This rudder is made of cloth ...

[GoldfishLord]

I agree that the cloth and the light wooden frame are both materials, but I think that the cloth stretched the light wooden frame is the rudder itself and the final product and is not a material anymore
How is the cloth stretched the light wooden frame a materal?

 

[emsr2d2]

I agree that the cloth and the light wooden frame are both materials, but I think that the cloth stretched over the light wooden frame is the rudder itself and is the final product, and is not a material anymore.

Note my corrections above.

How is the cloth stretched over the light wooden frame a material?

Who said it was?

 

[GoldfishLord]

This rudder is made of cloth stretched over a light wooden frame.

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It would seem to me that this sentence describes structure/composition and therefore that "made up of" is a better choice of verb.
What do I misunderstand?

 

[emsr2d2]

This rudder is made of cloth stretched over a light wooden frame.

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It would seem to me that this sentence describes structure/composition and therefore that "made up of" is a better choice of verb.
What do I misunderstand?

You misunderstand the simple fact that we use "made of" in the way that you seem to want to use "made up of".

I'm going to have one final try to explain it:
Think of "made of" as "constructed from" and "made up of" as "comprising" or "consisting of".

I honestly can't make it any clearer than I and several other people already have. I am going to respectfully suggest that you move on. This thread has run its course.