New Hybrid: Small diesel/ Big OB in 33' S/D Hull.

Discussion in 'Boat Design' started by rustybarge, Oct 28, 2013.

  1. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Motor boat Magazine did a reference test with a 28' 4ton planing boats rigged with a 370hp Volvo diesel....proper calibrated instruments measuring real fuel consumption in a real boat, on the real sea.....scientifically.

    My target is 1gal/hr at 7-8kts with75hp, the Volvo 370hp burns 3 gal/hr!!!

    [​IMG]
     
  2. eyschulman
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    eyschulman Senior Member

    What makes you think a 75 hp motor will not burn 3 gal/hr trying to push that particular boat? 1 gal. diesel= 20 hp with a 28 ft boat you probably will not get 7 K it is above LWL displacement speed. I personally think your ideas are way off. If you wish to prove me wrong build the boat and build an identical boat with a single diesel and see what kind of economy each gets. Its not the motors that make the big difference in fuel economy. The motor people are getting about as much energy out of a gallon of fuel as they can. Its what you put the motor in that counts and that's what marine architects spend a lot of time dealing with. I don't think the combination of a small diesel and a big OB is going to change the game for anybody if anything the combo of a big diesel and a small OB makes sense and is very commonly used its called a trolling rig.
     
  3. rustybarge
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    rustybarge Cheetah 25' Powercat.

    lets compare perol engines.
    Which is more economical around town, a Dodge viper, or a toyota 1.6ltr hatchback car?

    lets compare diesels:
    Which is more economical around town, a 400hp diesel truck, or a toyota 1.6ltr diesel car?

    so big diesels are nearly (not quite) as un-economical as big petrol engines at slow speeds.
     
  4. rustybarge
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    rustybarge Cheetah 25' Powercat.

    here's some spec's which match the article in MBM:

    yanmar 75hp, 1200revs min rated speed, 1ltr/hr......1/4gal/hr
    http://www.yanmarmarine.eu/theme/ya...easure - datasheet/English/Yanmar-4JH4-TE.pdf

    Yanmar 440hp, min rated speed:1400revs(off the graph) 10ltr/hr.....2.5gals/hr
    http://www.yanmarmarine.eu/theme/ya...tasheet/English/Yanmar-6LY3-STP datasheet.pdf

    From the horse's mouth:

    Engine damage.

    Diesel engines can suffer damage as a result of misapplication or misuse - namely internal glazing (occasionally referred to as bore glazing or piling) and carbon buildup. Ideally, diesel engines should be run at least 60% to 75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.
    Internal glazing and carbon buildup is due to prolonged periods of running at low speeds or low loads. Such conditions may occur when an engine is left idling as a 'standby' generating unit, ready to run up when needed, (misuse); if the engine powering the set is over-powered (misapplication) for the load applied to it, causing the diesel unit to be under-loaded, or as is very often the case, when sets are started and run off load as a test (misuse).
    Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since this relies on the gas pressure to force them against the oil film on the bores to form the seal. Low cylinder pressures causes poor combustion and resultant low combustion pressures and temperatures.
    This poor combustion leads to soot formation and unburnt fuel residues which clogs and gums piston rings, causing a further drop in sealing efficiency and exacerbates the initial low pressure. Glazing occurs when hot combustion gases blow past the now poorly-sealing piston rings, causing the lubricating oil on the cylinder walls to 'flash burn', creating an enamel-like glaze which smooths the bore and removes the effect of the intricate pattern of honing marks machined into the bore surface which are there to hold oil and return it to the crankcase via the scraper ring.
    Hard carbon also forms from poor combustion and this is highly abrasive and scrapes the honing marks on the bores leading to bore polishing, which then leads to increased oil consumption (blue smoking) and yet further loss of pressure, since the oil film trapped in the honing marks is intended to maintain the piston seal and pressures.
    Unburnt fuel then leaks past the piston rings and contaminates the lubricating oil. Poor combustion causes the injectors to become clogged with soot, causing further deterioration in combustion and black smoking.
    The problem is increased further with the formation of acids in the engine oil caused by condensed water and combustion by-products which would normally boil off at higher temperatures. This acidic build-up in the lubricating oil causes slow but ultimately damaging wear to bearing surfaces.
    This cycle of degradation means that the engine soon becomes irreversibly damaged and may not start at all and will no longer be able to reach full power when required.
    Under-loaded running inevitably causes not only white smoke from unburnt fuel but over time will be joined by blue smoke of burnt lubricating oil leaking past the damaged piston rings, and black smoke caused by damaged injectors. This pollution is unacceptable to the authorities and neighbors.
    Once glazing or carbon build up has occurred, it can only be cured by stripping down the engine and re-boring the cylinder bores, machining new honing marks and stripping, cleaning and de-coking combustion chambers, fuel injector nozzles and valves. If detected in the early stages, running an engine at maximum load to raise the internal pressures and temperatures allows the piston rings to scrape glaze off the bores and allows carbon buildup to be burnt off. However, if glazing has progressed to the stage where the piston rings have seized into their grooves, this will not have any effect.
    The situation can be prevented by carefully selecting the generator set in accordance with manufacturers printed guidelines.

