Current Tables: Reading the Traditional Format
Before digital navigation transformed our industry, mariners relied entirely on printed current tables—thick volumes published annually by NOAA containing predictions for thousands of tidal current stations along the coast. While apps like Mariner Studio now display this data digitally, understanding the traditional format remains essential for several reasons: it teaches you how predictions work, it helps you verify digital data, and frankly, electronics fail at the worst possible moments.
The traditional current table format has remained remarkably consistent for over a century because it efficiently communicates everything you need to know about water movement at a specific location. Learning to read these tables transforms you from someone who checks current predictions into someone who truly understands tidal current patterns.
What traditional current tables tell you
Current tables predict the velocity and direction of tidal currents at specific locations called current stations. Unlike tide tables that predict vertical water level changes, current tables predict horizontal water movement—the actual flow that affects your vessel.
Each current station represents a specific geographic point where measurements have been taken and predictions can be accurately calculated. These aren’t randomly chosen locations—they’re strategically placed at critical navigation points like channel entrances, narrow passages, harbor approaches, and anywhere current velocity significantly impacts vessel operations.
The two types of current predictions
Slack water predictions indicate when the current changes direction. At slack water, velocity approaches zero as the current transitions from flood to ebb or ebb to flood. These are critical reference points for timing passages through narrow channels or areas where strong currents make navigation dangerous.
Maximum current predictions indicate when the current reaches its peak velocity in either the flood or ebb direction. These predictions include both the time of maximum flow and the velocity in knots. For passage planning, maximum current times define your absolute limits—conditions that must be avoided or carefully considered.
The relationship between slack water and maximum current follows a predictable pattern. After slack water, current velocity gradually increases, reaching maximum flow roughly three hours later, then decreases back toward slack over the next three hours. This six-hour cycle repeats twice daily, though the specific timing varies by location.
Reading the table format
Traditional current tables organize predictions chronologically, typically showing one month per page. Each day lists all predicted slack waters and maximum currents in time order. Understanding this format allows you to quickly find the critical information you need for passage planning.
Example: Deception Pass, Washington – January 2025
Day Time Event Velocity
--- ---- ----- --------
01 0118 Slack, Flood begins -
0427 Maximum Flood 3.2 knots
0723 Slack, Ebb begins -
1034 Maximum Ebb 4.1 knots
1342 Slack, Flood begins -
1648 Maximum Flood 2.9 knots
1937 Slack, Ebb begins -
2241 Maximum Ebb 3.8 knots
02 0203 Slack, Flood begins -
0512 Maximum Flood 3.4 knots
0808 Slack, Ebb begins -
1119 Maximum Ebb 4.3 knots
1427 Slack, Flood begins -
1733 Maximum Flood 3.1 knots
2022 Slack, Ebb begins -
2326 Maximum Ebb 4.0 knots
Interpreting the columns
The Day column shows the calendar date. Tables typically show predictions for the entire year, organized month by month. Always verify you’re looking at the correct date—using yesterday’s predictions can put you in current moving the opposite direction from what you expect.
The Time column lists predictions in local time (often local standard time, not daylight saving time). This is crucial for passage planning—NOAA predictions are published in local standard time year-round, so during daylight saving time you must add one hour to the predicted time. Digital tools like Mariner Studio handle this automatically, but when reading printed tables, you must make this adjustment manually.
The Event column describes what’s happening. “Slack, Flood begins” means the current has stopped flowing and will soon begin flooding (moving in the flood direction for that station). “Maximum Flood” or “Maximum Ebb” indicates peak flow in that direction. The descriptors “flood” and “ebb” are location-specific—what constitutes flood at one station might be ebb at another, depending on local geography.
The Velocity column shows predicted current speed in knots at maximum flow. Slack water entries show no velocity since the water is essentially stationary during the transition. These velocity predictions represent average conditions across the channel—actual current can vary significantly based on your exact position within the waterway.
