Help me get my head around variable pitch prop propulsion design

Discussion in 'Props' started by Annode, Mar 1, 2020.

  1. Annode

    Annode Previous Member

    After reading this thread:
    Controllable pitch prop, effective range https://www.boatdesign.net/threads/controllable-pitch-prop-effective-range.62796/

    I have seen some calcs that assign as much as 50% to losses in getting engine power to the water.

    the reduction of gearboxs is roughly around 3:1? Diesle motors run at <2000rpm

    So I am not clear if a vairable pitch prop can completely eliminate a gearbox. Obviously "shifting" into reverse is possible, but can the blades spin at 1500 or 2000 rpm if they are lightly feathered to produce thrust efficiently for a full displacement 30m steel hull?

    The main reason for consideering a VPP is to be able to feather the prop when under sail. speed will be around 8-10kts max, so this is an efficiency question mostly

    If that were possible, a much smaller powerplant could be specified making the entire propulsion system smaller. lighter and most importantly, cheaper.

    If gear reduction is required to keep prop tip speed lower, then what engines are availble that can make full power at <1000rpm.
     
    Last edited: Mar 1, 2020
  2. jehardiman
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    jehardiman Senior Member

    First, while VPP and CPP are used interchangeably by most people, there is not really anything that is a true VPP. This would mean that the propeller blade changes its radial pitch as well as root pitch by some means. There are only a few types of props that do this, most which are composite material fitted to less than 5 kw trolling motors with highly skewed blades that twist off to limit torque at speed. This is not what you mean.
    For minimum drag in a sailboat situation I would not consider a CPP, but would consider a folding prop. Even if a CPP is fully feathered, drag is considerably more than a folding prop. The main advantage of a CPP over a folding prop is in near instantaneous change between ahead and reverse. The main disadvantage is weight, complexity, and lower ahead efficiency.
     
  3. Annode

    Annode Previous Member

    Thanks for that info. I am not sure that thre is a folding prop available for ~300hp pushing 200t of steel tho

    I am also trying to eliminate the gearbox losses using an ICE. a pipe dream perhaps
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Not quite accurate what Jehardiman said.
    First the gear boxes. To go without one you need lower rpm engine than modern ones. There used to be Sabb-Lister engines with a "gear box" without reduction instead having the mechanics to control the propeller pitch. With higher rpm engines the tips of the propeller blades would have too much speed causing severe cavitation damage..
    CPP's are the main type of propulsion in fishing vessels from the North sea to Barents sea. There are many producers like Nogva and West Mekan in Norway and some in Denmark I don't remember. All of these have no plastic or composite parts. They have some models (mostly West Mekan) for intended use for sailing vessels and use of conventional gear boxes in between the drivetrain. Their pitch is user controllable.
    VPP's, like Brunton's autoprop adjust their pitch automaticly by the lift created on the blades.
    Low rev engines? Look up Gujarat "high speed" diesel engines from India. Good old tech from the time when India was the gem of the Crown. At 300kg two cylinder engine 700rpm 28hp if I recall it right ;)
     
  5. goodwilltoall
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    goodwilltoall Senior Member

    What kind of boat do you want to propel, original post said sailing boat then next post says it is 200t
     
  6. Annode

    Annode Previous Member

    its a motor sailer in steel. 25 - 35m. its 120 - 200t loaden yes, Its a design project

    Some good answers. Thank you. I had not considered the maintenance aspect of a CPP
     
    Last edited: Mar 1, 2020
  7. goodwilltoall
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    goodwilltoall Senior Member

    That will be a big sailboat. I like your thoughts on using 1-2hp per ton.
    Only manufacturer I know of in that size is hundastad, well know for long lasting and top quality at same time dearly expensive
     
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  8. jehardiman
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    jehardiman Senior Member

    Yeah, 200 tons is not a sailboat, that is an auxiliary sailing ship. So what prop I would choose would depend on how the stern shafting and tube is arranged. Lots of options there, so you really can't say anything until the run is arranged.
     
  9. TeddyDiver
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    TeddyDiver Gollywobbler

    Hundested!
     
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  10. Niclas Vestman
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    Niclas Vestman Senior Member

    I have read that one reduction gear causes less than 2% loss. Meaning, better to get the highest efficiency engine. Would probably yield higher performance than some odd engine without reduction gear.
    Espescially since a prop is sensitive to optimum revs.
     
  11. KeithO
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    KeithO Senior Member

    Annode, I think you making a conceptual mistake. Most of the losses are at the actual prop to water interface.

