Major prop walk in forward

[Wainui]

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

[tkeithlu]

Well, the direction is right - a left handed prop pushes water against the starboard side of the aft hull or rudder, pushes the stern to port, and sends the bow to starboard. BUT, in forward gear and with a space between the prop and the rudder, you should be hard pressed to notice because the water has little to hit. Something is wrong here, but I don't know what it is.

[SVTatia]

Two questions:
1) are you sure the rudder center position and the wheel at center are synched?
2) if you dont touch or correct with the wheel and let the boat in forward, which way does it tend to go?

[DougR]

You are experiencing the water rotation within the prop wash impacting the rudder, causing pressure which wants to turn the rudder and thus turn the boat. It’s as common as dirt.

Compare the old setup with the new.... the 4-108 was maybe ( on a good day) 50 hp at 4000 rpm, and the Beta is 50 hp at 2800 rpm. You don’t tell us what the old reduction ratio was, but I’ll guess the current prop is larger than the old one, as it must be to absorb the greater torque of the Beta. Add to the probable larger diameter prop the fact that is it over pitched, it’s reasonable to think that there is a heavier prop wash with the new engine than with the old. And that’s why you feel it now but didn’t feel it with the 4-108.

How to solve it? Many designers offset the engine/ shaft line slightly to one side or the other to cancel the effects of LH or RH prop rotation. My Cal 39 has the shaftline offset with the front of the engine to stb. to counter a LH prop. Of course, it’s too late for that now. But we know that your engine is currently over propped, so probably the first thing I would try would be to take some pitch out of the prop so that the engine can obtain rated RPM at full throttle. This should reduce turbulence in the prop wash somewhat at cruising speed and lower pressure on the rudder. Hopefully that will be enough to make the situation livable.

[s/v Jedi]

I love prop walk, it’s like having free side boosters.

Once you make a little forward speed, you should not have any effect anymore. If you want to go forward without prop walk, pulse at 50% throttle for a couple seconds in gear, then back to neutral to have the rudder bite and correct.

I always try to moor so that prop walk will be helping when departing.

[nuku34]

With respect to all the posters, "prop walk" IMHO covers 2 separate, but related, phenomena. All that follows assumes we are talking about prop walk in forward.
Pure prop walk does not involve the rudder or the hull. It is just the tendency of the prop to act like a paddle wheel and want to move sideways as it turns. Left handed props want to move toward the left, right handed props to the right. This moves the stern of the boat to one side or the other as the boat moves forward causing the boat to turn.

As some posters mention, the prop wash moving aft is like a rotating tube of water, when this hits the rudder, it moves unequally over the rudder's sides and pushes the rudder one way or the other. That's not the same as the first case of "pure prop walk" although it feels the same, i.e. you feel pressure on the rudder.
Anyway, this is my own interpretation, I'd like to hear arguments against...

[grahamj]

Originally Posted by nuku34

It is just the tendency of the prop to act like a paddle wheel and want to move sideways as it turns.

Doesn't a paddle wheel work because one half is pushing against air the other water?

My understanding is that propwalk results from asymmetric thrust caused by the prop shaft angle inducing different angles of attack, hence differing thrust and drag. in the upper and lower prop blades.

A saildrive's propwalk is negligible because its thrust is lateral with the prop's path through the water.

[Kd9truck]

I took a owners class from a Yanmar east coast supplier and stayed in a Holliday Inn Express last night Soooo? I asked about repowering a Displacement vessel with a larger motor. He said only do it if you have no other option, and that there are a great many “things” that have subtle relationships with each other on the underside of the hull.
More HP And a bigger prop creating more prop walk sounds right, have you tried the old prop? Why did you repower? Has the pitch to the shaft changed at all? Even a degree up/down left/right ? Has the distance relationships of the prop/rudder/keel changed at all ? Is the motor sitting on motor mounts in the exact same position? We’re you able to use the same transmission? Is the new one in the exact same position?
These are all the questions you have probably asked yourself before getting started, I just took a class that went over maintenance and troubleshooting issues, I know nothing I am just asking obvious questions because sometimes as humans we overlook the logical. I am sure there is a very expensive solution, it seems those are the only ones I can come up with on my own vessel.

you could just Tell the Autopilot to start working out and turn the music up, no?

