Marine Mystery

Marine Mystery For anyone who's ever looked at a ship and wondered how the hell it stays afloat. Built by a maritime nerd, for maritime nerds.

We break down marine engineering, structures, and maritime mysteries — the real stuff, explained simply.

The depth on the chart is not the depth under the keel.Chart datum (most Admiralty tidal waters ≈ LAT) is the level soun...
09/13/2026

The depth on the chart is not the depth under the keel.
Chart datum (most Admiralty tidal waters ≈ LAT) is the level soundings are reduced to.
Present depth ≈ charted sounding + height of tide (± residual / surge).
Drying height is height above chart datum.
UKC = present depth − draught − squat and allowances. Never treat the printed sounding as UKC.
Lights, land heights and bridge clearances use a high-water datum (often MHWS or HAT) — not chart datum. Read the chart title / NP5011.
Study aid only. Use the official chart, tide tables and the datum printed on that chart.
— Marine Mystery

09/12/2026

A ship doesn’t stay put because the anchor “hooks” the seabed.
The chain does the real work.
Hundreds of metres of rode. ~160 kg per link. 240+ tonnes in the system.
It sags into a catenary, lies on the bottom, and keeps the pull horizontal — so wind and swell get absorbed before they rip the flukes out.

Overtaking, head-on, crossing — the three situations that decide who keeps out of the way.Rule 13 — OvertakingComing up ...
09/12/2026

Overtaking, head-on, crossing — the three situations that decide who keeps out of the way.
Rule 13 — Overtaking
Coming up from more than 22.5° abaft the beam. The overtaking vessel keeps out of the way. If in doubt, you are overtaking. A later change of bearing does not make it a crossing.
Rule 14 — Head-on
Two power-driven vessels on reciprocal or nearly reciprocal courses. Both alter to starboard and pass port to port. If in doubt, treat it as head-on.
Rule 15 — Crossing
The vessel that has the other on her own starboard side keeps out of the way, and should avoid crossing ahead.
Give-way: Rule 16. Stand-on: Rule 17.
These steering and sailing rules apply when vessels are in sight of one another.
Study aid only. Always check the official COLREGS.
— Marine Mystery

From a harbor tugboat to a 400-meter container ship—every vessel has a unique role at sea. 🌊⚓But which ship size surpris...
09/12/2026

From a harbor tugboat to a 400-meter container ship—every vessel has a unique role at sea. 🌊⚓
But which ship size surprised you the most?
Follow Marine Mystery for fascinating facts about ships, oceans, and maritime technology.

09/11/2026

You have the con.
Target on your starboard quarter. Bearing 118°.
CPA 0.4 nm. TCPA 6 min. Relative vector almost parallel.
Overtaking or crossing — A, B, C or D?

If she’s pointing at you, flip the picture.Your left = their starboard = greenYour right = their port = redYou are looki...
09/11/2026

If she’s pointing at you, flip the picture.
Your left = their starboard = green
Your right = their port = red
You are looking at their face, not standing on their bridge.
Save this for night watch.
Marine Mystery

09/10/2026

Nearly 300 years ago, John Hadley changed how sailors found their way.
The sextant has no batteries, no satellites, no screens — just mirrors, a 60° arc, and a double reflection of light. Move the arm 20°, and the image jumps 40°. Line the sun up with the horizon, read the height, check the day’s tables, and you have your latitude.
Optics and ingenuity. That was enough to cross an ocean.

Most of the collision rules are the same. The exam traps are not.This poster is the split at the COLREGS demarcation lin...
09/10/2026

Most of the collision rules are the same. The exam traps are not.
This poster is the split at the COLREGS demarcation line (33 CFR 80):
Seaward = International. Landward, Western Rivers, Great Lakes = US Inland.
Constrained by draught (three reds / cylinder) is International only. Inland Rule 28 is reserved.
Special flashing light: yellow, 50–70 flashes a minute, forward on the tow. Inland only.
Towing astern is the same both sides (yellow over white).
Pushing or towing alongside is where people fail: International keeps a white stern. Inland puts two yellows on the tug and a flashing yellow on the barge.
A rigid composite unit is one ship — Rule 23 lights only.
Whistle signals too: International = what I AM doing. Inland = what I PROPOSE, and the other vessel must agree.
Study aid. Check 33 CFR 83 and the official COLREGS.

Night-time collisions are often a lighting mistake, not a steering mistake.This poster is the 8 lights that actually mat...
09/10/2026

Night-time collisions are often a lighting mistake, not a steering mistake.
This poster is the 8 lights that actually matter:
Masthead — white, 225°
Sidelights — port red, starboard green, 112.5°
Sternlight — white, 135°
Towing light — yellow, above the sternlight when towing astern
All-round — 360°
NUC — two red, no masthead
RAM — red–white–red
Constrained by draught — three red (optional) plus normal steaming lights
If the other ship is pointing at you, flip the picture in your head: their starboard is on your left.
Save this. Send it to everyone still saying “red over red means fishing.”
Marine Mystery.

09/09/2026

A ship does not stay put by chance.
Watch each mooring line appear from the ship to the quay — and what it is actually for:
Head line — pulls the bow in, limits forward surge
Forward breast — holds her onto the wall
Forward spring — the handbrake against going ahead
Aft spring — stops her dropping back
Aft breast — pins the after body to the quay
Stern line — tucks the stern in, limits aft surge
Breasts hug the wall. Springs stop the slide. Head and stern pin the ends.
Made fast.

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