2.10 Stability, Trim & Tonnage Basics for Masters

Stability Is Command Responsibility

On a six-pack boat, bad loading can still kill. On a Master 100 vessel with dozens of passengers under a COI, the same physics scale up: people move in groups, gear stacks on upper decks, icing builds on railings, and free water on deck after boarding seas can destroy reserve stability in minutes. Q160 tests the vocabulary and the decisions; the COI and stability letter turn those decisions into legal operating limits.

Displacement, Buoyancy, G, and B

A floating vessel obeys Archimedes’ principle: it displaces a volume of water whose weight equals the vessel’s total weight. That total weight is displacement — people, fuel, stores, and structure. Add weight and the hull settles until buoyancy again matches weight.

Two centers control righting behavior:

• Center of gravity (G) — the point where the vessel’s weight acts downward. Weight high (flybridge crowds, stacked coolers, ice on the house) raises G. Weight low and centered lowers G.
• Center of buoyancy (B) — the center of the underwater volume, pushing up. As the vessel heels, underwater shape changes and B moves toward the low side.

When heeled, the horizontal separation between the line of weight through G and the line of buoyancy through B creates a righting arm (GZ). That lever tries to roll the vessel back upright — if geometry still allows it.

Metacentric Height (GM) and Positive Stability

For small angles of heel, the metacenter (M) is where the buoyancy line intersects the vessel centerline. Metacentric height GM is the distance from G to M:

• Positive GM (M above G): stable — the vessel tends to return upright.
• Zero or negative GM: unstable — she lolls or capsizes.

A stiff vessel has relatively large GM (snappy roll, resists heel). A tender vessel has small GM (easy heel, slow hang at the end of a roll). Tender is a warning, not a comfort feature, especially with free water or high passenger loads.

Master takeaway: almost every operational choice that raises G — topside weight, icing, high deck cargo, people on the house — shrinks GM. Choices that lower and center weight improve GM.

Free Surface Effect

Free surface effect is one of the most tested and most lethal stability problems. Liquid free to move in a partly filled (slack) tank, in the bilge, or ponded on deck runs to the low side as the vessel heels. That shift moves weight in the direction of the heel and acts like a virtual rise in G, reducing effective GM. The worse the free surface breadth, the larger the effect.

Operational controls:

• Prefer tanks pressed full or empty, not half-full, when operationally possible.
• Dewater bilges and decks aggressively; keep scuppers and freeing ports clear.
• Treat firefighting water accumulating high as both top weight and free surface.
• Do not "live with" a flooded cockpit or fish hold full of free water on a passenger trip.

List vs Heel vs Trim

Use the words precisely — the exam and the bridge log both care:

A list that appears after loading is a weight-distribution problem to fix before departure. A heel in a turn may be normal but must not ship water or alarm passengers into a worse rush. Trim by the head can bury the bow and hurt steering and freeboard; trim by the stern can reduce forward visibility and change prop bite. Masters correct trim with fuel, ballast, gear placement, and passenger seating plans — not with hope.

Icing, Topside Weight, and Passenger Distribution

Icing on rails, masts, houses, and gear is pure high weight. In freezing spray, masters must consider heading, speed, and de-icing before GM collapses. The same logic applies to temporary topside gear: kayaks on the house, full flybridge bars, and stacked coolers all raise G.

Passenger weight distribution is a daily Master 100 problem. Classic scenarios:

• Entire rail rushes to port for wildlife → sudden list, reduced reserve stability if a wake or gust hits.
• All passengers forward on a head-sea day → trim by the head, wet decks, slower recovery.
• Standing on cabin tops or overcrowding one side on a tender hull → capsize risk even within total headcount.

Control measures: seat people low and inboard, brief them not to all rush one rail, stage viewing so weight stays balanced, and never exceed the COI/stability-letter passenger or weight envelope just because the weather is flat.

Gross Register Tonnage vs Displacement

This distinction fails candidates who confuse license limits with weight:

• Gross register tonnage (GRT / GT) is a regulatory measure of enclosed internal volume (historically 100 cubic feet per register ton). It is used for documentation, inspection thresholds, manning, and credential tonnage limits.
• Displacement is the actual weight of the vessel and everything aboard, equal to the weight of water displaced.
• Net tonnage is related to volume after certain deductions (e.g., machinery spaces) and matters for documentation trade rules, not for "how heavy is the boat."

Your Master Less Than 100 GRT endorsement authorizes command of self-propelled vessels of less than 100 gross register tons. That is not "vessels weighing less than 100 tons." A vessel can weigh many times 100 long tons of displacement and still measure under 100 GRT — or look small yet approach the GRT limit based on enclosed volume. Always read GRT from the COD/COI, not from a bathroom scale or a length estimate.

Stability Letter and COI Implications (Subchapter T)

Most Master 100 careers center on inspected small passenger vessels under 46 CFR Subchapter T (and related small-passenger frameworks). The Certificate of Inspection (COI) states route, maximum persons, manning, and conditions of operation. Many vessels also carry a stability letter or approved stability/loading instructions from the Coast Guard or a naval architect review. Those documents may specify:

• Maximum passengers and total persons
• Weight limits, ballast conditions, or tank status
• Restrictions (e.g., weather, open doors, limited upper-deck occupancy)
• Special operating conditions after alterations

Treat the stability letter as an operating limit, not advice. Exceeding it is both unsafe and a compliance failure. If the owner wants more passengers or a different route, the vessel needs Coast Guard approval and an amended COI/stability basis — not a waiver signed by guests.

Master’s pre-departure stability scan

1. Confirm persons aboard ≤ COI / stability letter.
2. Check tank status for unnecessary free surface.
3. Keep weight low and centered; manage upper-deck occupancy.
4. Brief passengers against rail rushes and unsafe topside standing.
5. Clear scuppers/freeing ports; bilges dry.
6. In icing conditions, reassess route and loading before topside ice accumulates.

Stability knowledge without obedience to the letter and COI is incomplete. The physics and the paperwork are the same job.