[Rumrace]

You would think a huge advantage to avoid prop was with an sterndrive or out board since there is no hull to slide against. Not the case. Counter. Rotating stainless cleavers no prop walk.
The props which angle down to accommodate the hull design get the worst prop wash.
Sounds more like a bent rudder. A folding prop with a damaged blade with pull like a mule.

[hpeer]

Both my boats have it, the 44’er with a 72hp Yanmar and the 33’er with a wee 13hp Volvo.

Lots not known here.

Did it occur with the old engine? MAYBE OL just bought a new to him boat and repowered??
Was the prop changed? Likely but not stated.
What were the old prop specs?
What are the new prop specs?

[sinnerman]

Originally Posted by grahamj

Doesn't a paddle wheel work because one half is pushing against air the other water?

My understanding is that prop walk results from asymmetric thrust caused by the prop shaft angle inducing different angles of attack, hence differing thrust and drag. in the upper and lower prop blades.

A saildrive's propwalk is negligible because its thrust is lateral with the prop's path through the water.

I see the paddlewheel version as a simplified way to teach the concept to people so it is easier to understand.

Prop walk occurs on angled prop shaft configurations as you said above. I always envisioned it as occurring because the water flow at the top of the propeller is "pinched" between the prop and the hull. This water impacts against the hull creating a reverse flow. With a saildrive, there is no "pinching".

Prop walk occurs BOTH in forward and reverse (unless you have a saildrive as mentioned); however in forward you have the water flow over the rudder to counter-act it.

I have terrible prop walk on my boat. I found that the propeller is severely UNDER pitched (6 inch pitch vs the required 12-13" pitch. ). The prop is producing little thrust and acting more like a blender than a propeller. The engine likes it as it does not have to do any work, however I struggle to make headway in currents.

I believe that they installed a propeller from the GASOLINE version of my boat onto the diesel. Or perhaps the engine was converted from gas to diesel and they kept the original prop.

I am waiting to install a correct 12-13" pitch propeller.

Too high a pitch would bog (overload) down the engine. A sign of this is excess black smoke (diesel) .

Maybe on the subject boat, they re-installed the original propeller onto the new engine/gearbox and the gearbox has a different ratio. The GOOD news is you can take a fixed pitch prop to a shop and they can modify the pitch for way less than the cost of a new unit. Bronze props are pricey.

My two cents.

Cheers

[hpeer]

There are also different SHAPE props. In my limited understanding power boat (planing especially) props are shaped to maximize forward thrust, there is little concern for reverse because there is so much excess power.

Sailboat props tend to be much more symmetrical, our forward speed is not that great and we need significant reverse thrust for maneuvering. Our boat has extremely little effective reverse thrust. I sent the prop out to be reconditioned, the prop shop said “You have a power boat prop Son.” They sold me a sailboat prop and now I have good reverse thrust.

The boat is far easier to handle in tight quarters. At the time I needed to do a 180° turn in tight quarters to dock. Almost impossible with the old prop.

BUT I also have more prop walk. It is a compromise. I have a long full keel with the prop in an aperture and a keel hung rudder. So the original prop made the boat a lead arrow. Different underwater configuration, different usage, changes everything.

[Wainui]

Thanks for the replies.
Gear ratio is the same as before.
Prop pitch was increased to accommodate extra hp.
Autopilot handles it fine.
I will adjust pitch by one degree to get to 2800 rpm.
Regards

[oregonian]

Hello, I understand that the Perkins required a LH prop, but does the new Beta? The Betas that I work on require a RH prop. When the Max-prop is assembled it can be set for either RH or LH. The problem you mention is exactly the same problem when the Max-prop is set for the wrong direction. I have now seen this in approximately a dozen situations. Also, the Classic Max-prop has adjustments for 2 degrees, not one. Good luck.

[Rumrace]

Frankly I’ve seen way more sailboats over propped than power boats. Sailboats have pretty crappy clutches as compared with a cone clutch with 450 pound torque on it.
Hull speed is hull speed. Nothing sensible with existing drivelines surpasses it.
Prop walk is inevitable reducing it has been done by most manufacturers by specifying a handful of props design to work with that hull and driveline. A broad blade like the cheap three blades prop to give you better slow control than a 2 blade and 1/2 knot faster at Full is likely hard on the clutch and causes increased prop walk.
If you try to turn the screw further forward than the hull will alloy in walks off. A shallow hull vs a Deep V you’d expect the deep V had an advantage but it’s worse.
Counter rotating props are pretty likely future ZF drives.