Fuel efficiency

[psneeld]

Let's back up a moment to the OP.


Loop boat with twins.


Lots of options...miles of slow speed, miles of slightly higher speed, miles of near unlimited speed.


remote ops in some areas....good reason for twins...but not necessary...


But if you have them...running on one in the areas of slow speeds...maybe a good idea.


Not only do you save fuel in some cases if the speed is in the right range, there is less wear and tear on engines, less noise, fewer oil changes/general maintenance...and more that I haven't bothered to think through right now.


So to answer the OP...it is something that should be explored...and as I posted before possible...but at some point you may never get a "good" answer without your own experimentation or buying a duplicate and operating it the same way of someone else did and you trust their numbers.

 

[Nomad Willy]

A rambling thought.
Rig stabilizers and drag one on the side that the engaged engine is on.
You could vary the length of the spars to the drag if the fish or whatever you use for drag. Large “fish” = short spar ect.
Silly? Perhaps, but the boat would go straight fwd w a neutral helm.

 

[koliver]

A rambling thought.
Rig stabilizers and drag one on the side that the engaged engine is on.
You could vary the length of the spars to the drag if the fish or whatever you use for drag. Large “fish” = short spar ect.
Silly? Perhaps, but the boat would go straight fwd w a neutral helm.

Eric: Likely a great way to straighten out the boat, but isn't the title of this thread "fuel efficiency"?
Or course you could hen run with both engines, rig an identical pair of fish and measure the fuel efficiency losses when stabilized, then equate those to running on one only of a pair.

 

[Comodave]

Wouldn’t dragging the fish cancel out the fuel savings of running on one engine? I have never had stabilizers so I don’t know what the fuel penalty for having the fish in the water is.

 

[koliver]

Wouldn’t dragging the fish cancel out the fuel savings of running on one engine? I have never had stabilizers so I don’t know what the fuel penalty for having the fish in the water is.

What I am suggesting is that putting a fish in on the side with a running engine, which will straighten out the boat, demonstrates that pushing from a corner uses more fuel than pushing from in the middle, ergo, no fuel savings, but rather a fuel penalty to use only one of your twins.

 

[Nomad Willy]

My idea dosn’t mean fish”. Just an analogy to get the idea across.
Just something to cause a bit of drag.
The longer you make the poles/spars the greater efficiency.
Would work better w props closer in toward the C/L too.
The whole thing was just mind wandering fun thought.
But Comodave you’re right it would cancel some of the efficiency.
koliver “pushing from a corner” ?
But it could work well.

 

[Portage_Bay]

Based on my limited experience of running twins on a single I wouldn't purchase a twin engine boat intendeding to run her on a single without some extensive trials.

My first encounter was a boat my employer was evaluating. On that boat the running single ran too hot at normal cruise RPM, the GPH readout was high but not 2x indicating fuel burn was being reduced a bit overall compared to running both. But the boat crabbed so much it was difficult to steer. Reducing the RPM to control temps and the boat was too slow and the crabbing even worse. It didn't take us long to determine that single engine running with that boat was not a good normal operating procedure. Of course it would work in an emergency.

My second encounter the boat behaved much better. We'd wrapped a crab pot line and couldn't get it free. 70 miles of open water to reach safe harbor it was call for a tow or run on one. She behaved very well, slight drop in speed, slight increase in GPH in the running engine but well below the combined GPH when running both. The crabbing was minimal and she wasn't too hard to squeeze into a tight slip on one.

Neither of these experiences were long term measuring actual fuel burned to give solid results. Because I don't have the #s this his post needs to be taken as an experience based generalization, it is meant to make the point that some twin engine boats will apparently run reasonably well on a single. And some will not.


There are also transmissions and dripless shaft seals to consider......

 

[bayview]

Add in accommodation costs for a 3 bedroom waterfront apartment in exotic locations x 1500 nights so far and report back.
Doing it in our boat has saved at least $300,000 in accommodation costs thus far

I agree but then you are not addressing the OP topic of fuel efficiency. Most boats would use similar fuel to do all that.

 

[psneeld]

And to repeat myself...there are times running on one for multiple reasons, including savng fuel can always be explored and can provive surprising results. Actually I dont believe fuel efficiency was the big deal to the OP....


Post #7.....


"Thanks to all for responses.Rather what I anticipated,that final result is minimal difference,but thought worth exploring.Actually reinforces that two engines are preferable(as mine is),especially given that the safety factor in event of engine breakdown plus greater docking abilities far outweighs small fuel savings."

 

[koliver]

My idea dosn’t mean fish”.
koliver “pushing from a corner” ?

Using a propeller that is not on the boat centreline, but rather is offset towards one side of the stern. This requires that drag you were proposing, or steering to compensate, both of which methods will require the use of more fuel, not less.