    Lets make an analogy here: look at the difference between propellor planes and helicopters. For the same hp engine, the static thrust of an airplane propeller is much less than a helicopter rotor, which has to lift the entire aircraft, engine and crew, whereas the propellor on a plane cant do even a fraction of that.

    The obvious difference is the size of the rotor and the aspect ratio compared to a much stubbier propellor. The rotor with its large diameter and slow speed (rpm) has a huge static thrust, whereas the smaller stubbier prop develops a low thrust and therefore needs a long runway to take off but might give 200mph in flight.

    So, if your hull produces a certain resistance to motion, it is going to take a certain size of prop to produce adequate thrust efficiently. The smaller you make the prop, the less efficiently it is going to work. Perhaps achieving hull speed only in calm conditions but not into wind and waves. Usually the required prop diameter cannot be driven at engine speed, which is why you have a reduction drive.

    A modern transmission should have an efficiency of 95% or better unless it has a built in torque converter, usually used on gas powered recreational boats (velvet drive). I wouldnt expect to see one on a diesel powered yacht.

    The other major advantage of the VPP is the ability to go forward, backward and stop all with the engine running at a constant speed. Just 1 lever to manipulate, perhaps 2 in the case of a twin engine setup. This can be a lot less stressful than having to shift forwards and backwards and risk the possibility of stalling an engine at close quarters to a dock, other peoples expensive boats etc....

    Annode, post: 870447

    I have seen some calcs that assign as much as 50% to losses in getting engine power to the water.
     
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  12. bajansailor
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    bajansailor Marine Surveyor

    Annode, re the 50% losses that you mention above, there is not an awful lot you can do to improve this - even the best propeller in the world is not going to have an efficiency better than 75%, and usually much less.
    Combine this with all the other areas in the drive train where you will have losses, and you are starting to approach that 50% fairly quickly.

    This link is a gem - it explains very well what goes on between the engine and the propeller.
    https://www.usna.edu/NAOE/_files/documents/Courses/EN400/02.07 Chapter 7.pdf
    OK, it is for big ships - but the same principles still apply to 'small' ships like yours.
     
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  13. Annode

    Annode Previous Member

    Thank you All. GREAT responses! A lot to research still.

    This is a 200 ton "sailboat". It does not have a yacht style keel.

    Motorsailer - LUNAR C115 - Conrad Shipyard https://conradshipyard.com/yachts/sailing/lunar-c115/

    Its really a motorsailer, but in the right conditions it could sail. I am not sure how they solved the prop issue, but they dod seomthing

    The 50% loss figure I saw somewhere. I forget where. and yes a lot of that is prop inefficiencies. Even so, stepping up from a 300hp to a 600hp powerplant means throwing away 300hp in fuel every hour. That is not inconsequential.
    Making a powerplant choice becomes a case of diminishing returns as each knot above 8kts cost exponentially more in power
     
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  14. bajansailor
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    bajansailor Marine Surveyor

    Re the Lunar C115, is this the type of boat that you would like to design and build for yourself?
    Yes, the Lunar is a 'motorsailer', but given a decent breeze she will stomp along very powerfully.
    I crewed on a 30 m. motor sailer (a Philip Rhodes design) many moons ago, on a passage from Antigua to Newport RI via Bermuda- like the Lunar she has twin engines, but unlike Lunar (which has a pair of Amartech CP propellers according to the specification) she has fixed pitch props.
    We motored the whole way up to Bermuda - not enough wind to sail. When we left Bermuda in the teeth of a F 7 gusting 8 gale, we put up full working sail, turned the engines off - and she took off. The log was steady at 10 knots, with a very comfortable motion. CP propellers would have given us a bit of extra speed, but we didn't really need it - 10 knots was trucking nicely.
     
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  15. Annode

    Annode Previous Member

    That was really interesting to read. Thx. 10kts is a respectable speed to drag TWO fixed props through the water at, but F7 gusting F8 is a substantial wind. What is your guestimate of the speed that configuration would manage at say F3 or F5?

    The Lunar is a proof of concept for me. It is beyond my budget, but the design isnt. Getting real world metrics for motor sailers is a lot tougher than I was expecting given the software and expertise out there. Like a hull trying to exceed hull speed, I seem to run into severe resistance at the suggestion of such a project with 2000 cast iron reasons why it cant be done and there is no point discussing the design in any meaningful way. Of course if I throw $10m at the idea, well then things like Lunar happen and surprise surprise, it can be done.

    What point of the wind were you sailing at when doing 12kts?
     
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