Marine Oil Cooling

Start here if the oil belongs to the engine itself and the evidence is oil in the expansion tank or a milky dipstick. Engine lube oil is the one oil duty tied to the engine data sheet, where heat to oil is published separately, and it rejects into raw seawater or the jacket circuit rather than into air. That makes it shell-and-tube work with oil on the baffled shell side and copper-nickel tubes with zincs in bronze bonnets, on frames from the B-401 up through the HC range. A pressure test comes before any head gasket is condemned.

This page covers deck hydraulics as a system: pump, relief valve, hose runs and the heat all of them leave in the oil. Where lube and gear coolers are sized from a builder's figure, a hydraulic cooler is sized from how the circuit is actually worked, with time on relief counted at close to full pump power. The page also carries the air-cooled Heresite range: A Series with AC motor fans, H Series with hydraulic fan drive, and DCS Series on 12 or 24 volt DC for small packs. The thruster and crane pages are specific cases of this duty.

Gear oil peaks in the harbor and settles on passage, the reverse of every other cooler in the engine room. Clutch slip during repeated engagements, not steady running, is the design load, so a gear cooler is chosen to absorb a maneuvering peak and hold the sump below the gear builder's published limit, which is a hard number. Like lube oil it sits on the seawater side, with gear oil on the baffled shell and one or two passes to keep oil side pressure drop sensible. Tugs, ferries and any boat that handles ships belong here first.

Two hydraulic duties that break the averaging assumption behind most cooler selections share this page. A bow thruster delivers short bursts near full pump power with long idle gaps, so the question is how many consecutive docking attempts the oil charge can survive. A zero-speed stabilizer runs the pack all night at anchor with the mains stopped and no incidental cooling anywhere aboard. Both usually live in a forepeak or lazarette with no raw water nearby, so the answer is normally an air-cooled Heresite core with a 12 volt, 24 volt or hydraulic fan drive, sized for the worst night.

Anything cooled where it stands on an open deck, in spray and sunlight, belongs here. Cranes, trawl winches and capstans sit too far from the raw water main to justify seawater pipe across a working deck, so the cooler mounts at the power pack and a fan does the work. The distinguishing problems are environmental: fin corrosion that removes capacity without a leak, recirculation when the core is boxed in, and matching the fan drive to the power available at the pedestal. A Series electric and H Series hydraulic-drive cores with baked Heresite coating cover nearly all of it.

Water carries about one BTU per pound per degree. Oil carries roughly half that, so for the same heat you need either more flow or a bigger temperature swing. Oil also forms a thicker boundary film on the tube wall, which is why oil-side heat transfer coefficients are an order of magnitude below water-side ones and why oil almost always goes on the shell side where baffles can disturb the film.

On a cold morning the oil in a deck hydraulic circuit may be several times as viscous as it is at working temperature. If the cooler produces excessive drop under those conditions, the relief valve lifts, the oil warms by throttling instead of working, and case pressures rise where seals cannot take it. Many circuits carry a thermal bypass for exactly this reason.

If the oil circuit lives in the engine room within reach of the raw water main, a shell-and-tube cooler with copper-nickel tubes is compact, quiet and cheap to maintain. If the circuit lives on deck, at a crane pedestal or in an aft winch space, running seawater to it adds hose, penetrations and winterising work for no thermal benefit.