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Low speed ver. high speed diesels, which is best?

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Bob Robeson

I am looking at re-powering our 1986 Catalina 36. The first thing I noticed when starting my research on different engines in the 27 - 35 HP range was the disparity of RPM's that ranged from 3000 to over 4000. My first reaction would be to go with the engine that produces its hp (torque) at the lowest possible RPM. Less noise, less wear, longer life, less vibration, better fuel economy are a few of the reasons why I think the lower RPM engines would be better. Is this a perception thing or are there inherent differences in the low verses high RPM diesel engines. Thanks Bob C-36 #586
 
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Justin - O'day Owner's Web

HP lesson

HP is a function of torque over time. Multiply torque by RPM to get the HP numbers. For a given HP, the engine with the higher RPM is producing less torque. This is not necessarily a problem. In fact, this is why my VW GTI could dust a remarkable number of bigger engined cars. :) Torque is largely a function of displacement. Cylinder bore multiplied by piston stroke. The bigger, the more torque. The problem is, the bigger the bore, the bigger the piston. The longer the stroke the heavier the connecting rod. As you go through the engine cycle, these parts go through some pretty extreme accelerations. Increase the RPMs and accelerations get worse. This is why ultra high RPM engines have very small strokes. In a boat engine, I would prefer torque to be available earlier in the RPM band to facilitate docking without having to spin the engine way up each time I need a burst of power. In my book, this means the lower RPM engine. It does mean a bigger block though. YMMV, Justin - O'day Owner's Web
 
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
Can you stay with the same brand engine?

Bob: I would think that you would try to stay with the same brand of engine if possible. This would possibly help to minimize the cost of the engine swap. I assume that you are looking for a larger (more powerful) engine or are you upgrading from a gas to diesel? The Yanmar people tell me that these engines are good for 12,000-15,000 hours (if they are properly maintained). Now that is probably longer than most will own their boat. These engines are "high" RPM engine but are considered to some of the best in the industry.
 
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Tom Senator

How many hours on a diesel? Wow !!

That is something !! New Diesels lasted a long time but 12,000 hours !?!....that's great. Just think about it . If I take out my boat 20 weekends out of the year (that is way on the high side)and I run the engine all day Saturday and all day Sunday which is 16 hours (that also is way on the high side) and lets say I sail my boat for 20 years non stop. Thats comes out to 6400 hours !!!! That's just broken in !!! Wow....no wonder my 1980 15 HP Yanmar (2QM15) is still running strong - I was worried that some day I would need a new engine.
 
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Bob England

Low vs. high RPM

After reading some stuff on the Internet and talking to some "old timers" it seems that low RPM diesels would be running at around 1000 RPM or some even less at full power. So, both 3000 RPM and 4000 RPM engines would be "high RPM" by that standard. David Pascoe has some interesting information about big boat diesel engines at www.yachtsurvey.com. He classifies "high performance" diesels as those producing near 1 HP per cubic inch. These run at high RPM and have intercooled blowers, etc. My little 2GM20F may run at high RPM, but at 16 HP out of 38 Cu. inches, it isn't in that fragile high performance category.
 
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
Tom Tom

Dear Senator: Some day you will need a new diesel. I hope you keep you boat that long se you can tell us when it happens. Our boat is going on 15 years and only has about 800 hrs. Based on this I should also be scattered on the blue before I need a new one (unless I screw up).
 
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Bob

Longevity

How long your engine will last may have little to do with engine hours. Seldom used engines develope corrosion on cyliner walls, rings and bearings which hastens their demise. A small marine diesel may last 12,000 hours with regular use but a seldom used one probably will not last nearly that long. Regular oil changes are the best insurance that internal corrosion is kept to a minimum. Never store your engine with old oil in it. Also, running your engine long enough to get it hot and evaporate any moisture that is built up in it is a good idea if you are not going to be using it for a week or two. The tendency for a lot a boat owners is to motor out of the slip, raise sails, then shut off engine without it ever reaching operating temperature.
 
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