Propeller calculations - HELP!

Discussion in 'Props' started by sailor0000, Dec 27, 2012.

  1. BertKu
    Joined: May 2009
    Posts: 2,521
    Likes: 47, Points: 58, Legacy Rep: 223
    Location: South Africa Little Brak River

    BertKu Senior Member

    Thanks Mike, I have transferred your reply to my thread " Anti clock wise or clockwise".
    Bert
     
  2. baeckmo
    Joined: Jun 2009
    Posts: 1,654
    Likes: 670, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    The Joker in your game is the speed. With a weight/power ratio of ~181 kg/hp I doubt that you get more than 7 à 7,5 knots, even with the slenderness ratio (LWL/displ^0,33) 5,6 of your hull.

    Let's say 7,25 kn; equals 3,7 m/s, and gives a Froude nr of 0,92, based on third root of displacement. This speed is just where the resistance in displacement mode starts to rise abruptly, telling us that beyond this you need far more power.

    Now, when using the Bp charts for propeller selection: A/ you must use the advance velocity, i.e. the real inflow velocity to the prop, B/ you shouldn't aim exactly at the best efficiency point (if you err on the wrong side, the efficiency penalty is big), C/ you should allow for some power loss, say 10 % to the prop.

    Here is an algorithm from the SSPA (Swedish State Towing Tank), that is easy to use:

    Ps= shaft power (hp)
    N=rpm
    Vs= ship speed (knots)
    D=prop dia (m)
    p/d=pitch to dia ratio

    D =14,31*Ps^0,225/(N^0,55*Vs^0,1251);

    p/d =9,439*(Vs/(D*N))^0,6211;

    This is a good approximation of data for a three-blade propeller with BAR 0,5, and includes a rough estimate of the wake factor. If we use Ps=22,5 hp and Vs=7,4 knot for your case, we get:

    D=0,549 m (21,6")
    p/d=0,72
    pitch=15,6"

    The best choice would probably be a 22" x 15", since it is better to increase pitch if underestimated.

    That said, I am slightly concerned about the selection of a 25 hp engine for this displacement, but that is another story.
     
  3. sailor0000
    Joined: Dec 2012
    Posts: 27
    Likes: 1, Points: 0, Legacy Rep: 18
    Location: Switzerland

    sailor0000 Junior Member

    Thanks ever so much! This is really helpful, this looks very promising. I love having a new formula, an additional method. I'm beginning to understand there's a lot of approximations in those calculations, so having several methods gives me a chance to figure out an average.

    About the small engine: I should have mentioned that this is an inland waterways cruiser. We plan to launch her on the "Jura Lakes" in Switzerland (lake Murten, lake Neuchatel, lake Biel, connected by canals) and ultimately mover her to the French canals. So we don't need reserve power for beating into big waves, or going against a gale or a strong tide. But fuel efficiency is very important, because we anticipate to have to buy diesel fuel at a car gas station quite often, and we don't want to kill ourselves hauling the cans to the boat.

    Gerold
     
  4. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 208, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member



    Just watch your power figure here Baekmo since the power was stated as shaft power and you are taking another 10% off when those losses are already accounted for.
     
  5. baeckmo
    Joined: Jun 2009
    Posts: 1,654
    Likes: 670, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Yes, I'm aware of that, I should have been more clear on that point. All calculations are valid for the "as new" state, with ideal fuel quality and engine room temperatures, a shining prop without dents and barnacles, a light hull, no wind and so forth. After a few weeks in the water things are entirely different and that's where the additional margin comes handy; it simply avoids overloading the engine.

    This little bugger will probably have very small margins, and a slight increase in resistance or propeller torque may bring the operating point close to the lower rpm limit (depending on the inclination of engine power curve and propeller dito). When the question involves great uncertainties, as in this case (we have no idea about the hull shape), I want my answer to be on the safe side.
     
  6. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 208, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Yes far too many unknowns to get very specific. And cold fresh water for the kinematic viscosity input :)

    It's a good exercise for anybody on propeller design to look at the array of props the power racers carry with them, changed to get the best match for the salinity and temperature on the course that day.
     
  7. Ad Hoc
    Joined: Oct 2008
    Posts: 7,773
    Likes: 1,678, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Wise words indeed, can't argue with that, may be because it's what we do too :D
     
  8. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 208, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    If Gerold was following Dave Gerrs method then he already derated the engine and taken losses to arrive at his shaft power. That's why it could be confusing to get too specific on numbers without more info.

    But I just borrowed Dave Gerr's book to see what his method is. And leafing through I have an issue with the suggestion of derating the engine by matching the prop at a lower RPM rather than a derated power curve.

