* Mariners' bespoke pension plan dates to 17th century
* Sailors can retire earlier than most workers
* President says existing system is unfair
* Pension reform a central pillar of Macron reform agenda
* For an explanation of what's at stake:
By Caroline Pailliez
LA ROCHELLE, France, Nov 29 (Reuters) - Tugboat captain Jean-Yves Lagarde should be only 10 years from retiring thanks to special benefits for mariners that date back to the 17th century. But President Emmanuel Macron's ambition to simplify France's Byzantine pension system may scuttle his plan.
The weather-beaten 45-year-old spent a decade traversing the world's oceans on cargo ships, encountering pirates in Asia and working in stifling temperatures below deck before returning to the port town of La Rochelle to do battle with Atlantic storms.
A career toiling in punishing conditions merits the early retirement accorded to mariners, Lagarde said. Angry he could lose the perk, Lagarde will join a nationwide strike on Dec. 5 called by public sector unions to force Macron to back down.
"After all, we have a pension plan that has stood the test of time for four centuries," Lagarde said, steering his tug towards a choppy sea.
The showdown against unions will set the tone for the second half of Macron's mandate, with more reforms including an overhaul of unemployment benefits lined up, and comes as discontent swirls in hospitals, the police force and in schools.
Sailors are covered by France's earliest pension regime, drawn up in 1673 during the rule of Louis XIV to look after seamen disabled at sea.
They are entitled to retire on a full pension aged 52.5 - nearly a decade earlier than the typical worker - if they have worked for 37.5 years.
It is one of a dozen 'special regimes' with different retirement ages and benefits that cover among others rail workers, dancers at the Paris Opera and comedians. In all, the pension system comprises more than 40 different plans.
Macron says the system is unfair. He wants a single, points-based system under which for each euro contributed, every pensioner has equal rights.
It will not be easy. Nearly one in every two people oppose Macron's pension reform, an Elabe survey showed this month.
As the government finalises the draft bill, transport unions are plotting to bring major disruption to France's rail network, ports and airports on Dec. 5th and perhaps beyond. Teachers, emergency room doctors and truck drivers are joining in.
After last-ditch talks this week, Prime Minister Edouard Philippe promised unions there would be no brutal transition from the old to new regime. But the government would hold its course, he said.
The last time a French president squared off against unions over special pension regimes it ended badly. In 1995, strikes paralysed Paris and prime minister Alain Juppe pulled the reforms in a stinging defeat from which he failed to recover.
After Greece and Italy, France is the third biggest spender on pensions among developed nations, spending 14% of national output on pensions, OECD data shows. In comparison, Germany spends 10%, the United States 7% and Britain 6%.
Mariners are particularly burdensome: the state largely covers company contributions, in some cases fully, financing 78% of total contributions in a sector with 39,000 active workers and nearly three times as many retirees.
Furthermore, a sailor who spends part of his career on the sea before taking a land-based maritime job can still earn full benefits - a cherished idiosyncrasy at odds with Macron's ambition.
"No-one argues that a mariner's job is tough. The question is how do we gauge how tough?," said an adviser to Macron's pension reform tsar Jean-Paul Delevoye.
As the wind whipped across the Bay of Biscay, Lagarde argued his benefits were costly but deserved.
"At forty-five," he said, "I'm at the age where I am starting to feel a weariness that's down to my work."
(Reporting by Caroline Pailliez; Writing by Richard Lough; Editing by Mike Collett-White)