Conference vicki::boats

    To risk telling you some things you already know.  
    
    1. 2 strokes tend to have twice the HP for a given displacement over
       4 strokes.
    
    2. 4 strokes get better fuel economy because of better air/fuel/exhaust
       flow through the cylinders.
    
    3. 2 strokes vs 4 strokes for a given HP weigh less.
    
    4. 4 strokes tend to run at a lower RPM thus 'extending engine life'.
    
    5. 4 strokes have more moving parts and are more expensive to repair,
       and there is a higher risk of problems due to more moving parts in
       the 4 stroke.
    
    6. Personally, I would go for the 4 stroke.  Fuel economy is pretty
       important in a boat.
    
    Jeff

I recently just happened to read an article about Detroit diesels. They
have had ongoing problems with crankcase pressurization and blowing oil
past the gaskets and out the crankcase breather, which results in a
grubby engine and potential fire hazard. Some clever fellow has just
invented and patented a device that solves most or all of the problem. 

    I've got a good friend with a 46' hatteras with a pair of the 6v92's. 
    He burns 40gph at about 18 knots.  They do smoke a little more than
    comparable 4 strokers, but he's never had a problem with them and
    fishes extensively offshore.  

    Some of the points made in reply .1 apply to gasoline engines,
    but may not be valid for diesels.  For example, while a 2 stoke 
    diesel may have fewer *internal* moving parts, it has more external
    moving parts. A physical requirement of a 2 stroke diesel is that
    it must be supercharged to pressurize the crankcase in order to 
    run.  The Rootes type supercharger or "blower" you see on many 
    drag race cars was originally designed for the GMC 2 stroke diesel.
    The extra complexity and cost of maintaining a supercharger would 
    probably wipe out any advantages of not having intake and exhaust
    valves.
    
    Rick

FWIW,

   I took a marine engine class with the US Power Squadron in Worcester, Ma
a couple years ago.  During the discussion about diesels it was mentioned that
the dreaded "diesel runaway" (where the engine continues to run after being
shut off by burning its lubricating oil) is applicable only to the 2-stroke.

  The occurrances of such things are EXTREMELY RARE these days, but the
possibility does exist.  Diesels typically are "shut off" by stopping the supply
of fuel, whereas in gas engines you stop supplying the spark. When the diesel 
starts running off its lubricant, the operator has to do some scrambling to
shut it down by blocking off the air, or shooting a bottle of CO2 into the air
intake.

Al

    Re .1>
    	
    	I agree with most of this reply but in the case of deisels
    fuel economy is not a factor. Gas 2 strokes loose fuel because some
    fuel/oil/air has to go out the exhaust port while purging exhaust
    gases with incoming fuel/oil/air. With Deisels the fuel is injected 
    in the cylinder and the exhaust is blown out by pressurized air so 
    fuel is not lost. Two stroke deisels don't need oil intermix either 
    because the fuel doesn't get sucked into the crankcase for lubrication
    like gas versions, they have a conventional oil pan/pump etc.
    	Because of these features it seems to me that 2 stroke deisels are
    far superior to 4 stroke, at least on the surface. The popularity of
    the 4 stroke however has me wondering what I'm missing. 
    	This is a great topic. I'm going to research my library a bit in
    preparation for any further discussions. 
    Regards,
    Paul
    P.S. I'll be travelling the next couple days so bear with me if I don't 
    participate right off.   

    I'd be interested to know what RPM the 46' Hatteras is running at
    40GPH.
    
    The specs on the 6V-92TA are:
    550BHP at 2300 RPM
    Displacement 552 cu in.
    Compression 17 to 1
    
    The 48' Jefferson comes in at 46,300 lbs. The 46 Hatteras is probably
    heavier so 40 GPH doesn't sound all that bad.

    re .7
    
    Just spoke to my friend with the hatteras.  I lied, he's got the 8/92's 
    that burn 40gph at 15-18kts turning 19-21k.  Thats with a full tower on
    the boat.  He said that they do drip into the engine room, but aside
    from that he has no complaints.  He also said that he loves the 
    dependability.   FWIW, that boat as all boats are, is for sale.
    
    Joe
    

    There is an interesting review in this month's Trailer Boats mag about
    the new Volvo (??) diesel engine which is both supercharged and
    turbocharged.  The supercharger has an electric clutch which engages
    a drive belt when you first crack open the throttle.  This gives
    instant boost to the diesel until the turbo can come up to speed.
    The supercharger was needed because the Volvo Duo-prop drive provides
    such a bite while accelerating (ie: no prop slip) that the engine was
    taking too long to wind up to the rpm's that were needed for the
    turbo to kick in.  Not to say that this little Volvo set-up would work
    in a 48 footer but it does show some of the research work that is being
    done.
    Wayne

re .16:

Well, in theory diesel fuel injection systems are simple and reliable. 
But even mechanical systems are not owner-repairable and reliability may 
depend on how the engine is used.

Since diesel fuel systems inject the fuel into the cylinder near the top 
of the compression stroke, the injection pressure is very high -- for 
our little diesel the injection pressure is about 2400 psi. All of the
injector pump internal clearances are minimal, something like a very few
ten-thousands of an inch. A result of this is that even slight wear or
corrosion will cause major problems, as I found out recently. Diesel
fuel injection system components can only be repaired by highly trained
specialists who have all of the necessary and expensive special tools
(for example, the tool needed to set the timing of our injector pump to
the calibration mark stamped on the mounting flange is some $1500). In
spite of three fuel filters and two water separators, there was enough
corrosion, varnish, and wear inside our injection pump after only 1100
hours of running in 12 years (infrequent use followed by long periods of
non-use probably worsens the effects of corrosion, etc) that it required
a major rebuild and recalibration (and, yes, this was very expensive and 
it could have been very, very expensive). On one hand, 1100 hours isn't
very many for a diesel, on the other hand 12 years is a long time. 

Adding the complexity of electronic fuel controls on top of this is a 
really depressing thought. More expensive parts that I can't fix. I 
probably couldn't afford (or wouldn't want) to buy spares and if the
electonics fail, the engine stops. 

re .19:

Certainly violates the concept of simple is reliable and inexpensive. 
Sounds like horrendous over-complexity for a minimal gain in 
performance. When it comes time to replace our engine, it will be
normally aspirated with no electronic controls if I have any choice in
the matter.