The following discussion is distilled from my correspondence with our own anonymous motor sailing test pilot, at sea on his own Antares 44i.

Some editing & paraphrasing has been done to protect the participants from accusations of being dumbass, pedantic and more boring than customary.

The pictures are added for the relief you will deserve if you are obsessed enough to read all this stuff.

Q: Stig

Has the following question been covered somewhere in the blog? I don’t recall, but it’s been a while since I read the first one.

Question: “Does running one engine whilst under sail really save fuel?” I suspect not, but maybe there are exceptional circumstances when it can. To me, the proposition “for” goes something like this:-

When motoring under sail, you can switch off one engine, and still maintain at least 75% of your boat speed.

I would imagine that running one engine would tend to turn the boat, so it would have to be balanced by turning the rudder (which slows the boat), or trimming the sails in such a way to counter the turn (which I suspect would produce a lower forward force from the sails). If only one engine is running, does it tend to burn more fuel than it would if the second engine was running?

Certainly the reduced speed when running one engine means a longer trip, which means more fuel will have to be burnt in that engine anyway. The only circumstance which gives me some pause for reflection, is in very light airs where the engine is helping to create apparent wind, and the windward hull engine is the one being run. But this is a tenuous proposition in itself, so not a robust cruising strategy, and certainly not legal in the racing arena.

Putting all this together, I suspect that “only running one engine to save fuel” is one of those layperson yachtie myths, but would appreciate your comment, or blog if you deem the issue worthy.

A: TC

Jeez Stig, I was reluctant to support a blog in the first place on the grounds that I didn’t want to generate a lot of backwash that I would have to try and answer rationally (tough).

Your subject seems like just the sort of thing to get any number of theorists going around and around in a spiral of acrimony. As long as we keep your topic to ourselves however, I should be safe, but you have to promise.  (owch, ed. you’re hurting my arm!)

I think your question, “Does running one engine whilst under sail really save fuel?” is accidentally kind of catawampus; you will save the most fuel by not running the engine at all.  Maybe we should re-phrase and ask two questions;

1) Can I motor along more fuel efficiently with the sails helping? 

2) If so, should I use one engine or two?

One of the factors pertaining to either question is, ‘When do you want to get to your destination?’  

In a non-planing (displacement hull) vessel you can always save fuel, regardless of how many engines you use to burn it, just by slowing down. In the displacement speed regime, power is used primarily to make waves, a smaller wake equates with less power consumption.

When you slow down, it takes longer to get there so you have to run longer but the rate of consumption goes down to your advantage in terms of fuel/mile.  To discuss the relative merits of various sail +power combinations, I suppose it is therefore rigorous to think in terms of some consistent speed to be achieved by using the various combinations?

Let’s try addressing the question 2  first;

The concept of running only one engine when two are an option is not confined to sailboats; I had to address it for the power cat fleet we built as well.  The sails just add another dimension.

When full power/speed is not required, the hypothetical advantage of running one of two possible engines is that; engines are less efficient when run at less than full power.  Running one at closer to its full capacity is possibly more efficient than running two at partial power.  Also, the parasitic loads associated with turning the engines and shafts are halved.  It’s these efficiency factors that motivate the discussion normally, creating a theoretical advantage over throttling back two engines.

In practice, the theoretical advantage is severely tempered in a twin engine power vessel by the necessity of dragging the one dead set of running gear, and having to apply excessive rudder to hold a course.  But we may suppose your sailing cat could run peg leg more advantageously since the idle prop will feather and create minimal drag.

Without some very accurate fuel consumption metering, (rather impractical on engines at this size) it would be impossible to confirm the actual fuel saving at any given circumstance.  To complicate things, the engines are propped to work the vessel up to speed together so a single engine trying to get the boat up to speed will be over-propped (like running in too high a gear with your car), but as long as you back down a few hundred rpm from the max rpm achieved on one engine, you won’t overload it.  The power level and its associated fuel consumption will vary according to the prevailing sea and wind conditions.

