Alright I think I have a pretty good question....

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Boz Mon

Boz Mon

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Lets say you have a boat. You put this boat in a lake with no current or anything restricting it other than water drag. Lets say the boat goes 60mph in the water. NOW lets say you put the SAME boat in a lake full of rubbing alcohol. Would it go faster because of less drag or would it go slower because it couldnt push as hard on the alcohol? :drool:
 
Hmm, thats a hard question. If there was any advantage drag wise, you'd have to factor in that there would be a disadvantage in the ability of the prop to "grip" the water. That might only affect acceleration though.
 
Well, that's a bit like asking if a car goes faster on ice than on asphalt since there's less grip from the tires holding it back, but then it'd be harder to get to that speed.
 
I hypothesize that It would go the same speed.

Thank you, though. Thank you TONS. You just gave me this year's science fair project.
 
Be sure to keep us informed, Omnis. Make sure you don't breath too hard around the test basin full of rubbing alcohol. :D
I think the problem is comperable to aircraft at varying altitudes (prop powered ones, anyway). I'm pretty sure the advantage for them of lower air pressure at altitude outweighs the loss of propeller pulling power. Or maybe that's only for aircraft with variable prop angle to compensate. Possibly not, what I know and don't know about aviation is as foggy as a landing by Humboldt Bay, CA. (...alright, I googled for that one.)
There's my .02$. Maybe some of the more aviationally inclined members will stop in to give their comperable 50 bucks.
 
Well, actually...alcohol might be slower. When working with drag, volume is king. Alcohol is less dense than water. This means that the boat will probably sink more into the alcohol, AND the propellers might not generate propulsion as well as they could in water. More drag and less thrust = inefficiency. That is, the boat will go slower.
 
Car on Ice = More Momentum
Car on Asphalt = More Acceleration, Braking, and Handling

More grip is better. The only place where a heavy car with no friction would win is on a downhill point-to-point ice trail. So I'd go with the lake without rubbing alchohol.
 
I will put the opposite into this.
When you put a boat in SALT water, it rides further out of the water. As a result of the reduced drag and slightly (very small amount) increased prop bite, the boat goes faster. I know this from personal experience. Our boat we used to have in fresh water would pull 3-4 mph faster. So, in the area of 23-25 mph up to 26-29. This is at the same RPM's, and with a similar load on the boat (fuel, people, stuff).
 
Good question. A real thinker.

After some tips from previous posts (the prop powered plane, and the salt water vs. fresh water) I vote that it will go slower.
 
The boat will sit lower in the alcohol so the drag will be at least the same as the water, but the prop will have less thrust in the alcohol than water.

The boat will go slower.
 
Speaking of propeller-driven aircraft, thrust and drag, even though the thrust decreases as air gets thinner they attain their maximum speed at about 20.000-25.000ft.
 
As far as i'm aware it's jet aircraft that attain their maximum speeds at that altitude. Prop aircraft need to be much lower.
 
DQuaN
As far as i'm aware it's jet aircraft that attain their maximum speeds at that altitude. Prop aircraft need to be much lower.
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Nope, most jet aircraft attain their max speed at 40000ft or more depending on the aircraft (I think some supersonic aircraft reach their max speeds at 60,000ft or more). Propeller driven aircraft do infact tend to reach their max speeds at 20,000ft ASL. But, since jets tend to travel much faster than propeller driven aircraft, the effects of drag are much, much more prevalent than for slower, prop aircraft. So, reducing drag has a greater effect than the extra power produced at lower altitudes.

As for the rubbing alcohol question, it will definitley travel faster in water with a propeller driven boat. Yes, there is lower drag, but lower drag is useless without adequate 'grip'. And Omnis, if you really want to make your science project interesting, you could also test a boat propelled by a jet. It would be interesting to compare how they perform.
 
A similar thing has been the topic of debate for a while now.

Can a man swim faster in treacle than in water? The theory goes that, though the treacle is a more viscous medium and impedes the swimmer by means of hydrodynamic drag, the swimmer is able to impart a much larger force on the medium and attain a higher speed per stroke.

