Do competitors in freestyle swimming have to use the overarm style, or (as the name implies) are they free to use any style they want?
I realise that in practice the overarm stroke will always be the fastest, but (hypothetically) if a swimmer found that he could swim faster using a butterfly action, or some other style, would he be free to do so?
FINA Swimming Rules
SW 5.1 Freestyle means that in an event so designated the swimmer may swim any style, except that in individual medley or medley relay events, freestyle means any style other than backstroke, breaststroke or butterfly.
The overarm stroke is called the “crawl”. Which is a bit ironic, as it’s the fastest stroke. It’s been modified and tweaked a bit over the years, as have a few of the swimming rules since I swam many years ago, but it still works about the same as always.
The Men’s Freestyle competitions I saw in this Olympics seemed to use a different stroke than the typical alternating overhead arm motion typical of the crawl. Used both arms over the head at the same time coupled with a dolphin kick. So the Crawl may no longer be the fastest stroke.
That’s the Australian Crawl, thank you.
I think you might have been watching the butterfly!
Swimmers only do the arms-together dolphin kick when they first dive in, and when coming out of a turn. They’re underwater and not taking breaths at this point. For the rest of the race, the Aussie crawl is still the fastest stroke.
Actually, I’d grown up hearing about both the Australian and American crawls, and since the Aussies and Yanks seem fairly closed matched these days, I didn’t want to give one preference over the other.
Made in Australia, improved in America. The stroke of champions everywhere. 
Nope, I watched the whole thing. I remember because it confused me. Two arms overhead (Not together overhead as in the underwater portion, but like two crawl arm strokes at the same time) and dolphin kick the whole way. And the event was called Freestyle.
OK. I figured it out now. Apparently I was drunk and the event was Called The Butterfly. I’m not sure how I was drunk, having not have any alcohol, but I would like something to blame my continued mistake on. So I am blaming beer.
Personally, I’d blame NBC. But you do what you want.
I don’t know about that. The swimming commentary on NBC this year has been excellent. They certainly would not have mistaken a 'fly event for freestyle.
The Australian Crawl, used as the dominant form of freestyle at the Olympics for the past 50 years or so, has legs held almost entirely straight, with the main movement coming at the hip. The bend at the knee is almost imperceptible. When your knees are bending more than a tiny bit, it slows you down quite a bit. I don’t of anyone who uses a flexible-kneed version, except beginner swimmers.
So I’m afraid Olympians still use the Aussie Crawl. Take a look at the underwater cam in the next freestyle event. Perhaps this American crawl is the dominant stroke for surfers?
I’d blame the chlorine fumes…
Duke swam in the 1920s. I can’t find a cite but given other data I did find, wouldn’t be surprised if they swam with entirely no flex in the legs. Here’s a link that gives very high level notes on the evolution of the crawl stroke. Some of the changes are seemingly minor but very important, like the number of kicks to stroke ratio. When I was learning to swim many years ago I was taught that the arms were the main difference, but can’t find any cites to support this. The link I’ve provided also infers a lot of changes to arm positioning, but doesn’t differentiate American versus Australian. Could have just been American jingoism in what I was taught.
Net, the crawl as originated in Australia (late 1800s) versus the crawl of Duke K versus the crawl of today, while essentially the same stroke, are appreciably dissimilar to the untrained eye (find an old video of Johnny Weismueller somewhere). I’m fine with just calling them all the Australian crawl.
When you get right down to it, having different strokes as different swimming events is kinda silly. Just put people in the pool and see who’s the fastest – if they all wind up doing the Australian Crawl, then so be it. Why would we want to see which athlete is the fastest while using a slow stroke?
The track equivalent would be to have the 100M-Hopping-On-One-Foot competetion, followed by the 200M Backwards.
With the exception of the medley events, that’s exactly what freestyle is.
As far as track & field equivalents are concerned, what about the hurdles or the Race Walk? The former involves who is the fastest while jumping over things, and the latter involves who is the fastest while keeping both feet on the ground at all times. As far as the hopping-on-one-foot event is concerned, that sounds a lot like the Triple Jump. 
Who needs that event when everyone can just compete in the long jump?
These track & field events are similar to the other three swim strokes–they all have specific rules that freestyle does not. If you’re curious, butterfly evolved from the breaststroke; under the rules at the time, butterfly was allowed in the breaststroke event. Since butterfly is faster than breaststroke, pretty soon everyone in the breaststroke event was swimming butterfly. Eventually butterfly was spun off into its own event, and breaststroke got more rules.
I’d say that hurdles and triple jump are good tests of different skills other than just running faster or jumping longer, so they have a reason to exist as events. But to take an athletic skill and had a handicap, and make that an event – doesn’t make sense. So you’re correct; race-walking is as artificial as the butterfly stroke.
Look at it from an evolutionary standpoint: the crawl beat out the breaststroke in the survival of the fittest. The breaststroke is being kept alive only as a museum piece.
New olympic sport: Speed Shuffle