It looks reasonable to presume that you can indeed run measurably more efficiently on one engine rather than two in the low propulsion power demand conditions you reflected on in your question, but the degree of advantage will be unpredictable due to the many variables.   (Hey, less noise, that’s a given anyway.)

To address question 1 : (do I win anything?)

Introducing the sailing under power aspect makes for a whole new spoiler in the equation, hmm.

I like the augmentation of the apparent wind thing, and as long as the sails are not back-winded, they may be presumed to be contributing something, though perhaps in some conditions not enough to overcome their own drag.  An analogy could perhaps be made with the aircraft wing which contributes its lift while the engines create the necessary forward motion and resultant wind over the wing to provide the lift.

You could never motor sail effectively in a dead calm as the apparent wind would always be dead on the nose but at some points of sail in the presence of wind at sufficient strengths you could expect to motor sail effectively.

 So, in simplistic terms;

• If you just want to get somewhere most fuel efficiently, you would sail.
• If you want to get there faster, the course is amenable, and you are willing to spend some fuel, use an engine and sail.
• If you want to get there even faster use two engines and sail?
• If the course is not amenable to sailing effectively, don’t use the sails.

I don’t really fathom the idea; “When motoring under sail, you can switch off one engine, and still maintain at least 75% of your boat speed.”  The ‘speed’ obtained will be the result of the total propulsive effort (sails + engines) overcoming the resistance and the real numbers are highly dependent on the boat, its engines, props, speed regime, wind speed, point of sail, etc.  Declaring a particular percentage seems to me to be rather bold.

Ted
P.S. There’s no way we are publishing this confusing stuff.

Q: Stig

The great thing about ignorance is; you don’t know what you don’t know, so you can proceed in blissful ignorance 🙂

I think I follow your explanations, so if I find myself so disposed, I will carefully start to experiment on this issue. If I learn anything useful from my experimentations I’ll let you know (but don’t hold your breath).

On a related issue, one last question (I promise).
My boat is fitted with a feathering prop rather than a folding prop model. I imagine the feathering props are more robust and responsive in fwd./aft evolutions, whereas the folding props are better with significantly reduced drag? Am I correct in my speculations? Could you put a figure on greater drag reduction of the folding prop? (is there a case for changing)

A: TC

Be not concerned, I never undervalue ignorance and cultivate a personal style in this regard, always “seeking my bliss” at every opportunity.

The Maxprops you have probably generate about the least prop drag you could achieve when they are feathered. The blade shape is compromised however in order to be “flat” and slice through the water when feathered.  Racers love them.  Also, they have killer reverse power in comparison with props that have blades maximized for forward thrust efficiency at the expense of reverse, (most other props).   This makes them well loved in the harbour.

The folding props we are using now were developed by Volvo and, as the investment would have been substantial, I have to presume they examined all the options and came up with their best effort. These props fold into a kind of shriveled flower configuration that looks like a turbulence magnet, but apparently it isn’t so. They seem to work well, are serviced by Volvo’s worldwide network, and are much simpler mechanically.

But, there is a more decisive reason to consider the option depending on the kind of boating you imagine yourself doing.  Unlike the flat blades of feathering props, the folding prop blades may be constructed with the curvaceous twisted blade surfaces that are critical for maximum efficiency under power.

The twist is necessary to make the blade surface conform to change in attack angles associated with the desired pitch at different points on the diameter is not possible with a flat blade.

The measurable power (fuel) efficiency advantage of a folder style blade may be more important to you than a minor increase in drag.

If you spend a lot of time motoring (or motor sailing) and have a fuel budget or range target, you may opt for a more efficient powering prop over a more efficient sailing prop.

Lately, fuel consciousness has been forefront in the minds of our new owners so we felt that the ‘standard’ prop offering would be the folder.  They are noticeably less effective in reverse than the feathering props however so if you are used to the Maxprops and are known to be a menace in the harbour, take your time, keep the fenders available, and cultivate the old manoeuvring skills.

