Navigation General Study Guide
Tides, currents, compass math, weather signs, anchoring, celestial basics, and more. Everything in the 50-question Navigation General section of the OUPV exam.
Compass Math — TVMDC
Compass math is the most heavily tested topic in Navigation General. See the full TVMDC guide with worked examples on the Chart Plotting Exam Guide.
Tides
Spring vs. Neap Tides
Types of Tide Patterns
Rule of Twelfths — Predicting Tidal Height
In a 6-hour tidal cycle, the water level changes in these proportions:
The tide moves fastest in the middle hours (3rd and 4th) and slowest near high and low water.
Anchoring
Scope Ratios
Scope = total rode length ÷ (water depth + bow chock height above water). Calculate from the bow chock, not the deck.
| Anchor Type | Best Bottom | Weakness |
|---|---|---|
| Danforth (Fluke) | Sand, mud | Rock, heavy grass |
| CQR (Plow) | Sand, mud, clay | Loose sand, soft mud |
| Bruce (Claw) | Sand, mud, rock | Weed, loose sand |
| Fisherman (Admiralty) | Rock, coral, kelp | Sand, mud |
| Mushroom | Permanent moorings only | Everything |
Weather Signs
| Weather Sign | Meaning |
|---|---|
| Falling barometer (rapid drop) | Approaching storm or strong wind — the faster the drop, the more severe |
| Rising barometer | Improving weather, clearing conditions approaching |
| Steady barometer | Continued current weather pattern |
| Red sky at night | Fair weather tomorrow — high pressure moving in from the west |
| Red sky in morning | Storm approaching — moisture and clouds moving in |
| Cirrus clouds (high, wispy) | Warm front approaching 24–48 hours away |
| Cumulonimbus (thunderhead) | Imminent severe weather — lightning, waterspouts possible |
Restricted Visibility Navigation
Safe Speed in Fog (Rule 6)
Safe speed is whatever speed allows you to stop or maneuver to avoid collision. In restricted visibility, this means:
- •Reduce speed significantly — half speed or less is common
- •Sound fog signals: 1 prolonged blast ≤2 minutes (power underway)
- •Post an additional lookout
- •Maintain radar watch if equipped
- •Have engines ready for immediate maneuver
Fog Signals (Rule 35)
Celestial Navigation Basics
Celestial questions on the OUPV exam are typically conceptual, not computational. Know these key concepts:
Local Apparent Noon (LAN)
When the sun reaches its highest point (crosses the meridian). The sun bears due south in the Northern Hemisphere. Shadow is shortest. Used to determine latitude.
Latitude by Polaris
Polaris (North Star) altitude ≈ your latitude in the Northern Hemisphere. At the equator, Polaris is on the horizon (0°). At 45°N, Polaris is 45° above the horizon.
Noon Sight
Measuring sun altitude at LAN gives your latitude. Latitude = 90° − meridian altitude + declination (requires Nautical Almanac).
Sextant use
A sextant measures the angle between a celestial body and the horizon. The reading corrects for index error, dip, and refraction to give Ho (observed altitude).
Frequently Asked Questions
What topics are on the Navigation General section of the OUPV exam?
Navigation General covers: compass math (variation, deviation, TVMDC conversions), tides and tidal current, weather (fronts, cloud types, barometric pressure), anchoring (scope, anchor types, anchor lights), basic celestial navigation (noon sight, latitude by Polaris), radar use in restricted visibility, and navigational aids (buoys, lights, daymarks). The section is 50 questions and requires 70% to pass.
What is the difference between spring tides and neap tides?
Spring tides occur during new moon and full moon when the sun, moon, and Earth are aligned — producing the greatest tidal range (highest highs, lowest lows). Neap tides occur during quarter moons when the sun and moon are at right angles to Earth — producing the smallest tidal range (lower highs, higher lows). Spring tides are approximately 20% larger than average; neap tides approximately 20% smaller. Spring tides occur twice per lunar month, regardless of season.
What scope ratio should you use when anchoring?
The minimum scope for normal conditions is 5:1 — 5 feet of rode for every 1 foot of water depth (including freeboard). In severe conditions, use 7:1 or more. For overnight anchoring, 7:1 is standard. Scope is calculated as: total rode length ÷ (water depth + height of bow chock). More scope = better holding because it keeps the anchor flukes flat against the bottom rather than lifting them.
Practice Navigation General questions
Compass math, tides, weather, anchoring — tracked by topic so you know exactly where to focus. 1,628+ practice questions across all OUPV sections.
Start Free — No Account Needed