RS21 Rig Tuning by Condition
J/70 included as a supplement throughout — see note on analogue relationship at the bottom.
RS21 Mast Setup (One-Time Shore-Side)
From the SF YC RS21 Fleet Rigging Guide (04 Boats/RS21/Information/RS21 Rigging guide.pdf).
Tools Required
- 50’+ steel tape measure
- Spinlock Rig Sense gauge (RS21 guide uses this gauge — NOT the PT-2 Loos used on J/70)
- Spinnaker halyard led aft as backstay guide
Mast Centre
- Hoist jib halyard to masthead, measure to gunwale marks port and starboard
- Adjust cap shrouds until measurement is equal on both sides
- Sight up the back of the mast track — it must be straight before any tension is added
Mast Rake
- Hoist a tape measure on the main halyard to the masthead
- Target: 7100 mm from masthead to the top back edge of the transom above the rudder gudgeon
- Apply backstay when measuring — the rake is set under backstay tension
Headstay Base Tension
- With backstay slack: 120 on Spinlock Rig Sense gauge
Base Shroud Setting
- Cap shrouds: 275 on Rig Sense gauge
- Lower shrouds: 155 on Rig Sense gauge
- Base is defined as 7–11 knots
- After setting: add Velcro shroud straps between shroud and lower bottle screws; tape all split pins
RS21 Wind-Range Tuning Tables
Two sources — both are valid but use different measurement systems and slightly different wind ranges. Read both together.
Source 1 — SF YC Fleet Rigging Guide (Spinlock Rig Sense gauge)
Base: 7–11 knots
| Wind | Cap turns | Cap tension | Lowers turns | Lowers tension | Jib car | Tack height | Vang |
|---|---|---|---|---|---|---|---|
| 0–6 kts | -2 | 200 | -1 | 120 | 1 | 2.5” | snug to max ease |
| 7–11 kts (base) | 0 | 275 | 0 | 155 | 1 | 2” | snug to max ease |
| 12–15 kts | +4 | 400 | 0 | 155 | 2 | 1” | snug + |
| 16–20 kts | +6 | 475 | +1 | 212.5 | 3 | 1” | tight |
Source 2 — RS Sailing Official Guide (Jake Sorosky / West Coast Sailing)
Source: RS21 Tuning Guide, RS Sailing
Base: 8–10 knots. Tension columns partially populated — focus on turns from base.
| Breeze | HS* | Cap turns | Cap tension | Lowers turns | Lowers tension | Rake (mm) | Jib car | Tack height | Vang |
|---|---|---|---|---|---|---|---|---|---|
| 0–5 kts | 0 | -1.5 | — | -1.5 | — | — | 1 | 2.5” | snug to max ease |
| 5–8 kts | 0 | -1 | — | -1.5 | — | — | 1 | 2.5” | snug to max ease |
| 8–10 kts (base) | 0 | 0 | 19 | 0 | 12 | 9550 | 1 | 2” | snug to max ease |
| 10–14 kts | 2 | +1 | — | 0 | — | — | 2 | 1.5” | snug |
| 14–18 kts | 4 | +2 | — | +1 | — | — | 2 | 1” | snug + |
| 18–22 kts | 6 | +3 | 27 | +1.5 | 18 | — | 3 | bottom | tight |
| 22+ kts | 7 | +3.5 | — | +1.5 | — | — | 3 | bottom | tight |
*HS = Headstay adjuster turns (forestay).
Notes on reconciling the two tables:
- The SF YC guide is coarser (4 ranges) but includes actual tension values from the Rig Sense gauge — useful for absolute settings
- The RS Sailing guide is more granular (7 ranges), includes headstay adjustment, and is more useful for on-the-water fine-tuning
- Both sources agree on: jib car 1 in light, 2 in medium, 3 in heavy; tack height 2.5” in light dropping to bottom/1” in heavy; vang from ease to tight as breeze builds
- The cap shroud tightening is more aggressive in the RS Sailing guide — more turns added earlier
RS21 Sail Trim by Condition
Source: RS21 Tuning Guide, RS Sailing (Jake Sorosky). This guide is primarily qualitative — the tuning table handles the numbers.