There are a number of prop performance comparison exercises published on the web (eg. Segeln magazine 2008), but the science may be tough to apply to a particular boat and circumstance.   There is a surprising variety of designs on offer, arguments will abound, (No, I don’t want to know!).

I wouldn’t switch out the Maxprops unless they become worn out or damaged. They are still a preferred item from a pure sailing standpoint and have survived very well in the market, (I installed them on racing boats when they were a novelty, early 1980’s?), presumably for good reasons.

Q: Stig

Remember that discussion we had about the value of running one engine versus two, and I speculated that there might be a benefit in light airs going to windward in running one engine? Guess what, I’ve met a fellow sailor of a Nautitech 40, (which has the same engines as my Antares), who is convinced that running the leeward engine works. He has played around measuring fuel consumption, (he’s careful with money, and he’s German). He’s attached his fuel lines to bottles with volume marks on them.  

Anyway, the most surprising thing he discovered was that the fuel consumption of the engines at idle and 2,000rpm was the same. And this was measured over a 4hr run period. Even as I write this, I am questioning it, so I’ll have to interrogate him further on his methodology. But the implication is clear, there is a fuel overhead in running an engine, even if it’s not doing any useful work. So in the light air sailing to windward scenario, the engine would work to initially generate speed, but once the sails started to power up, the engine would be less pressed, and burn less fuel (ie, same revs, but less torque). Or something along this line of thought. The benefit would appear to only be in this narrow window of light air and low rev’s. He routinely will start the leeward engine when struggling to do 3kts, and claims significant speed improvement at 2,000rpm, with little apparent turning effect, so minimal additional drag from helm correction, and miserly fuel consumption. Certainly a lot less that if he was running the windward engine at the same time. Fuel for thought.

A: TC

There’s nothing wrong with the theory.  It’s all simply explained by relativity.

The prop load will be reduced (along with the fuel consumption) by whatever contribution the sails make. At any time, the engine rpm will however have to be sufficient that the engine will contribute to the sailing speed or the prop will drag start to drag (heavily). Getting the same fuel consumption at idle would be expected if the engine speed isn’t high enough to give the prop some bite, it’s just going along for the ride.

As the vessel speeds up, the engine will have to speed up to contribute power, eventually reaching its governed max rpm and at some boat speed beyond, there will be no more contribution and the prop will drag from there on up as it will be spinning below the rpm required to produce thrust. (We’d be really cooking along by then!)

The prop pitch is the factor that prevails here; it is configured to match the engine’s maximum power capability to the prop load at a specific boat speed, such load being dependent on the relative velocity of the prop race. The pitch would have to be excessive for normal conditions if it were to be matched to possible high sail + engine vessel speeds.

This sounds more complicated than it is – an analogy; you can push a car up to running speed but not beyond, your contribution will become negative (feet dragging) once the engine starts and pulls it along faster than you can run. Your pushing contribution will diminish as the speed gets up to that point.  If your legs were longer, well…

Q: Stig

 20 knot motor sailing?  I’m up for it, as soon as you can spec me an adjustable pitch propeller that I can progressively wind up as the wind cranks,  unless you have a more cunning plan in mind?

A: TC

The variable pitch prop spec herein provided;

And…

When you add in the issues of relative wind and points of sail, I think you need a bigger blackboard. It’s time for an empirical study.

Q: Stig

Today I had a light and variable sail to windward so I applied the single engine + sails methodology and I tried the one engine strategy.  I had my light bulb moment as soon as I tacked.

I have always assumed (the motherhood of most cockups) that you should run the leeward engine, (I guess a sailor’s natural instinct to favor anything that tends to help you get to windward); however I found that the windward engine balanced better.

What I believe is happening, is that as the wind drops and the engine starts to really make a difference, the apparent wind moves forward and the boat wants to round up. With the windward engine powered up however, the boat’s tendency is to turn away from the wind, which is exactly what is required. With this setup, I noticed that the autopilot did a lot less work as everything stayed in balance much better, which presumably would translate into faster and more efficient running.

