Thursday, April 30, 2009

Brakes? What brakes?

Maneuvering Eolian at speed while under way in the open is easy and intuitive, even for the novice, recently come up from an O'Day 25.

Maneuvering in close quarters at low speed is another kettle of fish entirely. A review of our early log entries shows that this was an issue which received a lot of attention. And rightfully so.

First, with any vessel (unless equipped with a bow thruster), the helmsman has control only over the stern. The thrust is applied at the stern, and the rudder is at the stern, or nearly so. This is very different than an automobile, where the driver controls the front of the vehicle, and it takes a lot of getting used to. For example, if you find that you are too far out as you approach the dock, applying rudder will force the stern further out, in order to get it behind the bow to push it more towards the dock.

Eolian, like all vessels of her size, is an inboard - all of our previous boats were outboard equipped. Making the switch from outboard (or I/O) to inboard is a very significant conceptual leap. With an outboard, you steer via directed thrust, that is, the thrust produced by the motor can be directed to port or starboard by turning the motor (or outdrive). But with an inboard, the thrust line is fixed, dead astern. However, the rudder is positioned so that it is in the middle of the river of water being moved astern by the prop. Therefore, side thrust can be developed by turning the rudder. Tho this works, it is not nearly as effective as directing the stream of water itself. And it is completely ineffective when moving astern... the boat is not moving at enough speed so that the rudder movement thru the water has any significant effect, and the prop slipstream is blowing forward, where there is no rudder. (The flow of water coming into the prop is diffuse and provides no effective steerage.)

Then, there is the issue of prop walk. When the engine is engaged astern, the boat will move significantly sideways, as well as astern. There are a lot of explanations for this phenomenon - the way I visualize it is this: The prop is not operating in free water, where everything is the same everywhere. Instead, the top of the prop is close to the hull, while the bottom is in free water. This means that the sideways motion of water thrown from the prop tips is different at the top, where it is restricted by the hull, than the bottom where it is not. Therefore, the prop acts sort of like a wheel mounted sideways at the stern with very poor traction. If the wheel is turning clockwise (as viewed from astern), then there will be a tendency for the stern of the boat to walk to starboard (right). Whether or not this is an accurate rendition of the physics involved, it certainly helps me to understand how Eolian will react. The same phenomenon occurs when the engine is engaged forward, however it is largely masked because the prop wash over the rudder gives a much more authoritative response to the wheel. Eolian's propeller turns clockwise when engaged forward, and counterclockwise when in reverse. This means that Eolian's stern moves to port (left) when she is reversed.

Finally, there is momentum. Figuring Eolian at 50,000 lb would be about right. Nothing happens fast, and you live with the results of previous control actions far longer than you might think. And the consequences of a collision at even low speed are significant, because of the magnitude of the forces involved.

So, docking. The objective is to arrive near enough to the dock for Jane to step off with a line and tie off to a cleat. You should arrive at the dock with zero velocity, so that there is no damage, and so that there is no bounce. And although you are in control of a vehicle longer, wider, and heavier than a city bus, you have no brakes. If you use reverse to bleed off forward momentum, the stern shifts to port. If there is wind, it blows against the bow, which then tries to act as a flag and stream downwind. It isn't easy, therefore you do what you can to remove difficulties.

Because Eolian walks to port when reversed, we elect to have a slip with a port tie. This means that when I reverse to bleed off forward momentum, the stern walks toward the dock. One issue neutralized.

Because of the configuration of the breakwater at Shilshole, there is a significant current flowing towards shore, out at the end of G dock, whenever the tide is running. So we elect to arrive at the dock at or near slack water whenever the tidal change is large. Another issue neutralized.

We have chosen a slip which faces north, because the prevailing winds are out of the north in the summer, when we do the majority of our docking. Approaching the dock with the bow firmly on one side of the wind means that it will not suddenly fall off as we slow. Another issue minimized.

Nevertheless, when docking we always have all of our fenders out, and never, never, NEVER take anything for granted.

And finally, when the lines are tied up and the engine is shut down, we breathe a sigh of relief, and celebrate with a beer (even if it is 09:00). And we almost always do a post-mortem, trying to see if there was a way we might have done things better. Usually there is.

I almost left this part out because this didn't start out as a "dock life" posting, but it really needs to be. Almost always when you approach the dock, there will be folks there waiting on the finger pier to catch your lines. This is important because by tossing a line to someone on the dock, you can get a line on a cleat, handled by skilled folks, even if you are too far from the dock. And we all do this for each other. On a Sunday afternoon, when lots of boats are coming back in, we are all alert to the sight of a familiar mast moving by outside the breakwater, or of someone turning into the waterway, and hustle down to their slip to catch lines and help them tie up. And of course this provides the perfect opportunity to find out where they went, and how the trip worked out.

And then there is one more person to help catch the next set of lines.

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