Good mainsail trim has always been an art. Increasingly it’s becoming a science too, with the ever-growing use of technology. These days the instruments on the back of the mast are just the start of it, with all kinds of live feedback and post-race analytics to help exploit those tiny margins of advantage.

For professional mainsail trimmer, Chris Hosking, you still need to relate everything back to ‘feel’, to that instinct that you learned in small boats. Hosking started out in the ultra-competitive single-handed junior class of Australia, the Sabot, and was twice national champion.

As a professional mainsail trimmer Hosking has worked on everything from an RC44 and TP52 (Provezza) to superyachts, not least the J-Class yacht Svea. Chris has won world titles in the Farr 40, Soto 40, TP52, Maxi 72 and Maxi classes, and the season championship in the RC44 and TP52

RC44 racing in Portsmouth. Photo: Pedro Martinez / RC44 Class

The early years of small boat sailing in Sabots equipped Hosking with a talent for being able to find the feel in any boat.

“Having grown up in dinghies where you don’t have any instruments or anything like that, as a kid you have a really good feel of what makes a boat go faster without ever really knowing why.

Those feelings, those instincts, still help me today, even when I’m surrounded by instruments and all kinds of data.”

For Chris, good mainsail trim all comes down to the right balance between the art and the science.

“The data is super important, but if you don’t have any natural feel, you should probably look for another job.”

Balance

Balance, balance, balance! That’s your number one goal when thinking about mainsail trim – to create the balance in the rig that gives the helmsman the exact amount of helm that he’s looking for in the rudder.

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There is a centre of effort in the sail plan and a centre of lateral resistance in the appendage package. The trimmers’ responsibility is to balance the boat so the helmsman has an easy job of steering the boat quickly.

Work in order

There can be an overwhelming number of controls for adjusting the mainsail, but don’t lose sight of the fact that the mainsheet and traveller are your two primary controls. Spend time working out the relationship between these two and the different roles they play.

Then you’ve got the vang, the Cunningham, the outhaul, and the runners or the backstay. That’s before you even get on to changing the rake or the mast bend.

A square-topped mainsail

On a puffy and shifty day you want your sail shape to be really forgiving to make it easy for the helmsman. You probably can’t move the traveller fast enough to respond to the pressure changes, so your primary control will be the mainsheet.

You’ll want the traveller higher up the track than normal and slightly softer sheet tension to create more twist in the leech. Whenever a gust comes on it’s effectively a velocity lift, so the apparent wind angle goes wider. Instantaneously the main is over-trimmed. The boat’s going to want to round up, so the fastest way to respond is through mainsail twist before anything else.

Gear changes through mainsail trim

We don’t tend to race in less than 6 knots, so let’s talk about that underpowered range between 6-12 knots of wind. You’re searching for power, which primarily comes from sail depth. Soften your outhaul, backstay, get your traveller above centreline, and look to stall your top telltales with mainsheet tension.

As wind speed increases, depth and power needs to decrease, so as soon as your crew are fully hiking, say around 12 knots of wind speed, you’re looking to start to get rid of some of that drag. Your traveller is going down and your mainsail is becoming a little bit more twisted and flatter.

As the breeze increases you’re going to want to start to depower even more, dropping the traveller down further, twisting the mainsail more, pulling harder on the Cunningham, flattening the sail as much as possible.

Looking forward

My eyes focus a lot on the headstay, looking at the angle on the horizon – you can feel the boat through your backside and you become pretty well attuned to heel angle that way. As soon as I see or feel a change, I’ll be adjusting mainsheet, traveller or both.

Keep looking forward to see what is coming

You also need to look up at the leech telltales, particularly in light to medium airs. They’re an excellent way of telling you how much drag is coming off the leech of the mainsail. If your top telltales are stalled, stuck in behind the mainsail, then you’ve got a lot of induced drag.

If your leech telltales are streaming 100% of the time, then you’re quite twisty and will be low and fast. They’re a simple piece of technology, but for me they’re an absolutely critical tool.

Listen

You need to be involved in the communication flow. A person on the rail should be communicating our performance relative to other boats around us, and I’m also watching any boat to leeward for the same intelligence.

That information tells me whether we need to be footing for speed or pointing for height. Another person on the rail will be calling wind speed, gusts, lulls or any bad waves coming along. And I’m also using my ears to listen to the changing wind. All your senses come into play when you’re trimming for max speed.

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