Prop Shaft Systems.

Discussion in 'Inboards' started by Dhutch, Oct 18, 2008.

  1. marshmat
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    marshmat Senior Member

    I came across an interesting setup recently that seems to predate anything you've come up with, Rick....
    A '61 (I think) Pontiac Tempest. Transmission in the back, with a seven-foot spring steel driveshaft bent into a curve. "Ropeshaft", they called it, c/o DeLorean. And, apparently, it worked.
     
  2. Guest625101138

    Guest625101138 Previous Member

    Matt
    I did some googling and found many references to the ropeshaft but no specification. It seems it ran in a tube in the car and I am thinking it might be the same when used in a boat. It is claimed it provided a performance advantage by improving weight distribution.

    Curved shafts have been around for a long time and was one of many ideas suggested to me by one of the many people who I chat to about such things. I do not know if I actually have original thoughts these days.

    Even the unsupported prop was being done by another fellow before I copied it. He was using it on a small electric powered prop on a creek boat. He sent me videos of this thing just bouncing over logs and rocks while pushing the boat forward. That discussion evolved after I did the Murray River race and ran into problems with fixed prop getting damaged from hitting logs as well as getting stuck on sand bars that the paddlers could easily cross.

    The current 1/4" shaft goes through severe vibration at critical speed in air but there is no critical speed in water because the damping is so high. So I expect 99% of all prop shaft vibration that people experience is due to inclination. THere may be a rare example where the unsupported length in the hull is sufficient to have a critical speed.

    As I have said quite a few posts back you do not need very large diameter shafts to handle the torque with good safety factors. I expect most prop shafts are sized on the misconception and calculation for critical speed in air. This has nothing to do with the situation when in water. Sure if you incline the shaft it needs to be mighty large otherwise it will bend due to the high force imbalance.

    I could have run the numbers quicker than producing this post.

    Rick W
     

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  3. LyndonJ
    Joined: May 2008
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    LyndonJ Senior Member

    Rick
    This app of yours is interesting but its not really in the same ballpark. When you put significant power relative to overall drag into spinning the fan its going to create its own flow alignment and angle is not a problem. Theres a lower pressure area ahead and behind the prop and the flow through the disk will be normal. The more power the more aligned.

    If its behind a non planing hull its operating in the hulls mess and the effects of a powerfull prop considerably alters the flow over the aft sections.

    I think your observations are valid for your application but there's no way thin shafting is going to work in a seaway as someone has already said.

    Ducted props are a good solution for any reasons.
     
  4. Guest625101138

    Guest625101138 Previous Member

    As I said earlier I am designing for efficiency. That includes the whole system. I have velocity ratios as low as 1.01. The hull/s is/are long and slender so very little disturbance. My props operate in undisturbed flow.

    The idea has merit for sailing craft and modern low power displacement pleasure craft. I expect the proportion of high power pleasure craft to decline into the future as overall operating efficiency takes priority.

    The original question here was vibration in a sailing boat drive and how to design a shaft to accommodate it. My answer was intended to encourage thinking about the cause of the vibration rather than just beefing up the shaft.

    The more blades you have and the higher the power the less noticeable the problem. I cannot say for sure how the flow will alter as the velocity ratio goes up but you would expect the far field flow will have diminished consequence.

    I have explained what I believe is a practical means of using a thin shaft. It involves a thrust bearing shaft strut that is closely coupled to the prop. There is no way you can leave the shaft unsupported if you want something practical for operation in most boats. You may not have noticed it but in the photo of the catamaran there is a rope looped around the shaft running to the back of the seat. This is a crude tension strut that allowed me to support the shaft when reversing.

