if you have photoshop you can cheat. Make two duplicate layers of the picture. Cut out everything but the ships you want to blur. Leave the base layer alone. motion blur the ships in the second layer, selecting each ship individually so you can get the direction right, leave the top layer unblurred but set the transparency to about 75% so that a bit of the blur shows thru, but so does your texture. as well, this will extend the blur past the end of the object so that it looks like the blur is stretched out a bit. I find it effective, though it is a lot of work

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