Maxprop issues

[daddle]

Originally Posted by LeaseOnLife

Maybe this helps

Yup. The gear is the only thing turning the hub and blades. And if the blades are wont to turn a bit faster than the shaft the blades will feather until the speeds are balanced.

[Ex-Calif]

Originally Posted by ChuckSK

The maxprop is designed so that each blade pivots around a shaft projecting laterally from the central hub. The area of the blade aft of the shaft is greater than the area forward of the shaft. When the hub is rotated the blade encounters more resistance from the water in the rear portion of the blade than in the forward portion. This difference in force on the blade causes it to rotate on the hub. The leading edge of the blade faces into the flow and trailing edge trails. This affect occurs whether the hub is rotating slowly or quickly.

There are stops in the hub to keep the blades from rotating past the preset pitch.

Chuck

Exactly correct.

There is a pinion gear attached to the prop shaft and bevel gears machined into the blade hubs. When indexed properly the forward and aft pitch is established.

The larger area of the trailing edge of the blade is what allows the prop to move. With the engine off the larger aft ends trail and the prop is feathered. Under power water resistance causes the blade to rotate on the pinion until the stop is reached. Same in reverse.

I suspect that with a trailing sea and "surfing" a bit down a wave the prop is unloading - slightly different than cavitating which is separation of flow on the unloaded side of the prop.

The sea running behind the prop would unload it and I suspect would feel like the prop is out of the water. I guess the analogy is like coming over the crest of a hill in a manual transmission car. In 4th gear the engine unloads - at the bottom of the hill it loads up again. It should not need to be put in neutral to correct and I don't think repitching will help, 3100 at WOT seems about right.

[cal40john]

Originally Posted by ChuckSK

Let's start with a fully feathered prop that is not turning. When the prop shaft starts to turn, the water resistance will hold the prop back. For a short period the prop shaft turns while the prop does not. It is this difference in rotation that drives the gears that rotate the prop blades.

You were doing better earlier. The pivot point (axle of the prop blade) is attached to the hub and allowed to rotate. So when the hub moves the blades will move. There is more surface area behind the axis line than in front so the trailing edge is pushed in the opposite direction of the spin. The gearing keeps all the blades at the same pitch as they rotate and the adjustable stop sets the ultimate pitch.

The only way this prop will feather or reverse pitch is if the water is travelling in an odd direction faster than the shaft is driving the blades through the water around the hub. Merely having the reverse direction and come from the back of the boat won't rotate it.

Max-Prop Features and benefits of a feathering propeller - YouTube

[ChuckSK]

It is the torque between the prop shaft and the prop that turns and holds the blades against the pitch stops. Simple as that.

[cal40john]

Originally Posted by ChuckSK

It is the torque between the prop shaft and the prop that turns and holds the blades against the pitch stops. Simple as that.

Replace the blades with round balls and paint a dot on them somewhere. Place a torque between the prop shaft and the balls and which way does it rotate?

In the video he said "when you put torque on the shaft the blades rotate".
Earlier he said that when the shaft isn't turning the water flow pushes the blades to line up with the flow. Why do they do that? It's because there's more blade area behind the axis of rotation the blade to the hub. When the blades spin they are rotating at a speed much higher than the water flow past the boat, so the vector sum has the water hitting the blades primarily from the side (I need to find a way to describe that better.) Again the water hitting the aft side of the blade's greater surface area forces it to rotate.

[ChuckSK]

Originally Posted by cal40john

Replace the blades with round balls and paint a dot on them somewhere. Place a torque between the prop shaft and the balls and which way does it rotate?

In the video he said "when you put torque on the shaft the blades rotate".
Earlier he said that when the shaft isn't turning the water flow pushes the blades to line up with the flow. Why do they do that? It's because there's more blade area behind the axis of rotation the blade to the hub. When the blades spin they are rotating at a speed much higher than the water flow past the boat, so the vector sum has the water hitting the blades primarily from the side (I need to find a way to describe that better.) Again the water hitting the aft side of the blade's greater surface area forces it to rotate.

If that's the case then explain what causes the blades to rotate to a reverse position rather than to a left handed forward position when the shaft rotation reverses.

[cal40john]

Originally Posted by ChuckSK

If that's the case then explain what causes the blades to rotate to a reverse position rather than to a left handed forward position when the shaft rotation reverses.

Oops, I see my mistake, I'm wrong.

John

[Farlander]

It seems like a big wave from astern could push the blades into reverse and they could get stuck there while in drive.