Running a twin on just one - on purpose?

semi-planing said:

[QUOTE
Quote:
Originally Posted by Art

+1
a twin cruises at 8kts/1500revs with a nice load on both engines, so should produce no slow running probs....that's exactly why you're doing 8kts not 6-7kts!

In other words you can't go 6kts in a twin because your engines will be idling.

Well surely you should then run on a single engine at equally economical 1500 revs, which will probably get you 6-7kts and save 50% on fuel.

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Sorry to tell ya semi - Somehow post quotes got mixed up - that one's not mine! - Art

neworleansrich said:

My friend and I have been running our boats from New Orleans to the Bahamas for years. He has a 61ft Hataras with 12-71s. This last trip he put a feathering prop on one side, giving him a maneuvering/ get home engine on that side. The boat cruises at 8kts about 20-30% more efficiently running on a single engine with the feathering prop. Honestly, if you don't run long distances, it does not matter, but for our trips where we'll burn several thousand gallons over a cruise, it does add up. One of the biggest benefits on a long cruise is not needing to change oil on the feathering side.

On my Defever 48 I have 3208s. I will install a feathering prop on the port side next month when I haul out.

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Thanks for that info; those numbers sound right on the mark.

What sort of ball park figure is it for a feathering/folding prop for your size of boat?

Art said:

Sorry to tell ya semi - Somehow post quotes got mixed up - that one's not mine! - Art

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That's my quote, (putting on tin foil hat!)

OK, so I exaggerated a bit to get my point across, it should be 20-30% fuel saving according to post#29

"On my Defever 48 I have 3208s. I will install a feathering prop on the port side next month when I haul out."

Be sure the blade area matches the existing prop.

A difference can mean different engine RPM are required to motor ahead , which gets hard on your ears in time.

Are the shaft rotations such that you can switch sides every few transits?

semi-planing said:

This has also been discussed in several threads over the years. I believe Timjet gathered some real world numbers and I've played around with it myself.

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Yes I did post questions/comments on this several years ago. A member at the time came back with some very precise engineering data most of which was above my head.

To summarize in my case cruising at just below displacements speeds and for me that was 1300 rpm and 8 kts when I shut one down my speed dropped by only 1 kt to 7kts. So I thought that it was definitenly a fuel savings. In fact I thought that if you note your speed and rpm with both running and shut one down as long as your resultant speed is greater than half the speed with both running then there is a fuel savings running on one. Not so.

As I understand and perhaps some experts will chime in, a diesel engine will try and maintain a set rpm at a set throttle position. So when one is shut down, the other engine will increase fuel flow to maintain it's rpm, in my case 1300 rpm. How much more fuel is used can only be really determined with fuel flow meters.

Also hull design, prop pitch, rudder size and the amount of rudder deflection required to counter act the assymetrical prop thrust all come into play and are all different for each boat.

However on the other side of the equation any engine will require a certain amount of power that does not go to the prop, coolant pump, raw water pump, transmission, turbo, that all take power away from doing any work, - power and fuel that is required just to operate the engine but not making the boat go.

So bottom line you won't know for sure unless you have accurate fuel flow.

Do a search with author timjet around the 2011 time frame and you will see some excellant info on this subject.

I decided to operate on 2 engines, it's safer especially when running in narrow channels and fuel is cheap.
Don't worry about ruining a diesel engine by not running it hard enough. That doesn't happen to much, more likely is early failures due to over propping. With those 500 hp engines you mentioned you can get up on plane when needed, but save fuel and cruise at displacement speeds the rest of the time, that's what I do.

"As I understand and perhaps some experts will chime in, a diesel engine will try and maintain a set rpm at a set throttle position. So when one is shut down, the other engine will increase fuel flow to maintain it's rpm, in my case 1300 rpm. How much more fuel is used can only be really determined with fuel flow meters."


Absolutely correct - exactly what a diesel genset is doing talking it from no load to 100% of load. This is also the reason why changing props on diesels to run at lower rpms will most often yield little or no results.
Not so with gas powered boats. The rest of your points also make sense but this is the large contributor.

smitty477 said:

"as i understand and perhaps some experts will chime in, a diesel engine will try and maintain a set rpm at a set throttle position. So when one is shut down, the other engine will increase fuel flow to maintain it's rpm, in my case 1300 rpm. How much more fuel is used can only be really determined with fuel flow meters."


absolutely correct - exactly what a diesel genset is doing talking it from no load to 100% of load. This is also the reason why changing props on diesels to run at lower rpms will most often yield little or no results.
not so with gas powered boats. The rest of your points also make sense but this is the large contributor.

