The containership industry is being transformed from a conservative business attracting little outside interest to one with a growing speculative element, an analysis of the rapidly expanding fleet shows.

Investors are being lured by the relatively high level of earnings. A medium-sized containership has produced far better returns over the past five years than a comparable-size bulk ship, according to Clarkson Research Studies Ltd.A "Handysize" ship with a carrying capacity of between 1,000 and 2,000 20-foot containers would have earned an average of $13,200 a day since 1990, Clarkson estimates in its weekly market report. A Handymax bulk carrier of about 40,000 deadweight tons, which would serve in similar trades to a medium- sized containership, has earned an average of $9,640 a day over the past five years.

Over 12 months, the containership would earn $4.8 million; the bulk carrier would earn $3.5 million.

These figures translate into a 37 percent differential for ships that cost roughly the same to build. The extra money that can be accrued from containerships is altering the business as independent investors move in.

Until recently, no containership would have been built unless the owner knew that the cargo was there to support the ship on a particular trade lane. But all that has changed, with the containership industry large enough and sufficiently lucrative to warrant its own charter market as well as its own sale-and-purchase market.

The strength of demand for certain categories of containership is confirmed by Howe Robinson Shipbrokers, which reported recently that charter rates for vessels with container capacity of 1,600 20-foot equivalent units (TEUs) soared 19 percent in the second quarter of 1995 compared with a year ago, the strongest advance of any sector.

Oil tankers and dry cargo ships traditionally have sought business, and been bought and sold, on the open market. Much of this business is transacted through the Baltic Exchange in London. In contrast, containerships usually have been built by the liner shipping company that planned to operate the vessel, with a particular trade in mind at the time of ordering.

However, nowadays many lines charter containerships rather than own them outright. The charter business is particularly brisk for "handy" ships of 1,000 to 2,000 TEU capacity. The number of fixtures for ships of this size is running at about 40 a month, according to Clarkson Research, a division of the London shipbroking firm H. Clarkson & Co.

Clarkson brokers also report an "unprecedented" level of inquiry for secondhand "handy" container vessels, with 41 sales completed so far this year. This is only slightly less than the number of sales of similar-sized bulkers.

With a large number of containerships on order, the world fleet will have passed 2,000 vessels by 1997, according to Clarkson. This compares with a global containership fleet of just 750 vessels in 1980.

The tanker fleet, at about 3,000 ships, and the bulker fleet, at some 5,000 units, are either static or growing much more slowly. With the number of containerships expected to continue growing at a rapid pace, the fleet soon will be catching up with the other two sectors, Clarkson predicted.

Demand for containerships of 4,000-TEU capacity is rising the fastest, with 36 currently on order. But it is the smaller 1,000- to 2,000-TEU ships that are capturing the attention of independent investors as they study earnings levels.