At one point of sailing, with the windward engine at 1,600 rpm, I managed to sail faster than the true wind of 5knts. Normally when solely motoring we have both 29HP Yanmars at 2,000 – 2,400 rpm for 5 – 6.5 knts.

My conclusions on the subject are:-

1. Always use windward engine.
2. Effective true wind range is 3 – 10 knts – Obviously it works in higher wind and engine revs, but why burn the fuel when you don’t need to.
3. Effective true wind angle is 60 – 140 degrees – So it’s not a great tacking to windward strategy, more for a straight line track. Particularly in really light stuff, just motor straight to windward if that’s where you want to go.
4. Effective engine rpm range 1,500 – 1,800 – Match rpm to wind speed and angle. In light stuff it is easy to over rev, and be headed by the apparent wind. Above 1,800 rpm, the wind is usually strong enough to just sail.

A: TC

A very useful real world trial!

I think it is interesting to note that the engine rpm necessary to make a contribution is lower than what one might intuitively expect, (1,600 rpm motor sailing vs. 2,000 rpm when purely powering), to achieve 5 knots.

The prop pitch/rpm always has to create a theoretical tailrace speed greater than the boat speed in order to make thrust (and burn fuel).  The degree of speed difference will diminish as the vessel speed increases but apparently there is still sufficient surplus at 1,600rpm to contribute thrust while  at 5 knots.  Presumably the engine is working a lot harder when both engines are at 2,000rpm without the sails helping.

You have to love it when imaginings (theory) seems to play out in the real world.

Q: Stig

I spoke to my German friend again.

His engine test was 4hrs at idle, 4hrs at 2,000 rpm, but in neutral sitting in the harbor. Nothing to do with sailing as such! Still he claims identical fuel consumption. He can’t explain it, but would love to hear from someone who could.

A: TC

Love is in the air.  We can leave props and sails out of this one.

This apparent paradox of fuel consumption is related to a story I was told starring the great Rudolph Diesel himself as protagonist.  (Most certainly it is apocryphal since the invention of direct fuel injection post-dated him by about ten years.)

Nevertheless, here is the story;

A Riddle

The prototype diesel engine was banging away nicely with Rudy manually controlling a rack and pinion/plunger arrangement to precisely meter the amount of fuel delivered by a fuel injector for each combustion event. He thought it would be nice if he could let go of the rack at some low fuel rate position and let the engine idle on its own, so he devised an adjustable stop mechanism to hold it at an ‘idle’ point with the appropriate setting.

The engine was revved up and allowed to drop to idle speed at which point Rudy set his adjustment stop, correctly presuming that one perfect idle size drop of fuel would be consistently delivered per injection shot. He then revved up again and returned the rack against the stop, expecting the engine to slow back down to idle.

However, instead of slowing down it continued to speed up, took off and blew apart. Huh?

While he was in the hospital, Rudy gave the riddle some serious thought.

Rudy Considers the Riddle:

With a non-governor equipped gas engine, the throttle butterfly plate (#5 below) modulates the supply of a combustible fuel air mixture and at some throttle plate minimal position, the engine has to work to draw in whatever it gets to burn and it is starved of the means to run any faster (throttled).

It is still possible to over-rev the engine by holding the throttle plate open at no load but for many applications (like cars) you can sense the load and react accordingly, directly modulating the power output. Nicely, the arrangement will provide consistent idle speeds at a particular throttle plate position. Also, even if the throttle plate gets stuck in some unfortunate position, (too open for the applied load) and the engine over-revs, you can always cut off the electric spark ignition.  In any case with this arrangement, the operator directly controls the power output; the engine speed modulates according to the load.

The (modern) diesel engine has no throttle butterfly so it is free to ingest as much air as it wants, power being modulated purely by the injection of metered fuel. The ignition is initiated by the heat of compression so there is no need for spark ignition.

When Rudy set his idle stop he was presuming that the fuel injection rack would have the same function as the gas engine throttle stop, reasonably enough, but there is no restriction to the available air, and the rack controls only the fuel injected per combustion event.

