Operating a twin on one engine and fuel economy

[timjet]

OK guys, we've thrown out a lot of "facts" that may or may not be true, myself included. I've talked to several boat owners who've confirmed that operating single engine on a planing hull boat saves money. This is real word experience, not theory.But why?


Lets review:


Fact: Planning hull boats have much bigger engines than displacement hull boats.


Fact: A certain amount of the power produced by any internal combustion engine is used to power the accessories and an additional amount of power is wasted in internal friction. Power that is not used to turn the prop and that otherwise does no good other than to enable the engine to run.


Fact: A properly configured planning hull boat will use approx 60 to 75% of it's available power to plane the boat at it's design cruising speed. Same for a displacement hull boat.


Fact: Fuel flow above 75% total power available on any internal combustion engine goes up dramatically.


Let's not get into a pissing contest over these percentages, they may be off, but not by much.


When I run at 8.3 knots, my engines (each) are producing 113 HP (of it's available 330 HP) at 1300 rpm. I'm running 2 engines with a total fuel flow of 4.9 g/hr and a total HP of 226 (2x113)
Shut one engine down and my speed is 7.1 knots, fuel flow 2.45 g/hr and total HP 113.


Why do I only loose 1.2 knots when I cut the HP by half?


Answer: Because at this low of speed and HP output, so much of the power output of the engine is wasted, ie used by accessories and lost by friction. So perhaps you could surmise that it takes 2 gal per hour just to turn the engine and yet produce no usable power. Eliminating that engine and you save 2 gal per hour.


Percentages again: At low rpm 75% of the power output of the engine is wasted, only 25% usable and I believe that is the answer.*

 

[Nomad Willy]

timjet,

"Why do I only loose 1.2 knots when I cut the HP by half?"


Because the hull looses half of it's resistance. It takes half the power to drive your boat at 7.1 knots as it does to drive it at 8.3. AND it will take twice as much power to go 8.3 as it does to go 7.1. You're stuck in this rut of thinking 1/2 power should produce 1/2 speed.
I know my Willard quite well and it takes about 20 hp to cruise at about 6 knots and it takes about 40 hp to go 7 knots. I read about a Willard owner that had an 80 hp engine in his Willard and claimed 8 knots. If I ran my Willard w 10 hp it would probably do over 5 knots. Planing hulls are not nearly as economical as full displacement hulls but they are more alike than different. It will take less power to drive your boat 8 knots w two engines than it will w one. I read your post and agree that there are some advantages to running one engine but I think with all things considered 2 running is best.

 

[Marin]

Okay, I remembered totally wrong. Bob Lowe's tests showed that he increased his range when running on one engine. An interesting fact were the performance increases realized by running on the port engine only and the starboard engine only. They were not the same.

I just looked up the results table he put together in the archives of the GB owner's site but I can't reproduce it here with a copy and paste --- the formatting gets all mucked up. But if you're interested here is the link to his table of results. http://www.mv-dreamer.com/SpeedTrials.htm*

Remember, this was for his 45' Alaskan, which has a semi-planing hull.* Note the speed and fuel burn advantages-- not huge but still there--- of running with the prop freewheeling vs. the prop locked off.

Dreamer is powered with a pair of Ammarine John Deere 6404D diesels (404 cubic inches, working rpm range 1500-2200) driving through Borg Warner Velvet Drive Transmissions.

Here's a picture:




-- Edited by Marin on Saturday 23rd of October 2010 04:36:38 PM

 

[Nomad Willy]

Got these numbers from Marin's GB guy. The "7K *3.85 gph" is an average of the next higher and lower numbers.




Both engines running:

6.2K / 2.9 gph * * // * * 6.9K / 3.5 gph * * // * * 7.7K */ *5gph


Stbd eng only:


6.1K */ *2.5 gph * * // * * 7K */ *3.85 gph * * // * * 7.6 k */ *5.1 gph


Port eng only:


6.2K */ *3.1 gph * * // * * 6.9K */ *4 gph * * // * * 7.7K */ *5.5 gph


Looks to me as though it's closer than I thought it would be but if you average the numbers running both engines looks more (but not much) efficient. I posted the question on Boat Design and it's a huge site so should get some good response.

