Downwind Technique in Chop — RS21 and Oh! (Olson 30)
Conditions
- Boats: RS21 (sportboat) and Oh! Olson 30 (ULDB) — see boat-specific sections below
- Point of sail: Downwind
- Wind: 8–15 knots TWS
- Wave state: 1–2 ft chop from the aft quarter (waves travelling roughly in the same direction as the boat)
- Sails: RS21 — asymmetric spinnaker only. Oh! — symmetric or asymmetric spinnaker available.
The Physics — Waves from Aft Quarter
This is the critical difference from upwind sailing in chop. The physics are reversed.
Downwind: Waves travel in roughly the same direction as the boat. As a wave crest arrives from astern, it lifts the stern first, then the hull slides forward and down the wave face. The wave CAN push the boat — accelerating it. This is where surfing and wave riding happen.
When the wave lifts the stern and the boat accelerates:
- Boat speed increases → apparent wind shifts forward (hardens)
- Correct response: trim spinnaker in and steer up slightly to keep the apparent wind angle consistent
- This maintains drive, keeps the kite full, and prolongs the surf
When the wave passes and the boat decelerates:
- Boat speed drops → apparent wind shifts aft (softens)
- Correct response: ease spinnaker sheet, steer to maintain drive — do not go so deep the kite collapses
This is the opposite of upwind. Downwind, active wave riding is appropriate and productive. Upwind, the wave was a braking event — downwind, it is a driving event.
The Wave Speed Relationship
In 8–15 knots, 1–2’ chop from aft quarter has a wave period of roughly 1–3 seconds, giving wave speeds of approximately 4–9 knots. A downwind boat doing 5–8 knots is in the same speed range — the conditions for wave riding are present. At the lower end (8–10 knots), surfing is marginal. At the upper end (13–15 knots), active planing/surfing is achievable on the RS21.
Three Downwind Modes in 8–15 Knots
The J/70 (primary RS21 analogue) has well-documented mode transitions. These apply directly to the RS21, and by extension inform the Olson 30 approach.
Mode 1: Displacement VMG (8–10 knots, between puffs)
Sail as low as possible without losing speed. Waves may push briefly but the boat is not surfing consistently.
- Helm: Deep angle, maximize VMG
- Kite: Sheet eased until luff curls, then trim slowly back. Ease tack line to rotate sail to windward and project area.
- Main: Traveler down, vang eased (let leech twist), backstay off
- Jib: Furled — adds no downwind drive and disrupts kite airflow
- Heel: Flat or slight windward heel — helps kite project to windward and stay full
- Crew weight: As far forward as possible. Maximises waterline length, minimises transom drag.
“Displacement mode… the boat should be flat or a couple degrees heeled to windward. Body weight should be as far forward as possible.” — North Sails J/70 Tuning Guide
Mode 2: Lazy Planing (10–15 knots, marginal surfing)
The boat can plane in puffs but not consistently. This is the primary mode for the RS21 in 8–15 knots with chop.
- Helm: Head up slightly to build apparent wind. Drive down once speed builds. Seek the surf.
- Kite: Active trimming — trim in as boat accelerates, ease as apparent wind moves aft. Sheet trimmer reports pressure constantly.
- Main: Vang on to keep leech firm once planing. Traveler down.
- Heel: 10–15° for the J/70 / RS21. Flat when planing.
- Crew weight: Forward to catch waves, then move aft quickly when the bow starts to bury, then forward again for the next wave. Very active.
“Crew extremely active with fore and aft weight. Keep weight forward until boat starts to surf down waves and bow wants to crash into next wave, then move weight back quickly, turn bow up and move weight forward again to catch the next wave.” — North Sails J/70 Five Downwind Modes
“We try to stay high initially to keep planing, surfing down a wave once the apparent wind goes forward.” — North Sails, J/70 Speed Guide (Tim Healy)
Mode 3: Full Plane / Active Surfing (13–15 knots, strong puffs)
The RS21 is consistently on a plane. Waves are actively ridden.
