Click for Abeking Click for Nordhavn Click for Abeking Click for Delta Click for Perko

Diesel engine/prop/hull efficiency questions

Discussion in 'Engines' started by Dan Evans, Jul 15, 2008.

You need to be registered and signed in to view this content.
  1. Dan Evans

    Dan Evans Senior Member

    Joined:
    Jun 17, 2007
    Messages:
    124
    Location:
    Port Townsend
    I have always thought of a long range cruiser as a bathtub looking thing with one small diesel that tops out around 8 kn. So I was a little skeptical when I saw an artical on the 128' Global making these claims:

    max speed 30 kn
    range at 10 kn is 7,800 nm
    range at 12 kn is 6,000 nm
    fuel is 8,000 USG with 1,800 aux tanks

    Does this sound right to anyone who knows Browards (or perhaps owns one;) )? I looked around a little at other builders just to double check and it seems like the 128' has an amazing range for being able to push a 25 kn cruising speed. Are all Browards like this or is this just wishfull thinking?

    Thanks for all your input in advance!
    Dan
  2. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,311
    Location:
    9114 S. Central Ave
    It is some kind of thinking, not sure what kind though.

    7800 miles at 10 knots would take 780 hours
    9800 gallons of fuel consumed in 780 hours is ~ 12.56 gallons per hour

    According to MTU, the 16V2000 M93 engine (2400 hp @ 2450 rpm) consumes 14.2 gallons per hour while delivering 275 horsepower to the prop at 1200 rpm.

    This means that according to the specs, that boat can do 10 knots on 1 engine running at just above idle :rolleyes:

    This doesn't even count the fuel used by the generator(s) but maybe they use an air motor for hotel power. ;)
  3. Dan Evans

    Dan Evans Senior Member

    Joined:
    Jun 17, 2007
    Messages:
    124
    Location:
    Port Townsend
    Oh don't bring that up again!:D It looks like you have just disproved the range that they claim. I am still curious how far they really can go on a full tank(s) of gas. Every article that I have found for this yacht, as well as Browards website, gives the same info. Hopefully there is someone out there who has cruised on a Broward and watched the fuel burn...

    Dan
  4. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,311
    Location:
    9114 S. Central Ave
    "Every article that I have found for this yacht, as well as Browards website, gives the same info."

    That is a sad example of "cut and paste" reportage ... nobody questions, nobody researches, they just cut and paste what the press release said and call it a news item or boat review.
  5. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    14,432
    Location:
    Fort Lauderdale
    Most all yachts are like this if you run them slow. If you run most any diesel at 1,000 rpm's or even less they use a minimal amount of fuel. So if you run any yacht at hull speed, you should get tremendous range. A 16v2000 makes 2000 hp,yet at idle only burns 2-3gph. I was on a 75' sportfish that burned 150gph at 35 knot cruise with 16v2000's, yet at idle we did 9 knots and burned 6 gph ie. 1.5 MPG.

    Generators use very little, we have a 20KW Onan and a 27KW phasor and both of them will use .5 GPH loaded.
  6. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,311
    Location:
    9114 S. Central Ave
    "... we have a 20KW Onan and a 27KW phasor and both of them will use .5 GPH loaded.."

    Those are some amazing generators. Half a (US) gallon per hour is what a 20 kW diesel genset should burn between no load and about 25 percent. Loaded to 20 kW it should burn around 1.8 gph.

    What rpm are those 16Vs turning at idle to give you 9 knots? Three gph clutched in sounds a bit optimistic to me. I would expect about 3 times that much or around 9 gph per engine clutched in at around 600 rpm.
    Last edited: Nov 8, 2008
  7. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    14,432
    Location:
    Fort Lauderdale
    That is what the generators average over running days upon days 24hrs a day all of the time off of their own fuel tank. That is running 2 refrigerators, 3 freezers, 6 a/c's, hot water heater, and everything in between.

    The 16v2000's are at dead idle to push the boat 9 knots. It is a custom 75' sportfish with a very efficient hull design considering it tops out at 43 knots with 16v2000's. I think you should do your homework on custom sportfish because most of the large custom sportfish...... ACY, Merritt, Jim Smith etc. 70-85' cruise right at/around 35 knots and they all do about 9 knots at idle wih both engines in gear. And 16v2000's at idle in gear burn about 3 gph each, in an efficient hull design. Have you ever been on a yacht with 16v2000's or several of them? I have run 16v2000's, c32 CATS, c30 cats, c18 cats, 1350 mans, 1100 MANs as well as many others and they all have single digit fuel consumptions under 1,000 rpms.

    According MTU's website 16v2000's burn 13gph at 1200 rpm's, so what do you think they burn at 600 rpm's?
  8. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,311
    Location:
    9114 S. Central Ave
    "According MTU's website 16v2000's burn 13gph at 1200 rpm's, so what do you think they burn at 600 rpm's?"

