This calculator gives an area-weighted longitudinal centre and a signed CE-to-CLR coordinate offset. It does not predict helm angle, griping, handling, seaworthiness or a suitable sail plan.
Hermaphrodite Brig Interactive Calculator
Combine two entered area shares and longitudinal positions, then compare the weighted centre with an entered centre-of-lateral-resistance coordinate.
Equation Used
- Entered fore share and complementary main share weight two user-entered geometric positions.
- All positions use the same aft-from-bow percent-LWL coordinate.
- Entered CLR is not derived from hull geometry or hydrodynamics.
- No sail force, apparent wind, heel, trim, underwater force distribution, rudder, stability, structural or maneuvering model.
Coordinates are treated as percent LWL measured aft from the bow. Positive CLR−CE means the entered CE is forward of the entered CLR. The two additional arithmetic indices are unvalidated comparisons with no helm-angle or risk meaning.
Static area-weighted coordinate
If two entered area groups have longitudinal coordinates xfore and xmain and the fore share is f, their area-weighted coordinate is CE=f xfore+(1-f)xmain.
With positions measured aft from the bow, the signed forward offset of CE from an entered CLR coordinate is xCLR−CE. This is geometry only; actual aerodynamic and hydrodynamic force centres vary with conditions.
Use only as a preliminary coordinate comparison
The arithmetic can check a drawing's entered area centroid and coordinate convention. It cannot select a rig, predict handling or justify changes to sails, ballast, appendages, rudder or structure.
Weighted centre and signed offset
Combined CE is the area-weighted mean of the two entered positions. The signed offset is xCLR−CE because increasing coordinates run aft from the bow.
For offset L expressed as the numerical percentage of waterline length, the illustrative linear index is 0.5L−3. The band index is 100(10−L)/10 below L=10, zero between 10 and 14, and 100(L−14)/8 above 14, limited to 0–100. These arbitrary reference numbers do not establish a suitable lead range, helm angle or probability of a handling problem.
Transparent example
With equal shares at 33.3% and 66.7% LWL, the weighted coordinate is 50.0% LWL. If entered CLR is 55%, the signed CLR−CE offset is +5.0% LWL; if CLR is 38%, it is −12.0%.
Neither result predicts helm feel because the inputs are static geometric proxies, not operating force centres.
Scope and limitations
A defensible sail-balance study needs the actual sail plan and aerodynamic force distribution by apparent wind and trim, hull/keel/rudder geometry and hydrodynamic force centres by leeway and heel, plus loading, stability and maneuvering analysis. Sea-trial decisions require qualified naval-architecture and sailing judgment.
Frequently asked questions
What does the offset sign mean?
With positions measured aft from the bow, CE ahead of CLR has the smaller coordinate. A positive CLR−CE value means the entered CE is forward.
Does the linear index equal rudder angle?
No. Its formula has no validated physical mapping to helm angle; it is a dimensionless arithmetic comparison.
Is a particular offset range safe?
This calculator cannot establish that. A geometric coordinate comparison does not replace a complete vessel design assessment.
Do the drawn sail areas match the entered area shares?
No. The two sail symbols are fixed-size illustrations. Area share changes the weighted coordinate numerically; it does not resize the sails.
Historical reference and calculation basis
Royal Museums Greenwich’s 1832 Charybdis and related vessel plan identifies brigantine rig as also known as hermaphrodite rig. This is historical context; the illustration is not a scale plan of those vessels.
The useful geometric results here are an area-weighted coordinate and a signed difference, both using the same aft-from-bow convention. The additional linear and band indices have no established vessel-performance interpretation.
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