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Propeller size vs engine torque vs horsepower

Discussion in 'Props, Shafts & Seals' started by Seasmaster, Sep 13, 2016.

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  1. Seasmaster

    Seasmaster Senior Member

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    I have a propeller/propulsion dilemma on 49’ Elling E4, and would appreciate some guidance and input from the diesel & propeller engineer’s reading the forum.

    Boat specifics:

    LWL: 49’; D: 4’; B: 14’; Displacement: 41,000
    Volvo D6-435 I Rated 435BHP@3500RPM; SHP: 422; Gear ratio: 2.5:1
    Max torque: 772ftp @ 2500RPM, dropping to 644ftp @ 3500RPM.
    Effective mean pressure: 349.9psi @ 2500RPM, dropping to 289.4psi @ 3500RPM
    Max combustion pressure: 2843psi @ 2500RPM, dropping to 2756psi @ 3500RPM

    Data from Sea Trial at Survey: Using 4-Blade Prop – 27” Pitch: unk
    RPM's 680 1350 2200 2420 (WOT)
    GPH (from gauge) 0.3 1.1 5.6 21
    Spd (GPS 3.4 6.9 9.8 14.8
    Miles per Gal 11.33 6.27 1.75 0.70
    Range @ 95% of 400g 4306.7 2383.6 665.0 267.8​

    Data after propeller replacement 5- Blade Prop - Pitch: 27 x 21
    RPM's 1000 1500 2000 2500 3000 3500 (WOT)
    GPH (from eng specs) 1.1 2.7 5.2 9 14 21.9
    Spd (GPS) unk 6.3 8.3 9.5 11.5 15.5
    Miles per Gal unk 2.33 1.60 1.06 0.82 0.71
    Range @ 95% of 400g uno 886.7 606.5 401.1 312.1 268.9

    My observations from reading the data:

    1. With the 4 bladed prop, the WOT of 2420RPM’s is very near the max torque of the engine, however the fuel burn of 21gph is identical to running at 3500RPM’s of the 5 bladed prop.

    2. It is understood that the GPH determined from the gauge might be inaccurate.

    3. It is understood that the GPH from the engine manual for the 5 bladed prop might be inaccurate too.

    4. Based on the data above, it appears the 4 blade propeller is more efficient and offers much better range up to the hull speed of the vessel of about 9.2 kts.


    My question is:

    Will the engine be damaged by using the 4 bladed propeller vice the 5 bladed one, knowing that the engine will never reach it’s rated RPM of 3500? Most of my cruising will be done at hull speed. Although I’m not planning on running WOT often with a fuel burn of 21gph, I don’t want to risk engine damage by having to run WOT should I choose to do so.

    In advance, thanks for the information-and my apologies if I've placed this in the incorrect spot, as this is my first post to YF.

    John
    Last edited: Sep 13, 2016
  2. Capt J

    Capt J Senior Member

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    Yes, running a 4 blade prop that only turns 2450 rpms WOT on a motor designed to turn 3500 RPMs will absolutely lead to a shortened engine life.

    A 4 blade prop should produce more speed than a 5 blade prop, all other variables equal.

    You'd probably be best served with a 4 blade, 27x22" prop. Possibly a 27x23".
  3. Seasmaster

    Seasmaster Senior Member

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    Cap'n J,

    Thanks. I always enjoy your comments and suggestions. And in discussion with a couple of prop shops, they are coming in with sizes very close to what you indicated: 27" x 20.5", and 27" x 21".

    What is driving my query is the change of RPM's discovered by replacing the electrolysis damaged 4 blade propeller (used during sea trial) with the original 5 blade propeller that was on the boat, and discovering that WOT is now 3500RPM. As I dig deeper into "propeller science", it sends me over into "diesel science", and to me that's just a complete "black hole" unto itself!

    Although I'm not an engineering wiz, I'm trying to get my head wrapped around the declining torque curve above 2500 RPM's. That 2420 figure was near max torque for the engine, so I'm not understanding how it would cause premature failure by using the engine at lower RPM's (hence less piston travel) yet at the strongest torque.
    The spec sheets follow.

    Thanks again for taking the time to explain.
    John

    Attached Files:

    Last edited: Sep 13, 2016
  4. Capt J

    Capt J Senior Member

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    If an engine is rated at 3500 rpms (WOT), it HAS to be propped so that it achieves 3500 RPM's WOT, then cruise or 80% load should be around 2800 rpms which would put it near the top of the torque curve. Torque is what gets a vehicle moving from a stop, HP is what keeps it moving and acceleration. Having it only reach 2420 rpms is a severely overloaded situation, with 100% throttle maintain that rpm. We're talking a pleasurecraft HP rating here. Some commercial continuous duty engines (are usually detuned compared to pleasurecraft) can be run WOT all of the time, most likely not what you have in your boat.

