Power Driven 'Freewheeling Prop'

Discussion in 'Boat Design' started by rustybarge, Mar 4, 2015.

  1. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Hi All,

    There have been several discussions on the forum about running a twin engined boat on one engine leaving a freewheeling prop on the stopped engine, in comparison to twin engine mode on low power driving both props.

    The data seems to suggest that there is very little saving on one engine because of the drag from the freewheeling prop.

    So I was thinking.....

    When in single engine mode, why not drive the freewheeling prop with a small v-belt from the running engine's g'box output shaft to cancel out the free wheeling prop drag.

    Slip a v belt pulley onto the output shaft of both g'boxes, connect together with a loose v belt with about 20hp rating, place a spring loaded jockey wheel in the middle which will act as a clutch and connect to the helm position with a boden cable (like the drum drive on a combine harvester.)

    Is this perpetual motion, or would the power feed from the running engine cancel out the drag on the freewheeling prop?
     
  2. philSweet
    Joined: May 2008
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    philSweet Senior Member

    This would work. The power used by the off prop would be extremely efficient at the prop. Freewheeling props still produce huge amounts of drag on typical recreational craft. Even small positive power will produce a huge drop in drag. The props are built for a specific drop-angle and torque. When free wheeling, part will be trying to go faster and part will be trying to go slower. Often, it is better to lock the prop rather than let it free spin. Especially if you can hide the prop behind deadwood.

    There are a whole raft of asymmetrical drive systems that have been used for specific purposes at specific times. It you have a truely bimodal requirement, like a tugboat or a sport fisher, there are proven setups for those jobs. If this is just a case of running an older recreational boat more slowly most of the time, it is a bit harder to make general recommendations. But if you are running the ICW twice a year, it might be worth the effort.

    But I think I could buy a folding or feathering prop for less than the cost to jury rig a decent pto. There are some tranny and shaft wear issues to consider as well.

    What are you trying to accomplish? What is the potential annual savings?

    <edit> okay, you're in Ireland, so not running the ICW.
     
  3. Rurudyne
    Joined: Mar 2014
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    Rurudyne Senior Member

    It seems like it would be tricky to balance power applied vs drag without a controllable pitch or variable pitch prop.

    There are feathering props that that I believe may be less expensive that CPPs or VPPs.

    Depending on your boat or desired performance you might be able to find a pair of surplus sail drives (which don't strike me as normally being all that cheap but at least in theory they may be had used) and gain the steering benefits. Another option along those lines might be a pair of feathering prop with Glen-L's diy "pods" ... again it depends of what sort of performance, power or budget etc you've in mind.
     
  4. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    A few years ago I bought a Honda lawn mower; it was a 8hp self propelled 'push along' type if you understand what I mean! It had four wheels, but only the rear wheels were powered.

    It used the jockey wheel v-belt arrangement, and I was very impressed with the performance. You could hold onto the handle bars and stop the mower but the wheels kept on turning, and even on steep hill it had fantastic traction from a tiny 1/2 thick v-belt/jockey wheel.

    As you've probably guessed I was looking at bigger trawlers with large twin engines which are very reasonably priced, but guzzle fuel even at slow speeds.

    Rather than re-engining the boat at vast expense which can never be reclaimed when you resell, I was thinking of ways to running them on one engine efficiently.

    So....

    This v belt drive would only work at slow displ. Speed of say 7kts, so a fixed gear ratio could work?
    A couple of 8" or 10" inch pulleys might transfer sufficient power.
    If only a small amount of power is needed a very small jockey wheel with light spring tension would tighten they belt enough to create drive.

    This sort of set up would only cost a few £'s.

    If 2gal/hr or 40hp will propel most 12mtr single engined boats at 7 kts, how much power would be needed to drive the freewheeling prop?
     
  5. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    I've heard that variable pitch props cost lots and lots. Is it true that big prop for a big 45' twin 500hp boat would be more than $10k each?

    I'd like to keep standard shaft drive rather than the expensive to maintain outdrives or pods drive.

    I know that the Cummins 6 CTA 450 hp can run at 1000-1200 revs all day long consuming about 2gal/hr to give about 7kts on a 40' single screw boat (approx).

    I was hoping that might translate to a big twin.
     
  6. JSL
    Joined: Nov 2012
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    JSL Senior Member

    If you can't get a feathering prop then lock it, the drag will be reduced significantly. There are shaft brakes available (Kobelt makes them I think).
    Years ago I did some tests on a 40' sailboat with a 16" 2 blade prop. With the prop free wheeling (back driving is a term used by some marine gear people) the speed under sail was 4.1 knots..... when locked (we used gloves and a pipe wrench-not the best) we got 4.4 knots: and less chance of burning out the transmission.
     
