Semi Displacement at displacement speed

Discussion in 'Boat Design' started by HowardH, Dec 12, 2010.

  1. HowardH
    Joined: Nov 2010
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    HowardH Junior Member

    How well will a semi-displacement hull work at displacement speed? What would one give up in the way of performance and efficiency at displacement speeds with a semi-displacement hull?

    A boat to be primarily operated at displacement speed yet with the ability to double or more it's speed if the need arose is appealing.

    I understand the requirement for a more powerful motor but if fuel consumption is based upon horsepower produced; a larger motor, turning slower, should have minimal increase in fuel burn at displacement speed.

    20 hp is 20hp and every diesel will have similar gph numbers at 20hp regardless of what peak hp is correct?
     
  2. michael pierzga
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    michael pierzga Senior Member

    Its not fuel per hour..its fuel per mile. Big difference between 9 knots and 10 knots for a given fuel burn.
     
  3. HowardH
    Joined: Nov 2010
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    HowardH Junior Member

    Not sure I understand your point. If you are saying that a 10% approx, increase in speed will take more then a 10% increase in fuel burn I get that.

    However if it takes 30 hp to push your boat to 8 knots how much will fuel use vary between a motor with a rated hp of 30 vs a motor with a rated hp of 200?
     
  4. Submarine Tom

    Submarine Tom Previous Member

    Simply terminology and trade tradition Michael, convert to whatever units work for you.

    -Tom
     
  5. michael pierzga
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    michael pierzga Senior Member

    Well the boat Im on right now get Zero miles per gallon when motoring in nuetral , but its fuel usage per hour is not bad when I put her in gear and make 8 knots at 1400 rpm. When I reduce topside windage drag , trim the boat, clean the bottom and make 8.2...even better !!!!!!
     
  6. apex1

    apex1 Guest

    You donĀ“t see a lack of performance in displacement mode with a common semi. A full planing hull is different though.

    The question if the motor runs at efficient rpm range is a second one!
    Most probably the engine will not be in the "efficient operation mode" when designed / choosen to bring the vessel to top speed.
    As a example, a engine that operates at 30% of the max power will burn 320 gram kW/h and say at 80% it will burn 200 g kW/h , where the same engine with a lower power rating, in a full displacement boat runs at 80% max to achieve the same speed, but burns less, because the max power output is less, and it operates at the efficient point.

    But it is nothing wrong in general, with a semi in full displacement mode, having the ability to speed up when need be.

    Regards
    Richard
     
  7. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Every engine has a characteristic called Brake specific fuel consumption (BSFC). That is a measure of fuel consumption at various output levels. It is calculated as pounds of fuel per horsepower per hour. Gasoline engines run in the general range of BSFC = 0.44 to 0.50. Different engines have slightly different characteristics. The engine does not have a constant BSFC. That is to say that it will vary somewhat at different output demands. Diesel engine manufacturers are pretty good about furnishing specifications that include BSFC figures.
     
  8. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Good point, Richard. How would these compare to a third possibility - twin engine for semi-displacement speed, but only one (at a time) working in displacement mode, the other one freewheeling?
    I feel a bit lazy today. ;)
     
  9. Joakim
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    Joakim Senior Member

    Are these values even close to typical? You can find some partial load fuel consumptions for this Scania engine: http://webapp109.scania.com/i/IndustrialMarine/Spec_marine_workboats/DI12_65M_430.pdf

    Note that all the reported consumptions are 197-223 g/kWh although the power output range is 90-316 kW and at lower rpm even the 1/2 load values are 203-212, thus not much more than the lowest value.

    What you can't find are the very low load values really in use at displacement speed. Something about that you can find from the propeller curves of Yanmar engines. A similar power model: http://www.yanmarmarine.com/theme/y...ine_files/Technical datasheet/6LY3-STP_HR.pdf

    This seems to provide about 55 kW and 15 l/h at the propeller curve. Thus not more than 230 g/kWh. Then we compare this to a 55 kW engine: http://www.yanmarmarine.com/theme/yanmar/uploadedFiles/engine_files/PDF/4JH4-TE.pdf

    It has about the same consumption at the same power! This graph goes all the way to 1200 rpm, where the propeller curve power is a bit under 4 kW and consumption under 1 l/h, thus not much over 200 g/kWh at 7% of maximum power.

