Boom sheet location

Discussion in 'Boat Design' started by Upston, Feb 7, 2022.

  1. Upston
    Joined: May 2009
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    Upston Junior Member

    Hi All
    Just put up a new mast boom on a 50ft cruising sloop. New boom was end sheet but I would like to do a mid location.
    I see that there is more force required at the mid point but what will that do to the boom itself.
    Second should the mid attachment point be spread out in say three points VS one.

    Thanks for any help. Upston
     
  2. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Mid boom sheeting uses less line and sheets in faster.
    But requires more pulling force and increases the stress placed on the boom.
    Spreading the load is advised.
    Also check that the deck is up to the task at the new deck attachment place.


    Edit

    Line and speed references are only valid if pully arrange remains the same.
    Moving the apartment points may necessitate a change in the number of blocks required. That will change line length and speed again.
     
    Last edited: Feb 7, 2022
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  3. bajansailor
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    bajansailor Marine Surveyor

    @Upston can you post a photo or two of the boat and / or the area on the deck or coachroof where you will want the mid boom sheeting block to attach to?

    Is the reason simply to get the sheet out of the cockpit, so that you can have a bimini awning rigged while under sail?
     
  4. Will Gilmore
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    Will Gilmore Senior Member

    Upston,
    tell us about the boat. Often, sailboats with end of boom sheeting have roller furling booms. I discovered the Mariner 19, my boat, has a roller furling boom. A lot of members of the Mariner Class Association switch to mid-boom sheeting without realizing the have a roller furling boom. You can, of course, detach the sheet blocks and still use the roller furling feature, but you will need another way to secure the boom.

    Blueknarr makes some excellent points about the speed of sheeting and reduced line length. Keeping in mind the strength of the bedded foundation at the deck is also excellent advice.

    I am not following this precisely. Yes, there would be more force on the sheet and it's attachments, but I'm not so sure that would be true of the whole boom. Pulling the boom down and towards the center against the sail takes x amount of force. Doing so from the end of the boom means two points of downward force against the upward pull of the foot of the sail, the boltrope and the leech with both points as far apart as possible. Only the stiffness of the boom to resist the upward bending the sail is working towards. Putting the sheet mid-boom, means the downward pull of the sheet is half the distance from the two end points. This would change the dynamics of the forces. x would be transfered from the downhaul or Cunningham, if you had one, and the end of the boom to the center, where the forces on the boom would be partially reversed with a concentration at the sheet. The end of the boom would be pulled up on by the leech, but there is still some of the downward force taken by the downhaul or Cunningham. Moving the sheet a little aft of mid boom should provide the most support, other than a distributed load of separated blocks along the boom. Adding a vang would also relieve some of the load on the sheet.

    Most sailors modify their sheet location for speed of sheeting, such as for racing, or for cockpit convenience. If your rig is made light, such as for racing, it is an imprtant question to ask.

    -Will
     
  5. Blueknarr
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    Blueknarr Senior Member

    End boom sheeting has the forces from the clew and sheet very close together. Moving the sheeting forward places the sheeting as a fulcrum and uses the remaining boom as a lever against the clew's force. It is this increase of clew leverage that dictates the increase of force required by the sheet.

    When close hauled the sheet will overpower the vang. The vang is only effective when the boom is outboard of the sheets traveler.

    No well cut sail should ever load the boom except at the clew attachment. If the sail is fully bound to the boom, it should have a sufficient pocket sewn into it to prevent the boom from distorting its shape. This pocket will also prevent direct upward pull from the sail.

    The gooseneck prevents any transfer of load from Cunningham to boom.
     
    Last edited: Feb 8, 2022
  6. Upston
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    Upston Junior Member

    Thank you all for the time you took to reply.
    Coach roof is very stout so that is not a limiting factor if the deck blocks are properly installed I think. I am using a 2speed winch to sheet the main so no reduction blocks. Spar was from a 60 ft cat , very heavy but no info on the boat itself. Conservative rigging, double spreaders, double fore and back stays giant chain plates.
    I can end sheet the boom but would need to install traveler track on deck which is not popular with the Admiral but will override her if it compromises the boom.