Personally, I think it's knackers. Notwithstanding the fact the poor old bloke will be absolutely hammered after three strokes, drag increases by a square of the speed - and power required to overcome it increases by a cube - so if the swimmer goes faster, drag will increase, and even if he does manage to impart a larger force, it needs to be as large again as the increase in fluid viscosity AND the cube of the speed.


I would say that, unless the boat explodes or the driver gets totally smashed on fumes, the boat will be able to go much faster in rubbing alcohol than water, as the total power available to the vehicle is finite, but the decrease in drag by a square and power required to overcome it by a cube would permit higher speeds.

For reference, aerodynamic drag can be calculated by the following formula:

Frontal area (sq.ft) x 0.00256 x Coefficient of drag x speed x speed

(the amount of stuff presenting itself to the air, multiplied by the slipperyness of the stuff, multiplied by an air viscosity fudge factor, multiplied by the square of the speed)

Power (in hp) required to overcome it is

Drag x speed/375
 
Yes but wouldnt the boat be sitting lower in the alcohol? thus creating more drag, and also have less thrust?
 
Acceleration would be poorer, but absolute maximum speed ought to be higher.

I can calculate, if you'd like...
 
If the boat were to reach 60mph in normal water then it would be plaining across the water and very little of the boat comes into contact with the water except the prop and rear half would would be skipping.

I believe the boat would go slower in rubbing alcohol as it would have more trouble getting on the plain.
 
Hmmm... MAYbe you should test this in a sailboat, with identical wind speed?

Or is it a given that it is a motorboat?
 
A sail boat will go faster as there will be less resistance in the fluid but the same amount of propulsion.

They are wondering if it will be faster with reduced friction and propulsion.
 
VIPERGTSR01
If the boat were to reach 60mph in normal water then it would be plaining across the water and very little of the boat comes into contact with the water except the prop and rear half would would be skipping.

I believe the boat would go slower in rubbing alcohol as it would have more trouble getting on the plain.
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You're comparing behaviours at "x" speed.

As I've already said, the boat in alcohol will accelerate more slowly, but will be able, with the same fixed power, to reach a higher terminal speed. You're underestimating the value of reduced fluid drag.

Also, I pointed out earlier that the experiment has been conducted with humans and the thicker fluid lost - the increased buoyancy and higher available work per stroke were obliterated by the massive increase in fluid drag.
 
ah so even though the boat sits lower in the alcohol, when its up on the plain it will be in the same depth of alcohol as it would be if it was up on the plain in water, thus it will definetly have less drag.
 
i would say faster because it would be easier for the propellers to turn so would spin faster. BUT the buoyancy would be lower and more of the boat would be in the alcohol making it harder to push. The fastest way to go through water is to be out of it. Thats why racing boats are designed to jump.
So i change my mind and say it would be about same speed.



EDIT: oops. i didnt read the second page.
 
Yay. I knew Famine would come to the rescue. Good looking out, bud.

I might or might not actually do this, though. I mean, you can calculate the whole thing mathematically, given each substance's properties. Maybe i'll do something more anatomic....like something with lizard brains and stuff.

and ants. Ants are good, too. People judge you on that.

Happy boating.
 
Yeah famine's comment on drag does make sense, that's yeah engineers consentrate on aerodynamic drag more when they design cars right. We could reach miraculous speeds if there wasn't drag on moving objects. But I think there would be a big difference between a speed boat and a cruise ship.
 
The boat in water -those exhaust fumes and heat from the motor have to go somewhere....:P
 
the_undrtaker89
Yeah famine's comment on drag does make sense, that's yeah engineers consentrate on aerodynamic drag more when they design cars right. We could reach miraculous speeds if there wasn't drag on moving objects. But I think there would be a big difference between a speed boat and a cruise ship.
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Well, land, air, and sea travel are all different.

I'll just blow some stuff up.
 
the_undrtaker89
Yeah famine's comment on drag does make sense, that's yeah engineers consentrate on aerodynamic drag more when they design cars right. We could reach miraculous speeds if there wasn't drag on moving objects. But I think there would be a big difference between a speed boat and a cruise ship.
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Hence meteorites heating up and turning into fireballs when moving into the earth's atmosphere.
 

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