Anchor Scope

About Anchor Scope

Anchor Scope Calculator, safe anchoring made simple.

Dropping the right amount of chain (or chain-and-line) before you anchor is one of the most important decisions you'll make on board. Too little and your boat drags onto rocks, other vessels or the beach. Too much and you waste rode, restrict your swing room and struggle to weigh anchor cleanly. This calculator helps you get it right, in seconds, with no internet connection required.

WHAT IT DOES

Enter your boat length, water depth, seabed type and a few weather details. The app calculates the recommended chain length, the scope ratio, the margin of safety against dragging, and flags any compliance gaps against Spanish regulations (RD 339/2021) plus ABYC H-40 (US) and RYA Day Skipper (UK) recommendations.

KEY FEATURES

• Works fully offline: no network needed once installed, ideal for remote anchorages where signal is patchy or non-existent.
• Bilingual interface: English and Spanish, switchable with one tap.
• Two unit systems: metric (metres, kilograms) and imperial (feet, pounds). Internally everything is in SI; the display layer converts.
• Built-in regulatory checks: Spanish RD 339/2021 anchor and chain-diameter tables are validated automatically against your boat length. Recommendations (not laws) for US (ABYC H-40) and UK (RYA Day Skipper) are shown when applicable.
• Scope ratio warnings: under 3:1 is high risk, 3–5:1 is acceptable, 5–7:1 is good, over 7:1 is over-spec. The app tells you which band you're in.
• Bottom type compensation: mud and sand hold differently. Sand needs roughly 10 % more chain than rock for the same holding force; mud slightly less.
• Visual summary: chain value, effective depth, scope ratio and a one-line risk verdict in plain English (and Spanish).


WHO IT'S FOR

• Recreational skippers in the Mediterranean, the Caribbean and similar cruising grounds who want a quick second opinion before letting go the anchor.
• Yacht owners fitting out a new boat who want to confirm their ground tackle is up to the job.
• Sailing-school students revising for the RYA Day Skipper or PER (Patrón de Embarcaciones de Recreo) exam.
• Anyone who has ever been confused by the "5-to-1 rule" and wanted to know when it applies and when it doesn't.
• Clients of rental boats that want to ensure that they are using enough line.

THE ALGORITHM

The calculation follows the heuristic guidance in Robert E. Pritchard's "Sailing Tactics" and the Royal Yachting Association's Day Skipper handbook (chapters 8 and 12). The base scope ratio is adjusted for sustained wind, gusts, chop and seabed type. Effective depth is calculated from water depth plus half the tidal range plus bow height. Outputs include chain length, scope ratio, margin against the minimum critical scope and a risk band.

IMPORTANT NOTICE

This is an orientation tool, not a guarantee against dragging. Real conditions at sea vary in ways no formula can capture. Make a habit of observing your anchoring after you set it, set an anchor watch in busy or exposed conditions, and follow the relevant national regulations. Do not rely solely on the app in safety-critical decisions.

ABOUT

Anchor Scope Calculator is developed by Liberty Dreams LTD, a UK-based independent studio focused on practical utilities for recreational sailors and boat owners. We design tools that work offline, respect your privacy (no personal data is collected) and stay out of your way on deck.

SUPPORT

If you have feedback, a feature request or a question, please get in touch via the support link in the listing. We read every message.

Made with care for the boating community. Fair winds and a peaceful night at anchor.