    http://en.wikipedia.org/wiki/Diesel_generator#Engine_damage
     
  5. eyschulman
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    eyschulman Senior Member

    Dear Rustybarge you are constantly comparing apples to oranges. Yes the truck is a big heavy thing and the small car well a lot lighter less wind-age and different gearing. The difference in economy is not in the motor and that's a point you still don't get. If you have to push something through water it takes so much energy to attain a given speed no mater what size the motor the fuel burnt = the amount of energy needed and used. Putting a smaller motor in a boat does not change the laws of physics. As for motors dying and running into trouble due to low load use vs high load use tune in to Boatdiesel.com and check out the multiple past posts and opinions of real life diesel mechanics. I am not advocating running motors at no load or full time very low load. What I am saying is that with proper management a modern diesel can be run for long periods at 30-40% or lower loads so long as coolant temps are in spec and at the end of the day or intermittently the motor is brought up to 80% load for 10-15 min. as for glazing this is avoided by proper break in as rec. by motor builders and the intermittent higher loads. Managing a single diesel or multiple is much simpler and more direct than what you propose. Also if you intend to get 7.5K out of a boat on one gallon/hr you will have to make or get a boat that can do that , then if you want 15+ k you will have to apply lots of power. You can easily do that with a small light skiff where 20 hp can breakaway from hull speed but its a whole different ball game on a larger heavy cruising boat where it takes a lot more energy to exceed hull speed so you may need to start with a boat where hull speed is 1K > than 7.5K pushing you up to a heavier longer boat. The concept of mixing displacement speeds and planing has been addressed for years with many different approaches it has not become very popular probably because it is not to practical. If you truly want great fuel economy forget the motor thing and look to very light boats with hulls that have little resistance or very long waterlines and run at or just below hull speed. Also catamarans and motor tri if built light and right are capable of great economy. Some Years ago Yanmar made some diesel OB and as a stunt put three on a light tri and went around the world with great fuel efficiency. By the way a 75 HP diesel of any make will burn one gallon/hour when it is putting out 20hp so at 7.5 K and one g/hr so you are advocating low load use of your small motor. By your own reasoning you should be looking at a 25-30 hp motor.
     
  6. FAST FRED
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    FAST FRED Senior Member

    The point is a diesel MAY create 20 hp from a gallon of fuel, IF it is properly (heavily ) loaded.

    The peak HP/RPM is only of interest to speedboat folks.

    A gen set can be very efficient when run at 1200RPm , as the gen head will be designed to load the engine properly at THAT RPM.

    The rule I heard from the Cummins folks was 80% load at 90% RPM , FOR THAT LOAD

    This is why the 400hp engine that is operated at 5% -20% of its rated load can only create 10-12hp from 1 gallon of fuel.

    There are transmissions that allow 2 engines to operate a single shaft , one at a time , or both together.
     
  7. WestVanHan
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    WestVanHan Not a Senior Member

    This has been beat to death here many times.
    I have twin 640 hp Cummins and run them about 90% of the time at low loads,with short bursts to beat ferries or tidal loads.

    1500 hours-and doing fine.
     
  8. FAST FRED
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    FAST FRED Senior Member

    I looked at the 75 hp graph and while it is not designed to fool you , it like most is useless for most cruising information.

    The HP curve given is probably very accurate , but as usual the fuel consumption follows a mythical propellor loading.

    If you gear just the way they describe its fine, but if you try for efficiency and gear and prop to a set point , say2000RPM , the fuel consumption graphs are useless.

    The graph shows at 2000 the engine is able to produce over 40 HP , but the prop is only loading the engine to 12 -13 HP.

    Run the engine at 35 HP and the fuel burn may be 350% of what is published.

    What is needed is a Fuel Map , which looks like a set of clouds , as small one laying on a larger and another larger under.

    The small cloud is the HP/ RPM sweet spot that will give the best economy. The next larger cloud is another measuring point , less efficient , but easier to operate in.
     