 

[sunchaser]

Using a propeller that is not on the boat centreline, but rather is offset towards one side of the stern. This requires that drag you were proposing, or steering to compensate, both of which methods will require the use of more fuel, not less.

Yup, when running on one engine my rudders will automatically offset about 6 degrees to maintain a straight line.

 

[Nomad Willy]

keith,
More for the running engine but less for the boat saving so much fuel going slow on one engine.
My thought was to completely tame the asymmetrical component so that one could gracefully use the running one w twins stunt. The idea is to capture the running under half power (approx) fuel burn advantage.
I think running on one w twins is basically a stupid idea and my idea of dragging something to straighten the boat out is also a stupid idea.
But it’s a conversation point and w so many experienced skippers in the conversation something positive and useful may come to pass.

But re your #40 post it depends on how much the drag of the rudder compares to the drag of the “fish”. If straightening out the rudder reduces the drag from using rudder to keep the boat straight more than the added drag of the fish ???
Obviously this will happen at some point as the drag requirement for steering correction will be so small that overall efficiency will be increased.
However the distance out would make the use of such a rig to be more trouble than it would be worth.
The question isn’t “will it increase efficiency”. It will if the drag of the fish is far enough out to be small enough to gain overall efficiency by reducing the rudder drag.
It would take experimentation to produce the information required to call it worthwhile. I don’t think it would but there probably would be physics that hasn’t occurred to me or us that could come out in conversation. But not likely IMO.

 

[Irish Rambler]

The answer lies in the hull shape and what it is designed to do and there you'll find your answer.
Some hull shapes can utilize either a single or twins.
In the UK and Ireland people are moving away from twins due to running costs and the reliability of diesels.

 

[koliver]

keith,
More for running engine but less for the boat saving so much fuel going slow on one engine.
My thought was to completely tame the asymmetrical component so that one could gracefully use the running one w twins stunt. The idea is to capture the running under half power (approx) fuel burn advantage.
I think running on one w twins is basically a stupid idea and my idea of dragging something to straighten the boat out is also a stupid idea.
But it’s a conversation point and w so many experienced skippers in the conversation something positive and useful may come to pass.

But re your #40 post it depends on how much the drag of the rudder compares to the drag of the “fish”. If straightening out the rudder reduces the drag from using rudder to keep the boat straight more than the added drag of the fish ???

Eric:

I think this debate is repeating several others over the years, but here goes.

If fuel economy is the goal, pulling back on both throttles will accomplish that goal simply and effectively.

Look outside, observe your wave making. Reduce speed to eliminate all wave making and your fuel economy will be optimized.

Now, if you also want to see whether running on one engine rather than 2 further reduces fuel consumption, you have a baseline.

IMHO any further gains will be elusive, as in order to maintain heading and boat speed, you will now need some rudder or the offset prop pushing forward and the unused prop dragging will put you into a turn. With some rudder applied, in order to maintain boatspeed you will need more fuel.

Remember, this is a test, so boatspeed has to be maintained. You could keep the rudder neutral and add some drag on the powered side, just as you have suggested, but again, as soon as you add drag, you will need to add fuel to maintain boatspeed.

So no matter how you fiddle with factors affecting performance, you will need to add fuel. Now maintain the settings you have settled on long enough to accurately get some mpg numbers. Then turn on the second engine and do it all again, at exactly the same boatspeed. Note the reduced rpm and individually reduced fuel consumption.

Then you will be able to say whether there is any gain in fuel economy by turning off one engine. I would expect the consumption of 2 engines at very low rpm is the same or better than one at the higher rpm required to achieve the same performance.

 

[rslifkin]

Efficiency of running a twin on one engine depends on how badly your engine efficiency drops off at low load and what the typical engine load is at your desired speed.

For a gas powered planing hull idling along just below hull speed, you'll likely burn noticeably less on one engine.

With diesels where light load efficiency is better, the difference will be smaller.

For a slower boat where hull speed is much closer to full speed and engine load is relatively higher at that speed (smaller engines), it's unlikely that running on 1 will save much once the extra drag is factored in.

 

[tiltrider1]

It takes a fixed amount of horse power to move a boat at a given speed. If you reduce twins to one engine you will gain efficiency by having reduced drivetrain drag, you will loose efficiency due to offset rudders and prop drag. In most cases the prop and rudder drag will cost more than the gains from reduced drivetrain drag.

People keep talking about their experience on one engine but we all know how hard it is to do true comparisons. To be accurate you have to be at the same speed in the same sea/wind state and you have to have accurate consumption measurements. I still remember the guy who went up river on two engines and down river on one and noted he used less fuel down river so one engine had to be better than two.

 

[psneeld]

once again...set different parameters and get different results...as I first posted...around and around we go.....

 

[markpierce]

...


In the 35 to 45 foot range, I'd bet that the bulk of us would get between 2 and 3 mph for the most part at hull speed, twin or single.

