planning06

Time for another stupid question...

Why is one shape more stable than another shape?

Er....Struggled a bit with this.

Issac Newton discovered this stuff called gravity.

You throw something up in the air and if its heavier than air it comes back.

Fast.

Ow!

Time for two more experiments.

Try to balance a bottle on your fingertip, pointy end first.

Its difficult to balance it.

Now try it with a desert bowl upside down - try not to break it!

The desert bowl should balance much easier than the bottle as its sides hang down under your finger and help balance it.

If air has gravity, water has anti-gravity.

If you push something down under water that is lighter than water, it comes back up.

Luckily not fast enough to hit me as water is thicker than air.

Trying to push the bottle under water with your finger on the lid is nearly as difficult as doing it up the other way in air.

The bowl sits on the water and takes being pushed down in the middle without wobbling like the bottle does.

The shape of the hull in the water can help stop it tipping.

When you put a hull in water, it pushes some of the water out of the way.

Heres some water.....



...with a glass hull in it...



The water level in the jug has gone up as the bottle has pushed the water out of the way and occupied its space.

This is what people are talking about when they mention displacement.

When they say a big boat has a displacement of 1,000 tonnes, they mean it actually pushes 1,000 tonnes of water out of the way.

If you take the boat out of the water, the water runs back and fills up the dent left by the hull.

In fact, the water is pushing on the hull trying to get the space back all the time its there.

Here is a bottle I drilled two holes in and filled with water....



Because the water is heavy and there is more above the bottom hole than the top, the water comes out the bottom with more force.

So....The deeper a hull goes into the water, the more pressure there is trying to push it back out.

Needed to know roughly where the centre of balance is in a person, so I got Big Idea to lay over a gate....



His centre of balance is about where his belly button is.

It might have been higher if he didnt have big steel toe capped boots on.



When he "sat", his centre of balance rose up another 2 or 3 inches.

Tried it with his legs straight down, but he fell off the gate.

Still....

It was enough to have some Idea where his centre of balance is and how it moves if you sit.

Ok....

Back to the hull....

If the hull is round, the water pushes it from each side and the bottom.

This push doesnt change if the boat tries to tip, so the water just lets it carry on....



This changes if a person sits in the boat as they weight it differently.

Man sitting in bottom of boat....



The green arrow is his centre of gravity pushing down.

The black arrows are the waters anti gravity pushing the boat out of the water.

Man leans over, moving his centre of gravity....



As his centre of gravity is not in the middle, the boat must move or rotate....

The boat rotates.....



His centre of gravity still isnt in the middle of the boat or in line with the push back from the water, so the boat must move or rotate some more...



He suddenly doesnt look too happy.

Lets try sitting him on a seat....



He leans a little again....



His centre of gravity moves and the boat reacts....



As he is sat higher, he ends up unhappy much faster.

This time with a wider hull.....



He leans...



His centre of gravity moves and the boat must either rotate or move...



It rotates, but not nearly as much as the smaller diameter hull, because he is effectively lower in the hull.

This time it doesnt sink.

Time to try a seat again...



He leans...



It moves further as his centre of balance is higher...



I suddenly want to kick a hole in his boat as he didnt end up swimming again.

Now for a flat bottomed boat with angled sides....

He is sat on the seat...



His weight is being supported by the water pushing on the bottom and the sides.

He leans....



His centre of gravity has moved. The boat must either rotate or move.

It rotates...



This time, as the hull is a different shape, more hull on the right hand side goes into the water - making the hull wider on the side he leaned. This makes the water push back harder against that side.

The water pushing the bottom will push harder on the lowest part of the hull and try to right the hull.

The water on the other side has less hull to push on, so it pushes less.

The result is the hull stops tipping.

If he manages to put enough weight on the lower side that he pushes it under, it will still sink and just as fast as the round bottomed hull, but it will taek more weight to do it - even with him sat on a seat.

The hull doesnt need sharp corners, but it will have more static stability if the sides come outwards from the bottom up through the surface of the water to a height beyond where it will go under if tipped.

 

The sides of the hull can be in layers, or planks so that they hit the water one after another as the boat leans. This way you can make each layer help to push the hull upright again.

This is used by paddlers to tip the boat, which makes it easier to turn.

Turning the top plank inwards as in the last pic is called tumblehome. No Idea why.

Tumblehome also stops you belting your elbows on the sides as you paddle.

I want lots of tumblehome as I dont like dented elbows.