Treat every SFV topic as a decision under changing load: ask what the catch, fuel state, gear, and sea state are doing to your stability, your signal status, and your escape options right now. Study by working scenarios closed-book, scoring yourself against a fixed rubric, and logging observable checks such as roll period so concepts become habits you can execute under fatigue.
Why water on deck is a stability problem, not a housekeeping one
Water lying free on deck, or in any partially filled tank, shifts as the vessel rolls. That movement cuts the righting lever — the free surface effect — and consumes the reserve stability you need once heeled.
Free surface effect is a pocket of liquid sliding to the low side, effectively raising the centre of gravity and reducing righting moment. Its severity grows with the breadth of the free surface, so a wide, shallow layer of water in a fish pen harms more than the same volume in a deep, narrow tank. An empty tank contributes nothing; a completely pressed-up tank contributes nothing; the half-full tank is the worst case.
Reserve stability is the righting power left in the hull beyond small heel — the area under the GZ curve. Free surface eats it exactly where it matters, at moderate angles. On a fishing deck the risk compounds: pens, trap stacks, hoses, and hatchways mean that a heel which lets water reach a downflooding opening can convert a recoverable roll into progressive flooding. Keep openings dogged shut and free liquid minimised as one habit, not two.
- Free surface depends on surface breadth, not just volume — spread-out water is the dangerous shape.
- Full or empty is safe; slack is the worst condition for any tank.
- A downflooding opening converts heel into flooding — keep hatches and doors closed at sea.
| Concept | What it measures | Observable warning | First corrective action |
|---|---|---|---|
| GM | Initial stiffness near upright | Long, slow, easy roll compared with baseline | Move weight lower; reduce topweight |
| GZ / reserve stability | Righting power at a given heel | Vessel fine upright but alarming once heeled | Remove free surface and topweight before heeling grows |
| Free surface effect | Loss of righting lever from moving liquid | Sudden tenderness with slack tanks or water on deck | Pump out fully or press up completely |
| Angle of loll | Negative GM, hull falls off to either side | List flips sides with wave or weight movement | Add weight low and carefully; never heel it further to 'hold' it |
| Downflooding point | Heel at which openings immerse | Hatches, doors, or vents near the waterline when heeled | Secure openings; keep heel small |
Tracing KG, GM, and the GZ curve through a fishing trip
GM is initial stiffness — the gap between centre of gravity and metacentre. GZ is the righting lever at a given heel. Loading catch or gear high raises KG, shrinks GM, and flattens the whole curve.
Trace the chain in one direction: weight added high raises the centre of gravity (KG up), a higher KG shortens GM, and a shorter GM means a smaller righting lever through the heel range. A vessel that felt stiff at departure can be tender on arrival, because catch stacked on deck, full trap stacks high, and fuel burned from low tanks have all pushed weight upward while displacement stayed similar. Nothing about the hull changed — only the load did.
Compare two loadings of the same vessel: catch landed low in the hold with tanks pressed full, versus catch on deck in slack-floored pens with half-empty tanks. The first keeps a healthy GZ curve; the second can lose reserve stability before any dramatic list appears. Make roll period your onboard check: longer, slower rolls at the same nominal loading signal reduced GM. Log the roll character against cargo state each trip so you learn your vessel's own baseline rather than a generic number.
Fishing lights: when 'vessel engaged in fishing' actually applies
'Vessel engaged in fishing' means maneuverability is restricted by gear. Only that vessel shows the red-over-white fishing lights and shapes, and only that status earns the priority given to fishing vessels in encounters.
Separate two conditions. A vessel under way fishing whose gear does not restrict maneuverability shows ordinary sidelights and sternlight — no fishing signals — and behaves under the normal encounter rules. A vessel whose gear genuinely restricts maneuverability shows sidelights with all-round red over white at night and the basket or cone shapes by day. Getting your own status right is the first decision; getting the other vessel's status right is the second.
Worked scenario: you are drifting over pots. Your gear does not restrict maneuverability, but you show the red-over-white lights anyway and hold course, expecting a large vessel to keep clear. Meanwhile the other bridge sees a small, slow target and plans to pass ahead. The better decision is to show lights matching your true condition, treat yourself as an ordinary vessel in the encounter, and make an early, substantial, obvious alteration if in doubt. Priority rules fail when either bridge has misread the signals — early self-help beats contested assumptions, especially in restricted visibility where confirmation is slow.