    Rick W
     
  5. brian eiland
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    brian eiland Senior Member

    Belt or Chain Drive Prop Shafts

    I've long be a proponent of horizontal prop shafts over inclined ones. My original thoughts back in the 70's was to utilize what in those days were referred to as 'gilmer belts'. Fast forward to modern days and make use of those industrial power belts by Gates, kevlar and now carbon belts. I'd love to be involved with some of this development if there were a client willing to support the work.

    http://www.runningtideyachts.com/power/


    There have been some fits and starts on such projects, but not much carry thru. PYI products developed a chain drive that was quite interesting, but then have stopped production of their two different power units. In recent talks with them I have discovered they are willing to build the larger size unit on a custom basis. Here are a couple of reference postings I've made on this subject:
    http://www.yachtforums.com/forums/7567-post9.html
    http://www.yachtforums.com/forums/15453-post14.html

    ...and a chain-drive duo-prop arrangement...not near as complicated as the current duo-prop drives
    http://www.runningtideyachts.com/dynarig/Tennant_Hull_V_ChainDrive.php
     
  6. Guest625101138

    Guest625101138 Previous Member

    Brian
    The curved shaft is hard to beat.

    When you consider typical prop shafts you see that they are inclined so they generate a vibrating force. The shaft is beefed up to avoid bending under the out of ballance force, also to avoid its critical speed and strong enough to avoid accidental bending - solid and heavy contributing plenty of drag.

    With a curved compliant shaft you can run the prop shaft in line with flow at the prop. This avoids the vibrating force in most circumstances - have to take care when making tight turns. You can use the water damping to avoid any issue with critical speed. You have a compliant shaft that flexes over things to avoids damage when bumped. It can also be easily raised behind the transom so the prop is clear of the water if you want to get the best sailing performance.

    If you are only designing for the torque on a high strength shaft you find it ends up being surprisingly small diameter. It also provides torsional compliance.

    Rick
     
  7. brian eiland
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    brian eiland Senior Member

    Rick,
    But will it work on 300 plus diesel hp with a vessel that is driving out thru a hard chop and slamming quite a bit??

    I was just on a 35 Bertram the other morning, going out into the Atlantic for some tuna fishing. We were driving north into a chop created by an unusual (for this time of year) northerly breeze that was giving us a wave form that was just not well suited for that hull length. Fortunately that was a good solid vessel, but on a more flexible vessel I could just imagine what those shafts were going thru, particularly when those props grabbed a little air. Really makes you think about what forces are involved at both ends of the shaft, both in torque and thrust loading, and in implusive, non-steady state conditions.

    On another vessel I was taking from Fla down to Venezuela. I actually twisted off two 2" shafts just aft the strut due to a slight misalignment of the shaft line. I was lucky the props didn't fly up thru the bottom of the vessel, but rather deep-six.
     
  8. Guest625101138

    Guest625101138 Previous Member

    I am proposing the idea for easily driven hulls that are set up for efficiency. Like auxiliary in a sailing boat. Fewer people are able to afford to run a 300+HP these days.

    That said there is no reason it cannot be scaled. The main reason for poor prop efficiency on boats is draft. The curved shaft allows the prop to be easily elevated to reduce draft and work as surface piercing when in shallow water.

    Once you have a compliant shaft the alignment is less critical. The whole thing is more forgiving.

    The idea was given me by a fellow Vic Garza who made fibreglass shafts for electric propulsion in shallow creeks. He sent me the attached video. It shows how it just climbs over things. Going "with the flow" rather than fighting it so to speak.

    Rick W
     

    Attached Files:

  9. Guest625101138

    Guest625101138 Previous Member

    Another of Vic's videos.

    I go quite a bit faster in the Murray Marathon and my prop is much higher aspect with more fragile blades. I have damaged the prop but it takes lots of hits and mostly bounces out of the way.

    You can design props to be heavy and solid so they destroy most things they hit or you can make them lighter and compliant so they move around things that are not part of their normal function. For example outboards will usually survive a hit with a log but not a high speed inboard.

    Rick W
     

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  10. MikeJohns
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    MikeJohns Senior Member

    Consider that for a vessel (rather than a sheltered waters paddleable boat) the propulsion system is the most important safety device aboard. A thin shaft will whip itself into destruction the first time the prop is fouled under power.

    On an ocean going vessel a prop shaft should be robust enough to cause the clutches in the gearbox to slip or to stall the engine if the prop is jammed. This is the torque that should be applied to the shaft design.

    Corrosion allowance is sensible, thin shafts are going to be very prone to shear failure following a little pit corrosion.

