Register to reply

How can an vehicle move faster than the wind that is powering it?

by Topher925
Tags: faster, powering, vehicle, wind
Share this thread:
spork
#163
Oct22-08, 11:42 AM
P: 203
Quote Quote by Trond View Post
if the boat is drifting with the current in air which is still (lets say relative the moon...) there will be a wind relative the sails...
Yes, the wind relative to the sails will be at least hundreds of miles/hr given that the surface of the earth moves at about 1000 mph and the moon is moving in orbit a bit above 330 mph. I'd think about battening down the hatches.
spork
#164
Oct22-08, 11:46 AM
P: 203
Quote Quote by Topher925 View Post
..................... Is this correct?
No. But thanks for asking.
uart
#165
Oct22-08, 11:46 AM
Sci Advisor
P: 2,751
Quote Quote by ThinAirDesign View Post
Yes he should, and it's most definitely not true ... However, I have a bit of an issue with your explanation.



That depends on the definition of "bow wave and wake". With no sails up, the drag of the hull, mast and other will definitely move the boat downwind through the water. This movement will cause some "wave and wake", just not as dramatic as when the sails are up.

I'm sure we're on the same page, but just wanted to clarify that small point.

JB
Quote Quote by uart View Post
You should re-think that, it's definitely not true.

The boat drifting in the wind in still water will only have bow wave and wake if it is under sail. If you could make the hull etc to have zero wind resistance then it would not move correct. I only say this becasue for some inexplicable reason you chose to imagine that in the still water plus wind case that the boat is "feeiling the wind" wheareas in the current plus no wind case you chose to assume that the boat does not feel the apparent wind. Tell me, do you understand the concept of apparent wind?

I made it perfectly clear that I was taking the ideal case of zero wind resistance on the entire craft! I can't see how you missed that.

My point was that schoder seemed to be under the impression that in the other case (no wind and boat in current) that the apparent wind had zero influence on the boat. I was saying that if you take this erroneous assumption and apply it to the no current + wind situation then you'll also get no bow wave or wake. I hope that makes it clear :)
ThinAirDesign
#166
Oct22-08, 11:48 AM
P: 206
Quote Quote by Trond View Post
It's probably just semantics again, but if the boat is drifting with the current in air which is still (lets say relative the moon...) there will be a wind relative the sails. Exactly like a wind relative the still water. So in both cases you would actually sail if you have your sails up hence there will a wake..
Trond has hit the problem schroder appears to be having right on the head.

In BOTH cases there is the same wind over the boat. If the air is moving 10mph and the water still, the boat *sees* an ~10mph wind. If the air is still and the water is moving 10mph, the boat *sees* a ~10mph wind.

With the wind the same in both scenarios, any 'wake and wave' will be the same. Any windsock will show the same. Any physics experiment will be the same. The occupants of the boat will be powerless to determine which is moving and which not -- they only know they have wind.

JB
Topher925
#167
Oct22-08, 11:53 AM
Topher925's Avatar
P: 1,672
ThinAir,

In BOTH cases there is the same wind over the boat. If the air is moving 10mph and the water still, the boat *sees* an ~10mph wind. If the air is still and the water is moving 10mph, the boat *sees* a ~10mph wind.
Can you explain how this is different then what I stated in my last post.

Spork, do you care to explain WHY it is incorrect.
ThinAirDesign
#168
Oct22-08, 12:01 PM
P: 206
Quote Quote by uart View Post
I made it perfectly clear that I was taking the ideal case of zero wind resistance on the entire craft! I can't see how you missed that.
I didn't miss it uart, but you still didn't make it perfectly clear by any means ...

Here's the quote:
The boat drifting in the wind in still water will only have bow wave and wake if it is under sail. If you could make the hull etc to have zero wind resistance then it would not move correct.
If it has zero wind resistance and the water is still, it won't be "drifting" now will it? If it's "drifting", sails up or down, it will have wakes and waves to varying extents.

You clearly state above that the boat is drifting and you clearly state that it won't have wake with the sails down. Those points are mutually exclusive. Any drift or relative motion creates wake.


uart, I know *you* understood your point (as did I), I was only trying to make sure that it was technically correct for those who are having difficulty with the inertial frame references.

Best wishes.

JB
uart
#169
Oct22-08, 12:03 PM
Sci Advisor
P: 2,751
"Spork, do you care to explain WHY it is incorrect."

