Efficiency vs Effectiveness

Good morning everybody.
I am a curious person and I like to understand things (in my limits).
I want help to understand one thing, without waste time of anyone.

I say before: I did not make SOR because I am still far (and I am a dreamer), I am only checking if possible.
I want to build a displacement boat of 30/32 feet, steel, with medium/heavy displacement (D/L 320).
Use will be cruising, with long time onboard for one or two persons maximum.

Now I was studying (maybe better say imagining), like you know from my old posts, the propulsion system.
I will need about 60 hp.
Here start the world of engines.

My idea is the biggest and slowest engine possible, naturally aspirated.
Because in my head, if you put with the biggest propeller possible, with good compromise, turning slow, with low slip, you will have maximum economy and better efficiency.

From project, the boat will go between 6 and 6.4 knots, with request of about 15 hp.
My idea is engine turning about 1500 rpm (I look engines that make 2700–3000 rpm max).
This allow, in my opinion, to use less fuel at cruising speed, but still have possibility to face sea and wind in bad condition (propeller will lose efficiency of course).

Now the doubt.
My choice go to older engines (like Beta60) and not to new ones.
These engines are less efficient than modern engines, but I think more effective.
They can run at low rpm without suffer too much (yes, they suffer anyway, but with one “happy throttle” every 2–3 hours they should be ok).
They are simple, they are strong.

Modern engines exist, maybe turbo, maybe common rail, they are more efficient but must run higher rpm, and I think they lose much of their efficiency in real effectiveness.

I would like some reflections about this.
I don’t want waste time of anyone, but the nice of forums is also make discussion without precise final goal.

Thanks to who will participate.

 

A very interesting question, but in my opinion, to focus the discussion and avoid unnecessary digressions and wasted time, you should clarify what you mean by effectiveness and efficiency in a marine engine (and how to evaluate it), and whether you're referring only to the engine itself or the entire propulsion system. Furthermore, you should define the conditions under which efficiency and effectiveness should be evaluated (whatever that means to you).

 

Thank you for the answer.

Effectiveness = lower fuel consumption in real use conditions, lower maintenance costs, higher reliability.
Efficiency = lower fuel consumption in terms of BSFC, so an engine more efficient but not, at the end, more effective (more delicate, must run at higher rpm, so fuel use in L/h or L/nm will be higher), maintenance more expensive or delicate.

The use conditions for evaluation of effectiveness are navigation at 6–6.5 knots for 90/95% of the time (like everybody), and in the other 10/5% the possibility to face rough sea without care of fuel consumption, and stay safe.

 

That would be a gigantic and very heavy stationary engine.

 

Sorry Gonzo, but I don’t think my doubt is so deserving of sharp and mocking answers (I used the translator… )

I don’t see why my question cannot be discussed with arguments.

Thanks.

 

It is a statement of fact. That would be the bigest and slowest. Modern engines are smaller, lighter, and more efficient. There is no mocking involved.

 

Ok Gonzo, sorry if I misunderstood and thank you.

But that is exactly the point of my talk.

Modern engines are more efficient but not always more effective (effectiveness = L/h or L/nm).

 

I think the best approach is to consider the engine and the rest of the drivetrain separately. You are correct in that, at those speeds a large slow turning prop gives you the best propulsion efficiency. There's no getting around that.
But! There are a wide variety of reduction ratios available on the market, so your big slow prop doesn't tell you much about your engine. For best efficiency (as correctly defined in physics or engineering) you need to look at specific fuel comption in gm/kWhr. That is the number that matters. You need 15hp? The one that burns the least diesel giving 15hp is the most efficient. Again, using the actual meaning of the word efficient. An engine that can give 60hp in extremis while most efficient at 15 will be a sophisticated modern engine.

 

A boat of that length and SL would have been recommended to have an old Sabb 10hp. Not sure why you think you need 60hp. Lots of old heavy single bangers stll here in Sweden. I am a big fan of the old 3 cylinder 152 Perkins, about 35hp in marine form, but almost 3/4 of a ton in weight! Spares are global and cheap.

 

...from my immagination the displacement comes out to about 8–9 tons.
A 50 could be enough, I agree, but certainly not a 10…
Granted, a Sabb with a CPP would be wonderful, but I need to stay around 60 hp to achieve 6–6.5 knots cruising speed and reliable use in rough sea conditions.

 

The old John deere 4030 could be had down to 60 hp and would run low speed for ages. We had one on a generator that had been on as long as Id been alive in high school. Currently have a 4045, that I belive is available in an 80 hp slow turning version. Its been a phenomenal engine for the 26 years weve had it. It runs 20 hrs a day for about 6 weeks each summer and while it is getting a little tired its lasted 4x as long as we got out of the 4bt it replaced. Would think a slow turning 4039 or 4045 at 1800 or 2100 rpm would be about perfect idling along.

 

Let's get some facts straight to ease your doubts.

Common rail and rpm have nothing to do with each other. Example: Wärtsilä RT-flex96C, the largest engine to date, common rail and runs at a maximum of 120rpm.
Electronically controlled common rail engines actually have an advantage over any form of mechanical injection when it comes to running at partial power, there is no glazing, the engine doesn't "suffer".

Specific fuel consumption is baked into the engine design. A 60hp engine will never have its best fuel consumption when developing only 15hp, but, if you want to compare apples to apples you compare the fuel consumption at the specific power you desire. How fast the engine is turning at that power is irrelevant to you, because if you want to also use it at 60hp you must gear and prop the boat for the max load, not the partial load.

Duty cycle ratings. With your modest usual power requirements any recreationally rated 60hp engine will do. It doesn't matter at what rpm the max power rating is, for yor use case it's simply irrelevant, you won't get more hours out of a bigger engine. Max rpm is only relevant if the higher reduction gearbox is more expensive, but only if the entire engine + gearbox + prop combination proves to be more expensive, otherwise you save on the gearbox and spend on the other components.

Turbo: you need to check if the turbo even kicks in at 15hp, and if the manufacturer is ok with long periods of it beeing unloaded.

Now a few words about the "strong and simple" mechanical diesels.
If you want a stronger engine you buy one with a higher horsepower continuous duty rating. It will have bigger and stronger components from the factory, that's all it is.
Simple is horse manure. You are lucky if your average dockside mechanic has an injector tester. Most of them are at least honest enough to keep their dirty fingers away from the injection pump. When was the last time you had to adjust valve clearance on a modern belt driven DOHC engine? On a recreational vessel even belt change will be because of old age, not running hours.
Diagnostics are way easier with an electronic engine, you buy yourself an adapter for your laptop/phone and download the error library. No more searching for the one guru that actually knows something about your specific model and can diagnose it by ear.

There are indeed some things that can matter to a high hour user, a wet block can enable in frame rebuilds for example, inline pumps are advantageous if you want to run on salad oil, gear driven camshafts last longer then belts and chains, etc.

 

What you call effectiveness is the definition of efficiency.

 

I don't quite agree. The mechanical basics still have to be tested. For example, compression and leakdown. Many new mechanics trust the computer blindly and never manage to fix the issue. Years ago I worked on a gas Volvo Penta that had gone through at least four mechanics. The issue was that the computer was mapped for the wrong engine.

 

Will this be a sailboat, or powerboat?