How long should my gennaker sheets be, and what diameter should they have?
I need a new pair of sheets for my gennaker, but I'm unsure how much rope to buy and what diameter to choose. Is there a useful rule based on boat length, and do I need thicker sheets for cruising?
Answer
Start with two gennaker sheets, each about twice the boat's overall length, then check that the lazy sheet can pass freely around the sail during a gybe. For most cruising yachts of 30 to 40ft, start with 8 to 10mm rope and choose the final diameter to suit the sail size, rope construction, winches and blocks.
Check the sheet route
Establish how you gybe: between the gennaker's luff and the forestay, or outside the gennaker's luff. The lazy sheet must travel around the sail without coming tight before the new working sheet takes over.
For a conventional cruising installation with two separate sheets, start with these lengths:
- 8m boat: approximately 16m per sheet.
- 10m boat: approximately 20m per sheet.
- 12m boat: approximately 24m per sheet.
- 14m boat: approximately 28m per sheet.
These lengths are for each sheet, not the pair combined. A long bowsprit, an outside gybe or a long run from the stern turning block to the trimming position may require more. Allow up to about 2.5 times boat length if the route demands it. Twice the length will not fit every installation.
Run a spare line from the clew through the actual turning block to the winch, allowing for winch turns and enough tail to handle comfortably. Check the longest route the lazy sheet must take during a gybe. Add allowance for knots or splices before ordering finished sheets.
Choose diameter for handling
These are practical starting points for ordinary cruising monohulls, not a load specification:
- Up to about 27ft: 6 to 8mm.
- About 28 to 34ft: 8mm, or 10mm where easier handling is the priority.
- About 35 to 40ft: 8 to 10mm, depending on sail area and rope construction.
- About 41 to 46ft: commonly 10 to 12mm, with loads checked against the sail and hardware.
A powerful reaching sail, heavy displacement yacht or multihull can need a different specification from a lightly loaded cruising yacht of the same length. Check the rope maker's strength data and the sailmaker's recommendation. Diameter alone does not establish a suitable working load.
Check your self-tailing winch's stated rope range too. A thin sheet may be strong enough but slip in the self-tailer; an oversized sheet can run poorly through blocks. On many 10m cruising yachts, 8mm works well in light weather. A 10mm sheet is easier to grip but adds weight at the clew.