Sunday, June 30, 2013

Monitor pendulum lines and amsteel

[Update at end added Sept 27, 2013.]

I'm still working through some project posts that have piled up.


Luckness has a Monitor self steering wind vane, made by Scanmar.  Its awesome.  I used the Monitor to steer all of the time offshore and I really liked how it performed.

I won't go into any detail on how it works, other than to say that the Monitor at the back of the boat is connected to the wheel in order to steer the boat.  The connection is through lines which run beside the cockpit and then to the wheel around a hub.  Its a very simple setup and works well.  Most of the parts are very robust and sturdy - the exception being the lines themselves.

In my experience so far, the pendulum lines chafe continually.  My setup is pretty nice.  There is one set of blocks to turn the lines from the cockpit onto the wheel at a right angle, and one set of fairleads that turn the line around a stanchion.  The line seems to run freely at both places.  When I examine the line passing these points I don't see any problem, everything has a lot of clearance and there is very little friction.  However the nature of this steering is that it undergoes continual small adjustments and frequent larger ones.  The lines are just constantly in motion, moving back and forth around all the blocks and fairleads.  What would not be a chafe issue for normal lines run as halyards, sheets, reef lines or just about anything else simply accumulates and slowly chafes the pendulum line.  Its pretty brutal.

On my trip down to California in 2011, on the last night before I pulled into Drakes Bay one of the fairleads that leads the monitor pendulum line around a stanchion rotated and ended up chafing the line badly.  I started my sleep pattern at night with the winds in the low 30's with everything fine and by morning as the sun rose I noticed the fair lead problem.

Bottom fairlead has rotated, chafing through control line cover
The lines cover had been completely stripped off due to chafe.  When I saw this I thought it might break at any moment.  Since then I've examined that line in more detail - the spectra strands were in pretty good shape, chafe on them was minimal.  It was pretty amazing that the spectra survived, this was a pretty brutal test.

Here is scanmar talking about their line:
Custom Spectra Pendulum RopeThis 1/4" inch rope is custom made for the Monitor and included in the price. It has straight (not woven) SPECTRA on the inside to minimize stretch.
The outside is polyester which will protect the line from wear. You can do a complete circumnavigation if you make sure to periodically change the places that chafe. Just pull out a few inches at the pendulum, make a new knot, cut off the stump and discard it.
All of the strength of the line is in the spectra core.  Scanmar seems to be suggesting that the polyester cover is there as chafe protection.  Polyester is a poor material to be used for chafe protection.

Aside from the problem I had with the fairlead above, the monitor pendulum lines continually chafed on all of my passages.  I tried wrapping the chafe areas with low friction tape and that seemed to help, although on the longer passages the tape started wearing through so I ended up adding more.  It also started being a friction concern as the tape built up and started fraying off.  Not a very elegant solution.  I wanted to find a better solution to this problem - I wanted pendulum lines which were better suited for their application.

At this point I can't help but start talking about spectra, dyneema, UHMWPE and HMPE, it just can't be avoided.

Spectra and Dyneema seem to be essentially the same fiber, although there may be subtle differences.  They belong to a family of fibers called Ultra-high-molecular-weight polyethylene (UHMWPE).  Dyneema is being used in armor, netting, cut resistant gloves, tug tow lines, mooring lines for freighters as well as high tech sailing lines for halyards among many other applications.  The following is from wikipedia:
UHMWPE is odorless, tasteless, and nontoxic.[2] It is highly resistant to corrosive chemicals except oxidizing acids; has extremely low moisture absorption and a very low coefficient of friction; is self-lubricating; and is highly resistant to abrasion, in some forms being 15 times more resistant to abrasion than carbon steel. Its coefficient of friction is significantly lower than that of nylon and acetal, and is comparable to that of polytetrafluoroethylene (PTFE, Teflon), but UHMWPE has better abrasion resistance than PTFE.
Coefficient of friction lower than Teflon? That sounds like pretty good chafe guard material

I was initially thinking of adding dyneema chafe covers to my pendulum lines.  Where in the past I used tape as chafe protection, I could use dyneema as chafe protection instead.  This would be much better.

However, at this point I started considering what I would have.  The core of the line would be spectra or dyneema (essentially the same thing), then a polyester cover, and then a dyneema chafe guard cover in the critical areas.  dyneema-polyester-dyneema.  What exactly was the polyester contributing to this equation?

If you have to handle lines, polyester has a nicer feel than spectra or dyneema itself.  However for this application you really aren't handling these lines.  You set them up before a passage and then basically leave them alone - you trim the Monitor using a different control line and if you want to disengage the steering you do it differently.  My Monitor lines were attached for months and months without really ever being touched.

If you remove the polyester from the dyneema-polyester-dyneema equation you are left simply with dyneema.  I liked the sound of that.

If you simply want a dyneema line, amsteel seems to be a good one to pick.  I've been working with amsteel for a few years now and like it - it splices easily and is generally easy to work with.  The original line was 1/4" and amsteel is made in 1/4" diameters.  A plan was forming...

Close up of monitor line from first photo above and 3/16" amsteel
In the picture above note that the amsteel isn't 1/4", only 3/16".  In 1/4" the old Spectra strands and the strands of the amsteel seem to be the same size - meaning the new line has roughly twice as much of the good UHMWPE (dyneema) as the old.  Also note that the spectra above is the spectra line from the first picture - it spent a night sawing away at two right angles which totally ate through the cover - and yet the Spectra is in remarkable good shape - all six strands survived.  I like this material.