Understanding flood and ebb directions
Here’s where traditional current tables can initially confuse mariners: the terms “flood” and “ebb” don’t universally mean “incoming” and “outgoing.” Instead, they’re locally defined based on the dominant tidal flow pattern at that specific station.
At most coastal locations, flood current flows toward shore or up estuaries, while ebb current flows seaward. This aligns with rising and falling tides—flood current accompanies rising water, ebb accompanies falling water. But in complex waterways, especially those with multiple entrances or connections between bodies of water, the relationship becomes more nuanced.
Why direction matters for navigation
Getting flood and ebb directions wrong can completely reverse your passage planning strategy. Imagine planning to transit a narrow passage during what you believe is flood current, only to discover you’ve actually timed it for maximum ebb against you. The fuel consumption, vessel stress, and safety implications can be significant.
I once watched a sailing vessel attempt Hood Canal Bridge northbound, timing their arrival for what the skipper believed would be favorable current. They’d read “flood” in the table and assumed it meant northerly flow. It didn’t. At that location, flood flows south. They ended up essentially stationary despite seven knots of boat speed, then had to abandon the attempt when the bridge opened on schedule and they couldn’t make forward progress.
The lesson: never assume flood means the direction you want to go. Always verify the actual geographic direction of flow before planning passages based on current predictions.
Calculating intermediate current velocities
Traditional current tables only provide predictions at slack water and maximum current. But what if you need to know the current velocity at 1015 when the table shows slack at 0927 and maximum at 1238? This is where the rule of thirds becomes invaluable.
The rule of thirds recognizes that current velocity doesn’t increase linearly from slack to maximum. Instead, it follows a predictable pattern: during the first hour after slack, current reaches about 50% of maximum velocity. During the second hour, it increases to about 90% of maximum. During the third hour, it reaches its predicted maximum and holds briefly before beginning to decrease following the reverse pattern.
Practical application of the rule of thirds
Let’s work through a real example. Your current table shows slack water at 0723 and maximum ebb of 4.1 knots at 1034. You plan to transit at 0900. What current velocity should you expect?
First, calculate the time interval between slack and maximum: 1034 minus 0723 equals 3 hours 11 minutes, which we’ll round to 3 hours for practical purposes. This is your total current cycle from slack to maximum.
Second, determine where your transit time falls within this cycle: 0900 is 1 hour 37 minutes after slack (0723). That’s roughly 1.5 hours into the 3-hour cycle, or halfway to maximum.
Using the rule of thirds: at one hour after slack, current velocity is approximately 50% of maximum. At two hours, it’s about 90%. At 1.5 hours (halfway between), estimate about 70% of maximum. With maximum predicted at 4.1 knots, expect approximately 2.9 knots at 0900.
This calculation isn’t perfectly precise—actual current velocity can vary based on weather, recent precipitation, and other factors. But it provides a much better estimate than assuming zero current or maximum current. For passage planning, especially timing narrow passages, these intermediate calculations can mean the difference between a comfortable transit and a challenging fight against the current.
The relationship between tides and currents
Many mariners expect slack current to occur precisely at high or low tide. In reality, the relationship between tidal height and current velocity varies dramatically by location, and understanding this relationship prevents dangerous timing assumptions.
In open coastal waters and offshore areas, slack current typically occurs near high and low tide—the water reaches its vertical extreme and briefly pauses before reversing direction. The lag between tide and current might be just 15-30 minutes in these locations.
But in rivers, estuaries, bays, and complex waterways, the lag can extend to several hours. I regularly transit Deception Pass where slack current occurs roughly 3 hours before high tide at nearby Port Townsend. Mariners who assume slack coincides with high tide find themselves fighting maximum ebb when they expected stationary water.
This lag happens because current represents water movement between different bodies of water trying to equalize their levels. In a long, narrow channel, water might still be flooding toward the inner bay long after high tide has occurred at the entrance. The water must physically travel the distance, creating the delay between tidal height change and current reversal.