No I think it was basically correct. When the downwind component of velocity is equal to the wind velocity then yes the apparent wind will be in a direction perpendicular to the actual wind direction. And yes in this direction it may be possible to still produce lift.
uart
#170
Oct22-08, 12:05 PM
Sci Advisor
P: 2,751
Quote Quote by ThinAirDesign View Post
If it has zero wind resistance and the water is still, it won't be "drifting" now will it?
JB
Congratulations you finally got it. That was actually the point I was trying to make, that it wouldn't be drifting! That's what the words "then it would not move" mean.

I was trying to show the falsity of schoders argument so I used his words (drifting) and then went on to explain why it wouldn't drift unless you assumed some interaction with the wind.

I'm presuming that if I had of punctuated it better as in,
The boat "drifting in the wind in still water" will only have bow wave and wake if it is under sail. If you could make the hull etc to have zero wind resistance then it would not move, correct.
then you wouldn't have a problem.

Sorry but I find that level of nit picking very annnoying when you're trying to make a quick post
ThinAirDesign
#171
Oct22-08, 12:27 PM
P: 206
Quote Quote by Topher925 View Post
ThinAir,

Can you explain how this is different then what I stated in my last post.
I'm not sure what you need me to explain. My post was not in response to yours -- I hadn't even seen yours when writing mine. I was writing my explanation for schroder.

Same or different -- can't say as I wasn't referencing your comments.

JB
ThinAirDesign
#172
Oct22-08, 01:29 PM
P: 206
I absolutely love the "still air relative to the moon" bit. If I'm picking a frame of reference for sailing -- that's the one I'm using every single time. And hey, if the wind is still, relative to the moon, we'll only need a one foot mast and a hankerchief -- saves a lot of room down below.

This "physics" forum is a real kick.

JB
spork
#173
Oct22-08, 01:29 PM
P: 203
Someone made a comment about how diagrams are so "second degree university" - but there hardly seems to be 3 people here that understand the most basic high school physics concept of inertial reference frames. Discussing the prop-cart going downwind faster than the wind is beyond hopeless.

Heck, we've presented every possible type of evidence of what ice-boats are capable of - and all that has resulted in is silence.
Trond
#174
Oct22-08, 01:48 PM
P: 24
I wonder why.....let me know when you are ready to discuss the cart
rcgldr
#175
Oct22-08, 02:35 PM
HW Helper
P: 7,128
[QUOTE=Jeff Reid;1923648]Assuming the formula stated for a device with a Beta of 14 degrees, I made a graph of Vmg/Vt versus heading offset from true down wind. It peaks at about 2.56 at 38 degrees. http://jeffareid.net/misc/dwvhdg.gif[quote]I missed the obvious flaw that when the heading is 0 degrees offset from the wind direction, Vmg/Vt is 1.0, where it would have to be less than 1.0 because of drag. The Beta factor needs to vary based on heading.

Does anyone have an actual table of heading versus Vmg? Another issue is lift versus drag as it applies to sails. When heading downwind, aerodyamicd and ground lift are zero while aerodyanmic drag = ground drag. Also unlike wings, sails aren't designed to divert the air flow by 90 degrees. I need to do a rethink on this.
spork
#176
Oct22-08, 03:05 PM
P: 203
Quote Quote by Trond View Post
let me know when you are ready to discuss the cart
I'll let you know.

Quote Quote by Jeff Reid View Post
Does anyone have an actual table of heading versus Vmg?
I presume you mean measured heading vs. VMG? I don't have that. Certainly given the measured L/D of the sail and skates I can give you that data.

Another issue is lift versus drag as it applies to sails. When heading downwind, aerodyamicd and ground lift are zero while aerodyanmic drag = ground drag.
I'm not sure what you mean. Can you give me more detail?

Also unlike wings, sails aren't designed to divert the air flow by 90 degrees.
I don't understand what you mean here either. I don't think I've ever seen a wing that diverts airflow by 90 degrees. If you compare a windsurf sail to a hang glider wing, they're practically indistinguishable. And before you all tell me how ludicrous that statement is, I've owned plenty of hang gliders and windsurf rigs over that last couple of decades.


Register to reply

Related Discussions
Does a black hole cause light to move faster? Special & General Relativity 20
Why don't galaxies move faster then c? Astronomy & Astrophysics 27
Data can't move faster than the speed of light? General Physics 16
Force required to move stationary vehicle? Classical Physics 12
Why move faster? General Physics 12