  9. eyschulman
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    eyschulman Senior Member

    Its really not all that complicated. Diesel motors should be propped so that at wide open throttle clean bottom with a fully loaded boat the rated rpm of the motor is reached and often if not controlled by a chip etc an additional 100 rpm for future dirty bottom added. What fuel the boat will burn and under what load will vary boat to boat and has to be measured in each case. What we observe in real life regarding fuel burn has less to do with the motor and more to do with the boat gearing and prop given that the prop parameters are often fixed by aperture size and need to reach rated rpm wot that leaves most of the issue to the boat weight-resistance in water -beam -type of hull-LWL- windage etc. etc.
     
  10. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Lots of people talk about prop curves, loading, hull forms, displacement weight, deadrise,, etc etc........
    Me, I make the presumption that the designer has done his job, that the correct engine for the hull will meet these parametres:
    Diesel continuos cruise revs will be 2500( modern high speed diesel) at 20kts in a planing hull, and 15kts in a S/D hull.
    Outboard cruise revs will be 4000revs for similar performance.

    Motor Boat Magazine tested 6 indentical hulls with different engines, 3 in the 2tons/250hp 23' class from the three top engine manufactures, and 3 from the 4tons/350hp 28' range. All measurement tsken with high precision instruments on the same day same conditions.

    It turned out the Magazine had been under estimating fuel burn on all the engines by a massive margin over many years by about 30-40%..,,

    Here's the results at CRUISE:
    All engines in identical hulls....Aquador planing hull 28' 4tons:
    Volvo 370 D6 22kts 2500rev 37 ltrs/hr
    Cummins 350 v8 22kts 3000revs 38ltrs/hr
    Yanmar 370 8lv 16kts 2500revs 38 ltrs/hr

    so we can pretty well say that a 350hp engine burns about 38ltrs an hour!!!

    And the 250hp burns about 18-20ltrs an hour.

    so the question we ask is how fast can we go with 250hp/18ltrs or 350/38 ltrs, not the other way around.
     
  11. FAST FRED
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    FAST FRED Senior Member

    Your concept is ONLY for a motorboat that wants to cruise at WOT or really close.

    Fine for Pilot boats , Navies or deep pocket water thrill riders. Folks that dont mind huge fuel bills.

    For most cruising boats operating the engine for modest speeds is the norm

    How efficient is a 75 hp engine that can make 40 hp at cruise that only is getting 13 Hp worth of push at cruise?

    The boat would be more efficient with a 25 Hp engine , to create 13 HP.

    This is why many cruising sailors will install a Cruising Prop.

    No way can full rated RPM ever be reached , but chugging along at the same or slightly higher speed 600RPM or so below max rated power is quiet and the higher loading results in longer engine life. 1500 is loads easier on the ears than 2100.

    Engine life is really measured in total gallons burned before replacement , so any added fuel efficiency is added engine life.

    Yes, the operator must be knowledgable of the engine limits to cruise with an over propped boat..EGT gauge is a great help, about $100.
     
  12. jonr
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    jonr Senior Member

    I don't think that anyone seriously argues that different engines don't produce different BSFCs at different outputs. And that at some point, load can be low enough to be bad for an engine (sustained use). If you want to increase the range of outputs that an engine can safely produce, consider adding gears. This allows it to run at a higher load/lower rpm (note that load and output aren't the same thing), which will increase cylinder pressure.
     
  13. rustybarge
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    rustybarge Cheetah 25' Powercat.

    this the sort of gearbox you need, but for small boats:http://www.wartsila.com/en/gears/wartsila-gears/Twin-Input-Gears
     
  14. sharpii2
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    sharpii2 Senior Member

    From the original post, I get the main reason for the higher speed is to cut down on rolling.

    There is a simpler way to do this.

    Have a small sail rig of roughly 150 to 250 sf (13.9 to 22.3 sm).

    The sails will dampen the roll considerably and they will also reduce fuel usage.

    You would still have a powerboat, not a sailboat, and the engine will be expected to run all the time.
    For this reason, you will not have to add a keel or a bigger rudder.

    Now you can use a hull design which has maximum efficiency at displacement speed, instead of one that
    is more of a compromise.

    Just a thought.
     

  15. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    George Beulher seems to have a bit of a following with his displ. boats.

    But I'm just wondering about all that exra weight of a mast and boom on a S/D hull......

    [​IMG]

    so the design concept of the 'two speeds/two different engines' hull is to resolve the compromises of a rolling displ. hull with flopper stoppers, steady sails, bilge keels, stabilisers.....which are either very expensive, or virtually impossible to deploy single handed.

    The beauty of a two speed hull is you just speed up to S/D range to stop rolling, simple or what?

    I think there maybe a two speed gearbox available for small engines???
     
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