Why you must use both tide and current tables
Checking tide predictions tells you about vertical water level—critical for depth concerns, bridge clearances, and determining if you can access shallow areas. Current tables tell you about horizontal water movement—critical for calculating speed, fuel consumption, and timing passages where flow is significant.
Neither table can substitute for the other. I’ve seen mariners arrive at a bar crossing at low tide with adequate water depth, only to find themselves fighting maximum ebb because they forgot to check current predictions. I’ve also seen vessels plan for slack current but arrive to discover insufficient depth because they ignored tide tables.
Professional practice means checking both. Mariner Studio displays both tide and current data for stations in close proximity, making it simple to consider both vertical and horizontal water movement in your passage planning. When both factors matter—and they usually do—having instant access to correlated predictions eliminates the mental gymnastics of trying to remember which table said what.
Working with reference and subordinate stations
Not every location has a current station with direct predictions. NOAA measures and analyzes currents at hundreds of primary locations called reference stations, then uses these to calculate predictions for thousands of subordinate stations using time and velocity corrections.
Reference stations appear in current tables with full daily predictions—complete listings of slack waters and maximum currents. These are locations where NOAA has deployed current meters and collected extensive data over multiple years. The predictions for reference stations are considered highly reliable because they’re based on actual measurements at that exact location.
Subordinate stations appear with correction factors that you apply to a reference station’s predictions. A subordinate station entry might indicate: “Times: +0:45 on The Race” and “Velocities: 0.8x.” This means to find current predictions for this subordinate station, you look up predictions for The Race (the reference station), add 45 minutes to all times, and multiply all velocities by 0.8.
How to calculate subordinate station predictions
Let’s work through an example. You want to know predicted current at Plum Gut, a subordinate station that references The Race. The correction factors show +0:23 for time and 0.9x for velocity.
The Race reference station shows slack at 0542 and maximum flood of 3.8 knots at 0847. To find Plum Gut predictions:
- Add time correction: 0542 + 0:23 = 0605 for slack water
- Add time correction: 0847 + 0:23 = 0910 for maximum
- Apply velocity correction: 3.8 knots Ă— 0.9 = 3.4 knots maximum flood
The corrected predictions show slack at Plum Gut occurring at 0605 and maximum flood of 3.4 knots at 0910. These corrections account for the fact that current behaves differently at Plum Gut than at The Race due to local geography, depth, and channel characteristics.
Understanding velocity variations across the channel
Here’s a critical concept that traditional tables can’t fully convey: the velocity listed in current predictions represents an average across the channel or passage. Actual current velocity varies dramatically based on your precise position within the waterway.
Current flows fastest in the center of the channel where water is deepest and least restricted. Along the edges, shallow water and friction slow the flow. In many channels, the difference between center current and edge current can be 50% or more. When the table predicts 4 knots maximum, you might experience 5 knots in mid-channel but only 2.5 knots near the shore.
I learned this vividly transiting Deception Pass on a small vessel. The table predicted 3.2 knots maximum ebb. Following the recommended route through the deepest part of the pass, we encountered over 4 knots. On a subsequent transit, I experimented with staying closer to the Deception Island shore where water shallows, and current velocity dropped to less than 2 knots despite occurring during the same predicted maximum.
Using geography to your advantage
Understanding velocity variation lets you minimize current effects even during unfavorable conditions. If you must transit during maximum current, positioning yourself strategically can reduce the velocity you actually experience. Stay in deeper water when current is favorable, move toward shallow edges when it’s adverse.
This technique has limits—don’t run aground chasing lighter current, and some channels prohibit deviation from the marked passage. But in many locations, subtle positioning adjustments can significantly affect your speed over ground and fuel consumption. The traditional table gives you the timing; local knowledge and careful observation give you the positioning strategy.
Seasonal and weather effects on predictions
Current predictions assume normal weather conditions and average freshwater runoff. Heavy precipitation, storm surge, sustained winds, and other factors can significantly alter actual current from predicted values. Understanding these effects helps you interpret what you experience versus what tables predict.
