Boat stupid- Curious

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Jul 14, 2009
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Ok I was reading through the Marine engine section of BITOG, and I never thought before that Marine use could be so much harder on an engine than on land. I figured the running at constant RPMs would be easier on it, but apparently not. Ok, so let me get a few things out of the way that I think I know. 1. Marine engines run very rich, cold and wet. a. Dunno why they run rich---? b. They run cold for best power output c. Wet because of close proximity to water 2. Have very high shearing affects... Why exactly? Course it might be good to know what shearing means, so a one sentence explanation might help with that one too. So why else is marine use so [censored] hard on engines? Truly, I don't know. I hope y'all can explain it to me pretty good. Also, years ago we rented a 21' Bayliner Ski-boat, with a 120HP Mercury Marine Outboard 2-stroke. Would that have been a 3, 4 or a 6 cylinder? Thanks in advance!
 
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the merc outboard was probably a 4 cylinder - but 2 cycle motors are a whole different story. certainly the lower temps in a marine engine (152-160 degree thermostats) are tough on the engine and the oil. I believe that the lower temp is to reduce the chance of thermal shock since they are raw water cooled. When a boat is up on plane it lifts the boat several inches out of the water. my boat runs aq constant 3500-4500 rpm the whole time it is running; how often do we torture a car engine like that? they liken it to towing a heavy trailer uphill. the high load and high RPM will shear an oil. some of the high performance boats run superchargers and/or more rpm. both Mercruiser and Volvo Penta call for an oil change every 50 hours on dino oil; 100 hours on synthetic. and my volvo penta actually calls for 30W synthetic oil. also, it is apparently accepted that a boat engine will generally only last 1000-2000 hours. many actually need to be rebuilt or replaced sooner than that - due to neglect issues. I cant personnally speak to the life of a marine engine, as I only put about 600 hours on my last boat. the new one only has 200 hours. check back in ten years....
 
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Have you ever run your car for more than one minute at full throttle? Think about it. Every time I take a boat out, it's full throttle until I get where I'm going. That could be half an hour or more. Most run rich, because a cylinder that is lean will score on a 2 cycle engine. But the biggest problem is neglect.
 
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Boat engines run colder and richer for max power output . They are always going up hill and run at higher rpm per hour than a car engine.
 
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I am a marine mechanic that mainly works on inboard ski boats. In the past they were shipped with 143 degree thermostats supposedly to minimize salt deposits if run in seawater. Carbs that run rich during choke phase will result in fuel dilution. As noted, the toughest conditions in marine use is when the boat is run at high load for long periods. Most pleasure boaters don't do this, but oil recommendations must account for it. As far as engine life in boats, I can't think of any boats I have been around that needed to be rebuilt due to wear. They generally are abused by overheating, or a mechanical failure. My 1984 Ski Nautique has 1450 hours on it and I consider it middle aged. I expect to put 3000 hours or more before it wears out. This will be an over 50 year old boat when that happens. I doubt my engine runs in a wetter environment than cars regularly run in the rain, like in Seattle or England.
 
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 Originally Posted By: bepperb
Have you ever run your car for more than one minute at full throttle? Think about it. Every time I take a boat out, it's full throttle until I get where I'm going. That could be half an hour or more.
Sounds like the VW bus I had in my formative years.
 

PT1

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My SeaRay T380hp 7.4L Horizon Merc GM engines cruise at 3200-3600 rpm all day. WOT is 4400. The sump is 7.5 qts because of top side remote oil filters and the manual OCI is 100 hours for 25w40 Merc dino oil. I change at 50.
 

PT1

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Why do boats run rich? Most Merc engines use Delphi ECM's that automatically retard the timing 6-8 degrees when you throttle up to prevent detonation. The ECM will retard the timing and then add it back as you get up to speed and there is less strain on the engine. This will result in fuel dilution. Harley Davidson uses the same ECM system on their TC88 & 96 engines. It works well on low revving high torque motors.
 
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Marine engines are set to run rich because the engines are under load most of the time-no loafing along. They are also kept cool. 160º thermostats are the norm.
 
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