 

[timjet]

Eric,Marin's example is not a planning type hull, so consequently the engines are working much harder to produce the speed he is getting whether it is single engine or two. The percentage of the HP used to power the accessories is small.


However even in this case it is plane to see a huge increase in fuel economy by running single engine. I'm curious why at 1700 rpm increases fuel flow so much.


In my case, I have so much power, at the 8.3/7.1 knots, they are barely working and much of the HP they produce as a percentage of the total HP produced is wasted.


Again look at my numbers:
2 engine * 8.3 kts * * 4.9 gal/hr * *= * *1.7 mpg
1 engine * 7.1 kts 2.45 gal/hr *= 2.89 mpg


Now I did mention earlier that a post on a different forum indicated that fuel flow on the remaining engine will go up to maintain 1300 rpm. That was refuted by I believe you and Marin. Personally I don't know, but am inclined to think that perhaps fuel flow did go up a little, but certainly not by twice. Looking closely at Dreamer's figures seems to confirm this.


I'll be very interested to hear what the folks on Boat Design have to say. Please keep us posted.


-- Edited by timjet on Sunday 24th of October 2010 07:35:58 AM

 

[RickB]

timjet wrote:I'm curious why at 1700 rpm increases fuel flow so much.

In my case, ... much of the HP they produce as a percentage of the total HP produced is wasted.

You need to look at a propeller power curve and the relationships between speed and power.

An engine only produces as much power as the load demands. Saying that power you don't use is wasted is like saying money in the bank or fuel in the tank is wasted.

*

*

 

[Nomad Willy]

"Running on one engine is definitely saving fuel: friction losses in one engine, gearbox and auxiliary equipment is reduced by 50%. The actual gain is a bit less because prop slip increases due to the higher blade load and there is some extra rudder resistance, but is still significant."

"I made a habit of using just one engine on long trips because it lowered the fuel bills and added a second power steering pump because the port engine didn't have one."----------------------------------------------------------------------------------


The above comments were made by a gentleman from the eastern Mediterranean that I have read over time and have considerable respect for.


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"I've spent several hours at different speeds trying to answer that question. For my 42ft planing boat there is virtually no speed where running on one engine is more reliable, not even close.If you want to save fuel you do two things: Prep the bottom like a racing sailboat (600 grit finish) and keep the speed less than S/L = 1.1.On my boat if I want the best speed with good range a combination of 25-26 knots @ .8 miles per gallon and 9.4 knots @ 1.9 miles per gallon gives a 14-15 knot average speed and 1.2 mpg. (Running the boat at 14-15 knots is .7 mpg)."

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I don't know the 2nd person. He is from BC. Obviously they are 180 apart. And neither of them is even close to my take on the question (hardly any difference). I think Rick has a good idea but it dos'nt take into consideration the inefficiencies of running on one engine, however, it would consider the losses of friction on the dead engine. This question may not be a debatable question as all the variables may not be known. To use Ricks method one would need to know the drag of the boat * *...including running w a trailing prop. I think that probably Ricks method would answer the question IF there was no trailing prop and no asymmetrical thrust. Just considering the power to drive the boat and how much one (or two) engines would consume to overcome such drag would render the single engine operation the hands down winner. But to answer timjet's question one would need to know about the drag variables. I'll keep monitoring the Boat Design post.


By the way I think the GB guy should'nt be dismissed as the GB is more of a planing hull than a displacement, however, it's engines are much smaller.










-- Edited by nomadwilly on Sunday 24th of October 2010 10:04:18 AM

 

[RickB]

nomadwilly wrote:To use Ricks method one would need to know the drag of the boat

First off, I don't have a "method."

Secondly, when the boat is running on one engine or two, the power produced by the engine and the fuel it burns is what it takes to move the boat the speed it is going. There really isn't much room for discussion on that.* It is what it is and knowing the drag or the price of tea is totally irrelevant.

Don't you guys ever just enjoy driving your boats around? This discussion really is in the same category as angels on pinheads.

Who really cares if they save 12 cents an hour or spend 11 cents more? If your margins are that small you need to find a hobby you can afford.

If you want to go fast, move the throttle lever forward, if you want to go slow and save fuel, pull it back.

