Major prop walk in forward

[Montanan]

Useful reference for an explanation of the prop walk effect.

https://en.wikipedia.org/wiki/Propel...aw%20motion%29.

[David Mathis]

A bit over simplified but here's the easier to understand version: Prop walk is simply the result of torque. Let's relate to a helicopter. Every action has an equal and opposite reaction. The rotor blades are turned with torque (Yes, and horsepower but this is the simplified version.) which produces equal torque in the opposite direction making the helicopter rotate in the opposite direction of the blades. To prevent the helicopter from rotating, a tail rotor is mounted at the end of the tail which provides equal torque. So how does this relate to a prop walking boat? Think of the prop as the rotor blades. About 3/4 of the distance between the shaft and the prop tip is the center of thrust for each blade. This can vary due to several factors but for this explanation, we'll use the 3/4 distance. Now approximate the distance between the center of mass for your boat and the center of thrust for the blade on top then do the same for the blade on the bottom. The rotational force needed to turn the prop is torque. The distance between the shaft and the center of mass of the boat is moment which is like the helicopter tail boom. You have a longer moment (like a longer wrench) from the center of mass to the bottom blade than from the center of mass to the top blade. The difference in the moment creates the force which we call prop walk.

[s/v Jedi]

No, Montanan got it right.

[siamese]

A couple of responders have been right about what you're experiencing NOT being "prop walk".

I had a Catalina 28 that suffered from the same situation you describe. The faster I went in forward (higher RPM and greater boat speed), the more pressure I felt on the wheel toward port. Going straight, the wheel was centered. But if I took my hand off the wheel at cruising speed, the boat wouldn't just turn, it would immediately spin to port in not much more than it's own length. It was annoying. That constant tug on the wheel was tiring if I had to hold a course for a matter of hours. As far as I know, all was stock on my boat.

Under sail, there was zero tendency for the wheel to tug to port regardless of speed.

I think this is what you're describing, and it's not prop walk.

The most I was ever able to learn was that it was related to how the prop (prop wash) affected the rudder, as a couple others here have noted in this thread. I found vague rumors about Catalina having offered a rudder upgrade, and heard that it may be solved by changing the prop. If I'd found something more concrete perhaps I would have acted on it, but it was manageable, and had no other effect on performance.

So, I'm not much help here other than to say you're not alone, and to suggest that you take the term "prop walk" out of any future discussions you might have as you try to move forward to a solution. If you're talking to an "expert" and they insist that it's prop walk, I would not throw money in their direction hoping for a remedy.

[FPNC]

[QUOTE=Wainui;3603477]I have repowered from Perkins 4108 to a Betamarine 50 hp/QUOTE]

You are already over propped.

What transmission do you have?

What diameter propeller?

How many blades?

[Wainui]

Velvet drive 1.1
Walter v drive 1.87.1
3 blade max prop 17”
Pitch 20 deg.
I’m changing pitch to 18 degrees when I get back
Thanks everyone

[FPNC]

Originally Posted by Wainui

Velvet drive 1.1
Walter v drive 1.87.1
3 blade max prop 17”
Pitch 20 deg.
I’m changing pitch to 18 degrees when I get back
Thanks everyone

Ideally that size engine would turn an 18" diameter propeller using a 2:1 ratio gear. I am not familiar with Walter having a 1.87:1 ratio v-drive. They did have a 1.67:1 unit. Which could explain the situation you are in.

I would check your numbers and call PYI to get a recommendation. Also make sure that the engine can turn 2900 - 3000 in neutral. If not, you have a fuel restriction.

[Wainui]

EPNC you are correct.
The ratio is 1.67.1
I will drop the max prop down to 18 degrees which is 10.4”
Thanks for your help

[nofacey]

Yeah, had same on my last sailboat - increased steering pressure one direction under power, none sailing. Increased pitch will make it worse.
Fortunately I had hydraulic steering, so rudder stayed in position by itself despite pressure, and autopilot had no issues.
Not going to say I liked it, but it really didn’t effect much.

[bobatkins]

I had a Hunter 41 with a 50HP Yanmar and a MaxProp 3 blade that had been verified to be correctly assembled.

I had exactly the same problem that the original poster describes as well as siamese describes with his Catalina 28. The problem was related to the centerline thrust angle and was actually quite easy to solve by shifting the engine very slightly to port thus changing the overall thrust angle slightly to starboard. I was amazed at how little movement it took to completely neutralize the pull to the left on the wheel when under high power. The hard part was convincing a mechanic to help with the change! It was like pulling teeth because 3 mechanics thought I didn't know what I was talking about. They waved their arms and did their very best to instill as much fear and doubt as possible about changing what 'the engineers originally set for the thrust angle' - blah, blah, blah..