Light Air (0–8 kts)
- High emphasis on twist and keeping power in the rig
- Backstay is almost a full-time job — it is the primary power-management control
- Jib sheet tension relative to main is the key to maintaining speed and height
- Weight forward critical; slight leeward heel upwind seems fast
- Downwind: jib down is key for good manoeuvres; late boom jibes
Medium Air (8–14 kts)
- Playing the backstay to manage power through wind variations
- Driver trims main; trimmers play backstay — this division of labour is key on the RS21
- Weight placement becomes critical — moving bodies in and out makes a measurable difference
- The mast is highly responsive — small changes have immediate effect
Med-Heavy Air (14–18 kts)
- Vang sheeting becomes more important
- Use vang and cunningham to set main shape; backstay to power the rig
- Playing these settings is worth “the average tenths of a knot of boat speed”
Breeze On (18+ kts)
- Vang sheet hard — this is the primary depower tool upwind
- Ease jib slightly for puffs
- Cunningham and vang adjustments are the key controls
- Backstay still used but vang/main more dominant than in light air
Backstay — The Key Control
The RS21 mast is uniquely responsive to backstay adjustment.
“The backstay is a key component in making the boat go fast; in light to moderate breeze it is almost a fulltime job.” — RS21 Official Tuning Guide, RS Sailing
- Backstay bends the mast → flattens the main and tightens the headstay
- More backstay = less power in both sails; useful in puffs and medium-heavy air
- Less backstay = more power; essential in light air and lulls
- In light air: backstay nearly fully eased most of the time, adjusted for each puff
- In heavy air: backstay on firm; vang takes over as primary leech control
J/70 Mast Setup (Supplement — Primary RS21 Analogue)
The J/70 is comparable to the RS21 but uses a different gauge (PT-2 Loos). Both boats have carbon rigs, asymmetric spinnakers, narrow fin keels.
North Sails Base Settings
Source: North Sails J/70 Tuning Guide
- Upper shrouds base: 19 (PT-2 Loos gauge)
- Lower shrouds base: 9 (PT-2) — many fast boats run 6–10 on lowers at base
- Pre-bend: 2”–2.5” (56–63.5 mm) at spreader height — check with main halyard held to bottom of mast track
- Mast rake: 56.5” (1435.1 mm) — measured from jib halyard mark to forestay turnbuckle pin
- Pre-bend adjustment: if too much pre-bend, tighten lowers; if too little, ease lowers half a turn
“The base tension for the upper shrouds is 19 and the lower shrouds is 9 on the PT-2 gauge. While adding tension to get to the base setting, occasionally sight up the sail track on the aft side of the mast to ensure the length of the spar is straight.” — North Sails J/70 Tuning Guide
Quantum Sails Base Settings
Source: Quantum Sails J/70 Tuning Guide 2023
- Upper shrouds base: 22 (PT-2)
- Lower shrouds base: 14 (PT-2)
- Forestay length: 58 inches
- Mast rake: 56.5” (1435.1 mm) — same as North Sails
Note on base tension differences (19/9 vs 22/14): North Sails and Quantum both use PT-2 but quote different base tensions. Different production years, different mast batches, and different testing conditions all contribute to these variations. The key is to set your own reference point for your specific mast and track changes relative to that.
J/70 Wind-Range Tuning Table (Quantum 2023 — Most Comprehensive)
Source: Quantum Sails J/70 Tuning Guide 2023
Base = 10–12 TWS (Upper 22 / Lower 14 on PT-2). All turns relative to base. Jib lead = holes showing in front of car.
| TWS | UP turns | LO turns | UP (PT-2) | LO (PT-2) | Jib lead | Weather sheet | Traveler | Vang | Backstay | Jib furl |
|---|---|---|---|---|---|---|---|---|---|---|
| 0–4 kts | -7 | -4 | 9 | (3” slop) | 8 | Play | Max up | Off | 25% | In |
| 5–6 kts | -5 | -3 | 15 | 6 | 8 | Play | Max up | Off | 25% | In |
| 6–8 kts | -3 | -2 | 18 | 8 | 8 | Cabin top | Max up | Slack out | 25% | In |
| 8–10 kts | -2 | -1 | 20 | 12 | 9 | Cabin top | Up 8” | Just firm | 25% | In |
| 10–12 kts (base) | 0 | 0 | 22 | 14 | 9 | Cabin top | Up 6” | 25% | 25% | In |
| 12–14 kts | +2 | +1 | 24 | 16 | 10 | — | Up 6” | 50% | 50% | In |
| 14–18 kts | +3 | +2 | 26 | 20 | 11 | — | Up 4” | 75% | 75% | In/Out |
| 18–20 kts | +5 | +3 | 28 | 24 | 11 | — | Center | 80% | 80% | Out |
| 20–23 kts | +7 | +4 | 29 | 26 | 11 | — | Center | 100% | 90% | Out |
