Do tides affect hovercrafts?

[ari]

If you were in a small hovercraft doing 20 knots up a river with an ebbing tide of 3 knots, what would your speed over the ground be?

Just a random musing... /forums/images/graemlins/crazy.gif

 

[Lakesailor]

Surely it would depend on wind speed and direction. Nothing to do with tidal current, unless there was some boundary layer effect on the air above the water?

 

[Nauti Fox]

As they dont actually touch the water I would guess its the wind direction that makes the difference

 

[ari]

[ QUOTE ]
Surely it would depend on wind speed and direction. Nothing to do with tidal current, unless there was some boundary layer effect on the air above the water?

[/ QUOTE ]

Assume it's flat calm.

Your comment about boundry layer effect is what interests me, whether the fact that the surface it is running over is moving would have an effect?

 

[Nauti Fox]

Anyways up, what you akshully need are these
Oh yes, its a long video and in foreign.

 

[mainshiptom]

17?

 

[Kawasaki]

Having the attention span about the size of a gnat.
I've just given the prob 2 mins of My valuable time and My head hurts you tw8t!!! /forums/images/graemlins/confused.gif /forums/images/graemlins/tongue.gif

 

[David2452]

Yea but no but, a hovercraft has no "speed through the water" so the 20 kts is nebulas anyway. (20 kts through or over what medium do you mean)

 

[ari]

Alright, let me put it another way. You're doing 20 knots on the GPS across water in flat calm conditions, no wind, no tide.

1/2 hour later there's 3 knots of tide against you. Nothing else has changed. Is your GPS still reading 20 knots, or is the fact that the surface you are travelling over is now moving against your direction of travel at 3 knots having an effect?

 

[ari]

Incidently I should add that I don't know the answer to this one, it's not a trick question, I'm just genuinely curious. /forums/images/graemlins/smile.gif

 

[mikesyam]

I was lucky enough to own a Osprey5 hovercraft for a year and while I never played with a gps the flow of the tide never seemed to make any difference as you were on the water rather than in it if you know what I mean, great great memories though, it actually was one of the ones from the bond film.

 

[tangofour]

[ QUOTE ]
If you were in a small hovercraft doing 20 knots up a river with an ebbing tide of 3 knots, what would your speed over the ground be?

[/ QUOTE ]

20 knots assuming it is 'flying' at the time. If it is in dispacement mode then 'it depends'. If one interprets your English one way - 'If you were in a small hovercraft doing 20 knots up a river' then the answer is 20 knots /forums/images/graemlins/smile.gif however if you meant 'the hovercraft doing 20 knots through the river water that is flowing at 3 knots in the opposite direction then perhaps it would be 17 knots over the ground. That would of course depend on the flow rate of the river which you omited to mention - rivers generally don't do 'tides'.

I have now managed to waste a good 3 minutes of a particularly poor day in the office ............

 

[David2452]

Hovercraft instruments read in airspeed and they seem unaffected by anything other than largish waves slowing them a bit, so in flat water and still air I guess the answer would be 20 kts airspeed regardless of water speed or direction. Mind you an aircraft has no connection with the ground either but if it is going with the rotation of the earth then it will take longer to cover the same distance over the ground than if it were flying against the earths rotation as it's destination would be running away in the first case and running towards it in the latter, so really I don't know is the real honest answer having thought about it a bit more.

 

[epervier]

and when you've been upgraded from club to first class you don't care!

 

[Steveu]

No, I don't think that's so. If you were right in what you say then a balloon need only take off in still air for the world to keep turning underneath it & it will eventually land somewhere else. This doesn't happen and nothing in Newton's laws of motion suggest it should.
Back to the aircraft. Assuming the air is still all the way, an aircraft will take the same time to travel the same distance in any direction. The difference in flight time between Eastbound & Westbound flights is accounted for by the jetstream which gives a headwind (& turbulence) in one direction and a tailwind & smooth flight on the return trip. Now you could argue that the jetstream itself is caused by the effect of the earths rotation and no doubt there are others on this forum who could confirm whether this is so.

 

[David2452]

There you are then, told you I didn't know really /forums/images/graemlins/confused.gif /forums/images/graemlins/smile.gif

 

[jfm]

Yup, as Steveu states, the earth's rotation doesn't have that effect. The earth's atmosphere rotates with the earth - why wouldn't it, it's a mass the same as the planet and there is no friction with space rubbing on the outside edge of the atmosphere slowing it down

Anyway, the surface speed of the earth's rotation is about 1000mph at the equator (25,000miles in 24 hours), praps half-ish that at UK's latitude. That would make a hell of a difference to westbound versus eastbound flight times! And to wind strength!

 

[powerskipper]

the GPS measure how fast you're moving across the earth surface so if it says 20 knots thats what you are doing.
In a boat you would need to be doing 23 knot though the water to achieves 20 knots over the ground [GPS]with a 3 knot tide,

 

[ari]

Yeah, I know that Julie! /forums/images/graemlins/grin.gif

But in my example you're not in a boat, you're in a hovercraft. So you're above the water, but you're supported by it...

 

[Deleted User YDKXO]

Interesting question. The hovercraft is supported on a cushion of air which is in contact with the water. How much effect the flow of the water has on the cushion of air depends on the friction between the air and the water. I guess this friction is very low but certainly not zero. Think of wind blowing across water. A stiff breeze could set up a surface current of 1-2 knots.
Therefore, IMHO, the 3 knot tide will have a small effect on the forward speed of the hovercraft. Very difficult to estimate without some fluid dynamics theory but I guess a small fraction of a knot
Obviously, the situation would be very different if the hovercraft itself was in contact with the water eg the side skirts. Then the effect on the hovercraft's speed would be dependent on the friction between the skirts and the water as well as the cushion of air