Mystery expansion tank found in bow closet

[CruiseMissile]

Hi All, My boat was built as a 1980 EDMY 70’ and now is a 80’CPMY. Had a bitOver a year. In closet in Stbd bow bunk Room found a 3/4 full plastic expansion tank With blue about 1/2” hoses going downUnder the deck. I have No clue! Have changed all oil and filtersAnd coolant. My guess is hydraulic fluidFor the bow thruster?Thoughts wise people?Maintenance for this?Thanks. Dan

[doc g]

yes you are correct , it keeps a head of oil pressure on the bow thruster(ABT Hydraulic??) -in case the seals ever leak it does not allow water to enter the unit ......Pat

[CruiseMissile]

my buddy Ken check it out and the hoses went to the Thruster, but it is the gear oil, not the hydraulic oil. He did not know why, but you did And it is for reasons stated to keep a small head on the gear oil in the thruster to keep out the water. Mystery solved. Thank you!!