I'm printing since a while with PRUSA MK3S and PRUSAMENT PETG, and I've checked several articles about WARPING, but I please need your help :-). 

Target is a model boat which will be 1,8 meters, so I need to print in sections (you can find a total view in the pictures). 
The bigger sections also need to be divided in Left and Right to fit in the printer. 

I'm facing a problem on the front parts, which have quite a "Deep V" at the keel.
When I printed first with an additional complete vertical surface (see print result at the right resp. front), it's straight in the middle. 
When I tried to do without this surface (see print result in the middle), it's starting to warp - with a bow, which of course is wrong. 
Next I tried with a vertical "helper" (see print result on the left resp. in behind) - but it got worse. Strong bow on the downside and to the left.

Fun part:
- The endpoints fit to the next segment! So it must be a problem during printing. 
- all other surfaces seem to be fully correct. 

Temperature should be perfect (as requested on the filament), and as far as I can remember this happens inside the cabinet and also outside. 

Who has an idea please? Can it be a mechanical problem, although all other surfaces seem to be correct? Shall I simply try again with the full vertical surface? 

Thank you in advance!  
Gerhard