Leeway Allowance in Chart Plotting

TL;DR — Navigational errors accumulate whenever a vessel fails to hold her planned course; cumulative errors must be checked against fixes whenever possible. A running fix, obtained by advancing an earlier LOP along the DR track, is less accurate than a fix from simultaneous observations.

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What the Rule Says

The supplied authoritative sources address two directly relevant principles for chart plotting and leeway allowance.

Navigational error accumulation is addressed in Bowditch Ch. 23 §2301, which identifies five categories of navigational error: instrument error, observation error, plotting error, computation error, and the basic assumption that the vessel has held the planned course. That last category is the one most directly tied to leeway. When wind or current pushes a vessel off her intended track, the assumption that she has held the planned course becomes false, and the DR position diverges from the vessel's actual position. The remedy stated in the source is explicit: cumulative errors must be checked against fixes whenever possible. Bowditch Ch. 23 §2301

Running fixes and their limitations are addressed in Bowditch Ch. 7 §702. When lines of position (LOPs) cannot be taken simultaneously — a common situation when only one charted object is available for bearing — the navigator advances the first LOP along the DR track to the time of the second observation. The intersection of the advanced LOP and the new LOP is the running fix. The source is explicit that a running fix is less accurate than a fix obtained from simultaneous LOPs. The reason is that the advance of the first LOP depends entirely on the accuracy of the DR track used — including any uncorrected leeway or current. Bowditch Ch. 7 §702

These two principles work together: leeway introduces error into the DR track; that error propagates directly into any running fix that relies on that track; and the only remedy is to obtain and apply fixes as frequently as possible. Bowditch Ch. 23 §2301 Bowditch Ch. 7 §702

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Why It Matters on the Exam

OUPV and Master 100 GT written exams test chart plotting in a very practical way. Candidates are given a scenario — a vessel on a stated compass course at a stated speed, with wind and/or current acting on her — and asked to determine the vessel's actual track over the ground, her estimated position after a given time, or the course to steer to make good a desired track. Leeway is the angular difference between the vessel's heading (the direction her bow points) and her actual course through the water caused by wind pressure on her hull and topsides.

The exam tests whether you understand the following chain of reasoning:

1. The vessel's heading is what the helmsman steers and what the compass reads. 2. Leeway displaces the vessel's actual path through the water to leeward of the heading. 3. The course made good (CMG) over the ground is the result of combining the vessel's water track (heading corrected for leeway) with any current set and drift. 4. The DR position is plotted along the heading line at the vessel's speed through the water, without accounting for leeway or current. It is therefore an approximation. 5. Any running fix that uses the DR track to advance an LOP inherits all the error in that DR track, including uncorrected leeway. Bowditch Ch. 7 §702 6. Cumulative error grows with time and distance. The longer the vessel has been running without a fix, the greater the potential divergence between the DR position and the actual position. Bowditch Ch. 23 §2301

On the exam, a typical question might read: "A vessel is steering 090°T at 8 knots. The wind is from the north, causing 5° of leeway. What is the vessel's course through the water?" The answer is 095°T — the vessel is being pushed to the south (leeward), so her actual path through the water is 5° to the right of her heading. To make good 090°T, the helmsman must steer 085°T (alter course into the wind by the leeway angle).

Another common question type: "A navigator advances a bearing LOP by 30 minutes along the DR track to obtain a running fix. The vessel has been experiencing 3° of leeway that was not accounted for in the DR. What is the effect on the running fix?" The answer is that the running fix will be in error because the LOP was advanced along an incorrect DR track. Bowditch Ch. 7 §702 Bowditch Ch. 23 §2301

The exam also tests the relationship between leeway and traffic separation scheme compliance. A vessel using a traffic separation scheme must proceed in the appropriate traffic lane in the general direction of traffic flow, keep clear of the separation line or separation zone so far as practicable, and cross traffic lanes on a heading as nearly as practicable at right angles to the general direction of traffic flow. 33 CFR §83.10 If a vessel fails to account for leeway when transiting a TSS, she may set into the separation zone or an adjacent lane — a direct regulatory violation. The plotting skill of leeway allowance is therefore not merely academic; it has direct operational and legal consequences under 33 CFR §83.10. 33 CFR §83.10

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Common Pitfalls

Pitfall 1: Confusing heading with course. Heading is the direction the bow points. Course through the water is the vessel's actual path through the water after leeway is applied. Course made good is the resultant path over the ground after current is also applied. Exam questions frequently use these terms interchangeably in the stem to see whether the candidate recognizes the distinction. Bowditch Ch. 23 §2301

Pitfall 2: Applying leeway in the wrong direction. Leeway always pushes the vessel to leeward — away from the wind. If the wind is from the north (a northerly wind), the vessel is pushed south. Her actual track through the water is south of her heading. To compensate, the helmsman must steer north of the desired track. Candidates frequently apply the correction in the wrong direction, adding leeway to the heading when they should subtract it, or vice versa. A reliable check: the vessel's heading must always be upwind of her intended track.