    If it has a prop connnected to it, around 9 gph, look at the propeller curve and see how much horsepower it absorbs at 600 rpm. Just like the generators, it takes a certain amount of fuel just to turn the engine at idle, generating electricity or turning a fixed pitch prop takes more fuel. Look up the no load fuel consumption for a 20 kW Onan. I didn't dispute your claim of the speed of the boat at idle, that is irrelevant.
  9. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    14,432
    Location:
    Fort Lauderdale
    The speed of the boat at idle is part of the equation. The faster the speed at idle, the less engine load, and the less fuel needed for the motor to turn the propellor. Load is load. Browards are relatively flat hulls and all of the ones I have run have had really good displacement speeds. I can believe their range figures, considering I do 15k NM in deliveries each and every year and have done a lot of long slow ones where fuel range is an issue on a variety of different vessels.
  10. K1W1

    K1W1 Senior Member

    Joined:
    Sep 30, 2005
    Messages:
    7,388
    Location:
    My Office
    Hi,

    I think what Marmot has written has been misunderstood.

    No matter who built your engine or how much it's top end output end is there will be a fuel figure given in lbs per hp/hr or grams per kwh.

    This means that if say at idle you develop 100 kW your per engine consumption will be 100 times the given figure. This is not a linear calculation through the whole rev range because the developed power will curve upwards sharply as the engine reaches the higher speeds.

    I know of a displacement yacht that crossed the Atlantic some 24 yrs ago, in an attempt to save fuel it went on one engine- swapping every few days. They tried to run at their normal cruising speed. End result was more fuel burnt than if they had run on both at a lower rpm setting.

    A quick guide to the intricacies of pressure charged engines is that the higher the boost pressure you have the higher volume you are perceived too have and therefore the larger amount of fuel that can be burnt in that volume of air. Hence you will develop the most power and burn the most fuel when the boost pressure is the highest.

    As for your 20 kW Genset I don't have ONAN Books to hand but have some CAT info a C 2.2 developing 21 Kw at 1800 rpm ( 60 Hz) uses 1.98 US Gal/hr. I don't see how ONAN could have internal combustion engine package that develops the same sort of power on 25% of the fuel.
  11. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,311
    Location:
    9114 S. Central Ave
    "... there will be a fuel figure given in lbs per hp/hr or grams per kwh."

    That figure, the brake specific fuel consumption or BSFC curve is usually lowest at the design max continous output power. At low power and at very high power the weight of fuel burned to produce each horsepower is higher. So not only is there a minimum amount of fuel required to run at idle, the engine is less fuel efficient at that power setting.

    It doesn't matter how efficient the hull is, it takes power to turn the prop. Look at any propeller curve and that is very obvious. DD doesn't publish the graphs for some reason, but taking a page from K1W1's book, the 2000 hp CAT 3516 burns 8.5 gph at 600 rpm clutched in. Like the man said, I don't see how anybody else can do much better.
  12. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    14,432
    Location:
    Fort Lauderdale
    I understand that it takes power and fuel to turn a propellor. But the load factor still determines how much fuel is needed to maintain 600 rpm's. When you put a motor in gear, you can watch the digital fuel displays and they'll show a higher number say 9gph, as the boat gains momentum and reaches the speed it will travel at, it takes less torque to maintain 600 rpm's and the fuel consumption will go down. It is like taking a tractor trailer and running it up a hill loaded at 2100 rpm's, it is going to use a heck of a lot more fuel because you will have considerably much more throttle then if it is on even ground or going down hill at 2100 rpm's. Fuel consumption is determined more by load factor then the rpm's it is running at. Look at a diesels fuel display (consumption) at 1950 rpm's in neutral, compared to if you are underway.

    Anyways, I guess Broward Marine simply made up those numbers out of the sky and they aren't based upon anything scientific and you should get an attorney and sue them for false advertising.
  13. K1W1

    K1W1 Senior Member

    Joined:
    Sep 30, 2005
    Messages:
    7,388
    Location:
    My Office
    it takes less torque to maintain 600 rpm's and the fuel consumption will go down.

    Hi,

    This is because the load has gone down from the initial turn of the prop.

    Try turning the shaft by hand it will be harder to get going than to maintain the motion.
  14. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,311
    Location:
    9114 S. Central Ave
    Not to be too pedantic but load factor is a ratio. The amount of power absorbed by the propeller is the load.

    Since boats don’t go up or down hill on a road, they can only accelerate or decelerate as they are constantly doing in all but dead calm air and water. The propeller absorbs a certain amount of power to turn a given rpm in water. That amount of power is the same whether the prop is driving a barge or a sportsfish, the only difference is the speed the driven hull will obtain from that power. If you look at the propeller curve you will find the amount of power required to turn the prop at 600 rpms. I suspect that number will be around 70 horsepower in this case.

