Pulleys for Cable Steering system

Discussion in 'Boat Design' started by Dhutch, Feb 9, 2012.

  1. tunnels

    tunnels Previous Member

    The race boats we made Had the turnbuckles on the quadrent runing diagonally as to get good length to be able to take up more slack , But the soft cordage That was used hade vertually no stretch at all once it was spliced throught the eye and There really is not a lot of movement in a quadrent so groves and flats and the like is just using common sense Grove for wire flat for cordage . :p:p
     
  2. hoytedow
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    hoytedow Carbon Based Life Form

    Don't use nylon rope! It stretches. Use polyester rope if using "soft" cordage.
     
  3. Dhutch
    Joined: Oct 2008
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    Dhutch Junior Member

    Ok. Might be common sense to someone from a boat/rigging background but not so obvious to me.

    What sort of gauge of cord would be right. Would 5-6mm dynema or vactran be large enough? Hard to predict what max loading would be.

    We have a weekend 'workparty' on the boat in fortnight som im hoping to get some bits together to offer up in time for that even if we dont get around to actaully striking an arc and fabbing it all onto the baot that weekend!


    Daniel
     
  4. Dhutch
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    Dhutch Junior Member

    Right! Progress is being made.

    I did some sums based on how much force I think I can get into the rope/cable. Due the geometry of the wheel and chain sprocket if I put my whole weight on the wheel, I can get about 1ton of tension into the chain and hence rope. 6mm Dynemma has a braking strain of about 2.5 ton, so even with allowences for splices/terminations that should be enough, so I have bought a lenght of that. Which fits into the existing quatrent quite well as hoped.

    Based on Stumble's 1/5 ratio 6*5 give a 30mm target pulley/sheave size.

    Looking at sailing equpment most are a bit plasticy a have breaking stain of 250-500kg or 25/50kN which is a bit small for the predicted loads. Or else are really exponentially more expensive. So a bit more looking has lead me in in the direction of some rockclimbing pulleys as there appear to a fairly good range of pulleys/blocks in aluminum for a resonable price, like about £10 GBP. most are deisgned for 13-15mm rope, some of which have smaller diameter pulleys, but im not exactly sure how much this matters?



    Daniel
     
  5. Stumble
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    Stumble Senior Member

    Dhutch,

    The 1:5 is for a specific type of dyneema called Dynex dux, which is pretty stiff stuff. If using any other dyneema line pretty much any pulley is fine.

    In this application I would probably use amsteel blue, which doesn't need the larger turning radios. And might seriously look at Collegio marines Static lead blocks http://www.colligomarine.com/Purchase-Systems.htm that use the slipperiness of the line to best effect, and remove bearings, and moving bits in the blocks themselves. But note that this will only work with uncovered amsteel, if it has a sheath on it it won't work.
     
  6. Dhutch
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    Dhutch Junior Member

    Interesting idea. I have seen these in the past on boats, it sort of feels all wrong, but obviously works with the right materials and loads.

    In the end we have gone with 6mm polyester sheathed dyneema line and size3 Barton marine pulleys, which is the thickest line that would fit the quadrent without modification and a cost effective pulley that is avilable with a range of mounting options (we used a mix of upright, cheek, and conventional mounting) all be it slightly lower spec than I was hoping for.

    So far its appears to be working well, after the first few hours use there is a bit of squeek from the pulley which im going to try some light silicon oil on to quieten them. But the only other issue appear to be the line going slack over time, I shall retighten again as currently im still mainly putting it down to everything bedding in, mainly the securing of the ends, but if turns out to be creep there might be an eliment of 'back to the drawing board' required.

    As a possable substitute, does kevlar exhibit more or less creep than dyneema?

    Either that or we shall put up with the slightly more play that that slacker line gives and see how that does.


    Daniel
     
  7. SamSam
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    SamSam Senior Member

    This may sound a bit critical, but I do realize it's out of your normal sphere of expertise. I think you got mislead from the very beginning. I think you would have been fine to stick with metal cables and regular industrial pulleys. None of the system is exposed to the weather, the 2-3 times expense and questionable quality of "marine" labeled equipment was not needed. Industrial/commercial conditions can be way worse than the alleged horrific conditions of a marine environment. The only problem I remember you having was in a "hard lock" turn to port, the starboard cable would fall off a pulley. I can't see a reason to go to "hard lock" as the rudder isn't going to turn any more, all you do is compress the spring and create slackness. If you have to put your whole weight on the wheel and exert 1 ton of pressure on the system to turn the rudder, I would look into balancing the rudder a little. Or get a bigger wheel.

    Re-reading the thread, in post #1 you say you have 5mm cable that won't fit, but now you say you are use 6mm rope...? The system you had lasted 15 years, and now you have one you contemplate replacing every 5 years?

    Oh well, it's not that big of an issue, but it does point out thinking within the box (marine) is hard to get away from.

    Anyway, I'd like to see pictures of the boat (and maybe the rudder). I love those canal boats. What are the logistics to owning one? Is it costly? Permits and licenses needed? Do you have to own your own slip or dock? Will they let you live on it permanently? Could you run it over to Holland?
     
