rotating mast/fixed boom?

Discussion in 'Hydrodynamics and Aerodynamics' started by fpichel, Jul 19, 2011.

  1. fpichel
    Joined: Jul 2011
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    fpichel New Member

    hello

    I was wondering what the impact on sailing performance or overall functionality would be if the boom was "welded" to the mast. So that it was always at a right angle in relation to the mast and the mast was allowed to rotate freely.

    I have searched this forum a bit and couldn't find the answer to this question. Please excuse my ignorance. Thanks in advanced.

    frank
     
  2. messabout
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    messabout Senior Member

    There are such rigs in use mainly on dinghys. The OK dinghy and the Finn are or were examples I think. The rigs have been developed over many years and they are entirely adequate though not necessarily optimum.

    There are some advantages in causing the mast to rotate to a different angle than the boom with respect to the centerline of the boat or the wind direction.. Many racing style multihull boats do this to good advantage. Some monohull rigs also take advantage of this concept. There are several different types of mast rotation limiters that play into this scheme. The rig may have shrouds and still be rotateable. The windmill dinghy comes to mind.
     
  3. Gary Baigent
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    Gary Baigent Senior Member

    Mast/boom relationship changes depending on wind strength and course sailed, high pointing, full and bye, reaching and offwind - so adjustments have to be made depending on these factors. Also on wing masts the correct setup is to over rotate to get a sweet flow from the leeward side of the wing to the main and that results in the best performance. Instead, as is often the case, if the wing is adjusted to point directly into the apparent wind, then there is a pocket of disturbed air in the step created between mast and sail ... and the boat is losing performance. So a "welded"angle would be a compromised and too crude setup.
     
  4. eyschulman
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    eyschulman Senior Member

    There was a rig knoxing around a few years ago that had both forward and aft fixed booms and rotated. jib on forward boom like on a sprit. Sombody did some research and put it on some medium sized boats maybe some one on this site will know it.
     
  5. PAR
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    PAR Yacht Designer/Builder

    A fixed boom isn't a good idea, but you can arrange a boom to be self vanging and fixed in the lateral plane to the mast (of course the mast has to be able to rotate). I have several designs just like this as do others; Eric Sponberg comes to mind first.
     
  6. Steve W
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    Steve W Senior Member

    eyschulman, the rig you are thinking of is, i think, the Baelstron rig. A company in the UK gave it their own trademarked name, the "Aerorig" and marketed it for some years with some success building some quite large rigs, one circumnavigated antarctica. Forespar was the dealer in the US. As far as efficiency goes the Hirrondelle cat and a Prout model about 38ft were available new with either Aero rig or standard rig, a boat test that was reported in Multihulls magazine claimed the Aero rigged version of the Hirrondelle to be closer winded and faster on all points of sail than the standard rig with less sail area. I believe the mast was a round carbon section.
    Steve.
     
  7. fpichel
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    fpichel New Member

    Thanks everyone for the thoughtful replies, I'll google those boat names. I apologize for my novice exploration/thought experiments.

    My idea for a fixed boom came from looking at rigid wing sails. Despite their adjustable shape, their foot remains parallel to the deck of the boat. What would be the performance disadvantage of having a fixed boom with a fabric sail create the same effect?

    I sail a Hobie Cat 16', obviously not the most advanced boat, but the mast rotates to the angle of the wind or maybe even a little beyond that to the windward side of the boat. I imagine this creates a good lead edge shape of the sail - creating a smooth connection on the leeward side between the mast and sail, and creating a pocket on the windward side. But I believe this would happen anyway if the boom was fixed to the mast, through the flexing of the soft sail. ?

    My hunch is that I could point to wind better with a fixed boom.

    I am also interested in simplifying the rigging or, I guess, understanding the performance gains of having all the adjustability - of a down haul which is related to the boom, a boom vang, a traveler car on the main sheet. For example on a broad reach or running - it seems that the performance is better sheeting in, and having the traveler car further out, than having the traveler centered and the sheet out - raising the clew of the sail and creating more of a belly. Wouldn't a fixed boom create this effect without all the additional rigging?
    When my boat is overpowered on a reach I sheet out with the traveler centered - I believe this makes the sail shape less efficient and slows me down, which is a good option to have. Though perhaps moving the traveler would have the same effect. ?

    Thanks again.
     
  8. PAR
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    PAR Yacht Designer/Builder

    You don't have to employ a conventional boom Fpichel. You can use a sprit boom, which eliminates the need for a vang and this is the arrangement I have used on fixed boom with a rotating mast.

    It sounds like you need some more sailing experience, especially with different rigs and setups, so you can get a feel for the various arrangements.
     
  9. blisspacket
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    blisspacket Junior Member

    The Hunters Child waterballasted boats by Hunter Marine use a fixed boom, a loose foot to the main, and the clew can be tweaked to just so. The boom is fixed with a rod instead of the usual boomvang arrangement.
     
  10. Gary Baigent
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    Gary Baigent Senior Member

    There are some French race boats with the fixed boom attachment on deck, in line with the mast base, so that when the mast is rotated, the sail has enough "elasticity" or actually looseness, at the tack area, to compensate for the swinging movement of the wing mast.
     
  11. Frosty

    Frosty Previous Member

    Nothing new in this, there is a 60 foot cat in the Marina right now with this set up only it has 2 booms --one out the back and one out the front.
     
  12. michael pierzga
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    michael pierzga Senior Member

    Its a well proven concept. Study the "Aerorig"
     

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  13. Earl Boebert
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    Earl Boebert Senior Member

    Google on "model yacht" and "swing rig" and you'll see plenty of them.

    Cheers,

    Earl
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    I assume this applies to mast with an aerofoil cross-section, where the mast ideally rotates slightly more than the boom to create an optimum leading edge to the sail. Is this done manually or automatically using the sail in some way? Has a mechanical linkage been used for the same purpose?
     

  15. Gary Baigent
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    Gary Baigent Senior Member

    It's more than slightly, AK, here is setup, slightly eased sheets in very light airs. You can see the port side rotating double lines cleated loose to create the angle; can be adjusted to flatten or widen the angle between boom and mast.
     

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