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Old 15-04-2015, 15:27   #1
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Prop walk depending on prop size?

Hi,

We are over-propped and have a spectacular prop walk when in reverse, almost like a one way bow thruster. Does propeller size influence the strength of prop walk , and what other reasons could there be for such a nice prop walk?

Thanks,
Phil
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Old 15-04-2015, 15:56   #2
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Re: Prop walk depending on prop size?

Pitch certainly effects prop walk. I'm guessing size does, too, but don't have direct experience with that.

I know folks who intentionally over pitch their Max Prop so that it makes more prop walk. Since they have a large boat with no bow thruster. The Dashew's mention in The Cruising Encyclopedia that they use their adjustable pitch prop as a kind of bow thruster.
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Old 15-04-2015, 16:10   #3
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Re: Prop walk depending on prop size?

Pitch and size both definitely have an effect. Also, a heavy touch will increase it. Hard bursts of throttle will make it walk. Just being heavy on the throttle can increase walk. Once you have the boat moving, power down and you'll wonder where your prop walk went. More pronounced in reverse too.

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Old 15-04-2015, 19:28   #4
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Re: Prop walk depending on prop size?

I have always had a preference for two bladed props and always had great prop walk. Be careful of being over propped it will over heat your engine if worked hard and severely shorten engine life.


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Old 16-04-2015, 10:48   #5
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Re: Prop walk depending on prop size?

My 41 had a prop sized for a 2.1 to 1 reduction gear, from factory.
Not much prop walk.
But this particular boat came with 3 to 1 gear.
When I propped it correctly, i.e. bigger diameter, what a powerful difference.

In reverse, I would give it a good shot to get the boat moving
and then quickly slip into neutral and had the maneuverability that I wanted.

Easy throttle gave less walk when not desired.


The best, imho after 8 boats and 40 years, would be a very large diameter 2 blade, with corresponding clearance, not often seen, and a larger reduction gear.
I would install the largest diameter possible with 15% + clearance then pitch according to the other factors.
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Old 16-04-2015, 11:15   #6
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Re: Prop walk depending on prop size?

I think it's mostly pitch, I had a 17x16 three blade and had huge amounts of prop walk, after I got used to it, I liked it as it's variable after all.
Now I have an 18" Autoprop, no prop walk at all, I can back straight up, forever. I don't like it as much, used ot be I could do a standing turn with about three rev / fwd bursts, now I can't, it goes straight in reverse. Autoprop is bigger, but very little pitch in it until you get boat speed, so I'm thinking it's the loss in pitch that got rid of the prop walk.
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Old 16-04-2015, 17:06   #7
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Re: Prop walk depending on prop size?

Are you sure it is prop-walk not wash? How close is the prop to the rudder and keel? Wash of the rudder can push the stern either way (very useful) but with a prop close to the aft end of the keel you get a wash down one side and it always pushes the boat the same way, sometimes overcoming the helm. A big prop will definitely make this more noticable. If you tie to the dock and run in astern does most of the disturbed water come out one side? If so it is wash along the keel that is doing it not prop walk.
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Old 16-04-2015, 17:54   #8
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Re: Prop walk depending on prop size?

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Originally Posted by FamilyVan View Post
Pitch and size both definitely have an effect. Also, a heavy touch will increase it. Hard bursts of throttle will make it walk. Just being heavy on the throttle can increase walk. Once you have the boat moving, power down and you'll wonder where your prop walk went. More pronounced in reverse too.

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My experience is just the opposite. When first into reverse at engine idle speed (1,000 rpm) the boat walks to port going astern. To get going straight I must and do immediately increase the rpm to 1,500 to 2,000, the boat straightens out because there is [finally] water moving over the rudder. I only take it out of gear shortly thereafter because I'm simply backing across our fairway to turn to get out. FWIW, I turn to port to get out.

15x10 fixed 3 blade on an M25 engine. I believe it's a 2.7:1 transmission.

Most prop walk discussions I read about seem to get it worse at low speed, simply because the boat is NOT moving.

Indeed, in doing my three point turns, the BEST maneuver is to use very little throttle when wanting to back to port. I goose it to a stop after going forward. Then idle back to port since that's is when my boat/prop combo does its "best work."

Your boat, your experiences.
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Old 16-04-2015, 17:59   #9
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Re: Prop walk depending on prop size?

Good point Roland, I'm amazed by how often bank suction is confused with transverse thrust.