    Note for interest that you should always paint a thin shaft, a highly stressed component will fail relatively quickly from fatigue in immersed situations unless well painted. Ditto for stress-corrosion. The S-N curves are for dry shafts once they are wet fatigue failure is greatly accelerated.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    MikeJohns

    Also the design case of all the strain energy in the shaft suddenly being realised radially..hence straight shaft...does wonders for the whirling calc's too!
     
  12. Guest625101138

    Guest625101138 Previous Member

    I put staying afloat the #1 safety feature followed by staying upright then pointing ability. There have been many circumnavigations without a motor so difficult to see how the prop shaft relates to the most important safety device aboard.

    Like all things new they take development. I certainly did not know that a pushing shaft operates quite happily without being supported until I tried it. From what I have seen very few people realise this. This realisation opens new avenues.

    Setting torque limits are not difficult. With electric drives you just set it as you please. Similar with hydraulics and the pressure relief. A mechanical slip clutch may not be as precise in setting but it can be set to suit. The torsional compliance of a thin shaft gives it time to gently absorb the motor inertia before the limit operates.

    Corrosion resistance requires the right sort of design. I have not been able to get any material better than common spring steel. I paint it for my shafts but they are predominantly used in fresh water. I am considering a fibreglass wrapped shaft to get a more robust protection system. Just a matter of development time and effort.

    Try a big solid shaft in the creek that Vic was negotiating.

    Rick W
     
  13. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Rick

    "...I certainly did not know that a pushing shaft operates quite happily without being supported until I tried it..."

    Your "bent/curved" shaft will have a minimum 2 bearings, on on the prop and one on the transmission side. In this case, the geometric axis of the shaft is not coincident with the axis of rotation. As such centrifugal forces will tend to make the shaft deflect until they are balanced by the restoring force owing to the stiffness of the shaft. Ergo a low stiffness shaft is balanced by a low force, your peddling.

    The speed at which balance is achieved between these 2 forces is called the whirling speed of the shaft. All you have is a low stiffness shaft in a low force environment that is matched to its whirling speed.

    As soon as you increase the loading, the shafts stiffness must increase to balance the load. An increase in stiffness of the shaft renders a "bent/curved" shaft from easy to hard to difficult to impossible as the forces increase.

    Otherwise the out of balance force will create significant inertia loads that are impossible to contain and major damage is the result.
     
  14. Guest625101138

    Guest625101138 Previous Member

    Here are a few more examples of what can be done with curved shafts.

    The first shows a sketch of a craft that Mark Drela built and tested using a 3mm shaft that was supported at both ends but unsupported for over 8ft. This section transmitted the thrust force to the gearbox. These guys were capable of producing about 1kW or more at that stage of their life. Mark got decavitator to over 18kts for 100m so he was certainly fit and strong.

    One photo shows my V11J boat "beached" sitting flat on the ground and the s-curve of the shaft as it curves outward. This is the same shape it takes when I pull it up for inspection for weed.

    The second set of photos show a strutless prop on an 8mm aluminium shaft.
    The video shows this shaft operating at low speed so you can see how the shaft aligns with the flow. Watch carefully for the flash of the prop as the boat is turned. I have used that shaft to push that boat to 9kts, requiring 570 to 600W.

    Most people simply do not realise these things are possible but when you chase efficiency as Mark and I have done you look for novel approaches. You are forced to understand how things actually work rather than take the accepted practice. I design from basic physics as I have a solid understanding of the physics involved rather than relying on rule books or past practice.

    The boat that Greg K used to set the world record used a strutless shaft . It was only supported at the gearbox above the water. It was 1/4" 6ft long.

    Rick W
     

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  15. MikeJohns
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    MikeJohns Senior Member

    I was reading Brians 300HP+ Motor vessel post.

    OK from a pragmatic point of view, presuming a sound and well built hull with strength and stability adequate to the conditions meeting SOLAS requirements

    Robust and reliable propelling machinery is an equally important safety feature on power boats and the next equal on sailing vessels.

    Reliable auxilliary propulsion on sailing vessels has saved more lives and vessels than most people would have considered.

    Vessels founder principally on coasts which those same circumnavigators tend to avoid.

    As Ad Hoc said the stored strain energy is high relative to the shaft size.

    Its a great application for your boats but I'm concerned when you suggest it for a powered offshore vessel.
     
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