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O'Day 19 Boom "down haul"

Dec 7, 2010
2
S2 7.0 meter Bohemia River -Maryland
I'm thinking about putting a new bale on my boom about 18 inches from the mast and a strong pad eye just behind the tabernacle. I have an old boom vang (4:1) that would run from the bale to the pad eye, to keep the boom down when it is run out to one side. Has anyone done this? Is it effective or do you need the boom to rise and spill air when loosening the mainsheet?
 
Nov 9, 2012
2,466
Oday 192 Lake Nockamixon
Boom vangs mostly exist (in this type of boat) to hold the boom down and control main sail twist. When running deep reaches and runs, you do not want the boom to lift. Also, allowing the boom to lift allows the top of the main to twist off too far and create a sideways force (this is the cause of "death rolls" in small boats like the Laser.) In smaller boats with bendy masts, a very powerful vang holds the main flat, and acts like a ram forcing more bend in the mast, which also depowers the main. The most effective angle for the vang is about 45º to the boom, and it should be fixed at the same pivot point as the gooseneck. My vang attaches near the base of the mast to a D bail that is through bolted to the mast. The reason for this is that the downward pull of the vang doesn't change with the angle the boom swings out. A notable exception to this is the Star class sailboat, which has a circular curved track on the deck that the vang attaches to. This provides A LOT of down force on the boom, while allowing that force to be equal through the swing radius of the boom. On my 192 with Dwyer mast, I have a sliding gooseneck that is not fixed position. As such, I have a line attached to the bottom of the gooseneck and then to the same bail as the vang. This downhaul line is similar to a Cunningham, in that it pulls down on the luff, controlling the position of max draft in the main. Pulling on downhaul (or Cunningham) moves the point of max draft forward, which is important in reducing weatherhelm in strong winds.
IMG_1360.jpg


Blue line along mast is downhaul, grey line is vang. (Yellow is spinnaker halyard, red is jib halyard, and green is main halyard. Some have complained about my multicolored approach to line choice. :cool:)

Another thing I do with my vang is to have an easily adjustable topping lift. I can balance the boom between the topping lift and the vang, so called vang sheeting. Because the 192 (and your 19) are end boom sheeting without a traveler, pulling too much mainsheet to pull the boom in for sailing to windward pulls too much leech tension, and pulls all the twist out of the main. In lighter wind, it's important to have the top of the main twist out more than the bottom, because of different wind velocities and apparent wind angle going up from the water. In many cases of light air, the weight of the boom can even pull out the twist, so I use the topping lift to lift the boom and induce twist, and lock that position with the vang. This allows the angle of attack of the boom to be adjusted by the main sheet without affecting twist.
 

Joe

Jun 1, 2004
6,844
Catalina 27 Mission Bay, San Diego
Your header title "boom downhaul" does not describe your intended project, it's called a "boom vang" A boom downhaul's modern counterpart is called the "Cunningham" and is used to control LUFF tension on the mainsail.
I'm thinking about putting a new bale on my boom about 18 inches from the mast and a strong pad eye just behind the tabernacle. I have an old boom vang (4:1) that would run from the bale to the pad eye, to keep the boom down when it is run out to one side. Has anyone done this?
Yes. Many, many boats employ boom vangs (google it)
Is it effective or do you need the boom to rise and spill air when loosening the mainsheet?
Very effective.... the vang takes over the mainsheet's job of controlling mainsail leech tension when the boom is swung out beyond the sheet's range of vertical pull.
 
Last edited:
Sep 29, 2015
53
Oday 222 Lake N ockamixon, pa
Lots of good info here. Twist in the main-sail spills the air out. Yes your boom down-haul is the same as a Cunningham; and it controls the luff tension; that is, if you have a floating goose-neck. The Vang is so important that you should not leave the dock without it. It's a safety brake to prevent the boom from getting caught in the back-stay during an accidental jibe. It happens! Also you shape the main with it. Put 4 telltales along the leech of the main and adjust the Vang until they are all streaming away smoothly. You will need a powerful Vang. An open leech spills air. A closed leech captures air.