MAX EBB — "D
ocking a big boat is all about confidence." Those words came to mind as I maneuvered my friend's boat toward its slip. It was twice the size and five times the weight of my own boat, and I had volunteered to move it back to its marina berth from the boatyard while the owner was out of town. This was a single-finger berth, port side to, and there was some wind blowing away from the dock. A little bit of extra speed would be required on the final approach, to minimize the effect of the wind. "No problem," I thought to myself. "I'll hit reverse at the right time, and the prop walk will pull the stern to port." A voice in my head asked me to practice reverse thrust out in the fairway, but confidence overruled and the voice was ignored. I should have listened. The approach into the slip went exactly as planned, but when I applied power in reverse, the stern swung to starboard, not port, and the wind was blowing me against the neighbor to leeward, a large powerboat, with no room for backing and filling. Naturally, I had not put out any fenders to starboard. Fortunately, the boat to starboard was a liveaboard, and the owner appeared on deck just in time to help me fend off. Other onlookers ran to help, and it wasn't long before my friend's big yacht was safely pulled up against the port-side finger. "Must be a left-handed prop," deduced the liveaboard skipper to starboard after I explained what went wrong with my docking attempt. "Sure felt like it," I agreed. "I expected the prop walk to pull the
stern to port, like it usually does." "Not with a lefty propeller," he said. "It's all about torque and P-factor. Ask any pilot." "I understand torque," I said. "But what is this P-factor?" "Ask any pilot," he repeated. "And I just happen to have logged a couple of thousand hours." He didn't wait for me to ask but launched into asymmetrical flight dynamics 101. "P-factor is when one side of the propeller is producing more thrust than the other side," he began. "Think of a powerful single-engine plane with a big propeller. Especially a taildragger. The plane starts the ground roll with a high angle of attack. The propeller shaft is nowhere near parallel to the incoming air flow, so the blades on the starboard side of the plane, going down and partly into the incoming flow, see more wind and more angle of attack than the blades on the port side of the plane going up. That puts more thrust The primary source of prop walk is deflected upwash from the propeller in reverse, especially strong when the boat is still coasting forward.
Port side
Starboard side
esulting ow into propeller blade
Blade going down
Flow from propeller blade rotation
Blade going up
The 'P-Factor' effect: Port and starboard views of the flow into a right-handed propeller in reverse, with the boat making sternway. The blade going up, on the starboard side, pulls harder than the blade going down, on the port side. But the propeller diameter is small compared to the size of the boat, and the effect is reversed when the boat is still coasting forward. 0AGE s Latitude
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s !PRIL
BOTH IMAGES MAX EBB
Flow from vessel's aft motion
on the right side than the left, so you have to push down hard on the right rudder pedal just to stay lined up with the runway. After liftoff, during climb, the angle of attack is still higher than in level flight, so you still have that unbalanced thrust trying to make the plane turn left, and you have to keep pressure on the right rudder. In level flight, there are trim tabs and such to keep control forces neutral, but then when you approach the field to land, essentially gliding, you need some left rudder to fly straight." I could imagine what this would look like in a diagram. There was the airflow from the plane's forward motion approaching the propeller, and the airflow created by the propeller's rotating motion, and the apparent wind that would be the vector sum of the two. It took some mental effort to visualize, but I could see how the propeller blade on the downstroke had more pull than the blade on the upstroke, considering that the whole plane was pitched upward compared to its direction of flight. "On your sailboat," he continued, "in reverse, a normal right-hand propeller is turning clockwise viewed from the engine, looking aft. But the shaft is pitched down, not up, so when you're backing down, the blades going up contribute more reverse thrust than the blades going down. You get more thrust on the starboard side of the propeller, less thrust on the port side, so going backward it tries to rotate the boat clockwise and pulls the stern to port." "Except, like, that's not what really causes prop walk!" It was the last voice I wanted to hear, because it meant that she had witnessed my botched landing. Lee Helm was behind me in a single-person outrigger canoe, and she had probably seen the whole thing. Now, I understand that there will always be witnesses to a bad docking, but did one of them have to be Lee Helm? "Nice to see you, Lee," I lied. "Beautiful day for a paddle around the harbor." "And a good day for crashing into docks," she said. "But like, any docking you can walk away from is a good docking." Fortunately, my new pilot friend in the powerboat to starboard changed the subject. "OK, why isn't P-factor the cause of prop walk?" he challenged Lee. "Because it's backward. Prop walk
