Why is autostrut affecting performance?

[Klapaucius]

I just discovered autostrut, so I decided to go back and tweak my hydrofoil.  I had originally held the whole thing together with the standard struts.

Counterintuitivly, it performs less well. In the first image, you can see the original. I can simply throttle up with no SAS and the craft will get on plane with no more effort from me and it will do 24 mps.

In the second photo, is the exact same craft except I have used autorstrut on all the parts and removed the original struts. It barely gets to 22 mps and will not get on plane without me actively controlling the trim. It also wants to bury its bow.

Kerbal engineer tells me the autostrut variant is just under 1000 kg lighter. The original is 87767, and the autostrut version is 86867.  So thinking the balance was off, I added some temporary weight to the craft, bringing it up 87867 (so only 100 kg difference), but that had no effect.  

I understand in principle that you can strut to a grandfather part, heaviest part or root part, but I don't understand how that actually affects the physics in KSP. I wonder if my routing, which really had little rhyme or reason, is a factor.

Note: I did not turn on rigid attachment since my experience so far is craft seem to get very brittle, but that is probably lack of experience on my part.

 

ORIGINAL WITH STRUTS:

AUTOSTRUT VERSION:

 

[4x4cheesecake]

Just a wild guess which came into my mind: maybe autostruts will reduce the bendability of your craft more than regular struts (especially since it looks like they connect to the CoM of the parts instead to somewhere on the surface) but your craft may needs some slight bending to operate properly. Just a minimal offset of the thrust vector to lift the body a little bit out of the water. So, you can try to reduce the amount of autostruts to a minimum or slightly rotate the engines (so the thrust will slightly push down the rear end).

Edited June 29, 2018 by 4x4cheesecake

[bewing]

I think that in the 2nd version, your CoM moved downward vertically. So you need to move your engines down a touch to get your thrust vector in-line (or just below) your new CoM.