- Helm: Ride the surf — drive down the wave face aggressively. Head up to maintain apparent wind as boat accelerates.
- Kite: Trim in hard as boat accelerates down wave, ease fast when wave passes
- Main: Vang on firmly, main acts as trim tab
- Heel: Flat to slight windward heel — never heel to leeward while planing
- Crew weight: AFT — keep bow knuckle out of water, rudder deep for steering control
“Full Plane mode… Crew weight needs to be all the way to weather and aft so the bow knuckle is constantly out of the water.” — North Sails J/70 Tuning Guide
Steering Technique in Downwind Chop
The Basic Cycle
| Phase | What’s happening | Helm action |
|---|---|---|
| Wave approaching from astern | Boat in trough, slower, apparent wind aft | Hold angle or head up slightly — build apparent wind |
| Wave lifts stern — boat accelerating | Speed rising, apparent wind moving forward | Steer down to ride the face, keep kite powered |
| On the wave face — maximum speed | Bow could bury | Helm up to keep bow clear, steer to maintain surf |
| Wave passes — boat decelerating | Apparent wind moves aft | Ease sheet, hold angle until next wave |
Key Steering Principles
Head up to build speed, drive down to take VMG. This is the fundamental downwind rhythm in chop. The temptation is to sail deep and slow — it feels like VMG. But a slow deep angle in chop collapses the kite and kills both speed and VMG.
“You should always feel the breeze blowing across the boat — not over the stern. When you lose apparent wind flow across the boat, head up, rebuild speed and apparent wind, and slide down again.” — North Sails, Trim an Asymmetric Spinnaker on a Sport Boat
“Steer around waves and surf whenever possible. Finding a good rhythm with the trimmer and talking about the angles is the best way to identify the path of least resistance.” — North Sails, J/70 Heavy Air Tips
The mode indicator — use boat speed as the primary guide:
“The proper mode is indicated by a barely sustainable boatspeed. If speed is too easy to maintain, you are sailing too high. If speed drops off quickly or continuously, you are sailing too low. The goal is to sail as close as possible to the ‘cliff’ without falling.” — Sailing World, Building Blocks of Asymmetric Spin Trim
Minimise rudder use. Use heel and spinnaker trim to steer where possible. Heel to windward → boat turns down. Heel to leeward → boat turns up. Rudder creates drag and in chop loads the helm — a heavily loaded helm can lead to a broach.
“The goal of sail trim and crew weight coordination downwind is to achieve as neutral of a helm as possible… Body weight should be moved in and out and forward and back relative to the boat’s heel so that the rudder is used as little as possible.” — Quantum Sails, J/70 Tuning Guide
Asymmetric Spinnaker Trim in Chop
Core Principle
The asymmetric spinnaker must be trimmed continuously as apparent wind angle changes with boat speed. In chop, these changes happen every few seconds.