 

[FF]

If you want to go fast, move the throttle lever forward, if you want to go slow and save fuel, pull it back.

A grand opinion from a mariner that moves with OPM,

Other Peoples Money

 

[RickB]

FF wrote:

A grand opinion from a mariner that moves with OPM,

Sorry FF, wrong again as usual.

I pay for the fuel on my boats just like almost everyone else here, always have, always will.

*

*

 

[Nomad Willy]

This time I agree almost 100% with you Rick. I've been tempted to tell timjet to just get a smaller boat if he's so concerned w fuel burn but I love to pick questionable things apart to see if there's something new and interesting to learn. Yet there usually seems to be people taking part in the discussion while saying there's no point in the discussion. I don't understand why ther'e there. But since you're one of them Rick * ..perhaps you can explain yourself. If I was timjet I'd get a smaller boat but the market being what it is now may make that an undesirable option. If I really wanted to keep the boat and go slow I'd modify the stern into a displacement shape that would allow much better economy than a planing twin running single * * *....without the option of much more speed.

 

[RickB]

nomadwilly wrote:

But since you're one of them Rick * ..perhaps you can explain yourself.

If I really wanted to keep the boat and go slow I'd modify the stern into a displacement shape that would allow much better economy than a planing twin running single * * *....without the option of much more speed.

Explain what? I stayed out of this silliness until it got too ridiculous to ignore then I responded to your question,* FF's traditional stupidity, and your claim that I have some "method."

And if you believe rebuilding the stern of a pleasure vessel is a means to achieve economy then this thread really has become seriously ridiculous.

*

 

[Nomad Willy]

Here is the 2nd post by the gent from BC.

"Yes, I meant economical.

I have run fuel curves for my boat for speeds from about 7 knots to 10 knots. I tried running on one engine over the same speed range. There was no point where the fuel mileage was higher running on one engine. I ran the range with one engine stopped and with one engine idling in forward (to simulate a failsafe condition on one engine). The mileage was worse with one engine stopped than with one at idle in forward.*

The power requirement at a given speed is the same. When running on one engine the added drag from steering to hold course burns more fuel at the same speed on my boat.

My experience is not the same as ___. *I'm sure each boat and power package are different.

My boat is a Sabre 42 Express. Two Cummins QSB 5.9L 425HP engines and Zeus pod drives. 1.5 miles/gallon at 8.7 knots/1200 RPM and 1.4 miles/gallon at 9.2 knots/1300 RPM. Above 10.5 knots mileage is .7-.8 miles/gallon to 25 knots."
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So it appears this fellow has put a lot of effort into finding out what is and what aint. And his boat is not too dissimilar from tinjet's....except the Zeus Drive.
Here is another post. This one is from a man who posts very regularly and I've come to highly respect his views as well.


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"If you know the charter boats made by Delta Marine, the semi-displacement 43 and 50 both have guys that have seasonally commuted to Alaska and locked one shaft or removed one prop for the ride. They felt that it was of value. With fuel at the price level it is now, tho, I doubt that there are many commuters. Certainly here (Kenai Peninsula) there are no longer commuters.


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Sounds like some serious experience went into the history that this post represents. I hear a lot of credible stuff from quite a few smart people and much of it is not in agreement w the rest. This leads me to believe there's a lot of difference from boat to boat but I'm not see'in that difference. I am see'in a lot of disagreement about the elements of this question and the many related variables that lead us around the issue. Is this thing related to politics or religion? I'd like to know * *is it or is'nt it?


"


-- Edited by nomadwilly on Sunday 24th of October 2010 08:03:20 PM

 

[Nomad Willy]

"I stayed out of this silliness until it got too ridiculous to ignore then I responded to your question,"Well if it is silly I would think it would be a very good reason to stay out of it. You say it's reason to jump in. Well * *...here you are and I am trying to explain you because you won't.
You're "system" is as I see it from your posts is to look at the power and fuel consumption curves. But how can we get to "knows" from a wide variety of "unknowns"?
Help us out here Rick or be a lurker for a while.

 

[Baker]

I will just throw a fact in here as well. I just skimmed this thread. But do not assume power curves for diesel engines are linear....they are far from it. Especially turbocharged engines. I don't know if y'all have been looking at actual curves provided by the manufacturer or just assuming that at half RPM, you have half the power......which is totally false.

