Friday, July 27, 2012

Making boats: hull design

A friend recently invited me to her house to do a science experiment involving the stream that runs through her property. Well, that sparked about a million ideas! However, I thought we'd start with a fairly simple concept: boatmaking.

More specifically I wanted to focus on hull design: the watertight body of a ship or boat that meets the waterline. Hulls are generally divided into two categories: displacement hulls and planing hulls. Displacement hulls have a large underwater profile and tend to move slowly. Planing hulls are meant to ride high along the waterline, and can go fast.

Art:Most boats are designed with a displacement hull, which pushes the water aside as it rides along. The planing hull, characteristic of many high-speed boats, tends to rise up out of the water and ride on its heel as the speed is increased.

The shape of bottom of the boat is key to how it functions in water. Depending on what you want to do, you can build a boat with anything from a flat bottom to a rounded bilge to a hard chined V-shape. And of course, you can combine these basic shapes into a wide variety of composite shapes. Finding a balance that allows for a boat to carry the needed load while being able to move quickly and accurately is a tricky thing.

To further adjust the hull, a designer can add a keel to add stability or a rudder to help steer. You can also easily adjust the bow of the boat, the front or forward part of the hull. Changing this shape affects how well the boat cuts through the water, by reducing its resistance.

When you ask a child to draw a boat, they often focus on everything above water. After all it's what you can see easily. So getting them to think about the hull, and how it can affect the way a boat actually works, is a great change in perspective. And it makes for a fun engineering design project.

For materials I wanted something easy to manipulate, basically waterproof, and inexpensive. I like using things you can buy in the supermarket, because it gives kids the opportunity to keep experimenting at home without the need for special equipment. So I settled on wax-coated paper cups, an assortment of tapes and a stapler. I also provided some aluminum foil, but explained that they couldn't use the foil as their main design component.

I love working with kids, because you never know what they'll do. I assumed they'd cut up the cups, maybe use the flattened sides to build their designs. But instead they started creating displacement hulls and pontoon-style designs using the whole cups. Soon they were sharing designs, adjusting and improving their boats.

Once everyone was ready, we headed down to the stream to test our boats. First we used fishing weights to see how much load each boat could hold. Some were able to handle a pound or more! Then we let the boats take a run down an area of "rapids" while we timed the run with a stopwatch to see which moved most quickly. Several crossed the finish line in under 10 seconds, one in 7 seconds!

I was impressed with their determination. It started to rain when we headed down to the stream, but everyone stayed to test their boats. (What the heck, we were wet anyway!) However, the weather didn't give us a lot of opportunity to develop our boat design based on what we learned.

For students that want to extend the project, a soft wood like pine can be used to carve a hull.  They can also experiment with buoyancy by using various types of wood in the same shape and size to see how the density affects how they float.

There is a lot of physics to be explored with boat design. We only had the chance to touch the tip of the iceberg. But for an overview of all the amazing engineering involved in boat design, check out the video below.

Or if you have an older, computer-savvy child, they may want to give some CAD a try with this virtual hull designer.

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