Trim — learn to trim sails, not to guess

Three things a picture of a boat can't teach you

Most sailing apps animate a boat. This one solves it — apparent wind, lift and drag per sail, forestay sag, heel, leeway, rudder drag and added resistance in waves, recomputed around sixty times a second.

01 — Target

A target speed that is computed

The target isn't a lookup table. It re-solves the same physics with optimal trim for your exact wind, angle and sea state. So the percentage measures your trimming alone — not the boat, and not the breeze.

02 — Coach

Advice ranked by what it's worth

Every hint carries the drive it expects to add. You learn which controls matter where: upwind in breeze the backstay dominates, at 120° it's the vang and the pole. That ranking is the lesson.

03 — Shape

See the shape you're making

Cutaway sections at head, middle and foot show camber, draft position and twist as you pull the string. Colour tells you which part of the sail is off the groove — the top can stall while the bottom is perfect.

Read the sail, not the dial

Telltales are measured against optimal trim for the angle you're sailing, so they mean something downwind too — where every sail is technically stalled and an absolute reading would tell you nothing. Magenta is under-trimmed, amber over-trimmed, teal is in the groove.

Colour	What it means	What to do
Magenta	Windward telltale lifting	Sheet in, or bear away
Amber	Leeward telltale stalling	Ease, or head up
Teal	Both streaming	Hold it

Sail cutaway sections showing an over-eased headsail luffing at all three heights

Sections · headsail over-eased · 5.81 kn

Five boats that behave differently

Not one hull with different numbers painted on it. Monohulls get their stability from ballast, catamarans from beam — a different equation, which is why the cat carries 117 m² at three degrees of heel and warns you about righting moment instead of heel angle.

Boat	Disp	Beam	SA
40 ft cruiser-racer	8.6 t	4.0 m	94 m²
35 ft sportboat	3.8 t	3.4 m	74 m²
48 ft bluewater cruiser	18 t	4.5 m	132 m²
42 ft cruising catamaran	12.5 t	7.2 m	117 m²
45 ft performance catamaran	9.0 t	7.6 m	140 m²

3D view of a 42 ft cruising catamaran reaching

42 ft cruising catamaran · 18 kn TWS · 72° TWA · 9.10 kn

Weather you can read

The sea state is a wave field, not a texture: three components fanned around the wind direction with sharpened crests, plus short chop that gives the water its texture and shows the cat's-paws. Crests break where the wind is strong enough to break them, and the whole scene — sky, sea, cloud cover — greys down as it builds.

What you'll notice	Why it's there
The boat pitches and heaves	Attitude is sampled from the wave under the hull, bow to stern
Whitecaps appear around 12 kn	Breaking is driven by wind, not by the swell height
Sails look like cloth when right	Correct trim reads as dacron; errors bloom magenta or amber
Bow wave grows with speed	Kelvin wedge scaled off boat speed, with transom wash astern

A 40 ft cruiser-racer beating in 28 knots and a 2.8 m sea, whitecaps breaking downwind

28 kn TWS · 2.8 m sea · one reef · 6.31 kn

Everything below is free right now

Every feature listed here is open to everyone today — no account, no card, no trial clock. A paid tier will arrive eventually; when it does, the free trainer stays free and you will hear about it here first.

Feature	Included
40 ft cruiser-racer	✓
Cockpit and plan views	✓
Sheets, traveller, vang, outhaul, car	✓
Live target speed and % of target	✓
Coach hints ranked by gain	✓
English / Estonian	✓
3D rig view with orbit and camera presets	✓
Sail cutaway sections	✓
Five boats, including two catamarans	✓
Backstay, runners, cunningham, halyards, furling, pole	✓
Gusts, shifts, sea state, current	✓
Polar diagram and ideal-trim marks	✓
New boats and features as they land	✓

Open the trainer No account. Runs in the browser.

Crew and sailing-school use is fine and welcome — get in touch if you want it for a classroom.

Questions

Is this a game?

No. There is no course to race and nothing to win. It's a trim bench: set a wind and an angle, pull strings, and watch what the boat does. The scoring is a percentage of the speed the same boat would make with optimal trim in the same conditions.

How accurate is it?

The monohull polars land within roughly five per cent of published polars for boats of that size across most angles. The catamarans are close at reaching angles and a little optimistic dead downwind. It's calibrated well enough to teach trim relationships; it is not a routing or performance-prediction tool, and you should not plan a passage with it.

Will it make me faster on the water?

It will make you quicker at knowing which string to pull and why, which is most of it. It cannot teach you what a gust looks like coming across the water, and it will not replace hours on the helm.

Does it work on a phone?

Yes. The view stays pinned at the top while you scroll to the controls, so you can see the sail change as you drag a slider. The 3D view orbits with one finger.

What's the catch?

None. There is no account to make, no card to enter and no trial clock running. A paid tier will exist one day, and it will be announced here well before anything changes — but right now the whole thing is simply open.