Prop question

Discussion in 'Props' started by Kalur, Oct 14, 2017.

  1. Kalur
    Joined: Oct 2017
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    Location: Estonia

    Kalur Junior Member

    Hello,

    I got trawler that has 33" prop with 25" pitch, 4 blades kaplan in a nozzle. Engine is 215hp @ 2400rpm and it had 1:3 reduction.
    To order new trawl doors and fishing gear i had to measure bollard pull. Measring was quite easy with 10t crane scale and 50m rope. Result i got 1500kg at 1500rpm and knowledge that my engine is basicly heap of scrap metal, because it just couldn't go over 1500rpm in gear. Out of gear it goes bit over marked 2400rpm.
    I have left over engine that is 380hp @ 1800rpm and gearbox with 1:2,04 reduction to put into that boat, but now questions arise that i can't answer myself. At least without some help.

    So now i would like to find out propeller bollard pull capability?
    Most formulas and calculators tell me which propeller i would need, but i'd like to know how much thrust i can get out of it and at what rpm to know if that gearbox is suitable. It might be simple and there could be calculators for it but language skills and stupidity may stop me from finding it out.

    Ohh yeah boat data LWL 10,3m, BWL 4m, draft 1,2m and displacement 18t.

    Any help appreciated.
     
  2. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Hello Kalur and welcome to the forum. Actually, your engine may not be all that bad after all. The propeller is "asking" for more and more torque from the engine with increased resistance (read trawling pull) causing reduced speed. This means that you can never expect the engine to deliver max rpms and power at bollard pull unless you have a propeller with variable pitch.

    Without knowing the power and torque curve of your present engine it is hard to say for sure where it would have its operating point at bollard pull, but I do not find it too odd that your propeller/nozzle combination results in about 500 prop revs/minute. With a rough calculation the power absorbtion would be about 150 hp at that point and the thrust would be slightly compromized by cavitation.

    So first a few questions: when steaming lightly loaded, what maximum rpms do you get and what speed (18 tons are heavy for a 10 m waterline length, btw)? What engine is it? With this info it is possible to come closer to a truth "of sorts".

    Then your alternative engine: with its 2,04 reduction it basically needs a bigger prop, but you may compensate with increased pitch. But then you are back to an even greater torque increase at bollard pull and trawling, so that engine may bog down as well. In order to assist, it is necessary to have more specific info on the engine characteristics.
     
  3. Kalur
    Joined: Oct 2017
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    Location: Estonia

    Kalur Junior Member

    When just steaming it goes up to 1800rpm and i get round 7,5-8kts. Boat is pure steel with ice class so i guess that contributes alot to 18 tons (that number is also from crane that put the boat into water, papers say 14 tons). My biggest mistake was not looking everything up before buying that boat and assuming things like max rpm for engine being 1800, so now i'm trying to get as much info before doing anything stupid again.

    Engine is Volvo Penta TD70E at least from markings on engine block and that is also in papers. My best guess would be that this used to be truck or generator engine because i can't find any info about such marine engine, also it lacks any usual marine engine features like cooled manifolds and stuff. So i haven't managed to find power curves for it. Just found 1 site where it states that torque is 680Nm @ 1400rpm and power is 205hp @ 2400rpm tho in my ship papers it sais 158kW that is roughly 211hp.

    Replacement engine is Volvo Penta TAMD122A which power curves are freely available online. Torque is over 1600Nm @ 1280rpm, 1483Nm @ 1800rpm. Simple math sais that i would have lot more torque at shaft with 2,04 reduction that i have currently with 3. With 2,04 reduction i would gain 82 shaft rpm at max rpm (if current engine would go up to max rpm in gear), since that engine is only 1800rpm. But that 82 rpm brings up question about cavitation which to my understaing kills thrust and propeller. That's why i would like to find out what that prop can do because finding another used gearbox with different ratio is possible.

    Replacing prop with bigger one is not an option currently due to finances. Also fitting a bigger prop would mean losing nozzle atleast without major reconstruction and to my understaing nozzle is good for trawling since desired trawling speed is 2,7-3kts. As for steaming speed i'm happy with 7,5-8kts tho more would never hurt but that deffinately isn't my goal.
     
  4. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Ok, the figures make sense; your prop has too much pitch for the 70E engine. At 1800 rpm its Power output is about 178 hp, and this confirms the bollard Power to be 145-150 hp. This iron was used in Volvo trucks type F86 (or thereabout....). One solution might be just to reduce propeller pitch to suit this engine; would not cost a fortune, and it might thrive better both when steaming and trawling.

    I'll twist the numbers for the TAMD 122 to see what comes up, I'll be back on that later.
     
  5. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Right, here we go....:

    The TAMD 122 will work ok, provided the coupling, shaft and propeller can take the 50 % torque increase it can produce. No pitch correction necessary, you will almost double the pulling force; if the gantry and hull stability up to this, then ok. On the plus side is of course the marine components like watercooled exhaust and turbo.

    Good luck!
     
  6. Kalur
    Joined: Oct 2017
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    Location: Estonia

    Kalur Junior Member

    Thanks for doing the math!
    I had diver go and measure propeller but thickness of blades was not measured so that is unknown for now. When i get the video diver made it might be possible to get some rough estimation. Shaft is 80mm thick stainless steel but since there are no markings on visible part in engine room can't tell exact mark of steel.

