planning07

Another stupid question...

Why are some boats V bottomed instead of flat?

Er...Dynamic stability...?

If you cut a paddle through the water, it sort of hardens up the water on both sides, making pulling it at the same time very hard.

The same thing happens when a boat slides through the water. The moved water hardens, and although the boat slides through it, it sort of resists the boat dependant upon it hull shape.

If the boat has a flat bottom, it pushes the water downwards and the pressure sort of builds up under the middle of the hull - much the same way as the sugar formed a round shape in front of the square fronted raft. This does nothing to stop the boat rocking from side to side....



If the bottom of the boat is round, it pushes most of the water downwards, and a little sideways, hardening the water to the sides as well as the bottom. This side pressure helps keep the boat stable..



The third shape is the V bottomed hull. This pushes all the water out to the sides. The downward force of gravity works against the water pushing sideways and up and resists any rocking forces. You see this with powerboats that will rock from end to end as they power over waves, but little or no sideways rocking. This makes the moving hull as stable as a much wider stationary hull...



Canoes usually start V bottomed, pushing the water sideways, before flattening out and pushing the water downwards in the middle part of its length. This supplies stability and a better cut through the water with a shallow draft or depth, which is very useful in shallow water.



Some of the angled side is usually in the displacement area in canoes to give more dynamic stability.

Next stupid question...

If having a wide gentle curve from back to front helps the canoe go faster, why are the ends often very narrow?

Simple answer - Tracking.

With a row boat, you have a couple of oars and shove them both in the water at the same time and pull them. The row boat ends up getting propelled between the two flat surfaces of the oars in the water in a pretty straight line.

With a canoe, you often have only one paddle, oar and pull it on just one side, or a double ended paddle, which you pull on one side and then on the other. Either way, the pull on just one side tries to turn the boat. Having two sharp ends in the water acts as skegs and keeps the boat tracking fairly straight.

Wind is also a factor with canoes. The wind pushes one end or the other away from the wind. If you have the sharp ends in the water, the boat still tracks reasonably straight. Sometimes, if the canoe will not track straight in a side wind, the paddler will stop and put a large rock in one end of the canoe to get the end in the water as this makes it track.

Another stupid question...

Can you have too much end in the water?

Short answer - Yes.

If tracks too straight, you can have a serious problem trying to turn the boat. Not too much of a problem on a large open body of water, but can be a serious pain when you need to turn fast in white water when you are heading for a large rock.

Three ways to fix this;

More rocker,

Round the undersides of the ends, so the water can flow under it allowing the boat to turn.

Roll a wide canoe on its side so that the keel comes out of the water. This shortens its waterline length and makes the boat turn rapidly. If you are going to try this, you really need to design the boat to suit this style of paddling.

When you paddle the canoe in a straight line, it pushes the water out of the way to both sides.

This push on the sides is balanced in one side gets the same push as the other side.

The water gets pulled into the trough behind the boat from both sides. This uses power too, but is also balanced from side to side.

When you turn, one side stops pushing water away, and the other side pushes twice as much.

Behind the canoe, one side has to pull the water back, and can even end up pulling from the wrong side as its closer.

As you turn, one side has to push the water and pull it back, while the other side does nothing. It takes more power to overcome the waters efforts to keep the boat going straight.

This drawing is exadurate, but gives some idea of what happens if the water does not pass under the boat...

Stupid Question time...

How can I check that all that er....stuff is right.

Lol.

Easily. Easily as talking to yourself.

You will need a puddle or sink, a couple of desert bowls, a few elastic bands, a long pencil and somme duct tape, or pvc tape if you havent got any.

Oh, and a couple of teaspoons.

Sombody must have been very drunk when they designed the coracle.

Its a completely round boat.

It hasnt got pointy ends and so it spins round in circles - unless you are a genius and have learned how to paddle one.

Gotta go.

We will fix a coracle later so it tracks straight.

This will let you test that this er... stuff is right.

Just waiting for the tide to come in on Lake Sink....



The Coracle - complete with paddle...



If the hull of a boat does not force the water out of the way when it turns, it can turn fast.

The less resistance, the faster it turns.

So, if I put it in the water and spin it, it will settle to a speed fixed by its resistance.

The paddle will show us its spin as its pretty uniform...

If you give it some resistance - like an upright end, it will not be able to spin so fast as it must then push water out of its way like the canoe ends do.

You will need a piece of tape, a pencil and an elastic band...



Stick the pencil on the tape in the middle, but not quite square...



Fold the tape over like a flag...



Get your bowl and pull the elastic bands over the top and push the pencil through the ends under it...



Ready to be launched again...



We are looking at the speed they turn at - if I can manage it without spinning them into the side of the sink....

The modified one really doesnt want to spin any more.

Going to try it now with just the pencil...



Again, you can easily see just how much difference this amount of resistance makes....



On a 13 ft+ canoe, it will make considerably more.

Dont take my word for this.

Get hold of a couple of bowls, elastic bands, pencils and some tape and try it yourself.

The more you try this stuff, and get to actually understand it, rather than believing its probably correct in theory, the easier it will be to make the decisions you will need to make to successfully design and build your own craft.