Major prop walk in forward

[sinnerman]

Dumb question but maybe the yard installed the prop on backwards. (??)

The shaft is tapered, but the taper is slight.

The yard may have just slapped it on, torqued down the retaining nut and been done with it.

[Cole13351]

I don’t think you can install a max prop backwards


QUOTE=sinnerman;3650526]Dumb question but maybe the yard installed the prop on backwards. (??)

The shaft is tapered, but the taper is slight.

The yard may have just slapped it on, torqued down the retaining nut and been done with it.[/QUOTE]

[Rumrace]

Prop walk has a long list of factors.
It should be noted even counter rotating twins get prop walk but the waste balances out. The single turn of the screw exceeds the boats ability to move that far forward.
Under or over propping a power boat is much less likely than a sailboat. Seems more power boaters understand the importance of propping to the correct RPM. Just about every cast iron truck engine ever put in a boat blows up at 4800 RPM unless it had surface drives. So for ever 4400 RPM it flat out. Wanna go faster pitch up the but not past 4400 rpm and not under 4100. You may reduce pitch and increase blade shape or OD. When the big fast diesel boats of Europe where challenged by these old V8 gas Donzi beat them all 25 races.
Outboards and surface drives much higher rpm

Sailboaters like the improvement of a 3 blade over a two at the dock. Check the boats specs for the props. Don’t check out what your chandlery tells you will get another half knot.

Prop angle! SeaRay made a few hulls with pocket V drives or a Pair of a new bravo stern drive. That a sterndrive could handle a hull this big had not been proven. The stern drives were deeper 4’2” draft. Way faster than the pocket V drive with better bronze props because of the angle of attack. I had a 330 Sundancer with bravo drives SS props a couple mods topped out at 48knots. Few V drives made it past 36k.
It’s the very reason why the ZF sea drive in Sailboats has less prop walk the direct shove forwards rather than towards the depths. The prop angled down does not have a chance to its job properly so it walks.
In high performance boats the ZF SeaRex performs Mercedes like auto suspension to keep the prop exactly on the surface near a meter behind the boat cause any prop under a boat deflects off the hull cause deflection to one side.
So stick to your hull speed don’t over prop and burn your clutch.
A folding prop with adjustable pitch is great but a massive cost. Prop paint helps but plowing on power boats or over propping on Sailboats burn new paint off in hours.

Also OP check your alinement cause that’s what your prop walk sounds like.

[fstbttms]

Originally Posted by sinnerman

Dumb question but maybe the yard installed the prop on backwards. (??)

Not possible. However it is possible to set the pitch for a right hand turning motor. Or the wrong pitch for a left hand turning motor.

[valhalla360]

Originally Posted by 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.

This is the explanation why the helmsman on small boats sits on the starboard side.

The torque from the prop wants to roll the boat over. With a perfectly weight balanced boat, the torque will result in a list. By placing the helmsman on the high side, it creates an opposing torque to cancels it out. As boats get larger and heavier, it becomes less relevant.

This is one reason I loved the outboard power that steered with the rudders. You had control over which way the prop was pushing and pulling at low speeds when the rudders aren't providing control. (Obviously not easily retrofit to an existing inboard)

[donradcliffe]

I saw the same effect when I swapped out the old 4108 for a beta 50 with a maxprop. The boat pulled a bit under power, but the autopilot handled it just fine.

[Montanan]

Simply the interaction mechanisms of the vortices shed by a single-screw propeller with a rudder installed in its wake.

[Bowdrie]

Originally Posted by Montanan

Simply the interaction mechanisms of the vortices shed by a single-screw propeller with a rudder installed in its wake.

^ This, and the effect is exacerbated by asymmetrical thrust due to shaft angularity.

[Rumrace]

Okay it’s way less complex voodoo.

In a perfect world a 21 pitch prop any diameter moves the boat forward 21 inches. If it doesn’t it walks.
Props have a sweet spot where that 21” works. On a sailboat it’s often 2/3 throttle or near by. Any higher rpm does not return the same increase in speed and increase in fuel consumption.

A Verado 600hp V12 Merc has two sweet spots cause they come over pitched. They have a 2 speed automatic transmission you can’t feel shift once the boat is up to prop speed it shifts into second. There is a gentle shove from the prop walk the only signal you shifted other than Merc dash.

Think of a screw going in wood 21
Inches deep each turn simple.

[Montanan]

Originally Posted by Bowdrie

^ This, and the effect is exacerbated by asymmetrical thrust due to shaft angularity.