As boat accelerates (wave pushes from astern):
- Apparent wind moves forward → trim sheet in to match
As boat decelerates (off the wave):
- Apparent wind moves aft → ease sheet or kite stalls
The most common mistake is overtrimming:
“Ease, ease, and ease. Ease to a curl, pause and the curl disappears. Ease again. Carry a curl, and keep easing. Ease some more. Usually the sail stalls from being over trimmed.” — North Sails, Trim an Asymmetric Spinnaker on a Sport Boat
Sheet — Active Trimming
The trimmer drives the boat’s mode through communication with the helm:
“A lot of spinnaker pressure means the boat can sail lower; light pressure means the boat can come up a bit. Trimmer should constantly communicate pressure: ‘I’m losing pressure on the sail, let’s try a little higher’ or ‘I have a lot of pressure, let’s fall off a little.’” — Quantum Sails, J/70 Tuning Guide
In chop with waves from the side, the trimmer also acts as stability control — ease sheet when a wave rolls the boat to prevent sudden helm loading:
“Hold on to the luff a bit more than normal, because every time you trim the kite sheet in or out, it’s loading or unloading the rudder. So maintain a slight bit of oversheeting to keep the rudder load constant until the boat is successfully up on the plane.” — Yachting World, 5 Expert Tips: Asymmetric Spinnaker Handling
Tackline — Controls Sail Shape and Projection
- Tighter tackline: stabilises the luff, moves draft forward, good for sets, jibes, and very choppy conditions when the sail is unstable
- Eased tackline: allows sail to rotate to windward, projects better area, adds power — use in steady-state downwind sailing
“Tack-up soak mode… Letting a few inches off the tack line allows the spin to get deeper and rotate to weather more to maintain a lower course heading. If spin gets unstable, pull tack down and head up slightly.” — North Sails J/70 Five Downwind Modes
Mainsail — Trim Downwind in Chop
| Control | Setting | Notes |
|---|---|---|
| Traveller | All the way down | Always |
| Vang | On in planing/surfing mode | Keeps leech firm, increases power; ease for displacement sailing |
| Vang | On for gybes | Always — essential to pop battens |
| Backstay | Off | Keeps mast forward, luff of kite rounder and deeper. Pull on briefly in gusty chop to stabilise rig. |
| Main sheet | Play actively | Use as a trim tab — quick ease to depower if broach threatens |
“Backstay remains at 75% to 100% on [for active planing]. This will keep the mast back, the luff of the spin tight and the draft of the spin forward.” — North Sails J/70 Five Downwind Modes
Crew Weight Downwind in Chop
Fore-Aft
The correct fore-aft position changes with mode and wave state.
| Condition | Position | Why |
|---|---|---|
| 8–10 knots, displacement | Forward — as far as class rules allow | Maximises waterline, minimises transom drag. Bow knuckle in water. |
| 10–13 knots, seeking surf | Forward to get onto the wave, then quickly aft as bow starts to bury, then forward again | Dynamic cycle — catch wave forward, control descent aft |
| 13–15 knots, planing | Aft — keep bow knuckle out of water | Controls bow, keeps rudder deep for steering authority |
| In a puff (downwind) | Move aft | Prevents bow from burying as boat accelerates |
| Puff easing | Move forward | Catches next wave face, maintains speed |
“Weight back in the puffs and back forward in the lulls.” — RS21 Official Tuning Guide, RS Sailing
“With four on the boat downwind in chop, you do need to get the crew weight as far aft as possible.” — Test Sailing the RS 21, 48° North
“When the breeze picks up to 12 or 13-knots, the J70 starts surfing. The crew needs to stay forward to get on the wave and then move weight back enough to avoid crashing into the next wave.” — North Sails, J/70 Speed Guide (Tim Healy)
Athwartships
- Heel to windward — improves kite projection (kite lifts away from main), reduces leeward helm, reduces broach risk
- Pack crew to windward in displacement and lazy planing modes
- When planing hard: flat — excessive windward heel in strong breeze can cause a windward broach
“On asymmetric boats, heeling the boat to windward also helps the spinnaker separate from the main because it projects the luff to windward of the boat. This is why you see the top boats in an asymmetric fleet sailing downwind with the whole crew packed to weather.” — Sailing World, 5 Tips for Better Asymmetric Speed
Vertical
Keep weight low — same principle as upwind. High crew weight amplifies rolling in chop. Crew should sit or crouch, not stand, when not needed on deck.
