’t we just add toothpicks? Or use some tape?), they provided an opportunity to work within controlled conditions and compare their results!

Charlotte and Phoebe discuss their options for the foil boat challenge.

Like the more complicated Bevin’s skiff, Orange Band’s foil boats began with a sketch!

Roman works on his second iteration of the boat hull. It was a challenge to ONLY use a single sheet of foil for each boat.

How high should the sides of the hull be? What is the best shape for the bottom?

The pinched, oval-shaped hull was a popular choice. Kiddos discussed the need to make their boats “water-dynamic,” able to cut through the water with ease.


Amiya and Jeevan’s crafts were able to hold the most mass (over 300 g) – in addition to the shape of their hulls, they paid careful consideration to how they filled the boat, balancing the washers inside their craft as they added more.

These forays into how vessels carry mass that is greater than the crafts, themselves, set the stage for the Orange Band’s next explorations: What are the factors that affect the success of such crafts?

Next on the docket: deeper dives into density and shape of hulls and ships.

The Orange Band still has some more work to do in understanding the “how” and “why” of by-sea movement – at least before we take our designs out to the big blue waters…

Lest we encounter such dire catastrophes at sea, as Amiya envisions!