Not to mix metaphors, but when it appears the markets are headed for choppier waters - as was predicted by David Stevenson on these pages earlier in the week - then investors might want to seek the protection of a moat.

Just to reassure you, this isn't some kind of survivalist solution in ETF form. We're talking economic moats and an index investment proposition from VanEck which would appear to make a lot of sense.

Much chatter this year has focused on the long-in-the-tooth nature of the current bull run in the US. Much as investors love to celebrate the good times, though, the spectre of a bear market looms larger the longer the bull run. Or as Dimitri Klymenko, product manager at VanEck suggests, "valuations are tense". "Simply following the market means buying equities at valuation highs, which means that they are potentially overpriced," he points out. "This inevitably reduces the earnings potential."

Like a magician pulling a rabbit out of a hat, Klymenko and VanEck believe they have a solution. For the past three years they have a fund which has been pursuing a strategy of focusing solely on companies with first, a substantial and protectable competitive difference and second, they have what the Morningstar analysts believe is fair value characteristics.

Hence, we get the VanEck Vectors Morningstar US Wide Moat UCITS ETF which is the only fund available in Europe that follows the Morningstar Wide Moat Focus Index. The fund has been running for three years and has a decent track record, generating an annualised performance of round 17.5 percent, up to September.

For an example of a wide moat company VanEck points to beverage giant (and dentist's friend) Coca Cola. Such is the company's competitive advantage - in this case a secret recipe but in others it can be cost benefits, network effects or efficient scaling - that VanEck's system predicts its competitive advantage protects Coca Cola against competition for at least 20 years.

As Klymenko points out, the brand value of Coke is put at $73bn on the balance sheet, or roughly 40% of its current valuation.

Yet a moat is not the whole deal. To make such companies an attractive investment, Morningstar then carries out multi-tier analysis to provide a detailed analysis of future cash flows and thus a fair value.

This figure is then compared with the current valuation and if it there is a mismatch and on the low side - i.e. the current valuation is below what Morningstar consider fair value - then they have a potential portfolio entrant.

Ultimately, between 40 and 80 US stocks with a corresponding 'wide' moat rating and whose current stock price is the furthest below their long-term fair value make it into the Morningstar Wide Moat Focus Index. A rebalancing takes place every quarter.

Investors who have focused on this index with the VanEck Vectors Morningstar US Wide Moat UCITS ETF have in the past outperformed the broader market as represented by the S&P 500.

As Klymenko points out, this success is not a simple matter of course: after all, according to data provided by Standard & Poor's, over the past ten years around 90 percent of all actively-managed US large cap funds that track the S&P 500 as their benchmark have underperformed it.

Klymenko points out that given the current investment backdrop in the US of a market with high valuations, investors should consider whether they want to gear their portfolios to indices which consider and weight equities on the basis of market capitalisation. "As we see it, it is more advisable to choose models which work with a number of relevant criteria," he concludes.