Author Topic: Tensioning system  (Read 4257 times)

Rambot

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Tensioning system
« on: November 23, 2009, 08:26:57 AM »
Hello everyone,

Firstly, let me apologise in advance for any mistakes or foolish statements: this is my first post to this forum.

Secondly, while I am looking for something specific, if anyone has a better suggestion in general, it is welcomed.

I have been trying to tension the steering line in a motor boat. I have no specific knowledge in the subject, but since I feel I have a working knowledge of knots & the rope I have installed is replacing a length of electrical wire fitted for the task (which had snapped), I figured I was as well placed as anyone to do so.

The system consists of an outboard motor fitted with a pair of eyes to which are connected a pair of springs intended to tension the lines. The loop on each spring is fitted with a rope thimble (I think that's what they're called). To each side of the motor, there is a pulley wheel which roots each arm of the line forward to the cockpit. There are holes placed in various fitting for the lines to pass forward to a second pair of pulleys which direct the line to a drum, connected to the steering wheel, which the line is looped around.

The obvious solution was to loosely tie the rope to each spring, then tension it using a sheepshank, but the range of motion is only fractionally less than any free length of the rope: even the most compact sheepshank would either hit a pulley, bulkhead or the steering drum. Similarly, a taut line hitch would hit the pulley next the outboard motor. As such my approach has been as follows.

I've tied an adjustable grip hitch at one end. While not strictly necessary, I figured I may as well make use of the rope thimble, and this is the only knot I could think of which both fits snuggle up against the thimble and doesn't take up too much space, as would just about any loop knot (the adjustability isn't used, the knot is pushed right up). Before having looked up alternatives, I had used a munter hitch to allow tension to ratcheted, via many combinations of strategically placed blake's hitches, bars in the steering wheel and cleated off ropes: at best this was awkward & time consuming , at worst ineffective. I had considered loosely tying off the line with a "double sided hitch" (eg. clove), before tying a blake's hitch on the standing end with the working end, but again, this would take up quite a bit of space, limiting the steering range. I think that I've settled on using a garda hitch; it will require an extra carabiner, but at least it won't require any other messing around to tighten: just some well placed grunt. However, to maintain the tension, I figure I would somehow need to lock off the knot. I've tried a mule knot, but since this is intended to lock off a belay, it tends to need some slip to tighten it.

Is there a knot which can be tied around a taut standing end, but snug up against an 'obstruction' (in this case the carabiner)?

DerekSmith

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Re: Tensioning system
« Reply #1 on: November 23, 2009, 01:20:25 PM »
Hi Rambot, and welcome.

It strikes me that you have already done it.

Simply take the sliding grip hitch at the thimble and slide it up the line to put the spring under tension.  Alternatively, pull the thimble to put the spring under tension and to generate some slack in the line, then take this up with the sliding grip hitch.

Job Jobbed.

Derek

roo

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Re: Tensioning system
« Reply #2 on: November 23, 2009, 04:21:13 PM »
I have been trying to tension the steering line in a motor boat. I have no specific knowledge in the subject, but...

This doesn't seem to be the place to reinvent the wheel.  Why not investigate how the system is intented to be assembled?
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Dan_Lehman

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Re: Tensioning system
« Reply #3 on: November 23, 2009, 05:55:21 PM »
I should think that you could tie to the drum first and then tie
to the side-of-motor spring eyes with a Round Turn & 2 Half-hitches,
adequately overtightening them a little in anticipation of some
retreat in tension as the knot settles.  Beyond this, getting more
"clever" (one hopes not "by half"), one could incorporate two
more (short) lines which would be tied to the eyes and then they
--being adequately short-- would tie to your control lines with
the adjustable friction hitches (after adjusting, you would tie off
their tails just for neatness).

Realize that your rope will likely have noticeably more "give"/stretch
than that "length of electrical wire fitted for the task" (and was this
original material or another's replacement solution?).  You can
minimize such elasticity with pre-stretched polyester of HMPE or
aramid cordage.  Then, again, the springs themselves must give
some amount of "stretch" occasionally.


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squarerigger

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Re: Tensioning system
« Reply #4 on: November 24, 2009, 04:16:53 AM »
Hi Rambot,

A question first and then an assumption, based on my knowledge to date of steering systems:

Are the pulleys you mention thin in comparison to their diameter and do they have a V-shaped groove?  It seems likely that the wire you removed was the best solution, being relatively stretch-free, and that it should have space somewhere along its length to insert a turnbuckle - that's my assumption.  By wrapping the wire around the wheel spindle first and then leading each end away under some light tension, you should then be able to attach one end with a thimbled splice (or perhaps with a Molly Hogan splice),and then attach the other end to a tensioner such as a turnbuckle.  You are chasing the moon trying to do this with rope, I think, because those systems were seldom if ever intended to be using stretchy rope and they were invented before the era of 0.5% stretch lines.  In addition, the pulleys were rarely large enough for the right size of line.  Another thought - do you have room to insert a kind of bowstring tensioner to give a sideways pull on a length of line between two of those pulleys by inserting a floating pulley or perhaps to tension two lines at once by inserting a spring attached to two pulleys, one each end, which pulleys then do the bowstringing through the spring?

