Sailor checking one instrument record
Align STW, signed TWA, windward axis and averaging interval.
Enter the record plus a same-frame independent VMG reference.
Sees signed scalar projection and difference without losing downwind/leeward meaning.
Sailing Windward VMG Record Check
Compare a signed true-wind-angle and through-water-speed record with an independent water-relative windward-VMG reference.
Built for Marine Navigation1 documented formulaFocused single-task workflowPrivate in your browser
Use this result well
A quick decision brief for this specific tool
Inputs that matter
Aligned calibrated boat speed through water, signed port/starboard true-wind angle over the same averaging interval and an independent water-relative windward-VMG reference
Output to expect
Signed calculated windward VMG and signed/absolute difference from the entered reference
How it works
Projects STW onto the windward axis; positive means windward and negative means leeward, so downwind records are not silently excluded
- Align true versus apparent wind, water versus ground speed, port/starboard sign, windward/leeward axis and averaging; compare only with a reference using the same conventions.
- Use vessel-specific polars/trials and account for sea state, current, leeway, trim, sails, fouling and steering; this is not an optimum-angle, sail, reef, route or race-tactic recommendation.
Choose your path
Built around the job you need to finish
Reconcile a signed true-wind-angle and through-water-speed record with an independent windward-VMG reference while preserving leeward negative output.
Coach comparing a polar or trial point
Avoid treating one cosine as an optimum sailing angle.
Use vessel-specific polar/trial evidence and compare like conditions.
Can identify record mismatch without a universal performance grade.
Skipper choosing sail, reef or route
Keep sea state, current, leeway, trim and warnings above arithmetic.
Use qualified sailing judgment and current official marine products externally.
Does not use the worksheet as a sail or route recommendation.
Authoritative checks for this tool
Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.
- US Sailing Speed Guide ExplanationUS Sailing · Official sailing guidance on VMG relative to windward/leeward marks, polar angles, sea-state variation, leeway and performance checks.
- The American Practical NavigatorNational Geospatial-Intelligence Agency · Official vector magnitude/direction and relative-motion foundations for a signed scalar projection.
- National Weather Service Marine ForecastsNOAA National Weather Service · Current official wind, wave and warning context that one onboard VMG record cannot replace.
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Reference & details
How it works
Updated August 2026
How it works
Updated August 2026Signed Windward Projection
Entered STW is multiplied by the cosine of signed true-wind angle. Positive output is windward; angles beyond plus or minus 90 degrees remain visible as negative leeward VMG.
Reference Frames Must Match
STW, true rather than apparent wind, port/starboard sign, windward axis and averaging interval must align with the entered reference VMG.
One Record, Not a Polar
The result checks one scalar projection only. Sea state, current, leeway, trim, sails, fouling and steering remain outside the arithmetic.
Updated: August 2026
Example Scenarios
Enter aligned STW and signed TWA, then compare the calculated windward projection with the independent record.
Compare only the same wind definition, axis, vessel condition and averaging method; retain negative leeward values.
Use vessel-specific polars, current forecasts and qualified sailing judgment for optimum angle, sail, reef and route decisions.
Common Mistakes to Avoid
Common Mistakes to Avoid
Mixing apparent wind with true-wind angle
This contract expects signed true-wind angle and STW over the same interval. Do not compare with an apparent-wind or ground-relative reference.
Dropping the sign on downwind VMG
Relative to the windward axis, leeward progress is negative. Preserve that sign instead of relabeling every result as toward wind.
FAQ
The axis is windward. A record moving leeward has a negative projection rather than positive progress toward wind.
Equal-magnitude port and starboard angles have the same cosine, but the signed angle preserves the original tack-side record.
No for this contract. It uses STW and a water-relative wind axis; SOG introduces current and changes the reference frame.
No. One entered record is not a polar or tactical model. Use vessel-specific data and actual conditions.
Use the US Sailing speed-guide, NGA vector reference and current NWS marine products linked in the authority panel.
About Sailing Windward VMG Record Check
This tool reconciles one water-relative windward-VMG record. It is not a vessel polar, optimum-angle finder, sail or reef recommendation, race tactic, forecast, route decision or performance guarantee.