The Motor Boat

Because when you put it in the still water, you measured its speed to be 8km/hr.

 

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What does that have to do with how much time it takes for the boat to travel the 8km in CURRENT?

 

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You asked, "How do you know the boat speed relative to the water?" and that is what I answered. You tested the boat in still water and found that it travels at 8 km/hr relative to the water. When there is a 3.27 km/hr current in the water, we can still choose a reference frame in which the water is at rest, and that reference frame would be moving at 3.27 km/hr relative to the embankment. That is why the speeds compose like this (8 km/hr + 3.27 km/hr) for the downstream speed, as measured by the embankment frame. And that is also why the speeds compose like this (8 km/hr - 3.27 km/hr) for the upstream speed, as measured by the embankment frame. It's just like your own theory!!!!!

 

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Or, time it to go past a floating meter stick. Or use a chip log. Or a marine speedometer


MD, what is the difference between water being moved by carrying a pool, and water moving in a smooth current?
As far as the boat is concerned, there is no difference. The speed relative to the water for a given power output is the same.

 

I don't think there needs much discussion in this thread. Thinking too much can lead somewhere else... And that's what Motor Daddy did!!!

And also it is very tough to convince persons by threads..

 

Easy.. Assume that it tries to move 8km/h. But there is additional current..so its speed varies.

Don't think too much.

 

Ash, you have put your finger on the problem exactly.

 

What do you mean?

Problem has been answered by you,Farsight,Neddy Bete,Pete and everyone.

 

Neddy Bate, there is no 8km/hr in the test of the boat in current. The boat did not take 1 hr to travel 8km in the upstream direction in current, and it didn't take 1 hour to travel the down stream direction in current. There is no "8" in the test of the boat in current. If you want to measure the boat in current then you MEASURE the time it takes for the boat to travel upstream in current and you calculate the speed after the test is complete.

What NOT to do when you want to measure the time it takes the boat to travel in CURRENT WATER is to place a boat in still water and find the speed, and then measure the water and find the speed, and then do math and proclaim you have the right answer. That would be downright stupid!

 

You are downright stupid!

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That is a feasible scenario and I don't see any reason why not.

 

8km/hr is the reading shown on the boat's speedometer.

It gives the same answer, MD. Try it and see. Go outside and do the experiment.

This is how you navigate the old fashioned way - you measure your speed relative to the water, look up the expected current in your charts, then calculate their speed relative to the ground by adding them together.

 

So as the current increases to a billion km/hr, the boat travels at a speed of eight minus a billion upstream??

 

If it's so deadly accurate why is it now the old fashioned way?

 

Well, that would be a current faster than light , so no. Also, I don't think we could safely ignore air resistance in that scenario.

But if the current is a more reasonable 10km/hr, then the boat travels at 8 - 10 = -2 km/hr upstream, ie it goes backward at 2km/hr.

Are you arguing with marine navigation theory now?

It's the old fashioned way because GPS is easier, more precise, and more reliable. Marine charts only tell you the expected current for your position, time of year, time of day, and phase of moon, they obviously don't measure the actual current at the present time. Wind shear can also interfere. A storm blowing you off course can screw up your dead reckoning, but your GPS will still be right.

Unless it fails, of course... which is why navigators still need to know how to do things the old fashioned way.

But, the physics is clear. See Set and Drift, for example.

 

So what you're saying is that it's not linear?

If GPS is more accurate, then that makes the old-fashioned way inaccurate, right? Or does your GPS have a greater accuracy than 100% accuracy, which makes the lesser accurate old-fashioned way less accurate, but still 100% accurate?

 

Why do you think aeroplane pilots commonly speak of "airspeed"? What do you think it means? How do you think they measure it? Now think of the same thing in water. You can perfectly easily measure the speed of something in motion through a fluid, i.e. relative to the fluid.

 

No. I don't know what you're thinking.
Speed relative to water - current speed = upstream speed
8 - 10 = -2

No measurement is 100% accurate.
But, it's accurate enough. How do you think people traveled the ocean before GPS?
How do you think someone would bring their boat to berth through a foggy harbor on a running tide?

 

If no then it's eight minus a billion upstream, clearly!

Right, so your math is never 100% accurate. Garbage in garbage out. What you are saying is that there is only garbage for input, and I'm saying that makes your math garbage.

 

It looks like you've run out of meaningful arguments, so we're done.

 

So I ask again, how much time does it take for the boat to travel upstream to the finish line?