Things you always wanted to know the answer to [Vol. 2]
Discussion
DannyScene said:
Based off something someone said previously, if you are in a sailing boat and you want to head south but the wind is blowing north do you have to wait for the wind to change?
I don't understand at all how a sailing boat can go up wind although I imagine it's a simple answer
No need to wait for the wind to change - it can go upwind.I don't understand at all how a sailing boat can go up wind although I imagine it's a simple answer
Think of it like this:
Stick the sail out 45 degrees.
Like this:
Very roughly that would send the boat heading along perpendicular to the wind.
(In the picture it is going to go East with a wind from the North.)
Now imagine the wind drops completely and boat changes into a golf ball on a mini-golf course say.
It keeps heading along the course Eastwards.
BUT...
Then it hits a wall which is at 45 degrees to the direction it was going.
Sort of like this:
Obviously it will now bounce off the wall and head up towards the hole.
As a massive simplification this is the job of the centre-board in a boat.
As you point the boat into the wind - the centre-board "pushes" off the water allowing you to convert your Easterly motion into a little bit of Northerly motion i.e. into the wind.
Edited by walm on Thursday 8th January 16:22
Edited by walm on Thursday 8th January 16:22
VladD said:
Boats don't get blown, they get sucked. The sail forms a similar shape to an aircraft wing and uses the equivalent of lift.
If the wind is blowing from the north then you can head north east or north west quite easily. The wind pulls the boat and the keel resits the pull in that direction and gives you forward momentum.
This is 100% wrong.If the wind is blowing from the north then you can head north east or north west quite easily. The wind pulls the boat and the keel resits the pull in that direction and gives you forward momentum.
The aerofoil effect of a sail works against you when trying to beat into the wind.
Just think about the shape of the sail as you look at it on top.
Lift pulls you pretty much perpendicular to the sail - the WRONG way!!!
walm said:
VladD said:
Boats don't get blown, they get sucked. The sail forms a similar shape to an aircraft wing and uses the equivalent of lift.
If the wind is blowing from the north then you can head north east or north west quite easily. The wind pulls the boat and the keel resits the pull in that direction and gives you forward momentum.
This is 100% wrong.If the wind is blowing from the north then you can head north east or north west quite easily. The wind pulls the boat and the keel resits the pull in that direction and gives you forward momentum.
The aerofoil effect of a sail works against you when trying to beat into the wind.
Just think about the shape of the sail as you look at it on top.
Lift pulls you pretty much perpendicular to the sail - the WRONG way!!!
P.S. http://en.m.wikipedia.org/wiki/Tacking_(sailing)
http://en.m.wikipedia.org/wiki/Jibe
Edited by Vipers on Thursday 8th January 20:49
walm said:
Vipers said:
Tacking and jibing isn't it?
Well tacking and gybeing perhaps.Not sure about your point, exactly - sorry!
I was just saying that the (tiny) aerofoil effect from a sail pulls you to leeward not windward.
To suggest a sailboat is pulled not pushed is absurd.
Maybe they were not familiar with sailing boats tacking to saill against the wind. Your quote is fine.
walm said:
Vipers said:
Tacking and jibing isn't it?
Well tacking and gybeing perhaps.Not sure about your point, exactly - sorry!
I was just saying that the (tiny) aerofoil effect from a sail pulls you to leeward not windward.
To suggest a sailboat is pulled not pushed is absurd.
Without a keel the boat would have such leeway that it would slide downwind at such a speed that no matter how close to the wind direction you could point the summation of the vectors meant that effectively you were not making any progress to windward.
With a keel then that leeway is reduced.
In determining a dead reckoning plot you have to determine the result of all the vectors in respect of boat speed through the water, leeway, and effect of tidal current.
If you look at the old square riggers, they could not point in any degree to windward, they had no significant keels in terms of water resistance so made massive leeway. Thus they had to wait for what was called a fair wind. It's also the reason why so many became embayed, that is onshore wind and if they couldn't anchor all they could do was sail back and forth hoping for a wind shift to allow them to escape.
LordGrover said:
Some schmuck decided 8 glasses of water a day is 'needed' to be healthy.
Tabloid readers believed it.
Specifically, someone worked out in the total amount of water absorbed in a day.Tabloid readers believed it.
Someone else took the figure out of context, missing out the point that most of this water is contained in food.