Freshwater runoff from rivers can strengthen ebb currents and weaken flood currents. After heavy rainfall or snowmelt, expect stronger-than-predicted ebb as the volume of water flowing seaward increases. This effect is most pronounced in rivers and estuaries, less significant in purely tidal channels.
Storm surge from sustained onshore winds can delay or strengthen flood currents. The wind literally piles water against the coast, increasing water level and affecting tidal flow patterns. Conversely, strong offshore winds can strengthen ebb currents as they push surface water seaward.
Barometric pressure affects water level, with low pressure creating higher water levels and high pressure creating lower levels. Extreme pressure differences can shift current timing by creating abnormal water level gradients that enhance or oppose tidal forcing.
Common mistakes reading current tables
Even experienced mariners occasionally misread current tables. Understanding common errors helps you avoid them in your own passage planning.
Confusing slack water types
Tables list two types of slack water: “Slack, Flood begins” and “Slack, Ebb begins.” The current is zero at both, but what happens next differs completely. Missing this distinction means planning for a favorable current that’s actually about to run against you. Always note whether slack leads to flood or ebb before timing your transit.
Using yesterday’s predictions
Current timing shifts roughly 50 minutes later each day, following the lunar cycle. Using yesterday’s table because it’s already open on the chart table can put you in entirely different current conditions. Always verify you’re looking at predictions for your actual transit date—this seems obvious, but in busy, distraction-filled environments, this error happens more often than you’d think.
Ignoring time zone and daylight saving
NOAA publishes predictions in local standard time year-round. During daylight saving time (roughly March through November), you must add one hour to predicted times. Forgetting this adjustment means arriving at the channel an hour early or late—potentially the difference between slack water and maximum current. Mark your tables clearly when switching to or from daylight saving time.
Assuming slack means no current
Slack water represents minimum current, not necessarily zero current. In locations with multiple tidal influences or persistent weather effects, “slack” might still mean 0.5 to 1 knot of flow. The table shows when current is weakest, but local conditions determine whether weakest actually means stopped. Always approach critical passages expecting some current even at predicted slack.
Applying current predictions to passage planning
Understanding how to read current tables transforms from academic knowledge to practical skill when you apply predictions to actual passage planning. The goal isn’t just reading the numbers—it’s using them to make smarter navigation decisions that save fuel, reduce vessel stress, and improve safety.
Planning departure times
For passages where current significantly affects your vessel, optimal departure timing makes the difference between a smooth transit and an exhausting battle. Calculate your expected passage duration, then work backward from current predictions to find the ideal departure window.
Example: You’re planning a 4-hour passage through an area with significant tidal current. Current tables show slack at 0800, maximum flood (favorable to your direction) at 1100, and slack again at 1400. Ideally, you want to depart around slack water, allowing the building flood to assist you for the first half of your passage, then continuing with diminishing current assistance as you progress.
If you depart at 0800 at slack, you’ll experience building favorable current for the first three hours, reaching near-maximum assistance mid-passage, then face diminishing flow for the final hour. This timing maximizes your speed made good while minimizing fuel consumption—the kind of efficiency that separates professional from amateur passage planning.
Calculating speed made good
Current directly affects your speed over ground, and traditional tables give you the information needed to calculate expected speed made good before you even leave the dock. This allows realistic ETA calculations and helps identify whether a planned passage is even feasible given your vessel’s speed.
Your vessel makes 8 knots through the water. Current predictions show maximum ebb of 3 knots (adverse to your route) at mid-passage. Your speed made good during maximum current will be approximately 5 knots (8 knots boat speed minus 3 knots current). If you can’t accept this reduced speed for your schedule, you must either depart earlier to catch more favorable current or accept that the passage will take longer than expected.
Mariner Studio’s route planning feature automatically factors current into ETA calculations, showing you predicted arrival times that account for current effects along your entire route. This eliminates mental math and gives you realistic expectations for passage duration under actual conditions.
