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I think the single dashes (hyphens) should be placed between the numbers and units in this sentence:
...in a cylinder of 3-inch radius and 24-inch length.
Is that correct and should hyphens be added to this second sentence?
...making a trench nearly 15 feet wide and more than 30 feet long.
I believe this is a style issue rather than a question of grammar. That is, different publishers may have different preferences for when to use a hyphen.
In the style I learned (for academic technical writing), a hyphen is used when combining words to make an attributive. For example, "a fifteen-foot boat" or "a two-year assignment". A hyphen is not used when using a measurement as a noun, as in "the boat was fifteen feet long" or "the assignment lasted two years".
You can see that this style matches your examples.
