If I recall, the major building material for the original model was wood, so we can deduce that either
A) since the nacelles were wood, then the primary and secondary hulls are just differently shaped nacelles
B) since the primary hull is wood, then the nacelles and secondary hull are just differently shaped primary hulls.
Basing the function of the item just on it's material without taking in context and form clearly does not produce clear results.
Well, while that was cute... it's not accurate.
The main outer surface of the primary hull is made from plastic, not wood, though there's a wooden frame. The nacelle has a wooden front end but the aft portion is rolled sheetmetal. The secondary hull is mainly wood, though.
And, of course, the deflector dish is a fruit bowl. So the real question, I guess, is where the giant apples and bananas are on the ship?