Hull Speed Frustrations

Discussion in 'Props' started by DreamCatcher34, Sep 5, 2012.

  1. DreamCatcher34
    Joined: Sep 2012
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    Location: Salem, MA

    DreamCatcher34 New Member

    I need some . I’ll try to keep this brief so this is the situation. For some reason I do not feel that I am achieving the speed I should at the full range of RPM’s. I added some tables below for reference. What is frustrating is that my friends Pearson 34 will do 6.3kts. at 2500rpm. The Pearson has a shorter DWL & is heavier displacement. I believe she is running the same engine although it might be less HP.


    I really can't go any bigger on the Diameter. Is it possible that I am over pitched? I went from a 2 blade fix to a 3 blade thinking that I would gain speed. It was not the case. It's very hard to see boats that are technically slower blow by you under power. Please correct me if I’m wrong, but I thought that I should be able to reach hull speed at 80% power.

    The hull bottom is clean for those of you who are wondering lol

    My boat particulars

    Boat: Show 34
    DWL: 27’ 2”
    Hull speed on paper: 7.3kts
    Engine: Yanmar 3GM30 27hp@3600
    Gear: 2.6 reduction
    Prop: 3 blade fix; 14 x 11pitch


    rpm speed

    1000 1.6
    1500 2
    2000 3
    2500 3.9
    2800 4.5
    3000 4.9
    3300 5.5
    3600 6

    RPM PROP HP % of Power

    1000 0.6 2%
    1500 2.0 7%
    2000 4.6 17%
    2500 9.0 33%
    2800 12.7 47%
    3000 15.6 58%
    3300 20.8 77%
    3600 27.0 100%


    Thank you for your time
    Rob
     
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Your hull speed is about 7kt.

    I think you are under pitched. A 14x14 two blade would be my first choice. A 14x13 three blade would give better maneuvering but less efficiency at reduced power. I push a 38'er around with a 14x14 two blade folder. On the other hand, your prop would be good to motor through a storm with.

    Most of us want to motor at the sweet spot on the second wave. That is .7 hull speed. So 4.9 kt @ 3000 is okay, but you should be able to do it at 2800 no problem with more pitch. Fuel burn a bit over a liter an hour at 4.9 kts.

    That second table appears to just be a prop curve. It certainly has nothing to do with your boat. I doubt you need more than three Hp to do 4.5 knots.

    At any rate, it's a compromise. You have a great prop for maneuvering around, motoring through bad weather, or towing something twice your size- just not real good at extended motoring or motorsailing.

    One other question- can you hit 3600 rpm in forward while tied to the dock? If you can't, you will want to reduce the area ratio of the prop when you increase pitch. I'm guessing that you can do so easily with your current three blader and can maintain the present AR. You may end up not being able to hit 3600 with a 13 pitch, but the thrust will probably be greater.

    PS. The factory recommendations are usually pretty damn good. What do they spec??
     
  3. Jimboat
    Joined: Feb 2002
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    Jimboat Senior Member

    it sounds like you should try a prop with slightly more pitch. your switch to 3-blade from 2-blade would be to reduce slip or vibration....was this a problem with the 2 blade? Or, if not, what problem were you trying to solve by going to the 3-blade?
     
  4. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    I don't see how slip could have been a problem. He's only got 20ft/s advance at 3300 rpm with the 11 pitch and 2.6 gearing.
     
  5. johneck
    Joined: Nov 2011
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    Location: New England

    johneck Senior Member

    It looks to me that you are only actually absorbing about 17 HP at 3600 RPM. Your curve of RPM vs HP does appear to be a propeller curve based on a bollard condition (maybe). The change from 2 to 3 blades only effectively changes pitch by about 1/3", not enough to really absorb much more power. I think that you need a pitch of about 13-14" as Phil said to absorb all the power that you have. You need to decide if you want to do that since fuel burn will be greatly increased. You could probably use a 2 bladed prop, but the thrust loading is a little high and you will get smoother running with 3. The drawback for three blades is probably more drag when you are sailing.
     
  6. DreamCatcher34
    Joined: Sep 2012
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    Location: Salem, MA

    DreamCatcher34 New Member

    Thanks for the replies. I’m barely making 3600rpm with what I have now. I think more pitch will not allow me to reach 3600 RPM. Factor recommendations from Prop sites based on engine and gear is 14 x 11. If I over pitch I’ll put more load on the engine at lower RPM’s meaning higher exhaust temps. Is this correct in saying? I really can’t go any bigger on the diameter so the only thing left is to mess around with the pitch.
     
