Baby boomers' confidence in a good retirement is plummeting, according to a survey released Monday.

The Insured Retirement Institute survey found that 27 percent of baby boomers are confident they will have enough money to last through their retirement, down from 33 percent a year ago and 37 percent in 2011. Only 6 in 10 boomers report having any retirement savings, down from roughly 8 in 10 in previous surveys.

"Many boomers are probably realizing, 'Yikes, it's up to me,'" Cathy Weatherford, the institute's president and chief executive, said in a conference call announcing the survey results. "They're probably asking themselves, 'How can I make this money last?' Unfortunately, for many of us I think the answer is 'I'm not so sure.'"

Some are already in financial stress ahead of retirement. Almost a quarter of all boomers said they had had difficulty paying their mortgage or rent in the past 12 months, and a growing number are putting off retirement. Some 36 percent said they plan to retire at 70 or later, up from 19 percent in 2011. (Tweet This)

The role of 401(k)s

Boomers are also reporting less overall economic satisfaction. Despite two years of a strong stock market, overall economic satisfaction among boomers now stands at 48 percent, down sharply from 65 percent in 2014 and 76 percent in 2011.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Boomers' declining confidence is coming at a time when they are having to rely more on defined contribution plans, such as 401(k)s, to provide retirement income instead of defined benefit plans, such as traditional pensions.

"Most in the workforce now are those that have been using the 401(k) instead of the defined benefit plan," Weatherford said. Those people also believe that Social Security will not provide the same underpinning for them as it has for their parents, and low interest rates of the past several years have added further savings challenges, she said.

"Even if you use a target date fund, the growth in those probably wasn't what you anticipated," she said, and growth was even smaller for those who were spooked by market volatility and kept their money on the sidelines. "There is just little-to-no growth."

More from CNBC:

In a separate study, the Employee Benefit Research Institute found that while 56.7 percent early boomer households are on track to be ready for retirement, those who are not ready have average shortfalls of $71,299 per individual in a family, $93,576 for single men and $104,821 for single women.

Too little financial know-how

Part of investors' problem with retirement savings is that they generally have insufficient financial knowledge to manage that challenge. A survey by Voya Financial found that workers scored an average of 4.1 out of 10 on their retirement knowledge, planning and saving, and retirees scored an average of 5.5.

"Most people do not plan for retirement. They have never tried to figure out how much they will need to save," said Annamaria Lusardi, academic director of the Global Financial Literacy Excellence Center. "Only 46 percent say they have ever tried to figure out" what they will need in retirement, she said in a conference call.

Those nearing retirement, the late boomers, have a greater awareness, she said. But "if it is hard to support 30 years of retirement with a 40 year career, if you start at age 50, of course it is going to be really, really hard."