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Hatteras vs Viking Yachts

Discussion in 'Hatteras Yacht' started by Captain Dufy, Nov 8, 2015.

  1. PacBlue

    PacBlue Senior Member

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    You have not grasped the concept that the engine does not reach the lug down point until the prop demand exceeds the engines ability to provide the necessary horsepower. We are talking 8 knots, not 12, 14, 16, etc. This diagram should give you a better picture:

    upload_2017-11-20_9-1-15.png

    In short, you have ignored the reserve power available in all of your "analysis" as shown in this example of a theoretical power curve.
  2. PacBlue

    PacBlue Senior Member

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    Ralph, you make a very good point of the difficulty of steering a twin screw vessel on a single screw, certainly nothing you would want to be doing in traffic, going under a bridge, etc. unless you absolutely had to.

    Keep in mind that the hull speed for the 80' Hatt is 11 knots, so 8 knots is not too big of a reach on a single screw. Your 58' Bertram has a hull speed closer to 9 knots, so running it at 10 knots on a single screw is not really the same comparison as you are pushing your modified- vee above hull speed into the pre-planning region which is a very inefficient task for a single screw.

    Also keep in mind that the question centered around burning 8-9gph on a single screw. The 12V-71TI burns 8gph at 1200 rpm producing 100bhp, so we could guesstimate that he might make somewhere near 8 knots. I believe he is looking at getting the desired 1nmpg figure.

    But the point is well taken and would be the most limiting factor in regards to choosing whether to operate on a single screw as a long distance cruising mode.
  3. PacBlue

    PacBlue Senior Member

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    It finally took you this long to say something relevant even though you are just repeating what Ralph stated earlier. Yes, it will be difficult to steer a twin crew vessel on a single screw.

    I assume the Hatteras is a sistership to this one: http://www.**************/boats/199...Fort-Lauderdale/FL/United-States#.WhLZy3mWzIU

    No I have not been on one, but have run on single screw up to 125 feet. This was a triple screw MY that was designed to used the centerline engine for long distance passage as well as high speed cruising. No issues, no phantom "overloading", no doom and gloom as you fear. My point all along is that the owner could run his Hatt 80 on a single screw up to 8 knots. This has not changed even though you can not grasp the concept and choose to make statements that are not accurate to the posters question. Look back at what you have said, you go to maybe to absolutely not, and now ok but you will have steering problems.

    What a journey!
  4. Capt J

    Capt J Senior Member

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    That is not true. I have over 10,000 NM's with a Hatteras that is a sistership to the one mentioned a 75' which hull speed was 10.3, hull speed on this 80' will be 10.7 knots. Given the extension balancing fuel (weight) is critical and getting it off 50 gallons in the wrong direction and 10.7 knots will become 8.7 knots before your eyes on the GPS. The best economy he will see is running BOTH engines at 750-800rpms to get 8 knots at 7-8 gph. Your fuel figures are way off. Once you get over 1000 rpms on Detroits, they really start DRINKING it. 1000 rpms is a good compromise of the engines running pretty clean, temps being on the thermostats, and at hull speed, stability,tracking straight, and fuel economy. Those boats are most stable right at hull speed. 800 rpms with both engines will net you much better fuel economy but the engines will run in the 140F range and soot up more and rock and roll more, as well as veer a lot. Running this boat on 1 engine at 1200 rpms, the engine WILL be lugging and black smoking like crazy because at that point you're asking the engine to provide more HP than it can and the rack is just throwing tons of fuel which isn't getting burned because the turbo doesn't start providing any boost on the engine till 1350 rpms. Detroits need to spin rpms to make HP. Lugging is ANYTIME in the RPM range that you're asking the engine to provide more HP than it is capable of making at that RPM point, at that point the rack just throws a ton of fuel at it and it's unburnt and you end up with black smoke out the exhaust which in turn washes cylinder liners and dilutes the oil washing bearings out. These figures are with both engines in use. Running a single at 1200 rpms and you can double that fuel burn figure almost.......
    12v71TI (1 engine) this is exactly what you will see in this boat with typical 110 LPH injectors
    600 rpms 2gph
    800 rpms 3.5-4 gph
    1000 rpms 6 gph
    1200 rpms 12.5 gph
    1400 rpms 22-23 gph
    1600 rpms 30 gph
    1800 rpms 37-40 gph
    1900rpm 40-45 gph

