Being a caddie requires the muscle of a pack mule and the grizzled canniness of a coach. Since I have neither talent, my day at the Sentosa Golf Club is not going so well.

"Where's my caddie?" bellows Mr Peter Downie, a good 50m ahead of me.

The 20kg golf bag slung around my right shoulder bounces as I jog to catch up with him. The dozen or so clubs in it rattle. I curse it under my breath.

Mr Downie, 48, the club's general manager, is trying to give me a taste of the real deal. I do not like it. The bag is making a dent in my right shoulder. I fear I will be listing permanently to one side, like an abandoned tugboat.

"You're lucky you aren't carrying a tournament bag," he says. I believe the Scotsman, known for his bone-dry humour, means it sincerely.

In the bag, there are clubs, balls, water for both of us and a towel to wipe mud and grass off the gear. A tournament-ready bag will also have rain gear, and spare shirts and shoes for the golfer.

The most important thing for a caddie to learn, he says, is this: "Don't speak unless you are spoken to."

There are more rules: Rake the sand in the bunker after the golfer. Remove the flag before the golfer putts. Wipe the balls and clubs as necessary. I may have to smooth over divots, holes left in the ground by the golfer's swing. I can put the bag down when we stop to tee off or putt, but must never put it down on the delicate grass around the cup, called the putting green.

Do not stand in the golfer's line of sight and never move or talk when the golfer is making a swing. As far as jobs go then, being a caddie seems to resemble punishment at a particularly sadistic military detention barrack.

Some of the golfers in our group of about five men, led by Mr Downie, take bets on whether I can last all 18 holes. These men march, not amble, from hole to hole - usually with me in pursuit, panting.

When I give up after two hours, at hole 9, the winner whoops with happiness. He has won $10. I am happy for him.

Later, speaking with Mr Downie, who turns out to be only half as intimidatingly grumpy as he first appears, I am told that a caddie for a professional golfer is a friend, one who can keep the player calm and focused when things go awry during a tournament.

Pro golfers, especially the younger ones, are high-strung. It is easy to see why tension is that high, when one bad shot can cost thousands of dollars in prize money.

"They become like brothers," says Mr Downie. And they can become very rich together.

Steve Williams, the New Zealander who has caddied for American golf great Tiger Woods for about a decade, is a philanthropist in his home country. Pro golfers offer a small percentage of the purse to their caddies, which is enough to make men like Williams wealthy.

The Barclays Singapore Open, for example, offers prize money totalling US$6 million (S$7.7 million). It is due to start next month on the same course we are playing on today and will attract some of the top players from around the world. Three-time US Masters champion Phil Mickelson will play.

The brutal session with Mr Downie is just a warm-up.

One week later, back at the course, I caddie for Mr Mitchell Slorach, 23, a pro golfer ranked among Singapore's top 10. The Singaporean, born to Chinese and Scottish parents, is among the 204 players taking part in the Open.

The wiry young man has taken pity on me and has brought a thin nylon bag, which is sleeker and about 10kg lighter than the bulky leather beast Mr Downie made me carry. Thankfully, it is one of the newfangled ones, with two backpack-style straps, so I do not feel as if I would tip over like an overladen crane when I hoist it.

Golf is a tradition-bound sport, so golf carts are not allowed in pro tournaments, making caddies a necessity, explains the genial Mr Slorach. Laser rangefinders, electronic devices that tell the golfer how far away the flag is, are also not allowed.

Caddies are needed, therefore, to not only carry bags, but also to help with reading "yardage books", which list course distances.

Mr Slorach has a regular caddie, Mr Ken Lye, a 30-something Singaporean who is an old friend and is himself a seasoned player. Mr Lye can read the lay of the land and recommend which club to use and where to put the ball.

"A good caddie is a strategist," says Mr Slorach.

My strategy today is to last nine holes without passing out. That goal is achieved and, two hours later, I get a polite thumbs-up from him, though I doubt he will be calling on my services any time soon.

A few days before, I speak to a caddie, a man in his 40s who has been working at various courses for 20 years. He is a for-hire caddie, unlike the caddies who work full-time for the pros.

He says he is "too shy" to be named, but says he charges between $50 and $70 for each 18-hole game, which can last up to four hours. A tip of between $30 and $50 is standard. When the golfer has had a great game and is feeling generous, a bonus of $100 is not uncommon.

And the most important thing a caddie must do?

"Don't talk. And don't move when the golfer is taking a swing," he says. A good caddie is like a mime, but with a sherpa's stoic endurance, he seems to be saying.

The deeply tanned veteran, who understands the psyche of the golfer better than the players themselves, says golfers having a bad game sometimes throw a tantrum, directed at the caddie.

Acting as whipping boy is just part of the job, he says, with the weariness of a butler working for a badly behaved lord of the manor.

And does the golfer, perhaps feeling guilty after making a scapegoat of the caddie, throw in a big tip after the game?

The old caddie does not pause. "No. Never," he says.