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All new 737s use split-tip winglets, which are more efficient than the simpler blended winglets previously used.

However, for no clear reason, they use two different types of split-tip winglet:

• New 737 Next Generations (the 737-700/-800/-900)¹ use split-scimitar winglets, which have a main body shaped like a blended winglet, but with its upper tip hooked backwards, and also have an additional scimitar-shaped fin projecting outwards and backwards from the upbend in the winglet.

(Image by Mnts at Wikimedia Commons.)

• The 737 MAX series uses the eponymous MAX winglets, which have a simpler shape than the split-scimitar winglet, looking essentially like if someone decided to put a winglet on the bottom of the wingtip in addition to the one at the top.

(Image by Aka The Beav at Flickr, via Helmy oved at Wikimedia Commons, modified by Altair78 at Wikimedia Commons.)

I don’t get it - why go to the trouble of producing two slightly different styles of winglet alongside each other for two mostly-similar families of the same aircraft? Why not just go with the more efficient style of the two (be it the split-scimitar winglet or the MAX winglet), and use that on both the Next Generation and the MAX?

¹: The 737-600 also belongs to the Next Generation family, but it left production long before the Next Generations switched from the blended to the split-scimitar winglet.

Since a winglet is basically a sail generating thrust (lift with a modest forward-tilted vector) from the circulating flow around the tip, it's just adding another sail beside the first one to extract more of the available energy from the flow. Different engineering groups will do studies of different configurations and will say, "hey, if we do this, it'll be some little bit more efficient than if we do that, based on our particular analysis".

If you put different groups of engineers together to attack the problem, they are all going to come out with variations on what they think is the ideal configuration. And so you see seemingly endless permutations that are, really, mostly nibbling at the margins of the major benefit that was achieved when you put one there in the first place.

Plus there is probably a bit of "styling" going on as well. Just to be different.

The 737NG was originally introduced with with no winglets. A company called Aviation Partners worked with Boeing to develop the "blended" winglet, originally for the NG-based BBJ (Boeing Business Jet). Customers then had them installed after delivery for a while before Boeing worked their own version into the production line. There is a similar case now with the split scimitar, where Boeing is delivering the 737NG with the "blended" winglet and customers are having it replaced with the split scimitar from Aviation Partners Boeing sometime after delivery if they choose.

One reason for developing a new winglet on the MAX is that it helps differentiate it from the NG and has become part of its "brand."

There also may be a consideration for cost from certifying the new winglets. As the split scimitar and MAX designs are so similar overall, there may not be enough benefit of the MAX design to justify certifying it for the NG, where they needed to do extensive flight testing on the MAX anyway. The split scimitar is also somewhat a modified blended winglet, which may have made certification easier.