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Old 24-07-2016, 17:52   #1
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Controlling very large flexible spars with multiple lines

On a small sailboat, especially if it has a traveler, a single control line can hold the mast down and in position when running on port or starboard tack.

You have to suspend what you know about sailing aboard my vessel. Much of what I do is reach back to traditional rigging. Everything is very big and loads are enormous.

In my quest to sail at a leasurely pace without high loads, I have abandoned the Bermuda rig. Loads, and equipment costs, are too high.

Currently I have converted to a paraw rig. I have a short solid 35 ft mast with a foresail and crab claw sail behind the mast. James Wharram calls this Tiki.

The crab claw sail behind the mast is supported by two spars. Reefing is accomplished by drawing the two spars together.

See pictures of a small one at Blogbook of the Tiki 21 ยป Performance of a bamboo-polytarp crab claw sail on a Tiki 21 catamaran

The lower spar, which is kind of like a boom angles upward from the base of the mast at about 25 degrees. Behind the pilot house I currently have what I term a boom cripple. This is a 18.75 ft tall support spar 21 ft behind the mast. It is on a pivot.

My intention was to control the lower spar by tilting the boom cripple. I like my boom cripple because it helps prevent the long spars from hitting my radar tower and wind generator. It also helps carry the weight of the sail when furled.

The lower spar is currently 37 ft. Later I intend to install a 65 ft spar for racing.

The main problem is spar flex.

Wharram doesn't have the same problem as I do since his vessels are much smaller and spars far shorter.

I need to have control lines about every 8 ft along the spar.

Experience with a single line, that turned on blocks to and from the deck, taught me this didn't stop flex. Wherever the load was highest the spar was able to draw line from wherever the load was lowest. In theory I could add more lines to areas of high load, however, I run out of deck and do not want to attach to the roof of my pilot house. Lifting the roof of the pilot house off during a squall wouldn't be funny.

Individual control lines going to different sections of the spar needs to be my approach. 37 ft / 8 ft requires 4 control lines. Near the spar I can split each control line into 2 more which allows me to have 8 attachment points along the spar.

Still with me?

With a small rig it is possible to still use a single line and perhaps hold the spar at one or two points. The flex is tolerated and part of the crab claw look.

The problem with my boom cripple idea is how much it has to tilt when running. Running requires it to tilt past 45 degrees. The issue comes when trying to pull the spar back to the center line. Most of the load on the righting line puts the cripple in compression. So when running, pulling in the spar requires far more load than acceptable.

The second problem is how to deal with multiple control lines. As the spar travels across the deck , a line attached to the spar tip requires a lot of line. A line attached close to the mast requires very little line.

Getting a crew coordinated to release and haul in a single line to a foresail is problem enough. Releasing and hauling multiple lines on a spar is not something practical.

In my next post I will discuss my purposed solution. It is a device that controls multiple lines that let out and take in different amounts of line depending on position along the spar.

Has anyone wrestled with this problem before?
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Old 24-07-2016, 18:11   #2
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Re: Controlling very large flexible spars with multiple lines

Note Sir Thomas Lipton's America's Cup vessel must have dealt with some of these problems. The boom of this vessel is along the lengths I am working.

see HTTP://www.google.com.ph/search?q=Th...droid&tbm=isch
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Old 24-07-2016, 18:31   #3
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Re: Controlling very large flexible spars with multiple lines

Since the ratios would vary with the boom angle... its not something you can easily build an automatic system for.

I'd start with some bamboo skewers acting as mast models (appropriate shape piece of cloth glued on for the sail) and dacron thread building models of the rigging. I suspect that with a series of blocks (pulleys for the layman) you could get the result you want.
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Old 24-07-2016, 21:46   #4
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Re: Controlling very large flexible spars with multiple lines

Sets of blocks attached to a "yoke" is what I have in mind. A direct line to the yoke is 1 to 1 and would attach closest to the mast. A 4 to 1 or 5 to 1 ratio of blocks would enable the spar tip to travel 4 or 5 times as far.
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Old 24-07-2016, 22:36   #5
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Re: Controlling very large flexible spars with multiple lines

The problem is the spar you are using isn't stiff enough. You can use a larger section of aluminium, or switch to a carbon spar, or both.

Trying to reinforce the middle of the spar, while possible is going to be pretty complicated. You could add shrouds to the boom, and bically set up diamond stays which will increase the local stiffness of the section, but it adds a lot of compression load which may or may not be an issue depending on the section in question and the loads you need to contain.
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Old 25-07-2016, 15:57   #6
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Re: Controlling very large flexible spars with multiple lines

sew the tarps into a normal m ain shape /fold edge of tarp over a rope glue and stitch to make bolt rope do the same along the foot sew in a clew you now have a loose footed gunter main / to ad reef points you need strong eyes 2nd 3rd clue positions because of the loose foot/or use bamboo for boom with topping lift .
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Old 27-07-2016, 14:18   #7
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Re: Controlling very large flexible spars with multiple lines

Quote:
Originally Posted by knockabout View Post
sew the tarps into a normal m ain shape /fold edge of tarp over a rope glue and stitch to make bolt rope do the same along the foot sew in a clew you now have a loose footed gunter main / to ad reef points you need strong eyes 2nd 3rd clue positions because of the loose foot/or use bamboo for boom with topping lift .
Thank You!!!!

Based on your post I found the sliding Gunter rig.

For a good idea how my final rig will look see:

Starclassics

This solves all my long spar concerns + provides easier reefing.

New problem:

The most difficult part of the rigging is a sliding mast car for the upper gaff pole.

This pole is going to be about 35 feet long so a typical off the shelf whisker pole car will not handle the load.

Since the car only slides 1/2 up my mast I can use a car with wheels that impart the load to a large area of the mast. It will be held normally against the mast in compression. Tension mode will use multiple sliders. All told with attachments this car assembly will be likely about 16 inches long.

Anyone have a picture of something like this?
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