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bingo!!

As I had one engine down for an extended period last year, I learned this:
At the same rpm as before, my remaining engine burned the same fuel as before. My boat speed dropped 2 knots, from just over 8 to just over 6. I had no trouble docking on the side away from the operating engine. I had a lot of trouble docking on the side nearest to the operating engine.
Conclusions:
Fuel is cheap. I would rather burn more to get the extra 2 knots.
I prefer twins.

We recently had a bit of a scare on our boat when we had a runaway starter motor on the stbd engine, at a time when we were transiting a lock and had just gotten secured inside the lock.


Long story short, we ended up having to cruise home on one engine, a distance of about 20 miles. Here's what I discovered as far as speed/rpm is concerned:


Two engines--
to cruise at ~10.5 kts both engines are set to about 1050-1100 rpm's (depends on load(


Single engine--
to cruise at ~10.5kts, the rpm's on the port engine needed to be boosted up to about 1250 rpm's.


That got me to wondering if cruising on one engine, on purpose, would do any harm to the boat. I sent an email off to Sea Ray to pose that question to them.


When I get a reply I'll post their answer on here.

GFC said:

That got me to wondering if cruising on one engine, on purpose, would do any harm to the boat. I sent an email off to Sea Ray to pose that question to them.


When I get a reply I'll post their answer on here.

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The transmissions may need lubrication due to the free wheeling prop. Check your manual and call the tranny manufacturer.
Also your shaft logs, depending on what type you have.

delete double post

neworleansrich said:

My friend and I have been running our boats from New Orleans to the Bahamas for years. He has a 61ft Hataras with 12-71s. This last trip he put a feathering prop on one side, giving him a maneuvering/ get home engine on that side. The boat cruises at 8kts about 20-30% more efficiently running on a single engine with the feathering prop. Honestly, if you don't run long distances, it does not matter, but for our trips where we'll burn several thousand gallons over a cruise, it does add up. One of the biggest benefits on a long cruise is not needing to change oil on the feathering side.

On my Defever 48 I have 3208s. I will install a feathering prop on the port side next month when I haul out.

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That is interesting. What happens on your DF will be good to know. The 3208s are a lot of engine for the 48.

We normally cruise at 7.8 to 8.1 yielding 4.6 gph (tank fill basis) including genset and diesel heat. Engines are PS rated at 225 and we pull less than 60 hp from each on average. The vessel will cruise nicely at 8.8kts with a bump from around 1750 to 2000 rpm.

I rarely participate in threads like this, for the simple reason that the answer requires a lot more diligence than most folks are willing or able to give it. A person claims a certain improvement that lies within a range where environmental factors may have a greater impact, and therefore the person claiming has little ability to separate their desired conclusion. Many are well intentioned, and enthusiastic.

Then we have the multiple discussions, one topic examples. Since I have not heard it yet, I'll give the example. Eventually someone will claim that when they ran on one engine, their consumption was cut by almost 75%. Because when they went from 20 knots on both to 7knots on one engine... Actually saw that on another forum. Examples that will come from entirely different discussion points, all grouped into one analysis.

Suffice to say, the challenge here is not just that different boats behave differently, but that the amount of measurable difference is small. I think by the way, that's the general point and conclusion. Running on one engine, in an otherwise identical environment, will have an eventually measurable increase in efficiency for many boats, but the difference will be small. Small enough to be worth a more wholistic consideration as to its benefit in a given situation, such as wear and tear, etc.. That's not the clean cut conclusion most are wanting to jump to.

There is another thread I'm resisting contributing to on the topic of fire related boat statistics, as the data is so intentionally incomplete and poorly populated with dimensional slicers that anybody with a background in data analysis would raise a few eyebrows. Archetypical for all the wrong reasons.

I'm a data architect/data scientist and have spent 20+ years organizing and working with large datasets. The first clue when trying to figure out who knows what they are talking about with statistical data is to look for opinions with high levels of confidence. The louder and more confident that you hear a definitive conclusion, the more likely it is that you should ignore the persons opinion and look deeper.

Some say they burn less running single on a twin and some say otherwise.
But since it apparently is worth talking about indicates to me that it's a big case for running a smaller engine in either singles or twins.

When skippers run a single on a twin is essentially cutting the size of your engine in half. And when they do they predominately burn less fuel even with the asemetrical thrust and drag of the dead prop. It's obvious that without the two huge elements of drag the boat with less engine burns less fuel. So small engines running harder seem to rule.

ghost said:

The first clue when trying to figure out who knows what they are talking about with statistical data is to look for opinions with high levels of confidence. The louder and more confident that you hear a definitive conclusion, the more likely it is that you should ignore the persons opinion and look deeper.