Pitfall 3: Assuming the DR position is the actual position. The DR position is a dead reckoning estimate based on the last known position, the vessel's heading, and her speed through the water. It does not account for leeway, current, or steering error. Bowditch Ch. 23 §2301 explicitly identifies the assumption that the vessel has held the planned course as a source of navigational error. Treating the DR as a fix — especially when constructing a running fix — compounds this error. Bowditch Ch. 23 §2301

Pitfall 4: Underestimating the degradation of a running fix. A running fix is less accurate than a simultaneous fix. Bowditch Ch. 7 §702 The longer the interval between the two observations, the greater the distance the first LOP must be advanced, and the greater the potential error introduced by an inaccurate DR track. Candidates sometimes treat a running fix as equivalent in reliability to a two-bearing fix taken simultaneously. It is not.

Pitfall 5: Ignoring leeway when planning a TSS crossing. 33 CFR §83.10(c) requires a vessel crossing a traffic lane to do so on a heading as nearly as practicable at right angles to the general direction of traffic flow. Note the regulation says heading, not track. However, if the vessel does not account for leeway, her actual track across the lane will not be at right angles even if her heading is. In a strong beam wind, the vessel may traverse the lane at a significant angle, spending more time in the lane and increasing collision risk. 33 CFR §83.10

Pitfall 6: Neglecting to update the DR after obtaining a fix. Once a fix is obtained, the DR track must be restarted from that fix. Candidates sometimes continue to plot DR positions from the previous fix or from a running fix, allowing cumulative error to persist. Bowditch Ch. 23 §2301 is clear that cumulative errors must be checked against fixes whenever possible — the implication being that each new fix resets the error baseline. Bowditch Ch. 23 §2301

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Quick Check

Question 1: A vessel is steering 270°T. The wind is from the south, causing 4° of leeway. What is the vessel's actual course through the water, and what heading should she steer to make good 270°T?

The wind from the south pushes the vessel north (to leeward). Her actual course through the water is 266°T (4° north of her heading). To make good 270°T, she must steer 274°T — heading south of the desired track to compensate for the northward leeway. Bowditch Ch. 23 §2301

Question 2: A navigator takes a bearing on a lighthouse at 0800 and plots the LOP. No second object is available. At 0830 she takes a second bearing on the same lighthouse and advances the 0800 LOP along the DR track. What type of position does the intersection represent, and why is it less reliable than a two-object fix?

The intersection is a running fix. It is less reliable than a simultaneous two-object fix because the advance of the first LOP depends on the accuracy of the DR track. Any error in the DR — including uncorrected leeway — displaces the advanced LOP and therefore displaces the running fix. Bowditch Ch. 7 §702 Bowditch Ch. 23 §2301

Question 3: A vessel is crossing a traffic separation scheme. The regulation requires her to cross on a heading as nearly as practicable at right angles to the general direction of traffic flow. There is a strong beam wind causing 8° of leeway. If the helmsman steers exactly 90° to the traffic flow without correcting for leeway, what is the effect on the vessel's actual track across the lane?

The vessel's actual track through the water will be 8° off from a right-angle crossing — she will traverse the lane at an oblique angle, spending more time in the lane than necessary and increasing exposure to traffic. To achieve a track at right angles to the traffic flow, the helmsman must steer 8° into the wind (upwind of the desired track) to compensate for leeway. 33 CFR §83.10 Bowditch Ch. 23 §2301

Question 4: Name the five categories of navigational error identified in Bowditch, and state which one is most directly associated with leeway.

The five categories are: instrument error, observation error, plotting error, computation error, and the basic assumption that the vessel has held the planned course. The last category is most directly associated with leeway, because leeway causes the vessel to deviate from the planned course even when the helmsman is steering accurately. Bowditch Ch. 23 §2301

Question 5: A vessel not using a traffic separation scheme is navigating in the vicinity of one. What does 33 CFR §83.10 require of her?

A vessel not using a traffic separation scheme shall avoid it by as wide a margin as is practicable. 33 CFR §83.10 Failure to account for leeway when navigating near a TSS could cause the vessel to set into the scheme in violation of this requirement.