what makes a mini simmons such a turnable surfboard having that wide tail and those big keel fins way further back

so wide tail is harder to sink/turn

the further back you place the fins the stiffer the board gets

paralel rails fast down the line and not so easy to turn at high speed

how come simmons are such a joy

I’ve never ridden a mini Simmons. My guess is that the opposite keel is mostly out ot the water during a turn. Twins are squirrelly even with a narrow tail. Wide tails spread out the fin cluster to make a pivot point with a larger radius. 

The ones I made with the fins way back were stiff. The last one I made I moved the fins up to 6.5 inches and it worked much better for me. None of them worked as well as my fish so I have not made one in a few years. Mike

The original 1950s Simmons were longboard length and were intended to be surfed straight and fast.   The Kenvin reimagination into the Mini-simms were also intended to come up on plane quickly, but they were cut so short that their riders could man-handle them through their turns.  The corners hold until they abruptly break loose.   It’s a skateboard.   

Yes to what gdaddy says. Plus, I don’t shape like Baugess or surf like Kenvin. Just a guy goofying off in his garage and in the water.I’d move the fins up to 6 or 6.5.inches. Or, make a few of them and experiment.  Mike

Because they are to be ridden very short.  Your back foot has to be at the very edge of the tail.  As as said above, you horse them around.  Don’t expect a mid or long length Simmons to turn well.

all the best