    Using the figures supplied by DD for the 16V200-91 (2000 hp @ 2350 rpm) the engine consumes 104 US gallons per minute to produce 2000 hp. That translates to a BSFC of .364 lb/hp-hr or a thermal efficiency of around 38 percent. Not too shabby a set of figures. Back off to a cruise power setting of 898 hp at 1800 rpm the engine burns 44 gallons per hour for a BSFC of .342 lb/hp-hr giving an efficiency of 40 percent which is pretty much the highest in that class of engine. As the power is reduced, the fuel burn goes down to 13.3 gph at 1200 rpm to deliver 268 hp. This works out to a BSFC of .35 lb/hp-hr. As I wrote earlier, the best BSFC is obtained at the normal cruise setting and increases at either extreme of power output. The thermal efficiency at this power has dropped a percent to 39.

    Now, if we reduce rpm to 600, the propeller curve will tell us it absorbs somewhere around 70 hp … give or take a few horsepower. Please correct me if you have a curve for that boat, I don’t have one handy.

    Here is where it gets interesting. If you idle that engine in gear and it produces 70 hp from burning 9 gph as I suggest it does, the BSFC increases to .84 lb/hp-hr or more than twice as much fuel to produce each horsepower than at 1800 rpm. The thermal efficiency has dropped to a miserable 16 percent. This is not a happy place to run that engine/prop combination. The reason it is so bad is that as the power is reduced the amount of fuel required to just turn the engine over becomes a much larger percentage of the power produced, so just like a car sitting at a stop sign, the mileage goes to zero.

    If that engine only burned 3 gph as you suggest, then the BSFC at 600 rpm is way down at .28, far better than the “sweet spot” at 1800. The thermal efficiency at that fuel flow has magically increased all the way up to 49 percent, right up there with the best slow speed two stroke diesels in the world. And those big 2-strokes are the most efficient diesel engines ever made by man.

    (And for those who wish to do the math, I based the calculations on 7 lbs per US gallon diesel with a heating value of 18,390 BTUs per pound.)
    Last edited: Nov 10, 2008
  15. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    14,432
    Location:
    Fort Lauderdale
    You're forgetting one variable in your calculations. Engine efficiency will be much better at slow rpm's 1000 rpms and under because you are not using a considerable amount of the HP to turn the Turbo as well as cool off the increased air charge temperature from the Turbo's compressing the air into the engine (boost). At under 1000 rpm's, you are not even creating enough backpressure to spin the turbo's and the exhaust air is simply flowing freely through them. Turbo's use HP in order to make more HP. It takes a certain amount of HP to spin them and most figures are about 40% of the additional power they create.

    A sportfish has a lower load factor at idle and faster speed once it gains momentum then a tugboat would because you also have to factor in that you are using a different reduction in the gear case and thus you are going to have a different shaft speed as well. There is a lot larger speed range between idle and cruise rpm's and the hull is more efficient at lower rpm's (displacement speed) and takes a lot less HP to move it through the water then getting and maintaining planing speed.

    According to the digital displays (which were amazingly accurate) on the vessel with 16v2000's I am familiar with. At 1950 rpm's we burned 150gph (both engines) and cruised at 35 knots. At dead idle in gear with 1 engine we did 7.5 knots and 1 engine burned about 5 gph and the 1 engine in idle burned 1 gph. With both engines in gear at idle fuel consumption was 6-7gph for both engines and speed was 9-9.5 knots depending on current and fuel load. Top speed was 43 knots, 2350 rpms, 220 gph. Our most efficient speed above displacement speed was 1200 rpms where we were doing 22 knots and using around 28 gph (total).

    Efficient cruising speed totally depends on the hull configuration as well as the engine chosen. Some yachts at 1800rpm's would be at maximum plow and only doing 14 knots, and at 1950 rpm's on plane and doing 22 knots. Needless to say there are a few planing 100-130ft yachts that cruise at 20-24 knots and post impressive range at 10knots and 12 knots...... Westport's are ones that instantly come to mind. Most of these 100-130ft yachts end up travelling at 10-12 knots 97% of the time, even if they can do 20-24 knots.
  16. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,311
    Location:
    9114 S. Central Ave
    I am both amused and saddened by your description of the contribution (or rather deteriment) of a turbocharger and aftercooler to an engine's efficiency. Amused because of the naivete, and saddened by this illustration of the level of technical literacy so prevalent aboard so many boats. Bon voyage, Cap.
  17. YachtForums

