Here are a few figures for you. 1.7 feet, 2.5 feet, and 5 feet. What do they represent? How many feet are traveled in a one second period of time at 1 knot, 1.5 knots, and 3 knots.
For a very long time, I avoided the docks in Ayala Cove. All it took was one near catastrophe and I wanted nothing to do with that place. My primary concern was the current. A finger of land on the western side of the cove projects out into Raccoon Strait. During an ebb, that finger grabs the current deflecting it in a counter clockwise circle around the cove, directly under the dock. The current runs lengthwise down the dock, perpendicular to the direction of the slips, quite often at a substantial pace. This morning, the ebb in Raccoon Strait peaked at nearly 3 knots! I’m not sure how much of that 3 knots deflects under the docks, however, from past experience it’s a lot. Enough to create visible current trails coming off docks and pilings.
When reading about and discussing docking you often hear of the importance of having steerageway, which is maintaining enough speed through the water to steer with. But, how fast is the vessel really moving through the water to obtain steerageway? Every boat is different, however, if steerage way is obtained at 1.5 knots the vessel is moving through the water 2.5 feet per second. At 2.5 feet per second, it is going to take nearly 15 seconds for a 36 foot boat to travel one boat length. If you have a one knot current coming across your beam, what will the boat do during that 15 seconds? At one knot the current moves the vessel at 1.7 feet per second, for a total of 25 feet of sideways movement. A three knot current will push you sideways 75 feet!
If you are entering a “single wide” slip in a 1 knot current and can get the boat half way in the slip, you probably are ok. The current is going to push you into the dock finger the entire length as you motor forward. As long as you are well protected with fenders you are good. What happens if you are going into a “double wide” slip. You just ended up in the slip next to the one you were aiming for. Not a problem if there isn’t a boat already there or a piling in the middle.
The above scenario considers what happens from when the bow enters the slip. That’s not very realistic because you must also consider the approach to the slip. If you start 10 feet out, you are going to slide to the side 8 feet in a 1 knot current, nearly 25 feet in 3 knots, before you get into the slip. You are not going to end up where you want unless you plan for it.
What’s to be done? Start by knowing your boat! Know how much speed through water it takes to maintain steerageway. Know how far it takes with no current and no wind to turn 90 degrees (e.g. from fairway to slip). The only way to get that knowledge is practice.
As for the actual docking, plan your Location … Orientation … Transition ahead of time. Whenever possible, approach into the current. At Ayala Cove, that means traveling parallel to the dock and making a 90 degree turn into the slip. When you make the turn into the slip, make it “just a bit” later than if there were no current, allowing the current to push you “backwards and sideways” into the slip. How far is “just a bit?” It depends on how much current and how far off the dock you are. In a one knot current turning 8 feet later than normal might be just right. Get close enough to look for evidence of current. Are there current trails coming off the end of the dock fingers and pilings? What are the bubbles and debris in the water doing? Try measuring the distance the bubbles and debris move over a five second period of time. Your boat is going to do the same. Check the tide tables ahead of time and know what to expect before you get there. Try a practice approach or two or three. These are “double wide” slips. Is the current going to push you into the dock or away from it into the next slip?
In summery. Know what to expect ahead of time. Verify that conditions are what you expect and plan your L.O.T. accordingly. Don’t fight the current, you will lose. Allow the current to help you. Work with it to accomplish the goal you are striving for.