| 24–26 kts | +7 | +5 | 29 | 28 | 11 | — | Center | 100% | 100% | Out |
| 26+ kts | +8 | +6 | 30 | 29 | 11 | — | Center | 100% | 100% | Out |
J/70 Key Tuning Principles (North Sails / Quantum) — Applicable to RS21
Headstay Sag — Light Air
Source: North Sails J/70 Light Air Headstay Sag & Jib Trim (Tim Healy)
- Target 2–4 inches of headstay sag at the jib mid-stripe in light air
- Sag adds depth to the jib and improves pointing — the sagging headstay pushes the jib luff to windward, improving angle of attack
- Achieve sag by loosening shrouds in light air (uppers 1–2 turns off base)
- Light air base (0–6 kts): uppers ~14 (PT-2), lowers very loose (barely register on gauge), ½” mast side sag
- Light air + chop: still want some sag but pull on backstay briefly to stabilise the rig when chop makes it pump; release backstay when chop clears
“Rig tensions that are loose enough to create 4” of sag are good for light air and flat water.” — North Sails, J/70 Light Air Headstay Sag & Jib Trim (Tim Healy)
Cunningham — The Secret Weapon in Transitions
Source: North Sails J/70 Tips to the Podium (Zeke Horowitz)
In a wind range straddling the rig tension threshold (e.g. 8–16 knots in the same race), set rig tension for the lulls, then use the cunningham to survive the puffs:
- Puff arrives → pull on backstay hard to depower and tighten headstay
- Backstay may invert the main (overbend wrinkles) → whale on the cunningham
- Cunningham pulls the draft of the main forward, un-inverting the sail, restoring shape
- Can now pull even more backstay → headstay tightens → boat balanced
“More cunningham = ability to pull more backstay = more controlled headstay = FAST!” — North Sails, J/70 Tips to the Podium (Zeke Horowitz)
This principle applies directly to the RS21 which has the same rig character (carbon, highly responsive).
Outhaul
- Light air / chop: ease outhaul to add depth and power
- Heavy air / flat water: tighten outhaul to flatten and depower
- In transitions: outhaul changes often get neglected — treat it like the cunningham (active management)
Jib Lead Position — Condition Adjustments
J/70 (applicable to RS21 jib car settings):
- Light air: lead forward, more depth in foot
- Chop (any wind): lead slightly forward, foot should contact (or nearly contact) cabin house to seal the slot
- Heavy air: lead back slightly to depower the leech; ease inhaul/weather sheet to open leech
“In light air or chop, you want more power in the bottom of the jib which means more depth in the foot.” — North Sails, J/70 Tips to the Podium
RS21 vs J/70 — Key Rig Differences
Both are one-design sportboats with carbon rigs, narrow fin keels, and highly backstay-responsive masts. Tuning principles transfer well.
| Factor | RS21 | J/70 |
|---|---|---|
| LOA | 6.5m | 6.93m |
| Displacement | ~480 kg | ~680 kg |
| Rig gauge | Spinlock Rig Sense | PT-2 Loos |
| Spinnaker | Asymmetric only | Asymmetric (+ sym optional) |
| Hiking | No formal hiking (side deck) | Hiking with straps |
| Jib | Non-overlapping, 2:1 sheets | Non-overlapping, weather inhaul system |
| Backstay responsiveness | Very high (RS Sailing: “almost a fulltime job” in light air) | High — same principle |
Gauge incompatibility: RS21 guides use Spinlock Rig Sense; J/70 guides use PT-2 Loos. These gauges are not directly comparable. Do not transpose numbers between RS21 and J/70 tables. Use each boat’s own table with its own gauge.
Existing Vault Resources
| File | Location | What it contains |
|---|---|---|
RS21 Rigging guide.pdf | 04 Boats/RS21/Information/ | SF YC RS21 fleet guide — mast setup procedure, tuning table (Spinlock gauge), physical rigging instructions |
Rigging Guide from San Fransico YC.md | 04 Boats/RS21/Information/ | Links to the PDF above and the SF YC RS21 fleet page |
Vakaros Atlas 2 User Guide.pdf | 04 Boats/RS21/Information/ | Atlas 2 device guide — relevant for setting up the wind/speed sensors used to measure conditions for applying this tuning guide |
Sources
- RS21 Official Tuning Guide (Jake Sorosky / West Coast Sailing), RS Sailing
- SF YC RS21 Fleet Rigging Guide:
04 Boats/RS21/Information/RS21 Rigging guide.pdf— original PDF in vault - North Sails J/70 Tuning Guide, North Sails
- Quantum Sails J/70 Tuning Guide 2023, Quantum Sails
- North Sails J/70 Light Air Headstay Sag & Jib Trim (Tim Healy), North Sails
- North Sails J/70 Tips to the Podium (Zeke Horowitz), North Sails