Quote:
Originally Posted by bridaus
No you cannot, the engines will die during warranty, but they will not be fixed under warranty.
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Nonsense.
Marine diesel engines have different power ratings -- intermittent vs. continuous. Continuous duty ratings are important because power
boats, especially planing power
boats, often use their engines at maximum power for long periods of time.
The difference between the continuous and intermittent duty ratings is usally about 20%, sometimes less.
So as long as you size the engine so that you are not taking more than the CONTINUOUS maximum power rating, you can run it as long as you like.
But why? Why would you do this?
This whole issue is a big fat red herring. Maximum efficiency of diesel engines is usually at about 75% of maximum power. But the difference in specific fuel consumption between say 35% and 75% is less than 10% in the table posted earlier in the thread, and between 35% and 90% is about the same. You're going to have the same issue running a
generator to run a hybrid drive -- if you use the
generator wide open, it is NOT indeed going to be running in its most efficient regime, and anyway you're only chasing 10% or so -- and this is a waste of time. Neither with hybrid nor with conventional drive are you going to be able to easily get right into the maximum efficiency groove, and you certainly will never get there with a hybrid drive and intentionally undersized generator where you may need to add
battery power during periods of high power demand.
My single diesel engine is a 100 hp
Yanmar. I normally need 20hp to 25hp for relaxed cruising speed in good conditions. Specific fuel consumption is higher in this regime than at 75hp, but remember this is SPECIFIC fuel consumption -- per hp/hour of
work done -- and if you're using less absolute amount of power, you're going to burn less fuel, even if you are burning MORE fuel per horsepower produced. I don't often need all 100 hp, but when I do need it, I REALLY need it, and I wouldn't have this
boat with less power than that (that's about 4.5hp per tonne). Those 100 hp have saved my backside a couple of times.
With series hybrid drive, you are losing 10% or more in conversion losses right out of the box. You'll never make that up out of a better operating regime for the diesel even if you could be more sure of operating in a more efficient regime, which you can't.
So this is simply not an advantage of series hybrid. I am not speaking only from theory -- I have a friend who owns a 200 foot diesel-electric
research vessel. It has a whole bank of generators to run the big
electric motor driving a single giant screw, and the
engineer can run different combinations of generators to get them in their ideal operating regime. Nevertheless, the fuel burn is about 15% greater than a conventional four stroke diesel mechanical drive would be, and more than 20% greater than a low speed Wartsilla two stroke would be. That is despite the giant
wheel, something we can't put on sailboats, which adds to
propeller efficiency, taking advantage of
electric motor torque curve. I know all this because when the existing
propulsion system needed an expensive
overhaul, he commissioned an expensive engineering study to compare the advantages and disadvantages of diesel-electric vs. mechanical drive. He ended up overhauling the diesel-electric system rather than converting to mechanical, but it almost made sense to carry out very extensive modifications to the vessel to fit a mechanical drive.
There is a lot of pure wishful thinking in a lot of these posts. Using a small diesel generator which cannot produce as much power as the electric motor needs at full power is not a recipe for a long lasting generator, or for better fuel consumption -- it will be operating wide open a much greater part of its life than with a normal drive, burning more fuel and incurring more wear and tear per hour. What this might achieve is to make it possible, by using
batteries to supplement generator power for short term requirements for high power, to use a cheaper and lighter diesel engine. For a cat which needs two screws, there might be an advantage in using one bigger diesel compared to two smaller ones. Those might be good things, but the cost will be more fuel burned and faster engine wear.
The other measure which is often touted here is to simply reduce maximum power available. What's the point of this? You guys don't have bad
weather,
head seas, lee shores, where you sail? An undersized generator is going to be lighter and cheaper, but that's about the only advantage. If you're regularly using more than 75% power, you will be using more fuel than optimum. From a fuel consumption point of view, you might as well go the other way down the curve, regularly use 40% or 50%, get the same specific fuel consumption, and have a bunch of power in reserve when you need it. But I guess undersizing the generator in a hybrid system is needed to make up for the extra weight and cost of the
electrical gear. But this is a mere illusion.
Notwithstanding many claims in here, series hybrid simply does not make sense for sailboats. You pay more for more complicated machinery just to lose power and fuel converting from mechanical to
electrical and back to mechanical.
Parallel hybrid, particularly on a cat which needs two screws, might be a very different matter. With such a setup you could efficiently drive ONE of the props mechanically, and use that for long distance cruising. For harbour maneuvers or when you need short term max power, you can supplement with the other screw driven by an electrical motor. You could end up with only one diesel engine that way -- a considerable advantage. You might size it somewhat less than the motors needed for conventional drive because you can add
battery power for short-term high power needs. You could store
cheap grid power by plugging in overnight.
On the other hand, you could just have two smaller diesels like a normal cat, and forget about it. I would guess that of all of the various combinations discussed in this thread, a bog standard
catamaran cruising on one engine has the best chance of hitting the efficiency sweet spot at about 70% to 80% of max power, and gaining the 8% or 10% or whatever you get running the engine at that rather than at 40% or 90%. And with no conversion costs. Plus you have totally redundant drive systems and engines. You can generate domestic power with large second alternators so get rid of the heavy AC generator. Unless you sail in a way which allows you to take advantage of
cheap electrical grid power plugged in over night, for short term motoring to get in and out of harbours -- a very narrow use case -- I don't really see how you can beat a totally normal
catamaran for cost, robustness, fuel efficiency, or weight. At the
current state of the art. It's good of course that someone is playing around with hybrid technology -- otherwise the technology would not progress. I'm sure there will be breakthroughs, but will we live to see them? I'm not sure.
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