This means that at 500 rpm idle, Rudy was injecting 500 drops of fuel per combustion even at a specific rack setting (lets presume his idle stop position) which equates to 500 drops/min at this speed. When he goosed the engine up to 1,000 rpm and quickly returned the rack to the previously established idle stop position, (500 drops/combustion event), he inadvertently continued to deliver 1,000 drops/min which corresponds to the engine rpm. So the engine had twice as much fuel as it did at the established idle speed, giving it no inclination to slow down in the absence of a corresponding power load. In fact his engine would have needed the rack set to virtually the ‘no fuel’ position in order to slow down. Eureka!

Rudy Fixes the Problem

No human could be expected to constantly stand by the fuel rack and modulate it by hand at idle and various loads so a self-governing arrangement would have to be used in the fuel injection system. Governors were common enough for steam engines and self-regulating gas engines so that was not too tough.

With the typical mechanical governor, you ask for a specific engine speed by tensioning a spring which fights against a set of centrifugal flyweights spinning in concert with the engine rpm. When the spring wins, the fuel rack moves towards the full fuel position, when the flyweights win, the fuel rack moves toward the no fuel position.

For a specific spring tension (your direct input), the flyweights and spring come to an agreement, balancing the fuel injected to the power load. You have no direct control over the fuel rack position which could be anywhere at a specific rpm depending on the load.  But you do have control of the engine rpm within a prescribed range.

To prevent you from over-speeding the engine, there is a limit to how much spring tension you can apply, and to establish a fixed idle speed there is a minimum spring tension setting. The ultimate power possible is dictated by a maximum fuel rack setting which is calibrated to agree with the combustion air aspiration limits and physical capabilities of the engine.

Unlike the throttle control that modulates the power output directly, the manual control of a diesel requests the governor to run the engine at a particular rpm, and it modulates the power required to attain that speed.

This type of governing is used in all the small marine propulsion diesels which, due to their size, are not yet available with electronic fuel injection.  The more obvious on board example of diesel engine governor control is the gen set engine which always runs at 1800rpm to create 60Hz ac power, regardless of the applied load, the fuel rate automatically modulating appropriately.  Common gas engine governor applications we are all familiar with are typically lawn mowers and tractors with settable speed levers.

So, back to the answer you would love to hear (any time now); your friend was really measuring his engine’s ‘rolling resistance’ more than anything. His governor was delivering only enough fuel to keep the engine spinning at various speeds and the fuel/min difference would be low in the absence of an rpm dependent load, like a propeller. His alternator electrical output may have been equally satisfied at idle or higher rpm depending on state of charge etc. Presumably, the aspiration loads (intake exhaust of combustion air) plus water and oil pumping loads would increase with higher rpm but the vagaries of the system efficiency may make them of little consequence.  Coasting is just that.

If he had had a gas engine, your friend would have observed the same apparent fuel consumption paradox but he would have only had to crack the throttle a very small amount to keep the engine to revved up under no load.  The action of moving the diesel throttle levers significantly forward would not have confused the issue.

Our intuition incorrectly associates the throttle lever motion directly with fuel consumption, our sensibilities being informed by the interaction with automotive (throttled) engines.

Your friend was just blowing a lot of hot air around, but like that other German fellow Rudolph Diesel, he will figure it out eventually.

Are you more or less confused now?

Stig:

Bloody marvelous, you never cease to amaze. (ed. keep this in!)

Despite being on the edge of comprehension for most of the email, I believe I got it.

I’ll forward it to my German friend, although it may have to wait until a face-to-face meeting in a week or two, for me to badly explain it to him.

TC:

Well, that’s all she wrote, you made it to the end.  We wish ‘Stig’ every success in his motor sailing, and explaining.

Many Thanks to ‘Stig’ for generously revealing his mixed bag of thoughts, reasonings, and confusions; typical of those from which we all suffer but normally guard against sharing.  The subject matter is especially convoluted and we never would have gone there but for him.

There is no better way to straighten out your own thinking than trying to explain a rationale to someone else.  This quickly reveals the mushy spots in your own concepts.