    In the 3 screw you mentioned, it is different, because the center engine is propped based on only that engine to push the entire boat..... so it will hit it's rated WOT RPM with just that center engine running, apples to oranges comparison.

    Without the numbers attached to your reserve power graph it is useless. There is about 30% reserve power on the lower half of the RPM range, NOT 100% reserve power which is what you're asking this engine to do by running it on 1 engine and crabbing and dragging a dead propeller around and pushing a big HEAVY 80' motoryacht around.
    Last edited: Nov 20, 2017
  5. PacBlue

    PacBlue Senior Member

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    You just don't get it the lugging concept and probably never will. The graph without figures should school you on the concept first, the numbers would come from whatever engine you select. Just take out one of your boats and run it on two engines at hull speed then on one to get a better feel what you are talking about. Let me know which boat you choose and I will give you the hull speed.

    The factory fuel consumption data for the propeller load on a 12V-71TA shows the following:

    1200 rpm - 9gph
    1400 - 12.5 gph
    1600 rpm - 17.5 gph
    1800 rpm - 23.5 gph
    2000 rpm - 32 gph
    2200 rpm - 42 gph
    2300 rpm - 48 gph

    upload_2017-11-20_13-33-59.png
    This is the factory data used to analyze propeller / engine matching. In a single screw application you are falsely assuming that the rpm is held constant, it is free to move to the point where it can deliver horsepower to make 8 knots. This is not the same rpm that is applied in the twin screw application.

    Some other concepts for you - Hull Speed is a calculation given as 1.34 times the square root of the waterline. Using 70 feet as the waterline for an 80' Hatt gives you a hull speed of 11.2 knots, this is a very simple and basic concept to grasp.

    Amazed that you are going to try and tell me how the triple screw 125 MY was propped without any knowledge of the boat or the application. So to clarify, it was triple 3412 Cats and the centerline engine was propped to run with ALL three screws online as the owner liked a fast cruise. That is, it would hit its rated rpm with all three screws together. If needed, long distance runs were done at hull speed on the single screw, pretty basic concept.
  6. Capt J

    Capt J Senior Member

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    First off, your graph and numbers are for 12v71 TA's which is not what the OP is talking about. The OP is talking about 12v71 TI's. Secondly those are REAL WORLD numbers from a boat that's almost a sistership to the one the OP is talking about and taken over 10,000 NM's. At 1000 rpms we burned 12 GPH for both engines all day, every day, and even Sundays at 10.3 knots......1100 rpms was still 10.3 knots. Fuel burn depends on RPM AND load, at 1000 rpms you are at hull speed, after that you're pushing a heck of a lot of water from 1050 rpms-1600 rpms and burning more fuel than even your TA graph yet gaining little speed. I did the entire Great Loop at 1000 rpms 90% of the time.

    The Hatteras ran Covingtons, which had a screwy generator injector height setting of 1.46" and a retarded camshafts, so the fuel numbers you posted don't match the numbers the OP will see on a 80' Hatteras. Not to mention they're for a 900HP 12v71 TA and not a 12v71TI which doesn't have blower bypasses and such to increase efficiency and gain HP.