    That is instead of droping the power curve, shifting the prop curve left and intersecting it at a lower RPM.
    This would lead to an overpitched prop and engine damage if the engine was operated at full RPM for any length of time. Since at the very worst condition ( working flat out) the governor will dump the max amount of fuel into the cylinders to try and get the RPM up and it won't happen. The result can be melted pistons and burned valves [especially in modern high speed turboed engines].

    Has anyone else read his book?
     
    Last edited: Jan 1, 2013
  9. JSL
    Joined: Nov 2012
    Posts: 811
    Likes: 64, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    some comments:
    (a) Over the past 40 years I have done countless prop calcs. and checks, and have seen some monumental screw-ups. If you want calcs done 'free' your best bet is to go to the prop mfr, they usually know what they are doing, providing they have the correct input.
    (b) 10% tip clearance is very tight and might be okay for a slow rpm, a slow boat, or a prop in a tunnel.
    (c) If anybody says you can make up for insufficient diameter by increasing the pitch, check it out. Blade area (diameter) gives the thrust... not the pitch. ( The VicProp calculator refers to this in their comment about blade area)
    (d) getting the right prop the first time is not always possible (not an exact science) - sometimes your have to 'tweak' the pitch.
    (e) are you operating in salt water or fresh. Prop size may differ slightly.
     
    Last edited: Dec 30, 2012
  10. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 111, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "getting the right prop the first time is not always possible (not an exact science) - sometimes your have to 'tweak' the pitch."

    3RD time luckey is not uncommon.
     
  11. johneck
    Joined: Nov 2011
    Posts: 253
    Likes: 17, Points: 18, Legacy Rep: 117
    Location: New England

    johneck Senior Member

    Mike, I am not sure where you are looking. Gerr does talk about not designing for the max RPM/HP to allow for fouling, seastate, wind etc. He also talks about not running engines continuously at 100% rated RPM, but I don't think that he suggests designed for rated power at 90% RPM. But you are absolutely correct that doing so will likely result in overpitching and overloading the engine.
     
  12. sailor0000
    Joined: Dec 2012
    Posts: 27
    Likes: 1, Points: 0, Legacy Rep: 18
    Location: Switzerland

    sailor0000 Junior Member

    Thanks a lot for all the help, all the thoughts! Here's some additional information:

    About Dave Gerr: His Bp - δ method uses full power at max. rated rpm. His Crouch method uses 90% of full power at 90% of max. rpm. That's how I did it, too.

    About the small engine: OK, to tell the truth, the engine (Solé Diesel Mini 34) is rated at 31 brake hp @3000 rpm. Because fuel efficiency is very important to us, I want to mount the biggest prop that'll fit, within reasonable limits. So I'm aiming at 2500 rpm. At that speed, the power curve shows 26.5 or 27 bhp. So, in case I have an unpleasant surprise (not enough power), well, I can mount a slightly smaller prop and get maybe 28 shaft hp. I'm aware I don't want to overload the engine, have it "lugging" at full throttle. On the injection pump, there's an adjustment screw for the idle speed, and another one for max. speed. So I'll simply adjust the max. speed to slightly lower than whatever speed the engine can reach at full throttle, and this issue should be solved.

    About the hull shape: I'm attaching 3 photos and a drawing. I'm hoping this will answer the question.

    Also, all your comments and advice make me understand that all those calculations are based on "guesstimates" (a Dave Gerr word) and approximations. So I guess I'll have to test several different props and see what works best.

    Gerold
     

    Attached Files:

  13. johneck
    Joined: Nov 2011
    Posts: 253
    Likes: 17, Points: 18, Legacy Rep: 117
    Location: New England

    johneck Senior Member

    OK, you've got enough power to push this thing, but I would be concerned about the windage and the ability to maneuver or maintain way in a headwind.
     
  14. JSL
    Joined: Nov 2012
    Posts: 811
    Likes: 64, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    You might want to check out your skeg & shoe alignment. Keeping it on one plane can make hauling and blocking easier.
     

  15. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 208, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    On page 6
    "As we will see later .......a good idea to size the propeller to cross the engine power curve a bit below top rated RPM "

    On Page 46-47 Crouch's method there appears to be some confusion mixing RPM and power. Wants to operate at 75% RPM to cruise but applies that factor 0.75 to the power. see the calculation 182/0.75=242 HP

    It then says :
    "base pitch on operation at 90% of maximum RPM ....." but then it says this: " we will still be able to open the throttle up to get top revolutions when needed."

    But that depends entirely on the particular engines response.

    I haven't read through the BP method implementation yet, I think it's a bit misleading so far particularly if you were a novice. And it also presupposes as close relationship between power and RPM but many engines don't have such a relationship and can have a flat power curve .

    The prop should be matched shy of the max intermittent RPM and power the governor/engine can provide, not based on an arbitrary RPM. As the setup is de-rated the curves of Prop power and shaft power fall naturally on a lower RPM and the RPM should be taken from that plot for the prop design.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.