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Rigging a preventer

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SausalitoDave View Drop Down
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    Posted: 22 June 2018 at 14:49
I am planning to rig a preventer on my 505. I plan to use 3/16 Dynema as pendants from the aft-end of the boom and retain the with a cleat near the goose neck. I will then use polyester lines for control lines that run from the mast to the bow and then back to the cockpit. My questions are: (1) do I need thimbles at the goose neck end of the Dynema or is a simple eye slice sufficient? (2) should I use a locking metal shackle on the control line or is a soft shackle good?  Any suggestions from more experienced riggers appreciated.  Thanks

Dave
Festina Lente
Hanse 505-17

Edited by SausalitoDave - 22 June 2018 at 21:49
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fulanito View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fulanito Quote  Post ReplyReply Direct Link To This Post Posted: 17 September 2018 at 07:32
Hey Dave,
I have rigged a preventer in my 505 #110. By no means under size your preventer as it will save your boat and life of all on board. The preventer line, and pendant have to be biffed-up. The working loads on it and back-filled sail are well and truly in the order of 1700 to 2000 kg on 35+ knots and this is if attached well aft the boom. If rigged mid boom, as seen in many cases, the load increases exponentially and could be in the magnitude of 4000 kg causing severe damage to the boom and goose-neck. Sailing downwind in 30+ knots with main sail up is common in our south pacific region, and you would not gibe in this conditions, however the preventer is rigged to account those situations where in 30+35+ knots and accidental crash gibe can occur( i.e. wave action, incorrect steering or in our 505 boat a Jefa autopilot failure, or cable failure causing a sadden accidental turn/gibe that will back fill the main, then disaster!
So, 3/16 (5mm) Sk78 no cover is rated 2340 B.L., and w/braided cover SK78 is rated 2120 kg. So you will be very close to break your line with no safety factor (normally WL 1:4 or 1:3 BL)- Polyester line is surely "stretchy" so you need to ensure your safety factor for WL 1:3 Min. 
I have rigged my strop from aft end of boom to a separated Selden eye 511-572 located just aft the rear main sheet eye on the boom (just a bit forward than dodger) with 10 mm SK78 dynema (9800 kg BL) covered with anti - chafe sleeve including the strop eye that attaches to boom end. I use a bungy cord to tension it so it does not flap against the bottm of the boom. 
There is no issue with loading or chafe against the boom end. (see attached PDF). the fore end has a SS316 10 mm tubular thimble to protect the line from a single point loading. The control line has also a tubular 10 mm thimble is attached with 10 mm soft shackle to the strop.  The control lines are double braided dynema SK78 (9000 kg BL ratio 1:4 WL). These run to the forward cleats and passing through 2 low friction rings (WL8900 kg) attached with 10 mm Dynema wrapped around to each cleat. the lines passes through both cleats (or low friction rings) in the fore deck and run all the way to the winch in the opposite side of the the boat via double fairlead fittings. The reason behind the use of two cleats is load sharing and redundancy if  any one cleat fails.
If you are interested in what a failed autopilot and wrong rigged preventer can do, please read the Platino accident report (17 pages) on Maritime NZ website.

Cheers,
GG
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S&J View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote S&J Quote  Post ReplyReply Direct Link To This Post Posted: 17 September 2018 at 11:35
We had a crash gybe at night this spring. My fault but a contributing factor was the autopilot panel on the Zeus/Vulcan MFD. These have the 10 degree adjustment to top and 1 degree adjustment below. The Triton panel has these the opposite way round and at night the labels are difficult to see.
I'd got used to using the software panel on my port Vulcan and inadvertently requested a 40 degree adjustment instead of 4 degrees (to avoid a cardinal mark) when I swapped to the starboard wheel.
No damage done (other than a grumble from the crew) and a lesson learnt.

Edited by S&J - 17 September 2018 at 14:51
H385 #351 Ningaloo UK south coast (2018)
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fulanito View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fulanito Quote  Post ReplyReply Direct Link To This Post Posted: 17 September 2018 at 11:48
S&J

Our boat rules are non-negotiable when sailing downwind, either day or night- Downwing = Preventer On.
We have ours permanently rigged on. The main will not be moved until the Preventer is ON.
Out of interest read the accident investigation report of "Platino" in 2016 from Maritime NZ were two experienced sailors were lost in the South Pacific due to the autopilot failed in the worst moment  with wind 35+ in port quarter turning hard over to starboard causing a crash gibe that ripped off the traveler and became the boom of death on deck.

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Wayne's World View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wayne's World Quote  Post ReplyReply Direct Link To This Post Posted: 17 September 2018 at 12:16
S&J,

You can set the maximum course change limit via the Autopilot- Automatic Steering options - Navigation change limit - ie the max course change of say 20 degrees and more than this limit requires a verification. Might be a good option to set. We have ours set to 20 degrees.  
Wayne W
Cruising, currently in the Mediterranean.
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Scotchmist View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Scotchmist Quote  Post ReplyReply Direct Link To This Post Posted: 17 September 2018 at 13:37
Here's a thought. 

instead of risking a jibe in strong winds, how about taking the mainsail down and just sailing under foresail?. much more comfortable and just as fast!
Andrew

Scotch Mischief Dehler 38 by Hanse
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SausalitoDave View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SausalitoDave Quote  Post ReplyReply Direct Link To This Post Posted: 18 September 2018 at 02:39
My desire for a preventer is for sailing direct down wind wing-on-wing in low to moderate winds.

Clearly, if in heavy winds, a headsail only maybe all one needs.
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