....as you were.....


-- Edited by Baker on Monday 25th of October 2010 05:38:00 AM

 

[FF]

"I don't know if y'all have been looking at actual curves provided by the manufacturer or just assuming that at half RPM, you have half the power."

This is the key to the problem of attempting to re-prop or new build a really efficient boat.

The fuel map, or BSFC graph is always missing from all the sales material from the engine mfg.

No amount of asking seems to locate them for the common marinizations.

Easier to find the plans for a T88 nuke weapon than actual useful info from the engine folks.

 

[RickB]

FF wrote:"Easier to find the plans for a T88 nuke weapon than actual useful info from the engine folks.

And probably about as useful to a recreational boater.* You've used that excuse not to buy an engine for the past few years FF, it's begining to lose its impact.

Unless you plan to install a CP propeller with a computerized "combinator" to measure torque, calculate BMEP, control pitch, rpm, boost, temperature, timing, and fueling, you won't benefit from your elusive BSFC map. If you have*a few hundred thousand in spare cash lying around*to contract for the engine instrumentation,*tank testing and development of this one-off device that might save you*a few cents a day in fuel, then have at it.


Even the big boys who measure fuel consumption in tons and now run their engines down at 35 percent to save fuel don't use or need what you claim you can't live without. They do demand and base their operation and turbocharger selection on compressor maps so that the turbo recovers as much energy as possible through variable*turbine nozzels and such*but for the most part, unless the*engine is optomized for low load running, the fuel consumption pretty much tracks engine load, percent for percent. That is how engines work.*

As far as the rest of the group goes, trying to calculate this stuff is absurd. Take your boat out and run the thing to see what works best for your engine in your boat in what configuration. Who cares what some other guy claims he gets*out of*a different boat and engine?

 

[Codger2]

RickB wrote:
"As far as the rest of the group goes, trying to calculate this stuff is absurd. Take your boat out and run the thing to see what works best for your engine in your boat in what configuration. Who cares what some other guy claims he gets*out of*a different boat and engine?"

Whew, Finally someone said it! This exercise compares to the solar car race that is*
held annually. The race that has the lightest, skinniest, soap box derby wheels cars
on the planet. The race that endeavors to find the stingiest, energy consuming cars
in the Universe! I know, I know, I don't have to read it, but if you only knew how
"silly" this whole conversation is, as evidenced by the few members who
contribute, you would let it* die a fast death. OR, take up kayaking! That doesn't
burn much fuel either!

End of rant.








*

 

[Nomad Willy]

Here is the latest from Boat Design. This is by one of my favorite guys. I think he is really smart and usually able to see through smoke screens.-------------------------------------------------------------------------------------------------------------
"Quote:
<table cellpadding="6" cellspacing="0" border="0" style="width:100%;"><tbody><tr><td class="alt2" style="font:normal normal normal 10pt/normal verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif;background-color:#dedede;color:#000066;border:1px inset;">Originally Posted by*______.

The power requirement at a given speed is the same. When running on one engine the added drag from steering to hold course burns more fuel at the same speed on my boat.

My experience is not the same as CDK's. I'm sure each boat and power package are different.

</td></tr></tbody></table>
"That I cannot deny.
With the props far apart and a considerable rudder angle to hold course, the fuel savings effect is reduced.

My own experience is from the time I had twin Mercruisers close together. There wasn't much drag increase running on one engine in that case.
I had fuel flow sensors connected to a Navman fishfinder showing a reduction of approx. 30% if the speed was kept below hull speed. With increasing speed the savings were decreasing rapidly.*
How it turned out at planing speed I do not know: maintaining planing speed on one engine was not possible.