    Another issue is that typically you select a much bigger engine for a displacement vessel, unless it is a ship going well under "hull speed". At cruising speed you are not typically running 80% of power. More like 30-50% of power (=70-80% of max rpm) or even less.
     
  10. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Joakim, the graph you have cited are relative to WOT condition, only RPMs are varied. That's a condition similar to 80% throttle example given by Richard, and nubers are quite similar too (around 200 g/kw/h).
    A partial-load condition means not 30% RPM at WOT, but 30% throttle. You need a fuel consumption map, not just a single curve, to get the correct numbers for that operating condition.
    At 30% throttle (not RPM), a diesel engine is known to be much less efficient than at 80% throttle (in terms of g/kWh), and I think Richard's numbers are not far from reality.
    Cheers
     
  11. Joakim
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    Joakim Senior Member

    Yes, I know. The Scania specification has also the values for 3/4 and 1/2 load at several rpm (look at the top of the first page). Yanmar shows fuel consumption and power for the propeller curve, thus not WOT, but you have to calculate specific consumption.
     
  12. Joakim
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    Joakim Senior Member

    Here are specific fuel consumption maps for some VW car diesel engines (at the bottom): http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps

    If we look at the lowest one for 2009 model 2.0L TDI, it has a maximum effective pressure about 20 bar at 2000 rpm and it consumes then about 200 g/kWh. The effective pressure is linearly proportional to torque and thus power at the same rpm. At 30% of max pressure the pressure is 6 bar and consumption 230 g/kWh, thus quite far from 320 g/kWh and only 15% worse than the optimum. Even at 15% load at 2000 rpm the consumption is about 260 g/kWh. Since the engine can't reach its maximum power at 2000 rpm, this 15% is actually less than 10% of the absolute maximum power of the engine.

    Here is another map, which also shows curves for biggest gear, which are somewhat similar to propeller curves. Note that these curves are never at the optimum and specific consumption is rather constant above 1500-2000 rpm, as is shown also in the Yanmar engine propeller curves I linked earlier. http://media.photobucket.com/image/%2522bsfc map%2522 diesel/biodeez/ExcelBSFCMapwgearswaps2.jpg
     
  13. apex1

    apex1 Guest

    It gets boring....

    What are we talking?


    Figures, loads, displacement, Froud.

    Not available?

    Keep the comments provided as given. We do our business since a few weeks....
     
  14. apex1

    apex1 Guest

    Well, we played with that since about 100 years now. It is never more economic to operate on one engine than on both!
    BUT, the gear has to be choosen for the opportunity of slow speed, or a CPP has to be part of the game.
    Not trolling valves! Proper design!

    A efficient operated Diesel is happier at idle, than at shut down condition.
    It is part of the moved weight anyway, and operates better, contributing a bit to the general propulsion effort, than sitting dead down.

    Regards
    Richard
     

  15. FAST FRED
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    FAST FRED Senior Member

    "However if it takes 30 hp to push your boat to 8 knots how much will fuel use vary between a motor with a rated hp of 30 vs a motor with a rated hp of 200?"

    At the least I would guess double the fuel burn for the 200 hp producing 30 hp.

    Diesels LOVE to work hard 30 hp might nor enough to even get the 200hp engine to operating temperature , and it surely would suffer from under loading , slobbering exhaust , burnished cylinder bores , oil dilution and stick piston rings.

    Twin engines on one shaft is easy as pie.

    The USN (as well as other Navies) have dual engines turning one shaft on many work boats LST , that sort of boat.

    These engines can work together or either one can be disconnected , under way.

    Since its pure mechanical , both engines must be secured to pust the lever to re engage them. Series 1 bell housings are the norm.

    FF
     
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