  8. Dhutch
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    Dhutch Junior Member

    I hear what you are saying. One of the main restrictions was the diameter of pulley that could be fitted into the space avilable without considerable re-work, the 5mm wire was ok on diameter but the bend radus was much too large. More flexable steel cable might be the answer, but at the moment the system now works.

    The barton pulleys arnt the best by any means, but there no worse quality than what was there, and they are all mounted in blocks of various type rather than half of them just being plain sheaves in the end of a post as before. We also weld in all the new brackery 10mm low and mounted the block on shims, so if in future we want or need to change to a diffrent block we have that flexabilty. Otherwise, at £8 a pop if they fail after 5 years it a simple mornings with a spanner work to swap them out like for like.

    Its all far more complex than simply having a 'swan neck' and tiller off the top of the rudder post (which we have one of for backup) but the whole boat is a bit like that, including have a steam engine rather than deisal. More of a narrowboat shaped folly than a conventional narrowboat but we're suckers for punshment and love it.
    Standard swan-neck arangement on a new shell: Linky

    ##

    The are some photos on our slightly basic/outdated webpage at www.emilyanne.co.uk
    Like ours most are privately owned, although there are plenty to hire, and share boats. the cost of a new or secondhand boat varies wildly, most waterways are then covered by a single license simular to a car which givens you access to most of the canal network and permission to moor on the side for upto a fortnight in most locations bar city centers and the like with additional fees for certain areas. Most will have a permanant base all year, but we leave in spring and stay out moving every other weekend thoughout summer. There are quite a few drydocks and boatyards you can hire to paint or let you paint the underside, every 3-4years typically, ditto wet 'paintdocks' for topsides. Some do live on permantly, and its not uncommon for people to spend a year or two once retaired, but most just use the boat when they want/can at they arent huge inside.

    Narrowboats (7ft beam, 60-70ft lenght) can make it over the channel one a flat day, but there not really intended to be seaworthy and its very raire and holand would be out of reach realiscally. There are some very small barges that come over from holland to live in the uk on our larger canal/river and there is a bit of a fashion at the moment for 'dutch barge replicas' which though typically still fairly box-like under the water have a little more shape freeboard which are more suited to european canals and would allow coast hopping for the brave, although again, typcially not, or else they go over in the hold of something large!


    Daniel
     
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  9. SamSam
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    SamSam Senior Member

    Thank you for the photos! Very enjoyable! If I was to drive one, I imagine there would be a lot of banging around and scraping through tight spots. I suppose there are different combinations of canal widths and depths that limit where you can go, within one country and from country to country.
     
  10. Frosty

    Frosty Previous Member

    All these problems, tut tut tut whats wrong with a hydraulic job. Yes ive come in a bit late and apologies if its been mentioned but if it has it needs mentioning again.

    1 pump 1 cylinder --some pipe.
     
  11. Stumble
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    Stumble Senior Member

    Dhutch,

    Sorry I didn't get back to you sooner.

    The stretch you are getting is not creep, but what is called constructional stretch. It dyneema needs to be loaded up to around 10% of MBL before going into service to get all the fibers and splices parallel, then it will stop. Depending on length of the piece it should come out to about 2" or so (.75 from each splice, plus a fudge factor). Just keep tightening up what you have and it will stop shortly.

    Creep is the permanent elongation from high loads applied over time. Typically in standing rigging, dyneema is designed to keep the static load (permanent pretension) at less than 10% MBL. this will result in creep of .01" per year on a 50' long stay. But it takes both high loads and long duration to be a factor. In a steering cable it is unlikely you could apply enough force long enough to ever see creep.
     
  12. Dhutch
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    Dhutch Junior Member

    No worries, yes, there often only 6-8inchs clearance on the bridges and locks at the waterline, sometimes no more to the cabin, occasionally even less.

    Canal sizes vary from canal to canal here as they where all build by different companies at different times but very crudely the northern canals are 14x62ft the midland canals are 7x72 and some of the later 14x72, the latter typically used by 'motor and butty' pairs.

    As said earlier in the thread, simply because the feel and lack of feedback wasnt what we where after having used a number of different hydraulic systems on other boats.

    Daniel
     
  13. Dhutch
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    Dhutch Junior Member

    That the sort of thing I wanted to hear. I understand there will be some 'construction stretch' (or bedding-in as I called it) and what creep is, but wasnt sure which I was getting after the first tightening went slack again after a week or so. I will continue to re-tighten and see where we get to.

    Currently I have pulled up about 3inchs from the system which contains six joins (to each for the wheel, turnbuckle and quadrent) and total length of I think nine metres.

    Daniel


    Daniel
     
  14. Stumble
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    Stumble Senior Member

    Hmm, you might have some loose splices in there, and not pretensioning can cause some problems. But it's all pretty easy to sort out.

    Once the splices lock in you shouldn't have any significant stretch again. Personally I just tie the line to my truck, and pull against a tree to set everything. If you run out of travel on the tensioning system, just strip a splice out, and redo it a few inches further down. You don't have to redo the taper, just bury the extra, if anything is will just make the splice better.
     

  15. Frosty

    Frosty Previous Member

    Lack of feed back on a canal barge? At 4 knots is this important.

    I would suggest reliability for very confined manouvering and navigating low bridges rather than rudder feed back.

    If you hit some one because of a home made cable steering you may get more feed back that you would like.
     
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