Digital vs. traditional current tables
Modern navigation apps display current predictions digitally, but this convenience doesn’t make understanding traditional tables obsolete. Knowing how to read the underlying data format helps you verify digital displays, understand prediction limitations, and maintain navigation skills when electronics fail.
Digital displays in Mariner Studio present the same NOAA prediction data that appears in traditional tables, but we format it for rapid comprehension. You see slack water times highlighted, maximum currents displayed with color-coded velocity indicators, and clear directional information. The data source is identical—only the presentation differs.
When using digital tools, occasionally cross-reference with printed tables or NOAA’s online data to verify your app is displaying predictions correctly. Electronics occasionally glitch, time zones get misconfigured, or software bugs appear. Maintaining skills to interpret the raw data provides backup verification of what your screen displays.
Frequently asked questions
NOAA’s current predictions are remarkably accurate under normal conditions, typically within 20% of actual observed velocities and 30 minutes of actual timing. Accuracy decreases during extreme weather, unusual runoff conditions, or in locations with complex current patterns. For passage planning purposes, predictions provide reliable guidance, but always monitor actual conditions and be prepared for variations from predicted values.
Current stations measure horizontal water movement (flow velocity and direction), while tide stations measure vertical water level changes (height). Current tables predict when water flows fastest and when it stops, while tide tables predict when water reaches high and low levels. Both are essential for complete passage planning, and you can’t substitute one for the other since they measure different aspects of tidal effects.
If you’re seeing different times for apparently the same location, you’re likely comparing predictions for different current stations that happen to be near each other. Current timing can vary significantly over short distances in complex waterways. Always verify you’re using the station closest to your actual transit point, and note that station names might be similar despite representing different geographic positions.
No. While tidal patterns repeat annually, the specific times and velocities change each year due to variations in the lunar cycle, astronomical factors, and geographic considerations. Current predictions are calculated specifically for each calendar year. Using old tables can put you in dramatically different current conditions than predicted—potentially arriving at maximum flow when you expected slack water. Always use current year predictions.
Check the station description in the reference information section of the current tables, typically at the beginning of each station’s predictions. This describes the geographic direction of flood and ebb flow for that specific location. In digital tools like Mariner Studio, we display this directional information with each prediction so you never need to reference separate documentation to understand which way the water flows.
NOAA provides predictions for thousands of locations, but not everywhere has a current station. For locations without direct predictions, identify the nearest reference station and use professional judgment to estimate timing and velocity adjustments based on local geography. Better yet, use digital tools like Mariner Studio that include both reference and subordinate stations with corrections already applied, giving you predictions for far more locations than printed tables could practically include.
Putting it all together
Reading traditional current tables effectively comes down to three essential skills: understanding the timing relationship between slack and maximum current, correctly interpreting flood and ebb directions for your specific location, and applying the rule of thirds to estimate intermediate velocities. Master these fundamentals, and you can extract everything you need from even the most cryptic-looking printed table.
But here’s what decades of experience has taught me: reading the tables is just the beginning. The real skill lies in applying current predictions to practical navigation decisions—timing your departures to work with favorable flow, positioning your vessel to minimize adverse current, and adjusting your plans when conditions deviate from predictions.
Modern tools like Mariner Studio make accessing current predictions faster and easier, but they build on the same fundamental NOAA data that fills traditional tables. Understanding that underlying format helps you verify digital displays, catch errors, and maintain navigation skills that don’t depend on charged batteries or reliable cell service. The vessels I’ve served on always carried current tables alongside the latest electronics, and that redundancy has proven valuable more times than I can count.
Next time you’re planning a passage through an area with significant tidal current, take a few minutes to study the traditional table format for that location. Note the timing patterns, observe how maximum currents relate to the lunar cycle, and practice calculating intermediate velocities using the rule of thirds. These skills sharpen your understanding of water movement and make you a more competent mariner whether you’re reading predictions from a book or from a screen.