Was the job easy - not really because it does require sliding the engine to port by a small ( < 0.5" ) distance while maintaining perfect shaft alignment which was not possible. So, suffice to say - the final result took a couple of iterations and quick sea trials to check the results. But, removing the significant pull to the left from the wheel when cruising under power made the boat a joy to run under power.

[malbert73]

Originally Posted by bobatkins

I had a Hunter 41 with a 50HP Yanmar and a MaxProp 3 blade that had been verified to be correctly assembled.



I had exactly the same problem that the original poster describes as well as siamese describes with his Catalina 28. The problem was related to the centerline thrust angle and was actually quite easy to solve by shifting the engine very slightly to port thus changing the overall thrust angle slightly to starboard. I was amazed at how little movement it took to completely neutralize the pull to the left on the wheel when under high power. The hard part was convincing a mechanic to help with the change! It was like pulling teeth because 3 mechanics thought I didn't know what I was talking about. They waved their arms and did their very best to instill as much fear and doubt as possible about changing what 'the engineers originally set for the thrust angle' - blah, blah, blah..



Was the job easy - not really because it does require sliding the engine to port by a small ( &lt; 0.5" ) distance while maintaining perfect shaft alignment which was not possible. So, suffice to say - the final result took a couple of iterations and quick sea trials to check the results. But, removing the significant pull to the left from the wheel when cruising under power made the boat a joy to run under power.

Very interesting! How did you align your stern tube and cutless bearing (or are there 2?)

Seems like a major job and great result but a good autopilot solves the issue for me as I dont hand steer under power much

[bobatkins]

Originally Posted by malbert73

Very interesting! How did you align your stern tube and cutless bearing (or are there 2?)

Seems like a major job and great result but a good autopilot solves the issue for me as I dont hand steer under power much

The amount of angular change at the cutlass bearing was so insignificant (it is effectively the pivot point for the thrust angle) that no cutlass bearing alignment was needed. Same for the stern tube - insignificant shaft movement and more than enough space in the stern tube to accommodate. Remember the front of the engine was moved to the left < 0.5". Everything behind that front moved less. I was amazed at how little shift made such a dramatic difference.

Yes, I also used the auto pilot for years to compensate for the left turn pull. What I could not do was compensate (of course I told them about the issue) for someone else driving the boat and letting go of the wheel while going straight - not realizing that the boat would quickly turn left. Basically in one situation it almost resulted in a collision. That's when I decided that it would be worth the effort to properly fix.

[Robjl]

It's not prop walk.
I think it's more to do with the swirling water from the prop interacting with the front part of your balanced spade rudder. You could try adding to the tail of your rudder or reducing the fwd part in front of the rudder shaft... ie you reduce the "balance" of the rudder... but then the helm is heavier when sailing. I'd try reducing the pitch first. maybe less "swirl"?
Just get used to it is what I'd do.
Or, get an autopilot, steering under power isn't much fun.

[Cole13351]

Did you ever resolve this issue. I believe I’m having the same problem. I have a 3 bladed max prop. It was recently taken off and put back on by the boat yard. Now when I’m in forward gear, above 2000 rpm, the boat starts to pull to port. The higher the rpm the more then pull. At 5kts if I let go of the helm the boat will spin hard to port. Only under power, not under sail


QUOTE=Wainui;3603477]I have repowered from Perkins 4108 to a Betamarine 50 hp
When powering in forward at 1500 to 2000 Rpm I get pressure on the wheel turning the boat to starboard.
This is noticeable since repowering.
Left hand max prop
2.1 reduction
Distance from prop to spade rudder 5’
Max rpm in gear is 2500
Would reducing the pitch help stop this?
Does this problem sound unusual?
This does not happen under sail.
Regards[/QUOTE]

[sinnerman]

Originally Posted by Cole13351

Did you ever resolve this issue. I believe I’m having the same problem. I have a 3 bladed max prop. It was recently taken off and put back on by the boat yard. Now when I’m in forward gear, above 2000 rpm, the boat starts to pull to port. The higher the rpm the more then pull. At 5kts if I let go of the helm the boat will spin hard to port. Only under power, not under sail


QUOTE=Wainui;3603477]I have repowered from Perkins 4108 to a Betamarine 50 hp
When powering in forward at 1500 to 2000 Rpm I get pressure on the wheel turning the boat to starboard.
This is noticeable since repowering.
Left hand max prop
2.1 reduction
Distance from prop to spade rudder 5’
Max rpm in gear is 2500
Would reducing the pitch help stop this?
Does this problem sound unusual?
This does not happen under sail.
Regards

[/QUOTE]