Pirates are once again threatening merchant ships off the coast of Africa. But now, most of the attacks are coming not off the eastern coast near Somalia, but in the Atlantic along the west coast near Nigeria. Moreover, today’s new pirates’ weapons, tactics, and demands are far more formidable, dangerous and destabilizing.

Unlike their Somali brethren, who typically kidnap their victims and hold them for ransom, west coast pirates attack and attempt to seize ships as prizes. This past April, west-coast pirates killed one crew member and injured another during an attack on a German-owned tanker. Though the ship was being guarded by an armed security team, its force was outgunned and overpowered by the attacking pirates. As for any kind of naval protection, there was none.

So why is West African piracy up while East African piracy is down? Naval presence — or, in West Africa, the lack thereof. Today, the East African coast — where Captain Philips’s ship the Maersk Alabama was attacked — is patrolled by a task force led by the American Navy, working with ships from other nations including Denmark, Australia, Pakistan, South Korea, Turkey, and Britain. While there were more than 170 pirate attacks off Somalia in 2010, the number has fallen to a handful today. Presence matters.

Global maritime piracy is a crime of opportunity. Pirates ply their trade in the absence of navies. Although pirate activity is rampant and growing along many of the world’s major shipping routes, the United States Navy is rarely to be found. The reason is simple: we don’t have enough ships. This is not a question of multi-billion-dollar aircraft carriers; we simply have too few destroyers or frigates — much more modest classes of ships —capable of joining or leading international coalitions.

Current policy threatens to make a bad situation worse. Just as the Obama administration has reduced the size of the fleet by retiring ships, it has also slowed down the construction of new ones. Such a strategy threatens American security and international stability in three ways.

First, maintaining peace and stability around the world is much harder when a President — whoever he or she is — doesn’t have a credible, forward-based military presence to communicate our capabilities. Promises of pivots are less credible than the authority of being there. Leading economists of all stripes agree that world instability restricts economic growth and wellbeing. America is still the world’s leading global trader, and instability — pirates, conflicts, threats — has a disproportional impact on us. More than 85% of all goods in our stores arrive by the sea. If we want to maintain — much less improve and extend our standard of living to all our citizens — we need to take a disproportionately large role in maintaining the peace. That means fielding a bigger, more capable Navy.

Second, as a maritime nation, we are dependent upon ships. Yet we build almost no merchant ships domestically, and are building fewer and fewer Navy vessels. As a result, our shipbuilding industry, once the largest in the world, is now among the world’s smallest. China has the largest shipbuilding industry in the world with a 45% market share. South Korea is second with 29%; Japan third with 18%; and the European Union fourth with just 1%. America doesn’t even make the list. To allow our shipbuilding capability to atrophy is a strategic mistake.

Third, building ships — including Navy ships — is a remarkably cost-effective way to create good-paying jobs. The average pay of skilled, blue-collar workers engaged in ship-building is about $40-an-hour, plus benefits. For every $100,000 invested in shipbuilding, one of these able-to-support-a-family jobs is created or sustained. The Obama administration’s stimulus program, in contrast, generated jobs at $238,000 a job.

To be sure, building Navy ships isn’t always as efficient as it could be. The cost-overruns on the new Gerald Ford aircraft carrier are estimated to be about $2.4 billion over its original $10.5 billion cost. One reason for the unexpected increase are challenges with three completely new and experimental technologies that are part of this first-in-class ship, the first new aircraft carrier design in more than 40 years. When the Arleigh Burke-class destroyer was launched, the first vessel took 9 million man-hours to complete, the last but 5 million.

The second reason for cost-overruns is that shipyards lose skilled workers each time there is a lull in the building schedule, which is often the result of political budgeting tricks. Under the Obama administration’s stop-and-go shipbuilding policies, there is little predictability of continuous work. As a result, many skilled workers — welders are the best example — leave, often for the shale oilfields resulting in costly “rework.” Stability in construction orders is the most important variable in shipbuilding efficiency.

We need a larger Navy for peace and stability. As we approach the nation’s birthday, it is useful to remember that President John Adams pushed through a reluctant Congress funds to build our “Six Orginial Frigites,” to protect the free flow of commerce across the seas. The same is true today. ”Shovel-ready” special interest projects might serve short-term economic or political interests. But if we want to create long-term, well-paying jobs that promote our nation’s security and economic vitality, we should build more ships.

Mr. Cohen is an attorney at KDLM in New York, and a former Director of the United States Naval Institute. Rear Admiral McKnight (ret.) is the former commanding officer of Task Force 151 and is the author of “Pirate Alley.”