SR

capt larry

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Re: Tensioning system
« Reply #5 on: November 24, 2009, 04:33:52 PM »
Rambot,

I agree with SR.  Don't ever remember seeing one of these systems done with "electrical wire" much less rope.  You should be looking for some type of "air craft" wire, preferably stainless steel and plan on fastening them with mechanical devices - eg. nicro crimps.  Stretch is a problem here.  Dan suggests hmpe rope but that is very expensive and my guess is more expensive than suitable wire.  You should check marine supply sources and should find the wire and parts necessary.

You might find useful advice at this site: http://www.boatersed.com/forum/default.asp

CL

Rambot

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Re: Tensioning system
« Reply #6 on: November 25, 2009, 10:52:14 AM »
Hi everyone,

Very impressed & grateful for all your replies, esp. in such a short space of time.

Derek: that would be the simplest solution; but (I probably didn't make it clear in my original post) but this would foul up the nearest pulley. With full lock, the springs are almost drawn into the pulley, so there isn't any space for any 'dead line.'

Roo/Larry: what I have done thus far is basically a 'scrapheap challenge' esque rummage around the boat house for suitable materials to do this with, settling on a spare throw line. I think I had previously noticed & puzzled at what appeared to be electrical wire in the boat, but figured at the time (when I was less experienced) that the installer knew what he/she was doing, but once it had snapped (and the strands been retied with a granny knot to get back into the landing stages!) showing itself to be plan copper wire, I realised that anything would be better (copper is far from stretch free). But basically, I doubt that there will be much funding available to get anything else in - the wire must originally have been improvised, it has been there for the last 5 years at least and I'm yet to hear anything from the committee on the subject. But I'll take a look at the link.

Dan: as I've mentioned, I seriously doubt that the electrical wire was the original material. I suspect that the springs are there to maintain tension in the event that the line slips/stretches. However, there is very little space to complete the round turn within the loops in the springs, and between the friction that this would generate and the give in the springs when I'm tightening, I can't develop enough tension in the line. Your suggestion with the shorter lines is basically what I've already suggested, only with he working end of the main line providing the 'short length.' As I've said, I'm using a throw line for this, so I can't really cut off the end(s). While this would work, it would inevitably eat into the steering range. That's the problem here; the slightly unusual circumstances mean that many of the traditional approaches are either ruled out or less than optimal. As for the material, funds permitting, elasticity isn't too much of a problem - this would just add to the elasticity in the springs, making the steering a bit sloppy, but at least stable. The problem in this case appears to be that the rope stretches permanently (probably the fibres compacting over time). However, with the right tensioning system, this should be rendered mute.

Square rigger: I had to feel around for the front pulleys; I recall that they seemed to have semi-circular groves of approximately 5mm diameter; I'm not so sure about the rear ones, but they are definitely different. Either way, the wire definitely wasn't better: it was straight forward multi-strand copper wire, which is very ductile. The bow string tensioner may work, but again, cost is an issue and the extra spring would add more compliance to the system which isn't ideal. A turn buckle would work, except the free runs of the line are far to short compared to the range of motion.

Basically, at least for the time being, I am limited to solutions which use this rope and tension it purely from the end(s), without interfering with with any other parts of the line.

DerekSmith

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Re: Tensioning system
« Reply #7 on: November 25, 2009, 12:57:23 PM »
Hi Rambot,

Your system reminds me of a small river boat my father had for 'pootling' and fishing (basically any excuse for taking a pie and a pint for a lazy afternoon on the river).

The boat was powered by an outboard clamped to a steering frame with four controls - a throttle link, a forward / reverse link, and two steering lines from the wheel in the cabin.

The steering lines were waxed sash cord - a hempen core with a cotton braided waxed covering.  It had virtually no stretch, but then the springs which fixed it to the steering frame were not that strong and 'gave' long before the strength of the cord was called into duty.  In this arrangement, the cord was fixed to the thimbles at the springs by simple whipping with cobblers thread.  I have a faint recollection that dad tapered the core in the end before whipping it in place against the line going back to the cabin.  The cords were fed around brass pulleys and through small guard tubes back to the cabin and to the wheel.

The wheel was quite small ca 1ft diameter and behind it was a small wooden 'drum'.  The two cords were tensioned against the springs and then wound counter to one another on the drum and the ends simply pinned in place with sash tacks into the wooden drum.

Derek