Low on rpm, going from four to five blades?

Discussion in 'Props' started by Magnus W, Dec 3, 2018.

  1. Magnus W
    Joined: Nov 2017
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    Location: Sweden

    Magnus W Junior Member

    Background:
    I have a new engine in one of my workboats and I have a few problems, one being that the prop is too big and the other is noise/vibrations.

    Current prop is a 4-blade skewback, stainless steel and reinforced for ice. It's stamped 22 x 29 and the pitch has been measured to range from 29,1 to 29,5 between the blades by a prop shop. Since we're so close to the stamped 29 I take this as an indication that 29'ish is correct. It's also well balanced.

    Boat weighs circa 5500 - 6000 kg lightship and I'm getting 26,3 knots (average speed in both directions) at 2810 rpm (according to the test computer so true engine speed).

    Since the engine is flat rated at 350 hp from 2100 to 3000 rpm we can reasonably assume that 26,3 knots is what the boat will do with this power at the test run mass. We were 3 persons on board and circa 300 liters of fuel (max is 400 liters) and given how I use the boat this would qualify as lightship. The boat is equipped for 12 passengers and one crew.

    I care nothing about top speed but I need to get the rpm up to reduce the strain on the gearbox which is the limiting component in the drivetrain.

    To begin with I need 190 rpm in lightship, then some more to compensate for higher mass.

    Then I also need a smaller diameter since the prop clearance towards the hull is too small (which may well be a source of noise).

    I have been told that a 5-blade skewback with higher area (about 106 percent) has a good chance of being quieter than the current 4-blade with circa 85 percent area. And since I don't need the heavy ice capabilities I can get away with "simply" a nibral alloy.

    Numbers and specs:
    Engine: FPT N67, 350 hp between 2100 and 3000 rpm
    Tranny: PRM 1000, 2,03:1
    Prop: 22 x 29 stainless reinforced skewback
    Current top speed: 26,3 knots at 2810 rpm. Circa 5500 kg.

    Wish list:
    * 3000 rpm at about 700 kg higher mass than during the test run from which the data above came from.
    * Reduction in diameter from 22 (to 21?) inches.
    * Quieter design.
    * Not reinforced or stainless but more sturdy than plain brass, perhaps nibral.
    * Standard prop, not custom, for ease of replacement.

    Please share your thoughts.
     
  2. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    This being a very “nuts and bolts” issue, I’d suggest you work closely with a reputable prop shop.
    In general, though, going to more blades will not help you get higher rpms.
    Adding blades is a solution where prop aperature is limited, and power is plentiful.
     
  3. Magnus W
    Joined: Nov 2017
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    Location: Sweden

    Magnus W Junior Member

    Yes, I realize that blades only won’t get me where I want. And I am in contact with two prop shops but I’d like some input from the collected knowlesge here too.
     
  4. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Afraid you are making a mental saltomortale here. To start with, you say top speed is not an issue, then what is the wanted speed light and fully laden? The point is, the propeller power consumption will vary roughly as rpm^2,5 and speed in the planing regime as power^2.

    In rough numbers, if you reduce rpms to 2600 (with the present prop.), the power will be something like 288 hp, the estimated speed ~23 knots and the "power per 100 rpm" used by PRM will be 11. This is below the leasure craft limit, but too high for "light commercial). So, on it again, now with 2400 rpm, giving 236 hp, speed 20 knots and power per 100 rpm is 9,8. Just take your pick, then note the turbopressure and exhaust temp after turbine and use them as power guidance.

    There is an additional upside with lower rpms in this case: cavitation reduction. Going to a smaller diameter and higher rpms is increasing the blade loading, leading to more intense tip vortices and reduced efficiency (=increased fuel consumption).
     
  5. Magnus W
    Joined: Nov 2017
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    Location: Sweden

    Magnus W Junior Member

    Perhaps I wasn’t clear enough so I’ll try to explain (although I may well be making a saltomortale anyway).

    I’m not targeting a specific speed at all. I’m just stating that the boat in the current setup and during the conditions at the time of the test reached 26,3 knots @ 2810 rpm.
    I’m also stating that the boat, in said conditons, will reach 26,3 knots regardless of rpm between 2100 and 3000 since the engine output is 350 hp between 2100 och 3000 rpm. If we disregard details like prop efficiency I could fit a larger prop and reduce the rpm whilst still reaching 26,3 knots (this until the prop is so big that it would limit the rpm below 2100 ie limit the power). At least this is how I understand it.
    (The engine is a FPT N67 450 that’s been derated to 350. FPT has simply (crudely) chopped of the power curve from 350 and up, thereby creating that flat rate.)

    My main problem is that I need to reach 3000 rpm at a 700 kg higher mass than during the test run. This will lead to lower top speed but that is not a concern.

