C08
Well-known member
Do outboard engines exhibit prop walk and if so does it depend on prop size i.e. do large props have walk and small ones not?
Outboard Engines & Prop walk
- Thread starter C08
- Start date
C08
Well-known member
I used to have an outboard that I used to tilt a little to induce some prop walk. It did work but only a little, perhaps because it was only an 8.5" prop. My new outboard engine is a 12.5" prop and I am wondering if this may exhibit more prop walk when tilted a little but still be well under the surface?
stelican
Well-known member
Best try it and see but bigger blades should help alternatively vector the outboard.
Sounds like alot of faffing around more useful for fixed shaft boats.
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C08
Well-known member
Difficult to swivel my outboard to steer but it would be useful when berthing to have some prop walk-if it was predictable and was in a useful direction. I am saying that because an outboard slightly tilted has the shaft tilted upwards wheras a fixed shaft is normally tilted down so I am wondering if the prop walk would be to starboard with a rh prop rather than to port as would be normal with a sloping fixed drive shaft?
Leighb
Well-known member
I am sure someone will correct me if I am wrong, but AFAIK the direction of propwalk is entirely related to the prop rotation and not to the angle of the shaft, so for a RH prop it will be the same on an O/B as on a shaft drive.
C08
Well-known member
As I understand it a horizontal shaft does not have any prop walk; an inclined shaft has prop walk because the upwards thrust and downwards thrust are uneven due to the tilt. Because an outboard tilted is an upwards tilt I am wondering if the uneven thrust is the opposite way?
jfm
Well-known member
Prop walk is more pronounced with big-pitch props. A 30 knot outboard boat might typically have a prop of around say 15inches pitch while a twin diesel big shaft drive P motorboat might have a prop pitch in the order of 45 inches, so you will have much more prop walk with the latter (at the same idle rpm of say 300-350rpm at the prop). All my numbers are ballpark, obviously.
jfm
Well-known member
That just aint correct. You get prop walk with a horizontal shaft.
Keith-i
Well-known member
I doubt an outboard would exhibit any appreciable propwalk. From my understanding the bulk of propwalk is attributed to the presence of a hull above the prop restricting the flow of water. An outboard sticking out behind the transom doesn't suffer the same effect.
Chris_d
Well-known member
Some odd myths coming out here, prop walk is better described as the "paddle wheel effect".
If you mounted two LH and RH props either side of a boat like an old fashioned paddle steamer and rotated them forward the boat would move forward, all a bit very inefficently and slowly, but the props are partially acting like the flat bladed paddle wheels would and generating forward thrust.
Hence as JFM stated the greater the pitch the more the paddle wheel effect, outboard props usually have a very fine pitch so generate very little paddle wheel effect, but there will be some but propably not enough to be useful compared to simply vectoring the prop to get the same effect.
So nothing to do with shaft angle or hull closeness the prop is just paddling or "walking" the boat sideways.
If you mounted two LH and RH props either side of a boat like an old fashioned paddle steamer and rotated them forward the boat would move forward, all a bit very inefficently and slowly, but the props are partially acting like the flat bladed paddle wheels would and generating forward thrust.
Hence as JFM stated the greater the pitch the more the paddle wheel effect, outboard props usually have a very fine pitch so generate very little paddle wheel effect, but there will be some but propably not enough to be useful compared to simply vectoring the prop to get the same effect.
So nothing to do with shaft angle or hull closeness the prop is just paddling or "walking" the boat sideways.
C08
Well-known member
I can understand the "paddle wheel effect" and why a bigger pitch prop will have more but I am still confused as there are many "experts" who maintain the shaft angle is part of the prop walk effect. See
https://en.wikipedia.org/wiki/Propeller_walk
and
http://www.castlemarine.co.uk/propwalk.pdf
https://en.wikipedia.org/wiki/Propeller_walk
and
http://www.castlemarine.co.uk/propwalk.pdf
Chris_d
Well-known member
The hull is directing the flow and amplifying the effect, so if makes sense that an outboard will produce much less prop walk but not none at all.
jfm
Well-known member
The wiki article says nothing of prop shaft angle. The castlemarine link says a lot about it, but is profoundly wrong and fundamentally flawed in its physics. The wiki article is also quite wrong, but not on the topic of shaft angle.
Don't believe everything you read on the internet. (Of course, you don't have to believe me either! But I hope you can see quickly that the analysis in the castlemarine article is really very badly wrong)
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MapisM
Well-known member
Correct, with the exception of a rather peculiar type of vessel.
Now, is anyone up for a virtual cigar?
MapisM
Well-known member
That's a case of overanalysis, if I've ever seen one.
Just try a single o/b powered boat, and it'll take seconds to find out that the propwalk is VERY appreciable, regardless of the rationale behind it.
And at any trim position, btw.
Croftie
Active member
Correct, with the exception of a rather peculiar type of vessel.
Now, is anyone up for a virtual cigar?
A waterjet boat?
MapisM
Well-known member
Well, that's impeller rather than propeller driven. I was talking of propeller(s) on straight shaft(s).
There's only one vessel with such propulsion which isn't affected by prop walk, as I was told by a Navy officer - just to throw in a hint...
There's only one vessel with such propulsion which isn't affected by prop walk, as I was told by a Navy officer - just to throw in a hint...
Croftie
Active member
MapisM
Well-known member
That's not what I meant, but shouldn't it be classified as a vehicle rather than a vessel?
Anyhow, if a hovercraft is a vessel, and if it isn't affected by prop walk (but isn't it? Prop planes are indeed affected, albeit in a somewhat different way), well, then make it TWO exceptions!
Anyhow, if a hovercraft is a vessel, and if it isn't affected by prop walk (but isn't it? Prop planes are indeed affected, albeit in a somewhat different way), well, then make it TWO exceptions!
Boathook
Well-known member
Never really noticed it on my boat, so no.