Turning a synoptic chart into a go/no-go and escape window
Isobar spacing gives wind strength, front orientation gives timing, and the angle between your track and the wind gives sea state on deck. Translate every chart feature into one operational decision before you sail.
Read the chart in a fixed order: tight isobars mean a strong pressure gradient and strong wind; a marked front means an approaching wind shift with squalls; a ridge means light, variable conditions. In the mid-latitudes, systems generally progress eastward, so position your ground relative to the front's forecast line. Note also whether wind will back (anticlockwise shift) or veer (clockwise shift) as the feature passes — that tells you which way your anchorage or ground will open or close.
Before departure, annotate the chart with three things: when the front crosses your fishing ground, which quadrant of the low you will occupy, and your escape window to shelter. Then plan gear around the weather, not weather around the gear — hauling traps in rising wind costs time you may not have. Remember the chart is a framework, not a forecast: it structures your reasoning, and the decision itself belongs with the official forecasts and warnings for the area you are working.
The first ten minutes of MOB, flooding, and machinery fire
Each emergency has a different opening move: MOB demands immediate maneuver and marking, flooding demands locating and stopping the inflow before pumping, and fire demands cutting fuel and air before opening anything.
For a person overboard, the tempting move is turning straight toward the person while shouting, and drifting while the crew scrambles. The reliable sequence: hit the MOB mark and take a visual reference — never trust the screen alone; turn away, then back through a Williamson turn; post a dedicated lookout with eyes on the person; and launch recovery gear only once roles are assigned. Drill it so each crew member has one job, because a skipper doing three jobs drops all three.
For flooding, the bilge alarm's first question is where, not how much: a leaking sea suction, a hull fitting, or a failed gland each needs a different response, and pumping before isolating the inflow just recirculates the sea. For a fire in machinery spaces, shut fuel and close air supplies before opening anything — an opened engine room feeds the fire. Then fix the reporting role: one designated person makes the radio call while others fight. Run all three as tabletop drills with timers, and compare your sequence against this priority order afterwards.
Log entries and watch handovers that record the decision, not the mood
A useful entry records what you observed, what you decided, and why — position, weather, sea state, gear state, traffic — so the next watch can continue your plan without reconstructing it from scratch.
Contrast two styles. 'Steady all night, weather fine' tells a reader nothing testable. A strong entry gives position and time, wind and sea state, course and speed, gear set and hauled with times, any near encounter and the action you took, and any equipment fault plus the workaround adopted. The difference is reconstructability: the strong entry lets another skipper rebuild your risk picture and judge whether it was still valid at handover.
Apply the same discipline to handovers: pass on the plan, not just the state. 'On autopilot, all fine' versus 'on autopilot, course 080, front due around 0300, haul the pots on the 40-metre edge before the shift, call me if visibility drops under a mile' — only the second lets the relief watch make your decisions. Exercise: write one simulated trip's log in this style, then hand it to a peer cold and ask them to state the current plan, the weather risk, and the standing orders. Their errors show you exactly what your entries left out.
A four-week rotating drill sequence with a self-check rubric
Rotate one block per week — stability, rules and signals, weather and passage decisions, emergencies and logs — and close each week with a timed, closed-book scenario scored against a fixed rubric.
Week one, stability: sketch GM, GZ, and free surface from memory, then work free surface examples until you can predict the effect of a slack tank before checking the answer. Week two, rules and signals: draw night scenes and name each vessel's status and obligation. Week three, weather: annotate synoptic charts with front timing, wind quadrant, and an escape window. Week four, emergencies and logs: run tabletop MOB, flooding, and fire drills with assigned roles, and write a handover script. Finish with a mixed scenario day combining all four blocks.
Score yourself against this rubric — these are learning milestones, not passing predictions: one point each for stating GM, GZ, and free surface in one sentence with a fishing example (3); correct status and obligation in 8 of 10 signal scenes (1); a go/no-go built on three annotated chart features (1); a tabletop MOB with correctly assigned roles (1); a log entry a stranger can reconstruct (1). Expected observation in the free surface example: pressing up or emptying the slack tank restores most of the lost righting arm, while adding a second slack tank compounds the loss — seeing that pattern on paper is what makes the habit stick on deck.
- Readiness check: you can classify any night signal scene in under ten seconds without notes.
- Readiness check: you can annotate a synoptic chart with three operational decisions, not three labels.
- Readiness check: your tabletop MOB assigns lookout, helm, and radio roles before recovery gear moves.
- Readiness check: a peer reading your log cold can state the plan, the weather risk, and the standing orders.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.