    Your hull formula relates STRICTLY to displacement hulls. The Hatteras MY we are talking about is a semi displacement, so the formula is useless as tits on a bull as it has an entirely different hull shape than displacement and especially with an extension on it and the Hatteras hull starts to try to pick up the bow below 11 knots and starts plowing water. The 80' is based on the 70' MY and the 75' I ran was based on the 65' MY but at the end of the day they performed about the same RPM/Speed. I've run over 200 of these old Hatteras' over the years and most all iterations except an LRC, I cut my teeth on them. But feel free to armchair away.
  7. PacBlue

    PacBlue Senior Member

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    If you looked at the yachtworld link, you will see that the 80' Hatt has 12V-71TA's, 870bhp, and without knowing the specific boat that is a safe base assumption.
    The factory numbers would show 18gph @ 1200 rpm for both engines and would most likely get to at least 11 knots. So the question has been can you get to 8 knots on a single screw at 8 - 9 gph and this seems absolutely feasible. Main deterrents are locking or cooling the off-line gear and difficulties steering, especially in close quarters. None of your rant provides information that this is not possible. I can ask Covington for their performance graph as well and when I get it we can compare notes. Your sample set of one 80 footer may just show that it was not really dialed in or was an overweight slug, which can be typical.

    I see you did your google engineering on hull speed. The Hatt MY are semi-displacement designs that are usually overloaded by the time they are outfitted with owners equipment. They predict closer to displacement assumptions than not, as substantiated by your claim of 10.3 knots hull speed, which is less than what the displacement calculation predicts - 11.2 knots. The formula is on a base principle level and can range from 1.3 - 1.4, taking issues with 1.34 is truly splitting hairs and once again misses the mark. Going closer to 1.40 only increases the potential hull speed, which is not practical for a Hatt MY, something you should be able to grasp with your sea time on then. The formula has already proven it's worth in this short example of theoretical vs your actual numbers, thank you very much.
  8. Capt J

    Capt J Senior Member

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    The Poster specifically asked me and specifically asked about 12v71 TI's in an 80' Hatteras. So If anyone is assuming way too many things, it is yourself. Most semi displacements hull speeds, without pushing water are not close to a displacement hulls, hull speed, but it mainly has to due with the transom area design. But to answer the question. You can get 1 NMPG at 8 knots burning 8 gph using BOTH engines without resorting to all of this nonsense of running one engine, locking the other shaft, crabbing, rocking and rolling all over the place AND putting undue stress on the running gear, steering, drivetrain, and only engine you're using and dragging a dead prop around. Why don't you recommend to the poster to remove the prop on the engine he doesn't plan on using to save drag? Here is the original post, #28

    "Capt. J, I am looking at a Hatteras 80 with DD 12v71 TIs ...my question is this. I am moving over from a large sail cat with obviously much smaller 100hp turbo yanmars. I nearly always cruised under power on one engine at about 8 kts. to conserve fuel and engine time. Is that a practical alternative on the Hatteras to cruise under one engine, alternating engines obviously but hoping to achieve a burn rate of perhaps 8 or 9 gph.? Your thoughts appreciated. SailorVee"
  9. Mark R Krebs

    Mark R Krebs New Member

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    Hi, You mean 8v92, I think. I have pretty hot ones s'posedly delivering 734hp. Thinking about the next boat, I've been comparing them to 12v71s (on paper) & noticed a recurring reliability theme preferring 12 x71cu-in cyl for their dry liners. For power, google "12v71 propped performance curve." These plots, sourced from detroit, are different for different HP ratings ('cause of variations in turbos & injectors) and you can do two very key things. First, figure your fuel economy or "specific fuel consumption" as lb(of diesel) per Hp-hr. I did that for a bunch of different points and got from 0.3 to .33. That's 10% difference, a lot! Worst was 870HP running flat out, and best was a 500hp tune running at 1600. All those numbers are pretty good efficiency: check on modern engines they won't do all that much better.
    SECOND: what really matters is boat speed. The prop curves show four curves, Two are for fuel and two are for horsepower. Of the pair of curves, one is max rated power (which you can't get without variable pitch prop, an airplane thing) and "propped power." The max rated power curves come out of a dynamometer test of the the engine, but the propped curves ASSUME a power which they calculate from the cube of boat speed. They assume you get just the right prop so you can hit max SHP at max RPM, below which speed the boat demands, and the engine will deliver, lots less power. CUBE means 2x the speed means 2x2x2 = 8x the power and fuel flow. (Sometimes they use a slightly smaller exponent to account for propeller slip, and planing hulls will do better at high speeds but skip that for now.) The point is if you want economy, you can slow down and your fuel flow will plummet gratifyingly. I have a 61' Tolly and I can get 1.4 gal/nmi at slow cruise, or 3gal/nmi running just 50% fa$ter. Generally, you can plan that your fuel economy will scale with the square of your speed. So, these are big numbers compared to the small (10%) changes in engine efficiency across motor brands, turbo setups or operating speeds. I was surprised to see in the detroit numbers 0.02 difference in favor of the less tweaked 500hp engine. More boost is supposed to be more efficient, so they must've really pushed the high power version it to get to 860. That's the engine I'm looking at, and it's making me a bit nervous.
    Last edited: Oct 1, 2022
  10. Mark R Krebs