Then the mineral water industry tried to claim that not only does all this water have to be drunk, but that water in tea coffee beer etc doesn't count and that even tap water is a bit dubious.
Even now there are people who claim that coffee dehydrates you because of the caffeine, ignoring the fact that you are taking the caffeine together with a large mug of water so still end up ahead on the deal.
FiF said:
It's a combination of the two. There is low pressure ahead of the sail which provides suction and pressure on the windward side of the sail.
I agree but I think Vlad is probably sucked in (DYSWIDT) by the equal transit time fallacy which is often used to describe lift on wings during poorly taught GCSE physics lessons.
The reason a wing forces an aircraft up is almost entirely due to Newton's third law.
There is a crap load of air being deflected down and the equal and opposite reaction is lift.
The speed of the air on the top vs. the speed on the bottom (Bernoull's Principle) is adding to the lift but the air deflection is by far and away the most important force.
I used to demonstrate this by asking people to stick their hand outside a car window as it goes along.
You can make a rough aerofoil shape with your hand while holding it horizontal and keep it there very easily.
Now try rotating it 45 degrees so you are deflecting a bunch of air downwards. Your arm will shoot up.
That's the force that matters.
It's all here: http://en.wikipedia.org/wiki/Lift_%28force%29
Dr Jekyll said:
DannyScene said:
Based off something someone said previously, if you are in a sailing boat and you want to head south but the wind is blowing north do you have to wait for the wind to change?
I don't understand at all how a sailing boat can go up wind although I imagine it's a simple answer
You can't go directly into the wind. But if you point at about 45deg to the wind and position the sails to bounce the wind over the back of the boat, then your keel prevents the boat going sideways and you travel forwards. So you can keep zig sagging and travel upwind.I don't understand at all how a sailing boat can go up wind although I imagine it's a simple answer
LordGrover said:
Some schmuck decided 8 glasses of water a day is 'needed' to be healthy.
Tabloid readers believed it.
I thought the amount came originally from NASA who calculated water requirements for astronauts who were only eating dehydrated food.Tabloid readers believed it.
Some people assumed they needed the same amount of water, on top of their normal intake of tea, coffee and normal food (which is 50% water or whatever).
walm said:
FiF said:
It's a combination of the two. There is low pressure ahead of the sail which provides suction and pressure on the windward side of the sail.
I agree but I think Vlad is probably sucked in (DYSWIDT) by the equal transit time fallacy which is often used to describe lift on wings during poorly taught GCSE physics lessons.The reason a wing forces an aircraft up is almost entirely due to Newton's third law.
There is a crap load of air being deflected down and the equal and opposite reaction is lift.
The speed of the air on the top vs. the speed on the bottom (Bernoull's Principle) is adding to the lift but the air deflection is by far and away the most important force.
I used to demonstrate this by asking people to stick their hand outside a car window as it goes along.
You can make a rough aerofoil shape with your hand while holding it horizontal and keep it there very easily.
Now try rotating it 45 degrees so you are deflecting a bunch of air downwards. Your arm will shoot up.
That's the force that matters.
It's all here: http://en.wikipedia.org/wiki/Lift_%28force%29
An understanding of that helped me, a "fatty", to maintain competitive boat speed in light airs against the 7 stone wet through kids in Laser races. They should have walked away from me in light airs and it used to piss them off that they didn't. Windy conditions though, yes it's tough hiking as a heavyweight but you don't half go fast.
VladD said:
walm said:
FiF said:
It's a combination of the two. There is low pressure ahead of the sail which provides suction and pressure on the windward side of the sail.
I agree
Then I thought, "he probably hates me and hasn't read this far anyway so I can get away with it".
Damn.
Sorry!
fomb said:
walm said:
The reason a wing forces an aircraft up is almost entirely due to Newton's third law.
There is a crap load of air being deflected down and the equal and opposite reaction is lift.
If this were true, the shape of the wing cross-section should be irrelevant.There is a crap load of air being deflected down and the equal and opposite reaction is lift.
fomb said:
walm said:
The reason a wing forces an aircraft up is almost entirely due to Newton's third law.
There is a crap load of air being deflected down and the equal and opposite reaction is lift.
If this were true, the shape of the wing cross-section should be irrelevant.There is a crap load of air being deflected down and the equal and opposite reaction is lift.
From the article I linked when referring to the Newtonian explanation:
"This simple explanation, while correct in as far as it goes, is not sufficiently detailed to support the precise calculations required for engineering."
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