Since this turned into a general discussion on mooring twin screw boats, I should maybe add that I’ve never actually walked the boat to the dock in the manner described. I have played with the dynamic while loitering for whatever reason, and it’s a fun thing to learn, but I consider it generally unsuited for approaching the dock, at least how it was for me. The bulk of my boat handling experience comes from my time as a yard hand, so I barely ever started a docking evolution with a clear idea of how the boat was going to respond, having never handled that particular type before.
Using diff thrust and rudder to translate laterally (“walking the boat”) is a fairly complex dynamic with a lot of finesse and individual differences between boats. Take for example that you never quite know if you’re going to get behavior A or C before you try; figuring out such basics on final approach is seldom a good idea. Added to this, it is a maneuver with a lot of power through the shafts and strong opposing forces, which sets the stage for things to go rapidly sideways if something happens. You get all kinds of fun and exciting failure modes if you for example suck a mooring line into a propeller.
In practice I used this approach for a starboard-to alongside mooring: Roughly one and a half beam out from my intended berth and two to three beams before it, I put the rudder hard to port and put the port shaft ahead, like this:
This causes the boat to move ahead while crabbing to starboard at a 30-40 degree angle. Additionally, one of two things may happen:
A: The rudder has enough authority to swing the bow to port, in which case I’m golden. All I have to do is correct to starboard, check my vectors, end with a slight swing to port which I arrest with starboard shaft astern as I land, stopping the boat dead.
B: The rudder is so small or the shaft offset so great that the bow swings to starboard. With certain types of boats (for example mid sized sports cruisers) this is more often the case than not. In this case I go until the swing has started developing, then arrest it with a firm kick astern on the port shaft. Water pressure on the hull assists with kicking the whole boat to starboard, and by alternating ahead and astern the boat can be translated sideways towards the dock.
So I guess there’s your proper answer: In practice I walk the boat by putting the rudder hard over away from the dock, then rocking the seaward shaft back and forth.
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That’s a good post.
If I understand correctly you’re using the forward momentum of the boat rather than an astern on the starboard engine to achieve the vector required to get alongside.
Or maybe said better using the momentum of the boat in general, exploiting the fact that a kick ahead or astern will change the heading.
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In a sense, yes. I allow the boat to creep forward rather than putting the starboard shaft astern, because it is more predictable. When handling a shiny fiberglass yacht that you’ve never seen before, predictability is king.
However, the bit with rocking the shaft back and forth can be done with very little head- or sternway. I’m not sure I understand it correctly either, but I think it goes like this:
Port shaft ahead swings bow to starboard while rudder action moves the stern by a limited amount to starboard. Port shaft astern arrests the yaw, while water pressure on the hull bottom pushes the stern to starboard (classic wheel walk). So I’m mostly exploiting moment of inertia.
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I would probably go in a straight line at an angle to the dock and then back hard on the port engine right before the bow touches, which if all goes well would both stop all forward movement and rotate the stern in towards the dock. Starboard engine as needed, you don’t want to go backwards at that point.
“Move that boat” was always fun on windy day with a boat you have never seen before too!
I need to try your way, that looks smooth if it works out.
It looks like three competing forces, the rudder wants to swing the bow to port, the offset thrust wants to swing the bow to starboard, and the overall thrust is moving the boat forward.
I guess if you get it all set just so the boat moves ahead at a 45 degree angle right into the dock.
That diagram is actually a worst case for my boat, I would have to go in the “wrong” way for the single prop, the two boats rafted block the easy way. I would need to be quick getting a midships or bow spring on.
The landing craft I learned on wasn’t difficult to make ‘walk’.
Tied up port side-to, to come off the dock it was twist to starboard (port ahead, stbd astern) with the wheel to port. It’d just slide, more or less, sideways off the pier.
The issues was the controls. The throttle and shift controls were mechanical. They were like brass shovel handles, were about thee feet long and mounted at deck level.
The throttles were controlled by twisting what would be the grip of the shovel handle. To go astern the handles were shifted from vertical to horizontal.
When ahead the throttles were at waist level but astern they were at ankle level and about three feet behind me when at the wheel.
This meant once I saw we were getting off the dock without hitting anything I didn’t much try to tweak things if it required ducking down to adjust the astern throttle.
Here’s a photo:
Never tried walking it in,. Didn’t want to risk hitting something. It had a ramp forward, just had to swing around so the deckhand could get across to the pier with a line.
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With a single screw vessel with a right hand screw and rudder the easiest berthing is port side to. A feeder container ship I was master of had a CP propeller and a Becker rudder. In calm conditions you could move the ship bodily sideways in either direction with neutral pitch and putting the rudder hard over and balancing the direction with the bow thruster.
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I’ve always used the same methods as Klaveness describes on the yachts I’ve operated. Except, once when transiting the canal in Panama, the 55’ motoryacht simply would not walk. We locked through by tying to a Canal Commission tug, and when the gates opened we were to peel away and let the tug run ahead and tie to the wall. Using the sticks and rudder would not walk the thing away from the tug and the line handlers would help push us away. At the third lock, the Canal Pilot turned to me and said “You ARE clear on the concept, aren’t you?” I explained my process and showed him that rudder hard to starboard and the starboard gear forward and port astern resulted in zero response. He was flummoxed by that result. I had a hell of a time not dragging the white topsides along the tugs black tire fenders during that transit…
I strongly prefer docking “good side out” as we say here. I find that it gives greater leeway for correcting mistakes. I’ll make a thread about it some day.
I don’t have much experience with this but reading a dozen or so post on a boat forum seems to confirm that some twin-screw boats just will not walk. Having to do with the underwater hull shape.
Boats with nozzles do not “walk”, you end up twisting in one end and then the other until you are alongside and can put a line out.
Boats with wheels set well up into tunnels do not walk.
Sometimes you can make a flat bottomed boat (or tug and barge) do something akin to “walking” by putting it into a bit of a spin and then twisting the stern in.
But not too much of a spin because she may not want to stop and you may end up doing a 180 or 360.
Chapman Piloting & Seamanship has an excellent section explaining the influences of the unequal thrust of the ascending and descending blades of the prop on a vessel with some angle of the prop shaft. Ultimately, there is some effect of the propwash on the deadrise/underbody and keel. I haven’t found an online source for the passages.
