A different water game, this one on my boat

[Foster]

I have a boat.

It has an engine that produces water at 170 degrees F after about 20 mins of use. The water temp starts at 50F and increases in a linear fashion until 170 and then levels off. The cold / raw sea water system kicks in and keeps the temp at 170 F

I have two water tanks, each holds 11 gallons of water

They have a 3/4 copper pipe in them, 6 feet long in a coil. The coolant water cycles through the tubing to heat the fresh water in the tank.

engine --> Tank 1 ----> Tank 2 --->  return to engine

If I start with 50 degrees F water in both tanks how long until the water reaches 120 degrees? I understand at some point both tanks will be at 170 degrees F, I'm interested in the 120 F mark.

If I replaced these two tanks by one tank that held 20 gallons and routed the water twice through the tank:

engine --> Tank Upper Loop ---> Tank Lower Loop --> engine return

How long would it take to get this tank up to 120 F?

(disclaimer: I have the boat, I've done the real world testing, so I know the answer to question 1 (two tanks). )