Historically, a sailing cutter was a small decked vessel with one mast and a bowsprit, commonly carrying a gaff mainsail and two headsails. Modern U.S. Coast Guard use of “cutter” is an institutional vessel designation and does not imply that sailing rig.
The Cutter Interactive Calculator
Enter total sail area, displacement, waterline length and a chosen headsail share. The calculator reports two conventional comparison ratios plus the corresponding area split; it does not predict speed, stability, loads or suitability.
Equation Used
- Sail area is entered in square metres, displacement as metric tonnes of mass and LWL in metres, then converted to the conventional imperial ratio forms.
- Headsail share is a user-selected arithmetic allocation; the calculator does not infer sail geometry or centers of effort.
- Hull form, righting moment, ballast, wind, sea state, appendages, sail shape, reefing, rig loads and crew limits are omitted.
What “Cutter” Means Here
The U.S. Coast Guard history office describes the traditional cutter as a small decked, single-masted vessel with a bowsprit, gaff mainsail and two jibs or a jib and staysail. The paired headsails and projecting bowsprit are identifying rig features, not a complete performance specification.
The Coast Guard also explains that its modern use of cutter is a service designation for vessels over 65 feet, regardless of rig. A modern Coast Guard cutter therefore need not resemble the historical sailing type shown in the diagram.
What the Calculator Illustrates
MIT's graduate sailing-yacht design materials use displacement, waterline length, sail area and nondimensional comparison parameters during preliminary design. This page performs only two conventional ratio calculations and a chosen sail-area split.
Those ratios allow comparisons between entered sail-area, displacement and waterline-length values. They cannot establish speed, pointing ability, seaworthiness, stability, reefing limits or structural capacity.
Conventional Comparison Ratios
D/L = Displong tons/(0.01 LWLft)3
The calculator converts the entered metric quantities before applying these conventional forms. H is an entered percentage: headsail area = SAH/100 and mainsail area = SA − headsail area.
Worked Arithmetic Example
For 106 m² of sail, 12.5 t displacement and 10.5 m LWL, the converted conventional ratios are SA/D ≈ 20.0 and D/L ≈ 301. With H = 50%, the entered allocation is 53.0 m² to headsails and 53.0 m² to mainsail.
This example illustrates the ratio calculation and the chosen sail-area split. It does not say that those values are appropriate for a particular vessel.
Model Boundary
- Calculated: unit conversions, conventional SA/D and D/L ratios, and an entered sail-area allocation.
- Illustrated: a simplified one-mast cutter profile with bowsprit, mainsail, jib and staysail.
- Not calculated: hull hydrostatics, resistance, speed, stability, righting moment, center of effort, sail forces, rig tension, scantlings, reefing, handling, weather limits, capacity or safety.
Frequently Asked Questions
Does every Coast Guard cutter use a cutter sailing rig?
No. The modern Coast Guard term is a service designation, not a rig description.
Does SA/D predict speed?
No. It is a comparison ratio and omits hull resistance, wind, sail efficiency and many other factors.
Does D/L prove seaworthiness?
No. It relates entered displacement and waterline length only.
Is the headsail percentage recommended?
No. It is a user-entered arithmetic split.
Does the diagram locate the true center of effort?
No. It is a qualitative rig illustration.
Can this size rigging or establish safe reefing limits?
No. Those require a vessel-specific naval-architecture and structural analysis.
References
- U.S. Coast Guard History Program, “What is a Cutter?”: historical rig description and modern service usage.
- MIT OpenCourseWare, Sailing Yacht Design 13.734: preliminary design, hydrostatics and nondimensional design comparisons.
- MIT 13.734 design report: examples of DLR and SA/D used as comparative design parameters.
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