The 5/16" will do fine with your nutshell I even would opt for the 1/4" as your dinghy
is very light. I pull my dingy (14' Achilles with 25HP OB and 10 gallons of gas and bow-bag with 10lbs anchor
, and 4 PFD's) across Lake Michigan all the time using a double 5/16" painter about 30' behind the boat. I also have the loop end at the boat and I have an extra second 1/4" safety line
I make fast running the bitter end of the painter thru both side bow "D" rings and than back to the boat knoting a second loop, I put both loops over/thru one of my aft cleats
or use both SB and Port cleat
. This enables the dinghy
to slide sideways keeping force off the center "safety" line
which is connected to the center tow/bow ring on the dinghy
I must use this set up as I can't lift the 250lbs dinghy
and/or the 140lbs OB on board (I should get davits, but the expense keeps me from it).
This seems to be working fine although I haven't used this set-up in 5 ft or more waves and if caught with these higher waves I would double my tow length if not triple it.
Last but not least never assume the dinghy is there, i.e. visually check at regular intervals.