"Heavy wind"

[JumbleDuck]

Go on, then. We're all sitting on the edge of our seats here, awaiting 'JumbleDuck's concise explanation of the General Theory of Relativity with particular reference to Gravity Waves.

Seriously? Well, if so, it's not gravity. It's just that very small water droplets, as in fog, are so light that gravity is overcome by Brownian motion. As they coalesce they get dense enough that gravity starts mattering and they fall, as rain.

 

[awol]

So it is as I have long suspected - my boat sails faster in winter for the same wind speed and feels somewhat lacklustre on the occasional hot sunny day.

 

[oldbilbo]

Seriously? Well, if so, it's not gravity. It's just that very small water droplets, as in fog, are so light that gravity is overcome by Brownian motion. As they coalesce they get dense enough that gravity starts mattering and they fall, as rain.

Huh. An 'idealised approximation'.... Rain is real. Scotch mist is, by definition, fanciful.

 

[davidej]

Cold air / wind is definitely more dense; in the old days of trading sailing boats the very experienced crews went by feel not an LCD readout.

Coastal trading boats used to send down their topsails each autumn and sail through the winter on mainsails and jibs.

Yes - but because of the greater likelihood of strong winds/ gales -not because the air was 'heavier'.

 

[lw395]

Yes - but because of the greater likelihood of strong winds/ gales -not because the air was 'heavier'.

The air is of course heavier when mixed with a good amount of southern ocean spray.

 

[Fantasie 19]

So, does that suggest that COLD air exerts more force on be sails than warm wind of the same strength?

I.e. As I suspected, the person I was talking to was wrong....?

When I used to windsurf a lot, we always knew we went faster in an autumn winter F4 than a summer one..... now I know why!

 

[winsbury]

So it is as I have long suspected - my boat sails faster in winter for the same wind speed and feels somewhat lacklustre on the occasional hot sunny day.

Water density also decreases with temperature so as sea temperature drops presumably the hull will move fractionally easier through it too.

 

[Tony Cross]

Perhaps the "heavy wind" humidity is caused by heavy water?

 

[KellysEye]

Cold air is heavier than hot air so yes cold air does exert more pressure on sails. You can easily notice the sail pressure difference in (say) 20 knot winds in England versus the Caribbean. You go slower in the Caribbean, been there done that.

 

[alan_d]

Water density also decreases with temperature so as sea temperature drops presumably the hull will move fractionally easier through it too.

Umm, why would that help? Pushing through a denser medium is harder than a less dense one (compare air and water, for example), or were you thinking that the vessel will float higher in a denser fluid, so reducing the wetted area? Your vessel will displace the same weight of water whatever its density (Archimedes), and I would have thought viscosity (which decreases with temperature rise) might be more important, but I suspect that the practical effects are small.

 

[charles_reed]

Umm, why would that help? Pushing through a denser medium is harder than a less dense one (compare air and water, for example), or were you thinking that the vessel will float higher in a denser fluid, so reducing the wetted area? Your vessel will displace the same weight of water whatever its density (Archimedes), and I would have thought viscosity (which decreases with temperature rise) might be more important, but I suspect that the practical effects are small.

Possibly, hot water is less viscous than colder water?

 

[winsbury]

or were you thinking that the vessel will float higher in a denser fluid, so reducing the wetted area?

Yup, less friction = faster even though displacement is constant. Though not sure what the effect on temperature versus salinity is or if hull shrinkage with reduced temperature are countering factors.

without a supercomputer to do the math I suspect you're right though, viscosity ultimately is king, especially as it increases with decreasing temperature until ultimately ice will block passage completely hence outweighs ALL these other effects at very low temperatures. They are probably all so small as to be insignificant but if I were a round the world ocean racer (I'm not) I guess it would be interesting to know the relationships and whether they need to be included in design calculations or ignored.

 

[JumbleDuck]

without a supercomputer to do the math I suspect you're right though, viscosity ultimately is king

I think wave making probably predominates, and certainly will at higher speeds.

 

[Seajet]

JumbleDuck,

is that ' wave making ' as in laminar flow / boundary layer drag ?

I've sometimes wondered about the NASA / McDonnel hull coating of riblets applied in skin form to the U.S. America's Cup racer ( Stars And Stripes ? ) in the races off Fremantle in the 1980's, this was supposed to smooth out/ break up the boundary layer drag with mini vortices as in some fighter aircraft, but as I've never heard of it since I presume it didn't work on boats !

 

[winsbury]

I believe it works on sharks and Australian swimmers, so why not boats ?

 

[Seajet]

Winsbury,

yes it was called ' sharkskin ' for that reason; don't know about Aussie swimmers, maybe that's their natural finish !

 

[JumbleDuck]

is that ' wave making ' as in laminar flow / boundary layer drag ?

Drag 101

There are basically four forms of drag ...

1. Friction drag is the longways component of shear stresses at the surface of <whatever> and depends on the viscosity of the fluid and the velocity gradient at the surface
2. Profile drag is the longways component of the direct stresses (ie pressures) at the surface and depends on flow pattern, velocity^2 and fluid density
3. Induced drag is the result of the bound vortex in a lift-producing shape changing the local angle of attack (other interpretations are available) and is inversely proportional to velocity^2
4. Wave making drag is the result of disturbing a free surface.

For a displacement boat, friction drag predominates at low speed (hence the effects of fouling on light wind performance) but wave making drag soon takes over. Beyond hull speed, wave making drag increases spectacularly, effectively limiting the maximum possible speed.

Induced drag is inversely proportional to lift coefficient, which increase with aspect ratio, which is one reason why a narrow, deep keel is much better than a wide, stubby one.

Boundary layers contribute to both friction and profile drag. Laminar is generally better than turbulent, but if you are going to have turbulent from it's often better to trigger it sooner than it would naturally occur - hence turbulator tape on the wings of gliders and dimples on golf balls.

Sorry, long answer to short question.

 

[Seajet]

Then there's parasitic drag too, I met an A-10 pilot who reckoned his aeroplane was badly afflicted by this, it didn't prevent him inverting at less than a wingspan up though !

 

[JumbleDuck]

Then there's parasitic drag too, I met an A-10 pilot who reckoned his aeroplane was badly afflicted by this, it didn't prevent him inverting at less than a wingspan up though !

Parasitic drag is generally the term used for everything except wave making drag. The terminology is not very standardised though, particularly across the Atlantic. What we (generally) call profile drag they (generally) call form drag, for example.

 

[Seajet]

' everything except wave making drag '; I know some Test Pilots and Flight Test Engineers who could do with that tatooed on their foreheads !