The ultimate solution for saving fuel at displacement speed is of course the variable pitch prop because only with that you can obtain the proper engine load at low rpm. But that is quite another topic."
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FF will like the comment about the variable pitch prop. And one of the biggest variables (distance between screws) is emphasized * *...w good reason (I think). Also put on the table was engine loading but at these very low loadings I suspect that's not much of an issue. Fly stuff or fly stuff X2.
John,
Yes and jimjet has had difficulty getting ahold of that. But at these very low speeds and low loadings it's probably closer than usual.
FF,
"Easier to find the plans for a T88 nuke weapon than actual useful info from the engine folks." Your joke was taken seriously. I have seen quite a few BSFC graphs but I look at 30 and 40 hp engines that are frequently used in stationary mode so that could be the reason. I didn't know you were shopping for a new engine??? Why would you do that as you've got the best engine ever built. They win me over w the sound.
This topic is getting to be like anchors. Shall we go for 100 posts or do as Rick suggests * * *... *just can it. I'm game for either.
Hey Walt,
Looks like your'e in your Baugh Humbug mode today. Looks like the thread has about the average number of posts per views. I think I see the problem w you and Rick and probably a bunch of lurkers (they are the majority aren't they?). I scanned the forum and I don't see any other interesting threads so you guys are stuck with this foggy thread about economy, and economy about something not too far different than "how do you get the best economy driving your Rolls Royce"




-- Edited by nomadwilly on Monday 25th of October 2010 10:12:28 AM

 

[RickB]

nomadwilly wrote:"Easier to find the plans for a T88 nuke weapon than actual useful info from the engine folks." Your joke was taken seriously.

I have seen quite a few BSFC graphs but I look at 30 and 40 hp engines that are frequently used in stationary mode so that could be the reason.

Unfortunately, it wasn't a joke, and you don't understand what he is looking for.

FF has been whining for years that he can't find a "fuel map"*for his dream engine. That map is far and away different from the graphs you have been looking at. They don't exist, as a rule, for marine engines that drive a propeller because the propeller laws allow a simple graphing of the relevant information but FF doesn't understand that either.

The fuel map he wants shows "islands of equal BSFC" for various loads and rpm based on BMEP. That form of showing "efficiency" works for spark ignition engines with throttles that drive multi ratio transmissions - like cars for instance. They*are developed for automotive diesel applications which use computer controls and have inputs for air flow, wheel speed, coolant temperature, exhaust gas temperture, inlet temperature and pressure,*and other variables which can be controlled by the computer but are not worth the manufacturer's time and expense otherwise.

See through smoke screens? Usually where there is smoke there is fire, this thread is just smoke with very little heat ...

-- Edited by RickB on Monday 25th of October 2010 12:09:08 PM

 

[Nomad Willy]

Thanks Rick for the clarification about BMEP and "fuel maps" that admittedly I didn't know what were. I may be miss'in some screws on sentence structure as well. I thought the nuke weapon and engine brochure was joke. I must be more careful about what I say as the "smoke screen" comment wasn't directed to anything you said Rick. Was loosely referring to all the widely differing views on this issue. When you refer to BMEP you should state once in a while what it means as most of the Guys here probably have never heard of brake mean effective pressure as I (other than on this forum) have never heard of fuel maps. There's several hundred guys and one or two women we're talking to.

 

[Marin]

Having run our boat on one engine (both port and starboard) and with the prop both locked and freewheeling due to precautionary engine shutdowns I can tell you that, fuel savings or not, it's not a pleasant way to run the boat. The non-powered prop puts out a lot of turbulence that buffets the rudder behind it which, depending on your steering system you may nor may not feel in the wheel. This same turbulence makes steering a rather jerky and non-precise affair, again depending on the kind of steering your boat has. We have cable-chain so whatever happens at the rudder is transmitted to the wheel.

Given that fuel is still one of the smallest percentages of the total cost of boat ownership it doesn't make any sense to me to try to eke out a few dollars in fuel savings by runing the boat in an unbalanced manner. If one was doing a long distance cruise where fuel use was really critical-- like the guy who did the GB stunt from Hawaii to California--- then there may be some reason to do it. But to run on one on the typical coastal cruise doesn't seem at all productive to me in the overall scheme of things.

The fellow on*the*GB forum*who put together the fuel use*and speed chart I posted the link to*comparing running on both engines to running on the port or starboard engine did this because he was curious what the actual numbers would be (for his boat) vs. simply speculation.* But he said he never actually ran the boat this way.* He always ran Dreamer on two engines, be it long cruises down the coast*to Mexico or short ones around the San Juan Islands.


-- Edited by Marin on Monday 25th of October 2010 03:04:55 PM

 

[RickB]

nomadwilly wrote:I must be more careful about what I say as the "smoke screen" comment wasn't directed to anything you said Rick.