1/4" amsteel has an average strength of 7,400lbs.  That's plenty for this application.  The Monitor just doesn't pull on the lines with that much force.  I would be surprised if the force was more than one or two hundred pounds, usually much less.  I could easily use smaller amsteel line, perhaps 3/16" and I would still feel confident in its strength.  However, since the system was setup for 1/4" line, having the extra material in the new line will simply give added chafe protection.

There is a concern about UV damage, as amsteel has no cover and is exposed to the sun.  Amsteel is being used in lifeline applications which is another application where it would be exposed to the sun continually.  I believe Samson is suggesting a 5 year life in the lifeline application which I suspect is conservative.  For a  Monitor pendulum line, it will probably be eaten by chafe before 5 years is up anyway, I don't think UV will be the limiting factor.

Scanmar constructs their line with the Spectra running straight, its not woven.  They claim to do this in order to minimize stretch.  Amsteel is advertized as having a 0.46% stretch at 10% of load - which would be 740lbs.  The monitor loads are much lighter than 10%, perhaps 2%?  So stretch will be on the order of 0.1%?  With a 17' line, that ends up being 0.2 inches, which is probably less than the slack in the system.  I don't rate this as a concern either.

When I went to buy the line I saw that 1/4" amsteel was $0.64 a foot, while amsteel blue (an even stronger material, with an 8,600lb average strength) was only $.10 more, so that is what I ended up buying - 180' of 1/4" Amsteel blue.  I have some left over, its handy stuff.

I decided to replace all of my monitor line with the new Amsteel, including the line around the hub at the wheel.  For the wheel line, I put a stopper knot in the middle and then a figure eights in each end.  I used figure eights rather than splices as I didn't want a long bury to make the line wider as it wrapped around the hub.  I did a short taper/bury on the line ends to finish off the figure eights though, that should be ok I think...and looks better than leaving the end of the line dangling.

I made the main pendulum lines pretty long and you can see some line wrapped up in the picture above.  As chafe appears I can do what Scanmar recommends and pull some line through at the pendulum to change the areas which chafe.

Here is a picture of the block - the new lines run through nicely.

I'm feeling pretty good about this new setup.  I'll try to remember to post an update in a year or two to report on how its holding up.  If anybody is curious after I start my passage making again on how the line is holding up, send me an email.

If anybody has any experience with using Amsteel as Monitor pendulum line, please let me know how it turned out - either through a comment or email.


I should also mention the store where I bought the line - ignore this if you don't live in Seattle.  There is a store called Seamar (Seattle Marine and Fishing Supply company.)  Google it, its down past Chinooks, south side of the ship canal west of the locks.  They have some amazing bargains.  Fisheries has pretty good prices, but their price for amsteel is $1.37 per foot.  Seamar's price is $0.64.  The place is a bit industrial, its cool.

Seamar is also an awesome place to buy Seine twine.  I've been looking for this for a while, its used in lashing, seizing and is generally useful to have onboard.  When you buy waxed nylon twine to be used for lashing from a sailing store its something like $15 for a few dozen feet - pricey.  Seine twine at Seamar comes in rolls up to 1000' for $10 - the length of the roll depends on the diameter of the twine, and they have a complete selection.  The Seine twine comes in tarred black, white and green, its seine twine heaven.  They also have a great selection of fishing gear, etc.  Its worth a visit.

[Update Sept 27th, 2013.]

After arriving on Coos Bay after my first passage with the new setup, I've made an adjustment.  I thought the knots at the ends of the control line and the hub lines would be sufficient.  I used the 'monitor knot' on the control line, which is a bowline along with a long tail which can be used in the lashing to the hub line.  This knot is nice, as you end up a lot of extra line on the control line which can be used to adjust the chafe points as they arise.  If the line chafe's then pull some through from the stopper knot at the base of the monitor and tie a new monitor knot on the other end.  But the knots slipped in the amsteel and the lashing I had used between the two lines was getting shorter and shorter over the four days I was out (only two of which I sailed.)  The control lines would slacken, I would adjust the lashing to tighten it again which would shorten the distance between the two knots.  And then later the lines would be slack again - I repeated this until the distance between the two knots was about 1 inch after a few days of sailing.  Maybe the situation would have stabilized, I don't know.

So I've now spliced eye's into the ends of the control line and hub line, with brumel locks and short bury's.  To two eyes are separated by 4 or 6 inches.  To join them I spliced eyes into two separate pieces of amsteel, cow hitched them to the control line eyes and then lashed them between the control lines and hub lines  This looks like its going to work well - but I've lost the option of adjusting the control line if it develops chafe points.  I could possibly end-for-end the control line if it develops chafe.  I have well over 100' of amsteel onboard so I can just make new lines as needed.  I'm still curious how much the amsteel will chafe, its not really been tested yet.


  1. You've tempted us with tales of cockpit locker gaskets...a task we're about to tackle. How did you do it? Was it anticipatory, or did you have actual leaks into the lazarettes on passage? The inquiring minds of fellow Crealock owners want to know...
    Good luck on your next sojourn, from John and Mary (sail repair seminar veterans.)

  2. Hi John and Mary - thanks for the reminder, I just wrote that post up. The technique I used is a bit of work, but I think the results will be good. I'll know in a year or two how well it works, but I'm hopeful!