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Under very dark skies, with plenty of aperture, swirls are visible, as well as curved bright stretches, but it's not a defined spiral even then.

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Mr. Hrundi,

Depends on what you consider plenty of aperture. In my 8" f/6 under gray skies, the spiral structure of M51 is all there with a vague hint of the bridge to companion, NGC 5195 at just 49x. Pending favorable sky conditions, the galaxy takes on high power well. It pops real nice if I push the mag. to just over 100x..or even better at 122x. Finding that nice sweet spot is key.

Viewing M51 in the 14" dob is really nice...
Viewing M51 in the 18" dob is extremely nice...
Viewing M51 in the 25" dob is out of this world.

Clear Skies to All!

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Plenty of aperture is entirely subjective, I agree. And M51 is incredibly nice from a dark site with a whole variety of apertures.
My point, that I was somewhat trying to make, was this though:
It took Lord Rosse pointing a 72 inch telescope at the galaxy for it to be recognized as a spiral. And even then the sketch that I've seen is quite a bit different from someone observing it today would sketch.
And, in my albeit limited observing experience, I've also noticed that while the detail that constructs a feature can be seen, it also takes plenty of study, as well as knowing what you're looking at to make this detail pop into that feature.

At least this is my experience and opinion. Milage tends to vary with different combinations of eyes, brains, skies and apertures.