If we are going to get into more hydrodynamic specifics, I'm going to suggest that we help people understand the differences.

Prop Walk, or more correctly transverse thrust is the result of the wheel turning, the upper portion of the wheel being higher in the water, in a more hydrodynamically disturbed area. This results in the lower portion of the prop having more dig or effect than the upper portion of the prop. So imagine a tire spinning in mud, it's pulling one way, but not necessarily with authority. There is slip. The upper portion of the tire has some traction in the air, but the lower portion has more traction in the slippery mud.

What we're calling prop wash here, or sometimes referred to as bank suction is an entirely different thing. Bank suction works the same as sails or air foils. You have a relatively higher velocity waterflow between the seawall and the keel, than say between the keel and open water. As we all know, relatively higher velocity results in relatively lower pressure. The vacuum created sucks you towards the seawall or bank (hence the term bank suction).

What happens, is if you are making a dock, and you realise your stern is closing too quickly with the dock, you pile on reverse to get the boat stopped. When you pile on the reverse you increase the relative velocity between your keel and the sea wall. People often think they are experiencing crazy amounts of transverse thrust when, in reality, it's half transverse thrust and half bank suction, which ultimately results in over rotation and smoking the stern against the dock AND the bow wondering off on you, possibly causing you to miss the dock, especially if the stern bounces hard and is also vectoring away from the dock.

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Old 16-04-2015, 18:24   #10
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Re: Prop walk depending on prop size?

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Originally Posted by Stu Jackson View Post
My experience is just the opposite. When first into reverse at engine idle speed (1,000 rpm) the boat walks to port going astern. To get going straight I must and do immediately increase the rpm to 1,500 to 2,000, the boat straightens out because there is [finally] water moving over the rudder. I only take it out of gear shortly thereafter because I'm simply backing across our fairway to turn to get out. FWIW, I turn to port to get out.

15x10 fixed 3 blade on an M25 engine. I believe it's a 2.7:1 transmission.

Most prop walk discussions I read about seem to get it worse at low speed, simply because the boat is NOT moving.

Indeed, in doing my three point turns, the BEST maneuver is to use very little throttle when wanting to back to port. I goose it to a stop after going forward. Then idle back to port since that's is when my boat/prop combo does its "best work."

Your boat, your experiences.
Hi Stu.

I think we are agreeing with each other, more flow over the rudder and less cavitation result in less prop walk.

So yes, speed will increase the effect of the rudder. However, I am also correct, a ham hand on the throttle will result in more cavitation, more slip and more transverse thrust.

I Believe we are agreeing with each other.

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Old 16-04-2015, 18:47   #11
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Re: Prop walk depending on prop size?

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Hi Stu.

I think we are agreeing with each other, more flow over the rudder and less cavitation result in less prop walk.

So yes, speed will increase the effect of the rudder. However, I am also correct, a ham hand on the throttle will result in more cavitation, more slip and more transverse thrust.

I Believe we are agreeing with each other.
Always good!!!
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Old 16-04-2015, 18:55   #12
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Re: Prop walk depending on prop size?

What causes the prop walk in the first place? P-factor. The steeper the angle of the prop shaft the more prop walk you will have. This is due to the difference of the ascending and descending blades angle of attack to the water.

P-factor is the term for asymmetric propeller loading, that causes the airplane to yaw to the left when at high angles of attack.

Assuming a clockwise rotating propeller it is caused by the descending right side of the propeller (as seen from the rear) having a higher angle of attack relative to the oncoming air, and thus generating a higher air flow and thrust than the ascending blade on the left side, which at the other hand will generate less airflow and thrust. This will move the propellers aerodynamic centre to the right of the planes centreline, thus inducing an increasing yaw moment to the left with increasing angle of attack or increasing power. With increasing airspeed and decreasing angle of attack less right rudder will be required to maintain coordinated flight.

This occurs only when the propeller is not meeting the oncoming airflow head-on, for example when an aircraft is moving down the runway at a nose-high attitude (in essence at high angle of attack), as is the case with tail-draggers. Aircraft with tricycle landing gear maintain a level attitude on the takeoff roll run, so there is little P-factor during takeoff roll until lift off.

When having a negative angle of attack the yaw moment will instead be to the right and and left rudder will be required to maintain coordinated flight. However negative angles of attack is rarely encountered in normal flight. In all cases, though, the effect is weaker than prop wash.
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Old 16-04-2015, 19:18   #13
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Re: Prop walk depending on prop size?