I understood that and took no offense.*

 

[Codger2]

Marin wrote:"Given that fuel is still one of the smallest percentages of the total cost of boat ownership it doesn't make any sense to me to try to eke out a few dollars in fuel savings by runing the boat in an unbalanced manner..... But to run on one on the typical coastal cruise doesn't seem at all productive to me in the overall scheme of things."
_______________________________________________________________
I thought about an insightful response to this thread, and guess what? As usual, Marin has stolen my thunder.

 

[Nomad Willy]

One more Canadian w 2 610 hp Cummins says he found no advantage running single. I think he's got a big Sea Ray.Walt,
Marin's a quick draw poster and to our good fortune he is an expert on the world at large.
His best stuff has been (in my opinion) on OTDE but he has beat me to the punch on the boat stuff too. Know'in what to say is good but know'in when to say it and say'in it is golden. Cheers for Marin.

 

[Peter B]

SeaHorse II wrote:Marin wrote:"Given that fuel is still one of the smallest percentages of the total cost of boat ownership it doesn't make any sense to me to try to eke out a few dollars in fuel savings by runing the boat in an unbalanced manner..... But to run on one on the typical coastal cruise doesn't seem at all productive to me in the overall scheme of things."
_______________________________________________________________
I thought about an insightful response to this thread, and guess what? As usual, Marin has stolen my thunder.

________________________________________
Dito, and spot on..... if I had two engines, I'd run them both, but not at WOT very often, I'd just rejoice quietly in having some real reserve power if I needed it, limp home capacity if the worst happened, and the extra manoeverability, but be darned if I'd be frigging round, weaving along, worrying what was happening to the gear box, or whether I had clamped the non-running engine properly, just to save a few bucks diesel.* And while I'm at it, may I remind you guys you still have the cheapest diesel in the world, so stop worrying.

*

 

[timjet]

nomadwilly wrote:

I've been tempted to tell timjet to just get a smaller boat if he's so concerned w fuel burn but I love to pick questionable things apart to see if there's something new and interesting to learn.

Eric,
I'm very happy with my boat. If fuel was the most important issue I would have chosen differently. My purpose in starting this thread was to get folks opinion and apparently this thread has taken a different course than I intended.
I can think of many reasons to be familar with the fuel efficiency configurations of your boat. Going to the Bahamas and back without refueling with their rotten fuel is simply one.



*

 

[timjet]

RickB wrote:Who really cares if they save 12 cents an hour or spend 11 cents more?

Well lets see, if my calculations are correct and the Cummins supplied fuel charts are correct then single engine I'm saving 75 cents per mile on one engine (see original post) at 7.1 knots which equates to $5.32 an hour.

*

 

[timjet]

Marin wrote:

The non-powered prop puts out a lot of turbulence that buffets the rudder behind it which, depending on your steering system you may nor may not feel in the wheel. This same turbulence makes steering a rather jerky and non-precise affair, again depending on the kind of steering your boat has.

Given that fuel is still one of the smallest percentages of the total cost of boat ownership it doesn't make any sense to me to try to eke out a few dollars in fuel savings by runing the boat in an unbalanced manner.

Operating single engine feels no different than 2 on my boat. Once you've applied rudder correction you would not know one was shut down. I have hydraulic steering, small rudders, and props than are relatively close together.

I agree fuel is still one of the lowest costs of boat operation. Let's hope it remains that way.

Perhaps when diesel rises to $5/gal we'll revisit this issue.

*

 

[RickB]

timjet wrote:Well lets see, if my calculations are correct and the Cummins supplied fuel charts are correct then single engine I'm saving 75 cents per mile on one engine (see original post) at 7.1 knots which equates to $5.32 an hour.

Yeah, you mentioned that in your original post. You stated clearly what your results were. and what the fuel savings are for you and your boat.

See the second and every subsequent post ... they talk about, and question how much, if any, savngs are to be had by locking the shaft or letting it freewheel. That is what the thread is about.

Hence the question "Who really cares if they save 12 cents an hour or spend 11 cents more?"


-- Edited by RickB on Tuesday 26th of October 2010 01:05:05 PM