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[Back
to the Site Directory] Why a Single Diesel Engine... Why would I prefer a single diesel to say a single gas, or twin gas...horsepower. This is where the most hot-rodders get lost, horsepower? How can a slow turning diesel put out more horsepower than a stout gas engine? Torque yes, but horsepower? This is because boating is based on "useable
horsepower" not total horsepower. The diesel engine provides boaters with
much more
useable power. The name of the game with any boat is cruise speed, not top
speed. Cruise speed is how you will judge your boat because this is how fast you
can go all day. Top speed is rarely used with a gas engine because no gas engine
can live very long at top speed, they will self destruct in short order. Take a
brand new Crusader, Mercruiser, PCM, etc. and run it at 4,400 RPM every time you
use it and you will have nothing left by 400 or 500 hours...if it last that
long.
These engines are great engines but they just aren't made to sustain rev's that high for very long. If you review the gas engine manufactures spec sheets and brochures you notice something missing. The advertised rated horsepower is reported at MAX "no load" RPM. But rated MAX RPM for most gas engines in marine service is between 4,200 RPM and 4,400 RPM, a few at 4,800 RPM....but not many. So what does the engine make at an RPM that it can survive, like cruise speed? Good question! An example of a modern gas engine might be the 6.0 liter MPI Chevy: Manufacture rated horsepower= 375hp @ 5200 RPM, but actual output at the rated 4,400 RPM is around 225 horsepower. What happened to the 375 hp engine you paid big bucks for? It's hiding in there never to be accessed. To make matter worst the 375hp gas engine is only making about 90-110hp @ 3,000 RPM, or cruise speed. How usable is 100hp? in a heavy sportfishing boat, not very. Gas engine owner's are constantly pushing the cruise RPM in the quest for higher cruise speeds, thus shortening their engines lives. It's a bad feeling to have 375hp and know you can never use it. Now lets look at a old, heavy Detroit. The 6V71N makes 287 hp @ 2,300 RPM with N80 injectors. What about cruise speed? Diesel engines are designed to be cruised at 80% of rated RPM all day long, forever. So 80% of the rated 2,300 RPM is 1,850 RPM. At 1850 RPM the heavy old Detroit is dishing out 175hp, almost double the useable horsepower at cruise speed of the 6.0 gas engine. It only takes basic math to figure out that it would require (2) of these modern gas engines to equal the "usable" horsepower of (1) of these old churners. Unlike the gas engine whose spec's only get worst the deeper you look, the diesel's numbers get better. That statement is based on the fact that you can cruise the old Detroit all day at MAX 2,300 RPM without damage. Of course the engine will have a shorter life (maybe ONLY 5,000 -7,000 hours) but that's still a long life by most boaters standards. These calculations and useable horsepower numbers are based on a "propeller horsepower consumption curve"...not the rated outputs of the various engines. The power consumption curve is used because all engines are propped at their MAX rated RPM, so you would prop a gas engine to be capable of spinning the maximum pitch at; say 4,400 RPM. Likewise you would prop a diesel engine to spin the maximum pitch at it's rated RPM. In the DD's case that would be 2,200. What about the weight? Good question because at this point the 2:1 usable horsepower ratio is basically 2:1 in a weight ratio (except reversed). The diesel engine typically weights "twice" what the gas engine weights. So take your pick...at 2000-2400 lbs you can have (2) gas engines that provide maybe 180-200 hp combined at cruise, or (1) diesel that provides 175 hp at cruise. The horsepower to weight ratio numbers are even better on modern diesel engines like the 6BTA Cummins, Yanmar, etc. These engines come very close to the weight of the gassers, but still have all the useable horsepower that a diesel's power curve delivers. If you run these numbers on the prop calculator software you will find that it's practically impossible to come up with a gas power boat that will provide the same cruise speed as the same diesel powered boat, assuming they are BOTH inboard boats and the boat can handle the added weight of a diesel. I noted "inboards" because the IO, or stern drive boats are more efficient than inboards and therefore typically enjoy higher cruise speeds, but this is at the expense of reliability. I won't even get into fuel economy because I really don't even believe this could need explaining, you already know the fuel economy answer. So basically a single diesel will do the "boat work" that it takes 2 gas engines to accomplish. The twin gassers do provide some edge in reliability but not much. The greatest advantage in reliability is actually not the fact that you have 2 engines but rather the fact that you have 2 drive lines. In the event of transmission failure or an impact to the shaft, strut or prop the twin set up will bring you home and the single may not, depending on the nature of the impact or failure. In my opinion from an engine only standpoint I feel that the single diesel is as reliable as twin gas engines....maybe, and I'm sure I would get some arguments on that statement. For me it's save the money that 2 engines would cost to purchase, install a good SSB radio and a satellite phone and you will still be ten's of thousands of dollars ahead....even if you add towing insurance it's still not even close. I have heard of mariners that sank, burned, shipwrecked etc. but at the time of this writing I have not heard any stories of mariners with engine failures that lead to their deaths, I suppose it has happened and someone will email me with a "single engine honorable death" story. The factors of "nasty" vs clean are another story altogether... In a perfect world I would run 2 smaller diesels and get best of everything! |
