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Dividend stocks can be the foundation of a great retirement portfolio. Dividend payments not only put money in your pocket, which can help hedge against any downward moves in the stock market, but they're usually a sign of a financially sound company. Dividends also give investors a painless opportunity to reinvest in a stock, thus boosting future payouts and compounding gains over time.

Yet not all income stocks live up to their full potential. By utilizing the payout ratio, or the percentage of profits a company returns in the form of a dividend to its shareholders, we can get a good read on whether a company has room to increase its dividend. Ideally, we like to see healthy payout ratios between 50% and 75%. Here are three income stocks with payout ratios currently below 50% that could potentially double their dividends.

CVS Health (CVS -0.62%)
This week, I figured we'd start off with a megacap stock that could very well boost its shareholder return by leaps and bounds over the coming five to 10 years: CVS Health.

There are two primary factors that have aided CVS's ascent and have the pharmacy and health products retailer growing its top line by roughly 10% per year. First, we have the Patient Protection and Affordable Care Act, which you'll know best as Obamacare. Since Obamacare became the law of the land, more than 23 million people who didn't have health insurance now have insurance. For CVS's pharmacy segment, this means more opportunity to fill higher-margin prescriptions.

Image source: CVS. 

The other factor here is the Walgreen Boots Alliance (WBA -0.06%) and Rite Aid (RAD 5.56%) merger. Some view the combination of the two as a potential problem from CVS since together, they'll have a larger chunk of pharmacy market share. However, I see this as a net benefit for CVS. Rite Aid has been a mess since its purchase of Eckerd, and asking Walgreen Boots Alliance to integrate Rite Aid's stagnant brand and $7.1 billion in net debt without any issues is, in my book, impossible. I believe the next couple of years could allow CVS to further separate itself from its peers.

It's probably also worth mentioning that CVS Health's dominant pharmacy market share and its efforts to push certain vice products out of its stores, such as tobacco products, make it appear even more attractive.

Currently paying out $1.70 on an annual basis, I believe it's within reason to expect CVS Health could raise its payout to $3.40 or more annually within the next decade. Wall Street is forecasting full-year EPS growth from $4.51 in fiscal 2014 to $7.43 by 2018, implying that CVS probably has dividend-raising potential in its future.

Marten Transport (MRTN -3.52%)
From a $107 billion company to one being valued just $550 million by Wall Street and investors, I'd suggest income seekers also consider taking a closer look at trucking company Marten Transport.

Image source: Marten Transport. 

Marten is one the larger providers of temperature-sensitive trucking in the U.S., meaning it's primarily a provider of food and other packaged consumer products. The reason Marten and much of the trucking sector have struggled can be tied back to the drop in oil prices. Sounds like a head-scratcher, right? You'd think lower oil prices would yield lower fuel prices, and thus lower expenses for trucking companies. Although this is true, it also lowers their ability to charge for fuel. Fuel surcharges for Marten were down nearly 50% in the fourth quarter from the prior-year period. These are high-margin charges that trucking companies are seeing reduced in a big way.

However, there are other things that could make Marten a force to reckon with over the long term (despite it being just a small-cap stock). For instance, historically low lending rates have allowed Marten to aggressively expand its truckload and tractor count for a relatively low cost. The company's tractor count grew by 340 in 2015 per its fourth-quarter report. As long as access to capital remains cheap, look for Marten to continue to boost capacity in order to retain or gain temperature-sensitive transport market share.

Another key point is that oil prices (at least in my opinion) are unlikely to stay under $35 per barrel over the long run. Even if oil prices don't return to $100 per barrel, they're likely to normalize at a substantially higher price than they are now once excess supply issues are worked out. For trucking companies, that means the likely return of healthy fuel surcharges, and thus juicier margins.

It also doesn't hurt that population growth also works in favor of food transportation companies. Food is generally inelastic during a recession. People need to eat, which means Marten's top and bottom lines are somewhat hedged against a dip in the U.S. economy.

Looking ahead, Marten Transport is forecast to produce $1.09 in full-year EPS in 2016, but it's only paying out $0.10 annually. With a minimal amount of debt on its balance sheet and the expectation that capacity growth should translate into top- and bottom-line expansion, I'd expect Marten Transport to double its dividend in the coming years.

Synovus Financial (SNV -0.03%)
Lastly, I'd encourage investors to take a deeper dive into Southeastern regional bank Synovus Financial, which operates 257 commercial and retail branches throughout five states.

Image source: via Flickr.

Most banks have taken a tumble in recent weeks, and Synovus has been no exception. The prospect of slower growth rates in the U.S., China's slowdown accelerating, and oil prices slumping are all giving investors a clue that banking growth could be challenged in the near term. Additionally, the Federal Reserve's plans to raise lending rates appears thwarted for now by stock market weakness, essentially cutting off banks' ability to quickly increase their net interest margins and interest-based income.

Nonetheless, Synovus' drop has been much tamer than many of its regional peers. To begin with, Synovus isn't a name synonymous with big energy loans, thus the credit quality of its portfolio is in very good shape. Total non-performing loans in Q4 dropped 25% year over year to $215.4 million, and total delinquencies fell to 0.21%, down three basis points from Q4 2014. Net charge-offs also dipped by nearly 50% from the sequential third quarter. Synovus' exemplary credit quality is one reason it should have no problem considering a dividend raise.

The other aspect of Synovus that makes it attractive has been its focus on technology advances. Synovus' mobile app, which was one of dozens reviewed by Consumer Reports, drew strong reviews from a customer service and banking aspect, while faring no worse (or better) than its peers when it came to customer complaints. A focus on new ATM technology and other banking conveniences should allow Synovus to achieve that personal touch needed by smaller banks to take customers away from national banks.

Currently paying out just $0.48 annually, Synovus' bottom line is projected to see rapid growth in the coming years. Full-year EPS is expected to cross $2 by 2017 and $2.50 by 2019, meaning its dividend, in my opinion, could double by the end of the decade.