When Oliver Hartwich suggested my next research note, I thought he was joking. It was a single question: What is a frigate? I had spent four decades in uniform. It felt like asking a chef what a saucepan is.

But Germans do not joke. And the more I thought about it, the fairer the question became. Most New Zealanders, and perhaps a few of the politicians who will soon decide, could not answer it.

Time is short. Cabinet must decide by 2027 whether to replace our two frigates, HMNZS Te Kaha and HMNZS Te Mana, worn out by the mid-2030s. They must ground national security in a strategic vision, and decide sooner.

Replacing the frigates will be one of the largest purchases any New Zealand government has made, and the choice will shape our standing as a trading nation, an ally and a friend to smaller island nations who look to us in times of trouble. Yet the public debate has barely begun.

My answer is now published as Adrift. It took a whole note, because the honest answer is larger than the ship.

A frigate is a warship that can cross oceans, fight in high-end wars, hunt submarines, defend itself and others from air attack, and keep going through a Southern Ocean storm. A patrol boat can do none of those things.

Grey paint and a naval flag do not make a warship. The difference matters because the same ship that delivers fresh water and army engineers after a Pacific cyclone must sometimes sail into waters where somebody may shoot at it.

Numbers matter too. With two ships, one is usually in dock. When the other breaks down, our naval combat strength is zero. Two frigates are not a capability, however you sell it.

Timing may matter most of all. If the old ships retire before new ones arrive, the loss is not a pause. Crews disperse, and their skills and knowledge fade rapidly. The know-how to run a combat navy takes a generation to rebuild. Once that is gone, you do not get it back by writing another cheque.

None of this settles which ship to buy, how many, or at what cost to other priorities. Those are questions for an honest public argument, which should happen before the contract is signed, not after.

Before Cabinet decides, New Zealand should have an open debate about its future as an island trading nation in a more volatile, uncertain world.

To learn more about John's research, read the full research note, watch the webinar and read about it in our latest NZ Herald column.