Here's a Thought About Propellors

[LittleSister]

Correct!

What matters is the size of the crumple zone -ie the space in which you can decellerate (spelling?). A car has a bit of one - a brick wall none.

What we are talking about is not whether there is a crumple zone, but how much force it can deal with. Run a car at 30mph into a brick wall and the brick wall will collapse or crumple. Run into it on your bicycle and it will not (you and your bike will!).

 

[LittleSister]

That's where I have to rely on my physics O'Level. My recollection is that it makes no difference whether the heavy object hits the light one, or vice versa. The effect is the same.

That's where I have to rely on my physics O'Level. My recollection is that it makes no difference whether the heavy object hits the light one, or vice versa. The effect is the same.

My 'O' level physics is now slowly coming back to me. The effect of two bodies one 100lb and one 1,000lb colliding in space may well receive the same impact, but that is not what is happening in the example we were discussing.

What we are talking about, I now think, is the kinetic energy of the two bodies. The kinetic energy of the 100lb, 100mph body is much less than that of a 1,000lb 100mph engine.

If the two are flying two space and collide with one another head on then the energy released will be their combined energy, and whatever the distribution between them (and you may well be right that it's equal) that energy will then be imparted in sending them back in the direction they came from (ignoring crumpling). If the energy distribution is equal then the 100lb body will now be going much faster than the 1,000lb engine (because it takes less energy to accelerate it).

By contrast what you have in the examples we discussed was the two scenarios
Scenario 1 - front engine) Where you have a now stationary 1,000lb front engine (its kinetic energy having been absorbed by the Earth moving away a teensy bit (ignoring crumpling), without the body being involved. When the body catches up with the engine, it is only the kinetic energy force of a 100lb, 100mph body that has to be dissipated by the belly (and any crumpling in the engine/movement of the earth ).
Scenario 2 - rear engine) When the 100lb, 100mph body hits the earth, the kinetic energy to be dissipated by the belly etc. is the same in the scenario as above. Unfortunately very, very shortly afterwards, there will be the arrival of a 1,000lb, 100mph rear engine, and the kidneys will have to dissipate the engine's much, much greater kinetic energy.

 

[LittleSister]

Quick rummage on ye olde internet suggests -
Kinetic energy = 0.5 x mass x velocity x velocity

With roughly equivalent 50kg body, 500kg engine, and both going 30 m/s my rusty maths suggests -

Kinetic energy of body = 22,500 Joules
Kinetic energy of engine = 225,000 Joules


Head on collision between the two = 247,500 Joules
Assuming this energy is equally shared (and no crumpling) result will be moving away from one another, body doing 70.35m/s and engine doing 22.25 m/s.


Scenario 1) Body hits stationary front engine (and Earth) - 22,500 Joules to dissipate.

Scenario 2) Body hits earth - 22,500 Joules to dissipate. Then rear engine hits stationary body (and Earth) - 225,000 Joules to dissipate.

(Must remember not to buy a mid- or rear engined car!)

 

[Lakesailor]

Hooray. Kinetic energy, not inertia.

Have you all lost grip of reality?
Whilst you are all sitting working out formulae has it occured you may be using the wrong ones?

Just visualise the scenarios.
In your mind.
Not using calculations.


Man behind engine less squashed than man in front of engine.

 

[Pye_End]

Quick rummage on ye olde internet suggests -
Kinetic energy = 0.5 x mass x velocity x velocity....

You are making the assumption that one body is at rest, and both bodies are at rest after the event.

I suspect that Twister Ken is assuming that in the case of a crash you have two moving bodies - one the engine and the other a person. A collision then occurs (so one must be going faster that the other - probably the heavier one due to inertia), and then after the event one or both bodies may still be moving.

In this case you need to use Newton's momentum law to work out the change in energy involved. If the crash were as simple as firing ball bearings across a table in an A level lab then I agree with Twister Ken. If the engine were doing 10m/s behind you and you were doing 8m/s the impact would be similar to you being behind it and doing 12m/s. The person will accelerate or decelerate by 2m/s, and then all you have to argue is that it lakes the same time to change speed to make the energy absorption the same in both cases.

However from a practical point of view the two bodies will be slowing down (in this type of crash), and like a feather and a stone dropped from a tower, the lighter body will slow down quicker, so if you are behind the engine you are less likely to hit it in the first place.

It does appear that being behind the engine is still your best bet.

 

[bignick]

Forward facing propellers....
No reason at all it won't work. Look up Volvo's podded drives. Or the propulsion of the Queen Mary 2. (Though the fines RR have had to pay may make people more wary of them in the future..)
Be slightly odd to have them at the front though.
Nick

 

[alant]

Has anyone designed a boat with a tractor propeller acting acting forward of midships?
Would it work for a small boat like a tender or fishing boat?
By mounting the power head backwards the recoil start and throttle etc would be available to the helm sitting behind it. It would need to protrude through a well in the hull or on a small cat between the hulls.
Would it be self defeating or could it work?

Just idly wondering.

The Poole Belles operating around Poole/Swanage/Bournemouth have 2 forward pointing props on pods, these situated at the stern side by side. Manouvering extremely interesting since these also act as rudders & turn 360 to gain astern power & quite difficult for anyone used to operating normal twin drives - schottel drives.

I Skippered for a season, to get my BM Licence.

One is now up in the Firth of Forth.
http://www.maritimejournal.com/feat...d-propulsion/repower_for_tour_boat_transplant

 

[davidej]

Voith Schneider propellor

Has anyone looked at these

http://en.wikipedia.org/wiki/Voith_Schneider_Propeller

They are like a horizontal rotating plate on the bottom of the hull with a number of 'rudders' attached which turn with the plate.

I went on a ferry in Canada so propelled. With one at each end (it was double-ended, so no stem and stern), the ferry could be propelled in any direction including sideways.

I don't know why it never caught on - a bit complicated and not very fuel efficient, I suppose?

 

[Lakesailor]

Losing the plot a bit.
My question was about a prop ahead of the the midships position.

It doesn't really matter if it's forward facing or not, but that it is placed ahead of the beam.

 

[alant]

Has anyone looked at these

http://en.wikipedia.org/wiki/Voith_Schneider_Propeller

They are like a horizontal rotating plate on the bottom of the hull with a number of 'rudders' attached which turn with the plate.

I went on a ferry in Canada so propelled. With one at each end (it was double-ended, so no stem and stern), the ferry could be propelled in any direction including sideways.

I don't know why it never caught on - a bit complicated and not very fuel efficient, I suppose?

I also skippered 'Bladerunner' in Southampton Water.
The smaller one - about 35m, had 2 schottels, but these were not props, but a form of ducted jet, with one up front & one toward the stern, rotating 360.

They could go in any direction apart from up/down.
When alongside for loading/unloading turbine blades (vestas ), they were held onto the dock by these units & no lines. Coming alongside, you started moving sideways, then slowly adjusted the thrust fore & aft, before hitting, which was a delicate operation.

 

[AngusMcDoon]

It does appear that being behind the engine is still your best bet.

In a light piston engine helicopter the engine is behind the passengers. The latest advice if having to autorotate into trees is at the last moment go nose up as far as possible. The helicopter will then fall through the trees to the ground hitting tail first, then engine, and the passengers on top of the heap. If the helo is level going into the trees it is likely to fall to the ground nose first, face planting the passengers into the ground who are then splatted by a 6 cylinder Lycoming lump. There are recorded incidents now of people surviving the nose-up fall backwards manoeuvre, whereas the nose down is usually fatal.

 

[BruceDanforth]

Either way will sting a bit.