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Grandpa Pencil
learns how to make a
Plasticine Flotilla



Why do the Plasticine boats float?

Grandpa Pencil's
List Of Contents

Front Page

Why is that?

Drop a ball of Plasticine into a bowl of water and it will sink to the bottom because it is heavier than the water.
Now mould the ball into the shape of a boat with a hollow centre and it will float. Why?

Archimedes discovered that floating objects have a certain amount of upthrust, or buoyancy, acting on them. The size of this upthrust is equal to the weight of the fluid that the object displaces.

Designing the hull

You will need to experiment for a while to get the shapes and sizes most able to float.

You will be attempting to get the the vessel lighter than the water that it floats on.

Try different sizes and thickness of hull along with different heights of the sides.

Try to make hulls that would be best for a sailing boat, a motor boat a work boat (that can carry a load) and a canoe.

Some experiments

Fill a suitable container with cold tap water and mark the side at the water level.

Dissolve a fair bit of inexpensive cooking salt in the water and note whether the hull rides higher or lower.

Rinse the container well and fill with cold filtered water.

What happens to your hull?

Add hot water to the container until its contents are quite warm, but not hot enough to melt the hull or you.

What happens?

What reaction will you get if you make a flat, barge like hull or a sleek racing sailor hull?

How do cargoes alter your craft?

Powering your craft

We are going to look at three methods of powering the craft for the moment.

These methods include:

Sail: Make a sail as the directions below (1) and attach to your craft.

In the wind you will notice that the craft might tip to the side and take on water.

Use one of the methods on the right to stabilise your craft.

Chemical: For this we are going to use moth balls as the directions below (2).

Air Jet: See directions below (3) and then experiment with other styles.


Stabilizing your craft

When we power our craft by sail we put it at risk of capsizing because of the push of the wind on the large sail area above water.

There are a number of methods that we can employ to stabilise our craft under these conditions including:

A Keel: A weight attached by a wing like structure under the water

An outrigger: Like a floatie sticking out from one or both sides of the craft or

Multi Hulls: Think of joining two or three boats together side by side.


1: An easy Sail

Cut a piece of stiff paper to the size that best suits your craft, make a hole in near the top and the bottom and push a straw through.

This sail and mast can then be set in your craft with a little more plasticine.

3: Air Jet

If you are anything like me you will love a bit of noise from time to time.

With this model you will need to make your vessel a bit bigger and a little bit thinner in the skin for the best results.

Your engine will be one of those balloon things with the whistle attached.

Set the whistle in the rear of your boat with the balloon on the inside.

Blow the balloon up, put the boat in the water and 'let her rip'.

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Published by Robin A Cartledge ~ ABN 19 924 273 138 ~ Low Head, Tasmania ~ Contact/Comment