    But still my original question sort of remains that how do you do it?
    In my (stupid?) theory it should be possible to have some nice graph that shows how much thrust you get at given shaft rpm.
     
  7. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    In my (stupid?) theory it should be possible to have some nice graph that shows how much thrust you get at given shaft rpm.........

    Afraid it isn't that simple, as there are double matchings to be done simultaneously (propeller inflow versus hull lines and engine power characteristics versus propeller dito). In addition there are different nozzle/propeller combinations, each with its own characteristics. Some of the trawling gear manufacturers have built their own data bases with relevant info on the most common combinations.

    This is why I asked for the max throttle performance in order to get one real operating point for your setup, in addition to the bollard pull data you gave. With that at hand I could let the computer do the nasty job and come up with the info in post no5. But that is just one operating point, and that is as far as I will take it here. The propeller shaft diameter seems to be ok at first glance, I checked it against the DNV 15 m workboat rules for AISI 316 steel.

    Now, if you want to dig deeper into the subject, start with manual calculations based on the so called Taylor propeller diagrams. That will give a feel for the process. There are some references to "Bp-diagram" in this forum.
     
  8. Kalur
    Joined: Oct 2017
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    Location: Estonia

    Kalur Junior Member

    Ah seems i need to go back to school or at least improve my english to understand that.

    But one big problem arised now that i got the video diver made. I somehow wrote down numbers wrong when they did measurements and calculated pitch wrong.
    Could you please let your computer do nasty job again with 31" pitch?
    Incase i even did pitch calculations wrong again i think it would be safer to give numbers divers took:
    Diameter - 83cm
    Blade lenght - 34cm
    Blade width at surface - 27,5cm
    Blade height difference - 8cm
    Propeller hub lenght - 16cm
    Nozzle lenght - 42cm

    Since quality of video isn't best it's kinda hard to even estimate blade thickness. My best guess from that video is no more then 3cm probably closer to 2.

    Video file is Dropbox - ADELE KRUVI https://www.dropbox.com/sh/t025f7181ddkzgx/AADUUTFQEKdXt6py-HPoeG7la?dl=0 if it gives any more helpful ideas.

    Damn feeling more stupid every day.
     
  9. baeckmo
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    baeckmo Hydrodynamics

    Damn feeling more stupid every day.....

    .....I'm very familiar with that feeling....., welcome into the fog!

    Your propeller pitch (provided the late measurements are ok) is 31", which makes even more sense with the performance figures.
     
  10. Kalur
    Joined: Oct 2017
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    Location: Estonia

    Kalur Junior Member

    Nice that something i write makes sense!
    How about post #5, is it still correct or bad are things happening with increased pitch?
    Also how much does that pitch increase change bollard pull? How close to 4 tons i get? Currently i have trawlwinch that has (had when new?) 5 tons pulling power and i would like to have at least 20% power reserve there.
     
  11. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    Your results are normal.

    A boat tied to a dock has no forward motion which is fine for calculating bollard pull for a tug boat.

    Every boats (besides rated tugs) prop expects to be moving in the water .

    A prop repitch should be less expensive , and would allow higher engine RPM , so more power , more fuel burned too.

    A 35 ft boat will be at hull speed at 7K , but you might be able to trawl faster if desired with a repitch. The pitch would be DECREASED to allow the engine to spin faster.

    A far larger engine will do little for your transit speed , except make bigger stern waves.
     
  12. Kalur
    Joined: Oct 2017
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    Location: Estonia

    Kalur Junior Member

    Maybe i should have explained backstory more.
    Current engine would need complete overhaul anyway, which means i have to cut open my boat to get it out. Spare parts only would cost in region of 2500 euros. Then i would have to take boat out which means ordering crane that would cost round 1500 euros and later another 1500 euros to put boat back to water. Then repitching, which i don't know if anyone can do in Estonia for that size prop. If i use that spare engine and gearbox i would still need to cut boat open but i can lift engine out with my own crane. Then take engine with frame to local metal shop along with spare engine and have them install new engine to frame so that coupling will stay in exact same location in reference to engine frame, would cost less then 500 euros.
    So it is much more economical to swap engine and gearbox then overhaul old engine and then replace/repitch prop. From that came questions what my prop can do and as baeckmo pointed out there is question if other items will withstand increased torque.
    That got me thinking of trawlwinch that has 5 tons of pulling power according to label on it, but since it is old and there are bound to be some losses in power, so question arised that how close to 4 ton bollard pull i get to have 20% winch power in reserve. If answer is i can no way go over 3,5 ton bollard pull i SHOULD be safe.

    And as i pointed out earlier transit speeds are not that much of a concern 7-8kts is very fine for me. Pitch increase came from original wrong data i gave about prop in my first post i said that pitch is 25" when actual pitch after revising measurements is 31" and that arised question about how much it changes calculations baeckmo did (still feel bad about giving wrong numbers at first).
     

  13. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Ok need to check status with 31" pitch and TAMD 122 for a thrust figure. Am out of office until wednesday next week, but you will certainly be able to use present propeller, possibly with a pitch decrease (note that I said decrease before as well.......).
     
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