...

Not in my experience. My boat consumes less than 2 gallon an hour at one knot below hull speed, but well over 3 at hull speed.

 

[markpierce]

Yup, when running on one engine my rudders will automatically offset about 6 degrees to maintain a straight line.

My single-engine boat requires a 3-degree rudder offset to go straight ahead under power.

 

[Nomad Willy]

koliver wrote;
“Remember, this is a test, so boatspeed has to be maintained. You could keep the rudder neutral and add some drag on the powered side, just as you have suggested, but again, as soon as you add drag, you will need to add fuel to maintain boatspeed.”

No. Not if the drag far out on the pole is less than the partially deployed rudder. At some point the drag of the fish device in the water will be so small that it will be less drag than the drag of the forces from the partially deployed rudder.

No I’m wrong. The drag of the stopped prop will not be overcome. I was distracted to the point of focusing only on the rudder drag. That could easily be neutralized by the device I’m talking about. But the overall scenario includes the stopped prop. And the drag of that dead prop cannot be neutralized. Like I said .. the whole concept is only applicable in unusual circumstances.
If you were to ferry a big twin screw boat to Alaska from Puget Sound and you had a strong and effective rudder/s and the prop was not too fat outboard it has been proven to save considerable fuel. But the prop must be removed from the unused shaft.

 

[Rufus]

This subject has been beat to death since the days of RickB...about ten years ago Back in the day member Tim B and I did some long runs on twins while operating both engines, and then with one shut down and the prop free wheeling. We used the same boat speed for twin and single ops. In my case, I used 8.5 knots as the test speed because that's where I like to cruise. Hull speed is around seven. We both established that for our boats there was a 5 to 10% fuel savings with one engine shut down (at our selected test conditions). Savings are undoubtedly more pronounced in boats with old tech fuel governing systems and a lot of excess installed power (mine, for example). Slobbering along at or near hull speed with big turbo twins is not efficient. Last I read, Tim was operating on one during long runs. I've been running on both.

 

[sunchaser]

My single-engine boat requires a 3-degree rudder offset to go straight ahead under power.

Maybe adjust your AP to yield "0" rudder offset when going in a straight line.

 

[markpierce]

Maybe adjust your AP to yield "0" rudder offset when going in a straight line.

You appear to be unaware of tropeller walk.

 

[ssobol]

Not in my experience. My boat consumes less than 2 gallon an hour at one knot below hull speed, but well over 3 at hull speed.

Then I guess your hull speed is not what you think it is. The common calculation is just a rule of thumb, not an absolute law like say Newton's Law or the law that says if you blow more than .08 BAC you get a ticket. The calculation makes no allowance for hull shape or beam, it is just length AWL. It is common sense that a 30' WL sailboat will have a different hull speed than a 30' WL jon boat (for example). Same goes for a 47' Nordhaven vs. a Great Harbor 47 (guessing that they both have about the same WL hull length).

The hull speed calculation only relates to the hull only. Anything hanging behind or under you boat affects the the most efficient speed you can run, but that is not hull speed.

 

[BruceK]

You appear to be unaware of tropeller walk.

Probably is. But I doubt Sunchaser has much to learn about propeller walk.

 

[SteveK]

Sheesh........
You all agree that one engie will operate the boat. However if one of two engines only operate then there is an offset drag coefficient to calculate in order to determine the fuel equation value of using only one of two fully functioning engines.

The solution is simple. Run a system of pulleys and chain drive from one drive shaft to the other and run both from either engine. Of course a disengage clutch contraption is inserted when both engines are operated. Remember the twins rotate opposite so you need something like this.



Or keep doing what works without overthinking it.

 

[FF]

"The solution is simple. Run a system of pulleys and chain drive from one drive shaft to the other and run both from either engine."

Simpler and far more efficient is an OTS tranny that couples one or two engines to a single center line shaft.

Used by the military since WWII.

Only question is large diameter (efficient) prop may increase draft.

 

[rgano]

I am purposely looking for a single. If you want to run at hull speed or close to it, why have two engines? More fuel, oil changes, general maintenance, shaft seals, anodes, etc. One good engine well maintained should work fine I hope. I also argued that you could run twins at hull speed just fine, but I learned from my snowmobiling that you cannot use what is not there. By this I mean if I had twins capable of planing I would likely being planing more than I thought and using much more fuel. I am trying to make myself slow down. Less is more in this endeavor.

Post 7 fully explains why OP wants twins.

 

[Tugalert]

If I were building a new vessel I would build a boat with two engines but one shaft. You can run or both engines. I know a small tug that has that set up.

 

[moparharn]

Which I answered in post #3. The post you reference is one where I stated and explained my own thinking on the matter. As you may have noticed there were many different opinions expressed throughout the three pages of posts. Why you picked mine? Who knows.

Post 7 fully explains why OP wants twins.