Unusaal Prop Size Delima?

Discussion in 'Props' started by Hotwater, Jul 8, 2008.

  1. Hotwater
    Joined: Jul 2008
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    Location: Tarpon Springs, FL

    Hotwater Junior Member

    I have an unusual prop, hp speed, problem. We have an ex Navy YP (80 foot, displacement hull) that has 4, 6-71 (165 HP) driving 2 shafts through Allison 2:1 gears. We spend most of the time ( except for storm or current issues) running on only two of the engines. The Navy propped her (and all 80 foot YPs) with 36 x 24, 3 blade "workhorse" style wheels. We wish to cruise most of the time between 9 and 10 knots. Running at this speed with 4 engines and these wheels puts us down into the 1000 rpm range (engine, not shaft) What are the thoughts on efficiency safety and economy? And, how do you calculate the best wheel size?

    Best Regards
    Bill

    Vessel particulars, St Michael (formerly YP 668 and CT2)
    LOA 80' 5"
    DWL 76'3"
    Displ full load 67.13 Tons
    Displ light 55.94 Tons
    Number of shafts 2
    Shaft size 2.5"
    Hp 165 or 330 per shaft
    Gears 2:1 Allison (hydraulically switchable between engines)



    Full Load Condition, Calculated data

    1) Full Load Displacement 67.13 Tons
    2) Area of Water Plane 947 sq. ft.
    3) Block Coefficient .460
    4) Prismatic Coefficient .630
    5) Midship Section Coefficient .737
    6) Waterplane Coefficient .774

    Light Load Condition, Calculated Data

    1) Full Load Displacement 55.94 Tons
    2) Area of Water Plane 912 sq. ft.
    3) Block Coefficient .438
    4) Prismatic Coefficient .616
    5) Midship Section Coefficient .720
    6) Waterplane Coefficient .765
     
  2. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    What speeds do you get when you simply de couple one engine from each tranny?

    I would think that two 6-71 should be fairly efficient (one per shaft) would be about right for economy cruise.

    Each at 1200 -1500 .

    Do you have a rpm/ speed graph for what you get with 2 engines and all 4 humming?

    This will be interesting! Although an 8K cruise would be perhaps the least fuelish.

    FF
     
  3. Guest625101138

    Guest625101138 Previous Member

    The current props will be giving efficiency around 83% at 10 knots. This is good. Props are well suited to the boat.

    Rick W.
     
  4. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "The current props will be giving efficiency around 83% at 10 knots."

    What is the HP input required for 10K?

    FF
     
  5. Guest625101138

    Guest625101138 Previous Member

    Fred
    The above number overestimates. I made an assumption that the props would be designed for that speed but they would have been designed for top speed at full power I expect. This results in larger blade area.

    These props at 10kts give 74% efficiency. Still quite good. Props would absorb about 25kW each at that speed.

    Rick W.
     
  6. Hotwater
    Joined: Jul 2008
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    Location: Tarpon Springs, FL

    Hotwater Junior Member

    which hp

    I appreciate the responses, but the question remains? Do I calculate for 165 hp per side or 330, or somewhere in between? What method did Rick and FF use to calculate?
    Regards,
    Bill
     
  7. Guest625101138

    Guest625101138 Previous Member

    I use my own software but it is very similar to Javaprop.

    Google Javaprop. Load the Applet. When you have it running you set the options to water parameters density and viscosity.

    If you have more detail on the props such as blade area, photo of props and hull etc it will help a little as well. Also very accurate revs for speed data would be useful. The 1000rpm for 10kts seems a bit low. What is the top speed and what rpm at full tilt with all engines running?

    You have not given the beam of the hull either.

    Not sure what you hope to learn though. These are reasonable diameter props and give fair efficiency at 10kts.

    What do you expect to do? Are you concerned that the engines are not working hard enough at 10kts?

    Rick W.
     
  8. Hotwater
    Joined: Jul 2008
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    Location: Tarpon Springs, FL

    Hotwater Junior Member

    Beam is 18 feet (17 feet at DWL) Yes, 1000 rpm is too low to run on 6-71s, and as I said I wish to mostly (90%) run on 2 engines not 4. Michigan Wheel "workhorse" three blade data is available via the web for those that are interested in foil shape and section.
    Regards,
    Bill
     
  9. Guest625101138

    Guest625101138 Previous Member

    Bill
    The workhorse in the literature seems to be a 4-blade prop.
    http://www.miwheel.com/MIWheel/products/inboard/com_prop.htm
    It also has large blade area 71% area ratio.