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Well said. The more data, qualifiers and disclaimers that accompany the data the more I'm interested. (As in:"Show your work")

And how about CALIBRATED fuel flow measurement for each engine without which this thread largely falls flat and reverts to opinions.

Tom,
Most just use hour meter readings and gallons they squirt in the hole. So much time is spent by most everybody at idle or other speeds slower than normal cruise speeds most gph postings are way off.

The only time I was close was running from AK to WA and keeping track of time run at less than cruise speed and subtracting that from total time. Then how much I squrited in the hole had real meaning .. but still off.

The difficulty of reaching any conclusion here is that people have tried different approaches. Commonly they try to maintain the same speed on one engine. In my case that was not reasonable. Too much steering offset. If I slowed down to a speed where steering was reasonable the fuel use declined measurably. Speed went from about 9 knots to ablur 6.5. fuel went from 2 NMPG to about 3 NMPG. A problem was going far enough to really measure use. My 120 Mile trip no doubt had inaccuracies in fillup measurements, nevertheless I found a benefit.
The unanswered question fundamental to the discussion is what would the fuel use be at the same low speed on two engines. No doubt it would also be significantly lower however at that speed both engines were just above idle so I felt it better to load one engine a bit more than run one at very low load.


My conclusion is that were I to make an extended offshore trip where range was paramount.I would run at lower speed on one.

ghost said:

I rarely participate in threads like this, for the simple reason that the answer requires a lot more diligence than most folks are willing or able to give it. A person claims a certain improvement that lies within a range where environmental factors may have a greater impact, and therefore the person claiming has little ability to separate their desired conclusion. Many are well intentioned, and enthusiastic.

Then we have the multiple discussions, one topic examples. Since I have not heard it yet, I'll give the example. Eventually someone will claim that when they ran on one engine, their consumption was cut by almost 75%. Because when they went from 20 knots on both to 7knots on one engine... Actually saw that on another forum. Examples that will come from entirely different discussion points, all grouped into one analysis.

Suffice to say, the challenge here is not just that different boats behave differently, but that the amount of measurable difference is small. I think by the way, that's the general point and conclusion. Running on one engine, in an otherwise identical environment, will have an eventually measurable increase in efficiency for many boats, but the difference will be small. Small enough to be worth a more wholistic consideration as to its benefit in a given situation, such as wear and tear, etc.. That's not the clean cut conclusion most are wanting to jump to.

There is another thread I'm resisting contributing to on the topic of fire related boat statistics, as the data is so intentionally incomplete and poorly populated with dimensional slicers that anybody with a background in data analysis would raise a few eyebrows. Archetypical for all the wrong reasons.

I'm a data architect/data scientist and have spent 20+ years organizing and working with large datasets. The first clue when trying to figure out who knows what they are talking about with statistical data is to look for opinions with high levels of confidence. The louder and more confident that you hear a definitive conclusion, the more likely it is that you should ignore the persons opinion and look deeper.

Click to expand...

It seems you occupy the moral high ground? But then you make statements without any data to support it, highlighted in red. Ironic that you go on to talk about jumping to conclusions!

In post #6 I attached a graph showing Dreamer's data (link to webpage also) and my own sea trials. The result is the opposite of your conclusion. My data was from the John Deere fuel flows that I log and cross-check against fuel fills. Two boat are not many, but the data is real.

Now there have been posts about boats with large DD engines running slow. Easy to believe their claims that running on one is more efficient (at the same speed) because the higher the load the less fuel those engines slobber. In conjunction with Eric's comment about smaller engines, this leads me to suspect that the more power a boat has installed then the more likely it is that running on one engine at a given displacement speed will use less fuel. Just a suspicion, I'd like to see more data before making pronouncements!

I won't be contributing here any more. This is exactly why. Thx

Maybe he was just a ghost.

Sure liked some of his past posts.

Indeed bigger engines would clearly have greater need (I use the ward lightly) to run on one engine. If they (engines) were small enough even running at light load would not be desired or likely.

With my own boat I'd have to run one engine (assuming two 20hp engines instead of one 40) full bore to achive my normal cruise speed .. if it was possible. And conversely if I had 70hp (twin 35's) I'd be very inclined to run on one. 36hp is what Willard provided for the boat and the W30 is one of the rare boats not overpowered.

But talking about power on a FD boat has it's limitations when most all the other club members run SD boats. But comparisons are interesting often lead us to truths.