    Mark R Krebs New Member

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    And don't cruise on one engine unless you lock the shaft. Tranny won't lube, and I don't like the high sustained reverse load on the thrust bearing. that can't be good.
  11. Mark R Krebs

    Mark R Krebs New Member

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  12. Mark R Krebs

    Mark R Krebs New Member

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    TA means Turbo, Aftercooled or Intercooled. Mine, and most I think are intercooled, that is, after the turbo but before the supercharger/blower. Still they usually seem to denote it "A" instead of "I" go figure. Maybe somebody else can clarify that one. The cooler chills and densifies the air charge so you can burn more fuel. They add tons of power, and fuel consumption close to the same, at the same power of course (which is what matters). People worry about reliability of higher power engine tunes on the same block and so do I. A life metric sometimes used is total fuel throughput which makes sense to me: more fuel, more power, shorter life.
  13. Capt Ralph

    Capt Ralph Senior Member

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    Welcome to Yacht Forums

    Nothing like loading up a 5 year old thread. Slow day in Seattle?
  14. Mark R Krebs

    Mark R Krebs New Member

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    Yah, ...sorrry? I think the conversations can be fun even if the orignal Q is answered, no?
    I came here after looking for 12v71 info & learned some things so that's good.
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  15. mapism

    mapism Senior Member

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    Seconded.
    I missed this thread back in the days, and only read it now.
    Some posts, particularly from PacBlue, made it very interesting indeed.
    Talking of which, he seems to have stopped contributing lately.
    Which is a pity, because factual comments based on first hand competence and experience (rather than useless web searches!) are the main asset of any forum.
    Does anyone know why he left?
    Regardless, I hope he's well and he'll be back.
  16. leeky

    leeky Senior Member

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    He checks in from time to time; last time was about a month ago. Lurking is less frustrating than participating is my guess for why he is silent now.
  17. mapism

    mapism Senior Member

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    Well, based on this thread alone I would think that your guess is a very good one.

    Anyhow, if and when he should have a look at the latest posts of this old thread, I would like him to know that I (as well as others, I'm sure) appreciated his factual contributions.
    Of course, in a forum there will always be someone arguing against even the most solid arguments, and it's annoying to deal with useless objections triggered by lack of basic understanding.
    OTOH, when the debate gets technical, it's not so hard to separate the wheat from the chaff, for anyone who doesn't live in la-la land.
    I'm saying this just in case PacBlue ever wondered, and in the (slight) hope that he will reconsider and decide to restart sharing his competence with us again.
  18. leeky

    leeky Senior Member

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    Me, too.
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  19. gr8trn

    gr8trn Senior Member

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    Me too. He has be more than helpful with my personal boat in the messaging on this forum. @PacBlue, thanks mate.
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  20. DOCKMASTER

    DOCKMASTER Senior Member

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    PacBlue is missed. I really appreciated the input from him when I did my repower. Hope he comes back someday. Sorry, off topic I know but wanted to say thanks to PacBlue if he is reading.