    Think of it as a bicycle or a truck (with manual transmission.) Now think of going up a steep hill at 15 mph, but in top gear. Yes you get further for each rotation, BUT can only maintain it for a short period of time because it's too difficult. Now shift to a much lower gear, and it's much easier to peddle or much easier for the engine in the truck to go 15 mph.
  5. Norseman

    Norseman Senior Member

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    Over Propping has been a hot topic for years:
    Some folks thinks it will increase speed at a lower RPM.
    (It will)
    The fuel flow, temps and engine life may go up, up and down however.
    I kept experimenting with it my self.
    Had 2 props for my Perkins diesel and kept taking them to the prop shop for adjustment and fine tuning.
    Then did a Bahamas cruise on each change, recorded RPM, speed and fuel flow. (And temperature)
    Finally settled on a pitch that would give me max rated RPM in the ocean and 90% RPM tied to a dock at full power.
    Then cruised at 70% of Max continous RPM: 3000. Max rated 4000.
    (The 4000 RPM should be considered war power only, but for pleasure craft it really was Max. Same engine on a Commercial boat was rated at 3000 Max)
    As said above and as experienced on my own boat over 14 years and 27 Bahamas cruises and numerous Florida Keys mini-cruises, the best combo was indeed the pitch needed to achieve rated RPM.
    Some folks will advocate a slight over-pitch and they may be smarter than me, but on my boats I won't do it.
  6. Seasmaster

    Seasmaster Senior Member

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    Many thanks to Capt J and Norseman for their helpful responses. Clearly they are speaking from their vast experience.

    Onboard the merchant ship I’m currently on, the senior engineers – the guys that speak diesel as a primary language 24/7 – are saying (with the caveat they don’t know the Volvo D6 dialect) that if the engine is run within the torque power band, there should be no downside. They obviously concur with not running WOT all the time, and concur with running at 80 – 90% to properly load the engine. That way there is not turbo-barking, and all the sub-systems are happy.

    The power band for the D6 taken from the manufacturer’s torque information [drawing a horizontal line through the graph’s bell curve] is from 2000 – 3000 rpm. At 2000rpm, it’s 100kpm(993Nm), rising to 105kpm(1047Nm) at 2500rpm, and decreasing back to 100kpm (980Nm)at 3000rpm. By WOT, 3500rpm, torque is down to 88kpm(873Nm).

    So, from an engine operating status, how is the apparent over propping bad in this case? If running at 2200rpms, going 9.8 kts, consuming 1.75 gal per mile, with torque about 3kpm less than max isn’t that better use of the engine’s power?

    I may be dense here, but I’m seeing the engine just using a comfortable amount of torque to thrust a lot of water astern, with minimum piston travel and fuel burn. The turbo’s kicked in – not working too hard & not barking – and life is “good” for the engine.

    If I’m not tracking this engineering situation properly, hopefully someone with a “PHD” in diesel engines, and with a specialty in Volvo D6’s, can realign my thought process,.

    - - Leaving deployment in Korea; next stop FL!!
  7. Norseman

    Norseman Senior Member

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    Aye mate, some if that stuff is above my pay grade, and my experience does not include turbo-charged diesels.
    The small normally aspirated diesel engines I know about likes 90% RPM at max power while tied to a dock.
    This, acording to the experts, and 100% RPM at max power, while running free, is an indication of a perfect prop:)
  8. Capt J

    Capt J Senior Member

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    Because it is overloaded at every RPM if it is not propped so that the boat runs 3500rpm WOT in foward while the boat is fully loaded with fuel,water,gear. You would be running 100% load with the current prop you're talking about. The motor needs to be propped so that it achieves 3500RPMs at WOT in gear and underway (or whatever the manufacturers rated rpm is at WOT). Call a Volvo dealer for further comments on what the manufacturer requires.
  9. ranger42c

    ranger42c Senior member

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    The Michigan Wheel website has a prop-sizing tool; might be useful to check that against your numbers.

    -Chris
  10. Capt Ralph

    Capt Ralph Senior Member

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    There was an artical in the PBB Mag a while ago that discussed over propping and leaning towards operating at the peek torque curve vs conventional wot rpm.
    I'm not a fan of this but there was an interesting argument with some numbers.
  11. Capt J

    Capt J Senior Member

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    And there is a neat gauge when you do that, called an exhaust gas temperature gauge, that would make most mechanics and engineers fall off their seat when they see what it reads.