VMG Angles in Chop vs Flat Water
Chop generally pushes the optimum downwind VMG angle higher (away from dead downwind). Three reasons:
- Sailing higher maintains boat speed — speed loss in a chop trough is very costly to VMG
- Higher angle maintains apparent wind — kite stays full and driving
- Sailing deep in chop collapses the kite as the boat slows — catastrophic VMG loss
“The RS21 is very comfortable sailing very high [downwind]. Looking to accelerate constantly ends up being counterproductive until its well into the 20 kt range. Picking puffs to get more speed and work down looks to be the way to go.” — RS21 Official Tuning Guide, RS Sailing
For the Olson 30 at 8–15 knots, the polar targets are:
| TWS | Best Run TWA (sym polar base) | Asym adjustment |
|---|---|---|
| 8 kts | 143–147° | Sail 10–15° hotter — approx 128–132° |
| 10 kts | 149–151° | Sail 10–15° hotter — approx 134–136° |
| 12–15 kts | 157–170° | Sail 10–15° hotter — approx 142–155° |
In chop: expect to sail 5–10° higher than these targets to maintain boatspeed.
Broach Prevention and Recovery
A broach occurs when the bow digs in or a wave loads the helm suddenly, the boat rounds up, and the spinnaker fills the boat to leeward. In chop, wave rolls can trigger this unexpectedly.
Prevention
- Keep the boat flat or slightly windward-heeled. Excessive leeward heel is the primary broach trigger — it creates weather helm that the helm can’t overcome.
- Ease sheet before a broach develops. If heel is building and the helm is loading, ease kite immediately. Ease main too.
- Speed before gybes. A slow, loaded-up jibe is the most common broach scenario. Speed gives control.
- Vang awareness. In gusty chop at the upper end of this wind range, ease vang if the boat is overpowered — it allows the main to twist and spill.
“If the boat gets overpowered and wants to broach, the vang must be eased!” — North Sails J/70 Tuning Guide
“As the breeze and boatspeed continue to trend up and waves start to come over the bow, crew weight must shift aft to keep the rudder deep in the water and maintain grip through turns. Too much helm load or too sharp of a turn could cause the rudder to cavitate and the boat to wipe out.” — Sailing World, Building Blocks of Asymmetric Spin Trim
Recovery
If the boat does broach:
- Ease vang immediately (allows main to twist and spill)
- Drop spinnaker halyard 8 feet quickly — this takes the driving force out of the kite
- Bear off once the boat is back under control
- Rehoist and resume
“If you do wipe out, blow the vang and quickly let the spinnaker halyard down about 8 feet. That should get the boat back upright so you can bear off, pull the halyard back up, and go for it again!” — North Sails, J/70 Speed Guide (Tim Healy)
Gybing in Chop
Key Principle: Speed is Your Friend
“The worst thing you can do is bear off and slow down and load the boat up right before a jibe. That is when you wipe out.” — North Sails, J/70 Heavy Air Tips
A gybe attempted at slow speed with a loaded helm in chop is high-risk. Build speed and approach the gybe at the top of your groove.
Timing
- Time the gybe for a flat moment between waves where possible — avoid gybing on the face of a wave as the boat accelerates suddenly
- If waves are regular, establish the pattern and pick the trough
Vang
- Vang on for every gybe — this is essential to pop the main battens through
RS21 Specific
- Late boom jibe — allow the main to come across late and under control
“Late boom jibes are the way to go.” — RS21 Official Tuning Guide, RS Sailing
Crew Movement
- During the gybe: crew stays low, central, and together — minimise the pitching moment during the turn
- After the gybe: immediately re-establish the new mode (weight placement, heel angle, trim)
Boat-Specific Application
RS21 (Sportboat)
| Aspect | RS21 in 8–15 knots, 1–2’ chop |
|---|---|
| Mode at 8–10 kts | Displacement — weight forward, deep angle, wave riding marginal |
| Mode at 10–15 kts | Lazy planing — active wave riding, very dynamic crew movement |
| Spinnaker | Asymmetric only |
| Jib | Down in medium air — improves kite angles and makes gybes easier |
| Apparent wind shifts | Very large and rapid — trimmer must anticipate aggressively |
| Crew weight | Most active boat on this list — forward/back cycle every wave in surfing mode |
| Heel | Windward in displacement, flat in planing |
| Don’t chase the plane | In 8–12 knots: pick puffs and waves, don’t grind trying to stay on a plane at all times |
“Don’t chase the plane if you don’t have to. Many weight placement adjustments as the speed increases and decreases.” — RS21 Official Tuning Guide, RS Sailing
Oh! (ULDB Olson 30)
| Aspect | Oh! in 8–15 knots, 1–2’ chop |
|---|---|
| Mode | Always displacement — no planing. But ULDB can quarter-surf (speed gain of 0.5–1 knot on a good wave). |
| Spinnaker choice | See below |
| Jib | Can stay up in displacement mode for apparent wind management |
| Apparent wind shifts | Present but smaller magnitude than RS21 due to mass and hull speed limits |
| Crew weight | Less frantic than RS21. Forward in 8–10 knots. Slightly aft in 12–15 when wave energy increases. Remove bow-heavy gear. |
| Heel | Slight windward heel aids kite projection. Flat in puffs. |
| Wave surfing | Active steering to catch quarter-waves worthwhile — bear down on the face, head up to carry speed |
Sym vs Asym on Oh! in Chop
In 8–15 knots with chop: prefer the asymmetric.
