Prop walk. Pysical reason why does this happen
- Thread starter Uricanejack
- Start date
Corribee Boy
Well-Known Member
Thread drift - Watching a programme on weapons last night, the presenter pointed out that the projectiles from a 40mm 'grenade gun' turned to the right in flight. I don't know if they were spinning or not. Is this part of the coriolis effect? Gyroscopic? anyone know?
Iain C
Well-Known Member
Right, so just so I'm 100% clear on this, a difference in density between your prop-forwards causes a turning force and you end up with moonwalk.
Got it.
Got it.
35mm
Well-Known Member
OK everyone talking about prop walk as an effect that happens mostly in reverse or when going astern. Maybe it's less noticeable when going ahead due to prop wash over the rudder, but it is still there. If I have to turn tight in confined space, I slow right down, then hard starboard rudder and full power and it virtually turns in it's own length. I always tight turn to starboard.
The effect is much less with a fin or lift keel down simply because the keel acts as a stabiliser fin - if you tried to fly a plane without it's vertical stabiliser it will loose all sense of direction and yaw all over the place. The keel probably also acts a bit like a rudder for the prop wash to act upon. My father used to have a Feeling 9.5 and with the centreboard up, it was completely uncontrollable in reverse. If there was enough depth, we would always reverse with the centreboard down and then it would be very controllable.
The effect is much less with a fin or lift keel down simply because the keel acts as a stabiliser fin - if you tried to fly a plane without it's vertical stabiliser it will loose all sense of direction and yaw all over the place. The keel probably also acts a bit like a rudder for the prop wash to act upon. My father used to have a Feeling 9.5 and with the centreboard up, it was completely uncontrollable in reverse. If there was enough depth, we would always reverse with the centreboard down and then it would be very controllable.
billskip
Well-Known Member
I wonder how many of you have actually taken a good look at a propeller.
I may be wrong, but there is one point I notice, that does not seem to have been mentioned.
I may be wrong, but there is one point I notice, that does not seem to have been mentioned.
Daydreamer
Well-Known Member
Its the Coriolis effect. Just Google it if you feel up to the challenge
Once again animations are useful.
C
catalac08
Guest
My main engine is an outboard and the prop is between the 2 hulls (and about 4ft from each hull) and is about 1ft astern of any part of the hull. With the engine in the normal vertical position there is very little noticeable prop walk but this increases dramatically as I tilt the engine upwards so that at about 25 degrees from vertical and still well immersed the prop walk is amazing. I am still trying to learn how to use this in real life close quarter handling as the amount of prop walk is so tilt dependent that I find it difficult to know how much to expect as I have no tilt level indicator fitted!
So the prop is well away from either hull, nowhere near either rudder and the angle of the prop seems to be the main mechanism creating prop walk in my case.
Does this help or just add to the confusion?
So the prop is well away from either hull, nowhere near either rudder and the angle of the prop seems to be the main mechanism creating prop walk in my case.
Does this help or just add to the confusion?
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lw395
Well-Known Member
I think it could be a bit of a side issue as your prop is operating in a quite different environment.
However, an earlier poster stated that he thought the tilt of the prop was significant in itself.
Since you tilted prop is tilted the other way from a typical angled shaft installation, does the propwalk go the other way?
I suspect what happens in you case is the top half of the prop cavitates, so the bottom half acts as a paddle wheel?
Corribee Boy
Well-Known Member
Its the Coriolis effect. Just Google it if you feel up to the challenge
Once again animations are useful.
Not prop walk at all, then?
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tinkicker0
Well-Known Member
I'm sure Pilotwolf as a rotorhead in particular and the other flyers on here have been watching the answers with interest.
Sorry PW since its Xmas and I'm a spoilsport I'm going to let the cat out of the bag.
Prop walk is caused by differential angles of attack of the prop blades as a result of the line of thrust differing from the line of drag.
Sorry PW since its Xmas and I'm a spoilsport I'm going to let the cat out of the bag.
Prop walk is caused by differential angles of attack of the prop blades as a result of the line of thrust differing from the line of drag.
Buck Turgidson
Well-Known Member
p factor is negligible on such a small diameter prop. Sorry to spoil you xmas
Just put the engine in revers and take a look over each side of the boat. The wash will be clearly visible on the up going side as the vortex is still intact whereas it has dissipated against the hull before reaching the other side.
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tinkicker0
Well-Known Member
p factor is negligible on such a small diameter prop. Sorry to spoil you xmas
Just put the engine in revers and take a look over each side of the boat. The wash will be clearly visible on the up going side as the vortex is still intact whereas it has dissipated against the hull before reaching the other side.
The more the thrust drag couple is increased, the greater the effect of prop walk.
I have a stern drive and have witnessed this myself at various trim angles, so sorry your hull theory does not hold water as at full out trim, the hull is nowhere near the prop thrust as the prop is always in clean water well beneath the hull but the effect is really excessive.
Since the effect varies with the angle of prop to drag, then this can be the only explanation.
It makes helicopters manoeuvre so it can also manoeuvre boats. As for size of prop, water is much denser than air and my prop soaks up 150 HP. Same as a small helicopter engine.
Time for a beer.
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Buck Turgidson
Well-Known Member
We don't suffer from P on turbojets as the cowl straightens the flow. But I'm pretty sure the effect your describing with 150 hp and angled motor is not the "prop walk" being discussed here.
But yes, it's definitely beer o'clock.
Cappen Boidseye
Well-Known Member
Do you get prop walk with ducted props? It would be interesting to try an installation which could be used ducted, then none ducted, just to see how much prop walk is reduced. My guess is that a ducted prop would walk far less than a naked prop.
C
catalac08
Guest
A good point that a tilted outboard is tilted the other way to a prop on a normal shaft. My outboard propeller is right hand and the more the leg is tilted, in reverse the more acute the prop kicks the stern to port. Is this what you would expect?
onesea
Well-Known Member
I would say your guess is correct...
JayBee
Well-Known Member
Correct. Ducted props, or props in kort nozzles, either fixed or steerable, do not have issues with transverse thrust.
tinkicker0
Well-Known Member
A cowl or duct around a prop modifies the drag vector and negates the effect. C'mon this is basic aerodynamics.
Buck Turgidson
Well-Known Member
Modifies which drag vector?
The blade drag or the hull drag? The blade drag is is a radial disk parallel to the aoa so unless something interferes with it it won't be the cause of prop walk.
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tinkicker0
Well-Known Member
The drag vector is always parallel to the direction of the airstream, so far as the prop is concerned, so fitting a cowl around an angled propeller disc redirects the flow and reduces the the thrust drag couple within the cowl to zero. No off angle means no pitch variation of the blades and no or little walk. Of course other external factors also affect flow relative to the angle of the prop disc such as currents get so unless you do have a closely ducted prop you will always get some amount of walk even if it has zero angle of incidence.
At the moment my pounding head will not dredge up technical terms like AoA without extreme effort and since I am describing the effect of a thrust drag couple on a prop, it seemed best to stick with drag vector.
I suggest that rather than taking my word for it, you dig out your books on aerodynamics and look it up for yourself.
Those without books can look up P Effect on the web now my addled brain has dredged up its technical term.
Edit . err that would be p factor
Edit 2 I have just noticed you mentioned p factor earlier BT and it did not register in my beery fog. Prop size is unimportant, it is the forces induced in it that does the work and the prop on my boat does the same work as that of a light aircraft despite its far smaller size. If it can handle the same power, it can produce the same effect.
Ever tried a bollard pull test with you being the bollard? I have with engine idling and could barely hold it back, the slightest throttle and I'd have been in the drink. Don't let size fool you.
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