    Can you provide the web reference for what you actually have?

    If you are getting 10kts from a 24" pitch at 1000rpm at engine then it is not absorbing much power. When you run on one engine per prop are they labouring or doing it easily? What is the concern with running at 1000rpm?

    I did a rough approximation of the hull but found I can get quite a wide range of drag by altering the transom. Wave drag really starts to kick in around 9kts. The data I found on the vessel indicates top speed of 12kts. I would just like to confirm some of these details.

    Rick W.
     
  10. Hotwater
    Joined: Jul 2008
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    Location: Tarpon Springs, FL

    Hotwater Junior Member

  11. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    For better efficiency you might consider using a smaller engine for LRC and decoupling it and using two 6-71's for faster progress.

    Cheapest would be a 3-71 or even 2-71 in place of one 6-71 per bank.

    The SAE bellhousings would match up .

    Which ever DD (or a different engine pair that) would give 25KWper shaft would be most efficient.

    Personally I think 25KW is low power , but if you would post the speeds you get /vs RPM we could work from your actual numbers.

    Speeds every 100 rpm ,700- to full bore , on 2 engines and from 1500 to full bore on 4 would be great.

    FF
     
  12. Guest625101138

    Guest625101138 Previous Member

    I have done a bit more modelling of the hull and props. There is a curve showing power for the hull based on 70% prop efficiency. The prop data shows this is close to their best efficiency.

    The 500rpm at 10kts (say 5m/s) does not quite jell. The props are only absorbing 25kW (say 33HP) at this rpm but the shafts want more like 200HP total (100HP each). Prop shafts should be doing more like 630rpm to do 10kts.

    With 660HP the top speed is going to be around 12.5kts so this fits reasonably well with the information I have found.

    If you tell me more about what you would expect to achieve with repropping I can do some more numbers.

    10kts is certainly the economic cruising point. Problem is that you can see how steep the power demand is as you start to push over 10kts.

    Rick W.
     

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  13. Hotwater
    Joined: Jul 2008
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    Location: Tarpon Springs, FL

    Hotwater Junior Member

    It is easy to believe that our tach info may not be accurate.
    The data book publishes the speeds as follows:

    1 shaft
    810, 6 knots

    2 shafts
    810, 7
    940, 8
    1070, 9
    1200, 10
    1400, 11
    1700, 12
    1800, 13+

    Since the Navy treated these engine pairs as 12 cyl, I have to assume the data is with both engines coupled to each shafts.
    Regards,
    Bill
    PS Fred, how is the 4106?
     
  14. Guest625101138

    Guest625101138 Previous Member

    Bill
    That data fits much better with what I determine. I am happy that the hull model is close.

    Now what would you like to do?

    With same diameter props but lower pitch and smaller EAR you could do 10kts with combined 120HP. Lets say 800rpm at the shaft and 18" pitch. Is this the sort of info you want?

    You could go to two 60HP diesels for normal use and retain two of the old 6/71s for backup or really heavy conditions. The new engines would need to be moderate rpm as well or geared suitably. The potential weight reductionb would also bear inclusion in the calculation.

    Rick W.
     

  15. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    3-71 will run forever at 60 hp,

    BUT (always that But)

    roughly each gallon of fuel run thru a DD will give 15 or 16hp, so you will have a fuel burn of 4gph /per engine.

    I'm not sure the 6-71 would use much more fuel (if any at that setting).

    So the question would be weather you will be doing enough running to make an engine that can produce 22hp-24hp per gal worth the cash for a swop?

    Since most modern engines that can run 60hp "forever" will be small , electronic injected and turboed , and probably wont have a SAE 1 bellhousing, it will be a job!

    So a "second" choice exists , De Rate a single 6-71 per side.

    Depending on the injectors now fitted,,,(N95?) with N 55 or N 60 installed , with advanced timing MIGHT make the existing engines ea$ier to live with.

    Our 4106 "the Sports Car of Coaches" is existing on the new crap Ultra Lousey diesel, but as in the trucking world the trash fuel costs over 10% of the mileage!

    All so the air will be cleaner in LA , hard to stomach in Maine.



    FF
     
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