    Wills post is what I was hoping was true

    ," but I'm not so sure that would be true of the whole boom. Pulling the boom down and towards the center against the sail takes x amount of force. Doing so from the end of the boom means two points of downward force against the upward pull of the foot of the sail, the boltrope and the leech with both points as far apart as possible."

    If this means what I think it does the only big downside of a mid sheet would be higher sheet loads ? The main is attached at the foot via slides and when refed is right in line with the mid point sheet location so the only concern is full sail ,sudden heavy gust or inattentive crew resulting in a boom failure.

    If the mid point sheet adds to the boom loading VS the sheet load , how do I quantify the difference between mid and end sheet moment/load/ bend ect ?
    I will try to post pictures Upston
     
  7. gonzo
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    gonzo Senior Member

    The force on the attachment point is proportional to the distance from the gooseneck. Torque = Force x Distance. Since the torque remains the same, as the distance decreases, the force needs to increase.
     
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  8. Blueknarr
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    Blueknarr Senior Member

    Will was correct in that the load from the sail won't change.
    What will change is the leverage that the load has.

    Simple math to find leverage.

    Assuming original end boom sheeting had sufficient mechanical advantage.
    Take the ratio of movement towards the mast.
    Reverse the ratio and that will be the increase in force required to equal the force applied to the boom end.
    So moving the sheeting location to 3/4's of the boom will require 4/3rds of the original force.
    1/2 will be twice as much
    2/3 of boom length gives 3/2
     
  9. Blueknarr
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    Blueknarr Senior Member

    It sounds like the new boom is stouter than the old one. You should be good to go.
     
  10. Upston
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    Upston Junior Member

    Thanks again for the help.
    If I use a webbing strap vs a rigid bail any ideas how that should look ,width ,number of turns ???
     
  11. Blueknarr
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    Blueknarr Senior Member

    Can't be answered without knowing the forces involved.

    Estimate the force you could apply with old sheeting system.
    I typically use 25 lbs as input from upper body. Multiply by the purchase of the sheet system. Ie a six - one purchase at 25lbs is 150 lbs of pull. Using a 24 winch provides 600lbs.

    Multiply by the ratio from moving the attachments.

    Multiply by a safety factor

    Is your original line strong enough?

    Webbing should exceed line strength!
     
  12. Will Gilmore
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    Will Gilmore Senior Member

    Keep in mind, these forces all change at different wind angles. Modern high performance boats spend most of their time with the boom near midship. The sheet angle is nearly straight down against the lifting force on the boom. This force includes the entire driving force on the mast where the sheet and sail can act to hold the mast up even without back stays. No traveler needed, in this case. Just strong anchor points.

    However, on a broad reach or running reach, the boom, on older designs, wings out, and not only is there less downward tension, because of the length of the sheet and the angle, but we generally don't want to keep the boom down and tight, because allowing more belly in the sail gives more driving force than a flat sail with these wind angles. With a mid-boom sheet on a traveler, that angle is reduced and the sail stays flatter or it can wing out too far and luff before the boom raises with much more camber. There is a tendency for less twist with mid-boom sheeting.

    With end of boom sheeting, the sheet angle can keep the boom from swinging too far out while allowing lots of camber and lots of twist, because the boom can lift without winging out farther. In this case, I would suggest a traveler the width of the transom, mostly to keep the sheet out of the helmsman's hair and add a vang to control the height of the boom, and consequently, the twist in the sail.

    One more point. Because the mid-boom sheet has less leverage, the pulley leverage needs to increase, so there really isn't a huge amount of difference in sheet length.
     

  13. Upston
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    Upston Junior Member

    Thanks for this information Will , The Wild Goose is not a performance boat but we need to make the best sale shape for the conditions, this will help,
    Upston

    ha
     
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