  7. johneck
    Joined: Nov 2011
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    Location: New England

    johneck Senior Member

    Yes, increasing pitch will increase engine load over the full range and increase temps.

    Bigger diameter would also increase load and there is no benefit in efficiency in this case, so even if you could you shouldn't.

    If you are already absorbing full power, then there is no room to increase. However, based on the numbers it does not seem reasonable that you are absorbing 27 HP.
     
  8. bit
    Joined: Feb 2008
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    Location: Trieste

    bit Student

    7.3 kn
    27 hp
    3600 rpm
    2.62 r/R
    3 Z
    0.35 Ae/Ao


    14x11 = 14hp = 25% cav.

    14x14 = 24hp = 30% cav.

    16x12 = 26hp = 18% cav.

    17x10 = 23hp = 10% cav.

    17x11 = 28hp = 13% cav.
    hi
     
  9. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    There's something fishy here:

    First: this engine with 2,62 gearing should swing a 15,5" or 16" propeller, and you say it is just coping with a 14"?? Checking prop data for a standard 14x11 screw, you get something like 18 hp out of that rig, so what is wrong with the engine/propeller/installation?

    A/Check tacho calibration
    B/Is the gearing correct
    C/Check air filter and that the engine is getting free, cold air
    D/Check engine room temperature
    E/Is the exhaust correctly dimensioned and built, check back pressure
    F/Check throttle cable and lever has enough travel
    G/Check fuel filter and f-line for dirt/air leaks
    H/Is your boat overloaded; check actual waterlines against design

    Second: Your tabula over propeller power is incorrect; it refers to power at constant advance speed, according to the affinity law (Power ~ revs^3). With a change in speed, the exponent is roughly 2,65 for a typical displacement hull. The Yanmar should run all day long at 3300 rpm, delivering 21.5 hp. But the resulting speed is depending on propeller efficiency; wrong propeller would easily eat all the power and give no thrust! So: what is limiting the prop dia?

    Third: The propeller "working environment" has to be checked. To that end we need pictures/drawings of the actual installation.
     
  10. bit
    Joined: Feb 2008
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    Location: Trieste

    bit Student

    Yanmar:
    [​IMG]
     
  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

    A 3 blade prop is less efficient than a 2 blade , because of the induced drag of the extra blade.

    IF your 2 blade has enough blade area to take the hp , no reason for 3 blades.

    With your LWL , you should be able to cruise at 6K (not stat mph) and burn about 3/4 gph .

    Is the bottom clean?

    FF
     
  12. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    I would add a few things to this excellent list:

    I/ Check that you get more than 3600 rpm on idle. I think the correct idle rpm is somewhere around 3700-3800 rpm. Otherwise the rpm limit in the governor is not correctly set and power may be limited by that.

    J/ Check the bollard pull rpm on reverse. With only 14" you should get close to 3600 rpm. If not, you don't get the full power from your engine. It may be a problem with governor maximum fuel output setting. I had that problem in my previous boat with 1GM10. After proper adjustment, I got 3480 rpm with 1:2.64 sail drive and 14x8 propeller at 6.4 knots and almost the same on reverse bollard pull.

    Now I have a VP 2020 (19 hp) which can turn both 16x11 and 15x10.5 propellers ~3400 rpm at 7.4 knots with 1:2.47. That's a 35 feet 6 tonne sail boat.
     
    Last edited: Sep 12, 2012
  13. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Using a bollard pull is seldom a good tool if the prop has sufficient blade area, and is proper for the boat..

    The reason is props are pitched to operate on an operating , under way vessel.

    This means for a 35 ft boat the prop "expects" the water to be coming at 5 or 6K.

    So it is pitched to shove the water at 10K assuming a 50% slip.

    Tied to a dock the pitch is excessive to operate at 0 K , so the engine will either overload and blow big black smoke , or the prop will cavitate proving nothing.

    FF
     

  14. johneck
    Joined: Nov 2011
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    Location: New England

    johneck Senior Member

    That is why Joakim suggested reverse bollard. The propeller loading will be much less and the engine should be able to reach rated RPM. If it cannot, then it would indicate that the engine is not supplying the rated power.
     
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