  7. Rurudyne
    Joined: Mar 2014
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    Rurudyne Senior Member

    Yep. Serious bucks. But no telling what your budget is like when I posted.

    And you never know how someone might score at an estate sale or some business clearing out the warehouse. My aunt got a nice 20 to 25' greenhouse for less than a thousand. Of course that sorta thing doesn't happen to me ... but it does happen to some folks.
     
  8. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    10% is a useful gain.

    My very rough rule of thumb:
    Up to a 34'/12' beam 6-7 ton single engine boat of S/d design will chug along at 7 kts on 2gals/hr to give 3.5-4 mpg.

    But.......
    Add 2' and twin engines and a 36' 10 tons boat will consume 4 gal/hr for 8kts!

    And when you get up into the 40' class with big DD's god know how much they burn....

    So 10% gain ain't going to make a big twin economical; drastic measures are needed!:D
     
  9. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    There's a 300hp. twin engined island gypsy here in Ireland asking €50k.
    Most people says it's a very hard hull to push through the water, hence the 600 hp To get it up on the plane.

    I'm guessing it will only do 2mpg at 7 kts or less.

    But if it was a single......?
     
  10. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    Don't forget most dual engine set ups will have counter rotating props. At the least, you'd have to cross the belt to reverse the drive.
     
  11. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Good point.
    The jockey wheel won't work on crossed belts.
     
  12. Rurudyne
    Joined: Mar 2014
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    Rurudyne Senior Member

    Maybe it's just me, but for the money they want for those CPPs a machine shop minded individual could just about buy a nice used milling machine, indexing head, small turret lathe, look up some vintage design for same well into public domain, buy some bronze and, well have some fun. Heck, with these 3D printers you can make nice patterns for lost wax casting for aluminum parts, or even more demanding fare with a call on a real foundry.

    Sure, you're still spending a bundle to get one, but the second and a store of replacement parts is the bonus end of it. And if your wife leaves you claiming emotional abandonment you have a new hobby to complement boat building/restoring.

    Even cutting gears isn't rocket science: with my Sherline mill small bevel gears or helical gears are within reach. Don't even need a mill for hobbing a workable worm gear (acme thread rod can be used).
     
  13. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    There's no doubt you could machine a very good variable pitch prop; lots of tech colleges have small CNN milling machines that can be used by amateurs after hours if you joint the local club.

    I my youth when I was young and foolish I had a small light aircraft, and I remember some of the well off guys had VP props on their planes. Quite a few failed the yearly test because of the close tolerances allowed on a/c constant velocity props.

    So maybe you could buy a second hand VP mechanism off a plane at scrap value and adapt it, it's streamlined but might still be a bit bulky.

    Looking at other tried and tested drive systems I notice that the Robinson helicopters use v-belt drive from the engine to the rotor; I know nothing about helio's but I guess they must be hundreds of hp with big torque loads.

    Looking at a worst case scenario we can estimate that a 40' 10 tons twin engined boat is going to burn about 4gals/hr at 7-8kts, so that's about 80hp.

    Distributing that 80hp with props into water from one engine to the other freewheeling prop with some sort of 'jury' rigged drive system should be relatively easy because water will absorb any shock loads nicely.

    If the v-belt/jockey wheel was specked at 80hp I think it would be enough as the power will be equally distributed to both props.

    How many twin boats have counter rotating props?
    Is it a common feature on sub 12meter boats?
     
  14. whitepointer23

    whitepointer23 Previous Member

    Just buy the island gypsy and use it as is. I thought they were semi disp and only good for about 10 knots. Twin diesels cruising along at 7 knots are not going to use a lot of fuel anyway. How much do you think this mod you want to do will cost, you could put that money towards diesel. If you are so worried about buying fuel maybe you need a sail boat. .. and all twin diesels I have seen have been counter rotating. Actually a motor sailor might be perfect for you. A fisher 32 type of boat.
     

  15. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    I asked this question on the trawler forum: which 32/34' single engined trawlers can do 15kts?

    Answer:
    Mainship 34' mk1-3 1980's on 250hp
    Nordic tug 32 on 270hp
    Latest MS 34' on 370hp
    American tug 34' on 370hp

    ...that's it: full stop.

    Either too expensive, or none in Europe.

    ..that's why I was looking at cheap twins: lot's around for small money
     
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