- Asym requires sailing at a higher TWA (10–15° hotter) — this maintains apparent wind and boat speed through chop better than going deep
- Deep dead-downwind angles in chop cause speed loss as the kite collapses and apparent wind disappears
- Sym: appropriate at the lighter end (6–10 knots) on very deep angles when polar targets call for near-DDW sailing, or in flatter water where the depth can be held cleanly
As a practical guide: if you are struggling to keep the kite full on a deep angle in chop, switch to asym or sail higher on the sym — the speed gain from the higher angle more than compensates for the extra distance.
Quick Reference — Both Boats
| Item | 8–10 knots | 10–13 knots | 13–15 knots |
|---|---|---|---|
| RS21 mode | Displacement | Lazy planing | Planing/surfing |
| Oh! mode | Displacement | Displacement (surfing on waves) | Displacement (active surfing) |
| Crew weight fore-aft | Forward | Forward + very active: forward to catch, aft to control | Aft (RS21); slightly aft (Oh!) |
| Heel | Windward | Windward → flat as speed builds | Flat |
| Kite sheet | Eased, ease to curl | Very active trimming | Active — trim fast when surfing |
| Vang | Off (twist) | On when planing | On firm |
| Backstay | Off | Off (pull on briefly in gusts) | Off |
| Jib (RS21) | Down | Down | Down |
Related Notes
260523 RS21 Upwind Wave Steering.md— wave physics and upwind technique (note the important contrast: downwind waves HELP the boat; upwind waves HINDER it)260523 Crew Weight and Fore-Aft Trim in Chop.md— upwind crew weight; downwind principles are a continuation of the same framework with reversed priorities
Sources
- RS21 Official Tuning Guide, RS Sailing
- Test Sailing the RS 21, 48° North (Joe Cline)
- North Sails J/70 Speed — Five Downwind Modes, North Sails
- North Sails J/70 Speed Guide (Tim Healy), North Sails
- North Sails J/70 Downwind Tips — Marginal Planing Conditions, North Sails
- North Sails J/70 Tuning Guide — Spinnaker, North Sails
- North Sails J/70 Heavy Air Tips (Allan Terhune), North Sails
- North Sails — Trim an Asymmetric Spinnaker on a Sport Boat, North Sails
- Sailing World — Building Blocks of Asymmetric Spin Trim, Sailing World
- Sailing World — 5 Tips for Better Asymmetric Speed, Sailing World
- Quantum Sails J/70 Tuning Guide 2023, Quantum Sails
- Yachting World — 5 Expert Tips: Asymmetric Spinnaker Handling (Cape 31 tips), Yachting World
- Olson 30 polar targets:
04 Boats/Olson 30/Racing Knowledge/Tacking VMG.mdand Tactician profile