So, the logic is that since marine engines are pushed much harder than a car engine, and operated above freezing, it needs a much thicker oil for protection. But, why wouldn’t you run 25w-40 in your car engine, which is basically the same, to give it more protection?. Wouldn’t it last longer if 25w-40 gives you more protection? Isnt it really the oil flow through the journal bearings that provide the separation force? Not necessarily viscosity and pressure? If that is true, why is it better to run a super thick oil. Just because the viscosity breaks down quickly?
Re:
"Wouldn’t it last longer if 25w-40 gives you more protection? Isnt it really the oil flow through the journal bearings that provide the separation force? Not necessarily viscosity and pressure? If that is true, why is it better to run a super thick oil. Just because the viscosity breaks down quickly?...Just because the viscosity breaks down quickly?"
No.
The workload seen by a marine engine (especially in a planing hull) and a car engine are as different as night and day. The first reason you wouldn't use a higher-viscosity oil in your car is because your car doesn't NEED more viscosity. Thicker oil will cause your CAR engine to crank slower in cold temperatures, increase (dramatically) the time it takes for oil under pressure to reach the bearings and valve-train after starting, and reduce gas mileage. Most
auto engine wear happens in the first few minutes of driving, while the engine is cold and oil is thick and is not circulating properly yet. This is particularly true in winter. Meanwhile boats and their engines are snug-as-a-bug-in-shrink-wrap.
Moreover...
no one jumps in a boat and blasts away from the dock with a cold engine, except for a very small minority of idiots -- and Darwin typically takes care of them in short order, at least in terms of their power-boating careers. So...a boat engine, running in warmer climates and with time to warm up, can afford the luxury of a higher viscosity, thicker oil film. That's a good thing, because most boat engine wear happens when the engine is hot and fully loaded.
Which brings us to your next misconception. The volume of oil flow
"through the journal bearings" has nothing to do with the
thickness of the oil film under load. Hydro
dynamics tells us that a higher viscosity ALWAYS means a thicker oil film under
dynamic (impact) pressure loads, like the explosion in a combustion chamber driving a piston rod down against the crankshaft. According to your reasoning, one could use isopropyl alcohol in place of engine oil as long as the flow rate was high enough. This is, of course, nonsense. Under pressure, thicker oil flows more slowly away from the pressure bearing metal surfaces (out of the bearing journals), resulting in a thicker oil film (on average) throughout the dynamic impact loading cycle. An auto engine almost NEVER needs to produce anywhere near it's maximum horsepower over sustained periods of time. A boat engine typically lives it's entire lifetime at or near the maximum torque/hp curve. Under this kind of workload, thicker oil provides a better cushion, which absorbs impacts and prevents metal-to-metal contact that thinner oils would not.
Look up the word 'viscous' if this is not intuitively true for you. Another word that might interest you is 'viscoelasticity'. If that fails, ask yourself this: would you rather dive from an eighteen foot platform into a 3-foot deep pool of water, or a three foot deep pool of molasses? Now you may appreciate the 'dynamics' aspect of 'hydrodynamics'.
Lastly...comparatively a car engine is VERY lightly loaded, producing less than 15% of max H.P.
about 98% of the time. Meanwhile...a
boat engine is
producing 60-75% of max H.P. about 85% of the time, and frequently needs to run at WOT for several MINUTES at a time.
Question: when was the last time you were in a car with the pedal mashed to the floor for even 30 seconds, much less five minutes or more?
This is what I mean when I say 'as different as night and day'. Compared to a boat engine,
a car engine pretty much sleeps it's life away.