    YachtForums Administrator

    Joined:
    Dec 22, 2002
    Messages:
    20,353
    Location:
    South Florida
    I think we have a little compressor confusion here? ;)
  18. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    14,432
    Location:
    Fort Lauderdale
    It does take a certain amount of airflow and power to turn the turbo, they do create additional power over and above that. Needless to say, I am not an engineer, nor am I an armchair engineer. I do however have a vast amount of real world experience with diesels and range, considering I do 15,000 NM's in deliveries each and every single year. And, I do know how to rebuild an engine, have built a few race engines, and have a working knowledge of how a diesel engine works. I am also quite versed as to how a turbo and aftercooler work as well as an intercooler. When a turbo (or anything) compresses air it creates heat, and it then passes through an aftercooler (usually cooled by seawater in a yacht application) and then helps to cool off the intake air. There are also many turbo diesels in yachts that do not have an aftercooler although that is not the case with the vast majority of newer diesels. It is still higher then ambient air that a natural diesel would take in, in a well ventilated engine room. Diesels with turbo's also go a LOT less hours between rebuilds then naturals. You're basically throwing fuel in there to cool off the air intake charge as well as slow down the combustion process and prevent detonation.

    Needless to say, the published range figures are believable and probably accurate. I also have experience with 16v2000's as well as many other makes and models. In the experience I have, one one engine at 7.5knots we burned 5gph on the engine in gear and 1-2gph on the engine in neutral. At 9 knots, we burned 6-7gph at idle with both engines in gear. At 1200 rpm's (our best cruise speed) we did 22 knots at 28gph, at 1950 150gph at 35 knots, and wide open 43 knots and 220 gph. Almost all diesels under 1000rpm's burn very little because it takes very little HP to move the vessel, compared to getting on plane and keeping the boat on plane. You also fail to realize that load figures can be different at idle between one boat to the next and vary with propellor type, gear reduction, hull design and such. A yacht has a reduction and propellor so that it achieves 100% load at rated RPM in this case 2350. From idle to there depends on many variables such as propellor type, cup, pitch etc. etc.

    Anyways here are some actual figures for you to chew on. I wasn't going to spend all day looking for more tests as the ones in other magazines don't show fuel burn below 1000 rpms for larger yachts and larger engines.

    Here's a 48' searay that burns 1.3gph at idle and 5.5 knots, thats 4 mpg, yet at cruise burns 1.5 gpm.
    http://www.motorboating.com/articleHtml.jsp?ID=1000066679

    Here's a 48 Fairlane similar scenario

    http://www.motorboating.com/articleHtml.jsp?ID=1000065239
    A 41' Rampage
    http://www.motorboating.com/articleHtml.jsp?ID=1000063633

    More reading:

    Turbochargers allow an engine to burn more fuel and air by packing more into the existing cylinders. The typical boost provided by a turbocharger is 6 to 8 pounds per square inch (psi). Since normal atmospheric pressure is 14.7 psi at sea level, you can see that you are getting about 50 percent more air into the engine. Therefore, you would expect to get 50 percent more power. It's not perfectly efficient, so you might get a 30- to 40-percent improvement instead.

    One cause of the inefficiency comes from the fact that the power to spin the turbine is not free. Having a turbine in the exhaust flow increases the restriction in the exhaust. This means that on the exhaust stroke, the engine has to push against a higher back-pressure. This subtracts a little bit of power from the cylinders that are firing at the same time.

    http://auto.howstuffworks.com/turbo.htm
  19. K1W1

    K1W1 Senior Member

    Joined:
    Sep 30, 2005
    Messages:
    7,388
    Location:
    My Office
    Hi,

    Before anyone has a fuel ( blood) starvation related seizure I want to know---

    Has anyone actually determined how far this Broward that was the original subject of this thread actually managed to go on a tank of gas?

    The turbine and compressor sides of a Exhaust Gas Driven Turbocharger will normally turn at all RPM.
  20. K1W1

    K1W1 Senior Member

    Joined:
    Sep 30, 2005
    Messages:
    7,388
    Location:
    My Office
    Hi,

    CaptJ,-I missed the big post you did and feel that I have to comment upon some of the inaccuracies and confusing things posted there.

    What exactly cools the Intake Air?

    Do you think that this might somehow be related to the amount of fuel being burnt in the hours those run versus the amount burnt in a NA Engine?

    Where did you read this?

    The article you referred to for this info is obviously used to dealing with small automotive GAS engines using figures like this.

    20 Yrs ago I ran The Engine Room of boat that had a pair of MTU 16V396 TB 94 Engines rated at 3500 Hp each at 2100 Rpm and this one used to plane quite well. We used to have a boost pressure of 5 Bar at WOT, That is some 75 PSI.

    Even the old and doddery CAT 3306 needs some 10 Psi to get it's 160 Ekw when used as a genset