    The reason is that I need the rpm in order to get within commercial limits of the PRM. Had I had another gearbox I wouldn’t have bothered (since I still get full power) but the gearbox is squeezed in tight in the keel below the engine (drop center) so I couldn’t find another option without relocating the shaft (and there wasn’t time nor economy to do that when we swapped the engine).

    Another issue is that there is very little clearance between the current 22” diameter prop and hull. I’d estimate about 15 mm. So while at it I’d like something smaller.

    (A side note about the test run. We weren’t able to log pyro and I don’t recall boost but it was as it should be according to FPT. During a 5 minute WOT run charge air temp never got higher than 27 degrees C and engine coolant was at 82. But I reckon that was to be expected since the engine is built to produce a 100 hp more (or may say 60 hp more given that we weren’t acutally reaching 3000 rpm).)
     
  6. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Well Magnus, you still don't understand the message. Due to the relation (propeller power) to rpms, ie Power is proportional to rpm^2.5, the torque is reduced quickly with reducing rpms. The prop does not know what is in the other end of the shaft, its power consumption follows its own curve no matter the engine characteristics! It is just a simple question of finding the acceptable operating point.

    Now in planing operation the Propulsion power is proportional to speed^2. This means that the engine power AND TORQUE is reduced quicker with a rpm reduction, than the speed (speed ~rpm^1.25) in the planing range. The PRM state a figure for power per 100 rpms for their transmissions, that is what I showed in my example; going down to roughly 2400 engine rpms (or slightly below), you are close to the light commercial limit. You must understand that the engine is only delivering the power swallowed by the propeller at the actual operating rpms and speed; the engine maximum setting has nothing to do with torque at reduced propeller shaft rpms!
     
  7. Magnus W
    Joined: Nov 2017
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    Location: Sweden

    Magnus W Junior Member

    I feel I don’t understand you.

    Assume that a given setup results in X shaft rpm with the engine producing a certain power.
    Now we change the propeller so that the setup results in 2X shaft rpm but with the same input power (ie engine turning faster).
    Is it not correct that the shaft torque is lower after the propeller change?

    Yes I understand that the prop is limiting the output of the engine. But if I reduce the WOT rpm by fitting a larger prop (with an engine that can produce the same power over a wide rpm range) I fail to understand how that could reduce the torque.
     
  8. Magnus W
    Joined: Nov 2017
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    Location: Sweden

    Magnus W Junior Member

    Also, I have never said anything about desired cruising speed. But since you mention it I cruise at about 20 knots which currently translates into 2400 rpm.

    My moderate cruising speed is also the reason why Newage approved the PRM1000 at 2,03:1 behind this engine for my use despite the fact that 12,64 hp/100 rpm is the leisure limit (and 9,98 is the light commercial).

    And of course the power and torque go down if I go slower. But the net benefit of a speed reduction is equally obvious related to the starting point.

    In this specific case, with 350 hp at 2810 rpm, it’s 12,46 hp/rpm.
    3000 rpm would give me 11,67 - which is the target I’m aiming for but at a 700 kg higher mass.

    And then I’ll reduce it even further by going slower. But that wasn’t a part of the original question.
     
  9. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Ok, here we go again. I am aware of the torque limit of the PRM, it is 700 Nm input. With the present propeller (red curve in diagram below) and full throttle, you have 350 hp @ 2810 rpm, point A. If you trottle down to 2400 rpm along the red curve you will get 240 hp (point B). This equals 700 Nm. Now if you install a new prop designed for 700 Nm @ 3000 rpm, that equals 298 hp (green curve, point C), which is the maximum you can use within the 3000 rpm/700 Nm limit of the transmission.

    This is where I was looking for your goals speedwise; you said "...care nothing about top speed....". With the present set-up (max 245 hp @ 2400 rpm) you will get something like 20 knot with 6,7 ton displacement. If you select a propeller for 3000 rpm you will be able to use maximum 298 hp within the transmission operating limits. Again with 6.7 ton that might result in ~22 knot; ie a difference of 2 knots. So "...caring nothing about the top speed...", you can just as well carry on with your old propeller.

    But if the noise situation at 2400 rpm is still a problem, then a five blade screw would be the way to go. But I would not aim at 3000 rpm due to cavitation (which probably is part of the noise situation now). What is the clearance with the 22" prop btw?

    Edit: Propellers designed to cope with ice have thicker blades and "blunter" edges. They often operate with more propeller noise and vibrations than normal props. The commuter ferries in "my" archipelago change from ice-props to normal as soon as the hard fresh water ice blocks coming from the river are gone, and that is a relief.
     

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    Last edited: Dec 4, 2018

  10. Magnus W
    Joined: Nov 2017
    Posts: 75
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    Location: Sweden

    Magnus W Junior Member

    Thanks b for the info and good talk over the phone - so much easier to understand each other that way.

    Could you drop me a pm with your email address?
     
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