I had an embarrassing prop walk experience a few years back. A friend had asked me to fill in for him operating a large tender/small ferry for a prestigious yacht club about 4 years ago. I'm a fair boat handler by most accounts.

The boat was a 65' small passenger ferry with about 50 wedding guests on board. The boat is 121 years old with fine lines and a large low pitch cast iron screw. I was making the dock with about a 30 knot off shore wind on an unprotected pier. I came in hot, maybe doing about 5 or 6 knots when my bow crossed where my stern should land. I was going fast for two reasons.

One was to maintain directional stability vs the wind.

The other was because I was trying to maintain a tight quarter hourly schedule with about a 1 mile crossing in between dockings, on a boat i had not operated.

When my bow was Abeam the dock stern cleat I backed hard to slow the vessel. The boat backed to port (away from the dock). As I did that my stern rotated to port (away from the dock) and my bow rotated to stbd.

I made a total rookie mistake and increased astern throttle (I had to get her stopped because of my speed too). The harder I backed the greater the over rotation.

What happened was my bow turned towards the dock due to over rotation, the harder I backed the more accute the angle became, the boat tracked right into the dock! I smoked the dock very hard in spite of the 30 knot offshore wind and my rudder being turned to port.

Non of the wedding guests lost their footing, but they all screamed as I drove the boat into the dock at about 30 degrees. As mentioned this was a very nice 100+ year old club tender. Very very embarrassing. I knew better, but- I still messed up the docking.



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Old 19-04-2015, 04:34   #14
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Re: Prop walk depending on prop size?

Hi everybody,

Thank you very much for the nice explanations. The reason I was asking is, besides pure interest, that we want to change our prop to get full power from our engine and increase its already long life (it's from '79).

It's very interesting to read about the difference of prop walk/ transverse thrust and the bank effect (which was completely unknown to me). I will have to observe more detailed from now on, but I do believe that in most situations I can rule out bank suction since we hardly ever dock on or close to a sea wall, it's normally pontoons or docks on wooden stilts. Besides that, I have never experienced the stern moving to starboard, always to port (except strong enough winds, of course).

Also, it's almost impossible to reverse in a straight line, no matter how hard or slow we drive the prop. Of course, once the boat moves and the rudder grips and we stop the prop from turning, we get steerage, but any prop turn in reverse will push the stern to port. This is, however, not noticeable when going forward, maybe because flow over the rudder is strong enough?

We use this all the time, the "ass kick" became our most reliable and favorite maneuver when docking or turning in confined space. We are afraid of losing this when fitting a new prop which fits to the engine, but hopefully exchanging it with other advantages such as actually being able to get the boat in hull speed!

There seems to be no real consensus on if it is pitch or size or blade number which influences prop walk most? This would be interesting to know when choosing a new prop.

Cheers,
Phil
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Old 19-04-2015, 08:01   #15
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Re: Prop walk depending on prop size?

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Originally Posted by phipseml View Post
Hi everybody,

Thank you very much for the nice explanations. The reason I was asking is, besides pure interest, that we want to change our prop to get full power from our engine and increase its already long life (it's from '79).

It's very interesting to read about the difference of prop walk/ transverse thrust and the bank effect (which was completely unknown to me). I will have to observe more detailed from now on, but I do believe that in most situations I can rule out bank suction since we hardly ever dock on or close to a sea wall, it's normally pontoons or docks on wooden stilts. Besides that, I have never experienced the stern moving to starboard, always to port (except strong enough winds, of course).

Also, it's almost impossible to reverse in a straight line, no matter how hard or slow we drive the prop. Of course, once the boat moves and the rudder grips and we stop the prop from turning, we get steerage, but any prop turn in reverse will push the stern to port. This is, however, not noticeable when going forward, maybe because flow over the rudder is strong enough?

We use this all the time, the "ass kick" became our most reliable and favorite maneuver when docking or turning in confined space. We are afraid of losing this when fitting a new prop which fits to the engine, but hopefully exchanging it with other advantages such as actually being able to get the boat in hull speed!

There seems to be no real consensus on if it is pitch or size or blade number which influences prop walk most? This would be interesting to know when choosing a new prop.

Cheers,
Phil
I think the single biggest factor is size of the prop, the more surface area, the more traction.

I love your term ass kick. Google "Back and Fill".

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