Shroud Angle

Discussion in 'Sailboats' started by SuperPiper, Oct 18, 2010.

  1. SuperPiper
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    SuperPiper Men With Little Boats . .

    There is lots of dialogue about the angle of the shrouds with the mast in the vertical plane. After a quick search, I couldn't see that there is a recommended angle back from the mast to the chainplates on a boat that does not have backstays. Is there a rule of thumb for dinghies and small keelboats for locating the chainplates?

    I see that swept back spreaders for larger keelboats are typically in the range of 15-20 degrees aft of the mast. Is it safe to assume that shrouds are typically at that angle too?
     
  2. Doug Lord
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    Doug Lord Flight Ready

    I think 5 degrees is what I found in Skenes-I'll check and correct this later.
    UPDATE: It is 5 degrees but p190 of Larsson & Eliasson(Principles of Yacht Design)-no or short spreaders. Athwartship angle=9 degrees minimum. I've used 5 degrees on my backstay-less rigs.
     
  3. Perm Stress
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    Perm Stress Senior Member

    minmum 9 degrees in front view, min 5 ddegrees in side view (anything less than 5 deg in side view should be considered =0 for mast strength); actual angle to the mast is better at min 12 degrees -it is consistent with mentioned minimums in side and front views; the greater the angle, the less the tension in the shroud allowing it to be smaller in diameter; ... much above 15 degrees can lead to too high compression forces in spreaders, causing unnecessary weight in the rig. However, on multihull or with deck spreaders or with very high modulus carbon conventional spreaders this could be OK.
     
  4. SuperPiper
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    SuperPiper Men With Little Boats . .

    5 degrees and single-digit angles just seem somewhat extreme. 12 - 15 degrees smells a little better.

    What is the significance of spreader lengths? You mentioned "short spreader rigs" and talk about "too high compression forces in spreaders". What is a short spreader rig as compared to a "long" spreader rig? What is considered too high a compression force in the spreaders? Do the shroud angles change for long spreader rigs?

    Hey, I thought this was an easy question . . .
     
  5. Perm Stress
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    Perm Stress Senior Member

    "short spreader" in Larsson and Eliasson mean spreader in almost neutral or in tension by the shrouds; a configuration normally used in dingies, with no lower shrouds at all.

    "short spreader" I mentioned mean spreaders on conventional IOR style masthead or fractional rigs; where spreaders are arranged in transverse plane, progressively longer going down the rig, with chainplates well inboard from the hull side.

    "long spreaders" start to appear when chainplates are at the deck edge, and shroud angle to mast is significantly over 12...15 degrees.
    This has several consequences:
    1 wide chainplates alone make spreaders 50..80% longer as IOR style, automatically requiring much stiffer (and heavier) sections for spreaders, if shroud angle is not changed
    2 but increase in length of spreaders normally do not mean wider spacing of spreaders on the mast => hence increased shroud angle; the devil is that when total angle of shroud on the spreader is ~10 degrees, only ~17% of shroud tension is translated in spreader compression force; while if this angle is ~20 degrees, compression will increase to ~34% of shroud tension.

    This way it is possible to get "too high" compression forces in spreaders. With resultant requirements for cross section and weight.

    Good guidelines on shroud angles are presented in Polski Rejestr Statkov Rules for sailing yachts. I do not have a copy at hand now, some years ago I used it a lot and still remember some key figures (and results of analysis for different configurations). Other classification societies generally give very similar numbers, only calculation procedures, definitions, additional details vary somewhat
    For backstayless rig it is best to have minimum angle between shrouds and mast 7 degrees in side view. When combined with 12 degrees in front view, it will translate in ~13.8 degrees total angle shroud-mast and 30 degrees sweep-back of spreaders.

    When there is a backstay, side-view-angle could be decreased to 5 degrees; when keeping similar front-view-angle, spreader sweep is naturally decreased to 15...20 degrees you mention in #1 post.
    Spreader sweep in this case simply make the mast more stable fore-and-aft, allowing a)much lighter section to be used and b) avoid the need for intermediate stays and checkstays (simpler handling and great benefits under ORC rating).
     
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  6. SuperPiper
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    SuperPiper Men With Little Boats . .

    Thanks for the good information.

    I looked at Larsson & Eliasson. I see the 5 degrees that you mentioned. The shrouds are more or less vertical from the chainplates to the spreaders and then turn to hit the mast at 5 degrees.

    I'll try that. I'm trying to keep the forestay tensioned on a small keelboat.
     
  7. Perm Stress
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    Perm Stress Senior Member

    You welcome

    Do your keelboat have runners and backstay?
    How low is the forestay attachment to the mast?

    With no backstays, or with backstay and low attachment of forestay you might need a 7 degrees angle in side view...
     
  8. SuperPiper
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    SuperPiper Men With Little Boats . .

    Follow my logic and then correct it for me.

    The boat does not have lower shrouds (many dinghies and small keelboats don't have lower shrouds). I'm trying to keep the forestay tight (without runners or backstay). I add a spreader to get a greater angle between the shroud and the mast at the hounds. The wind blows. There is tension in the shrouds. This puts compression on the spreaders. These flex the mast forward like a bow. The shrouds get straighter (= longer) and allow the mast to pivot forward. The forestay goes slack.

    Maybe the spreaders need to be in tension instead of compression and the mast needs some pre-bend?
     
  9. SuperPiper
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    SuperPiper Men With Little Boats . .

    Now that you have educated me, I see that I have a "short spreader" rig and I'm trying to make it behave like a "long spreader" rig. Can that be accomplished WITHOUT adding lower shrouds?

    I'm hoping to test SOMETHING/ANYTHING next Saturday. What do you recommend?
     
  10. Perm Stress
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    Perm Stress Senior Member

    Could you post some photos/drawings?

    It is too risky business to recommend something, based on conversation without any exact info.
     
  11. SuperPiper
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    SuperPiper Men With Little Boats . .

    OK, for the visual learners, here are some photos. These are Sandpiper 565s. If you look very closely at the flotilla picture, you may notice that the short spreaders are rather neutral when unloaded and some are cock-eyed and droopy if not properly taped and wrapped.

    These rigs don't do a good job of keeping their forestays tight. I've been looking to enhance the spreaders to stiffen the mast. Some skippers are convinced that the cabintops are flexing and have installed mast compression posts. I just think the masts are flexing too much.

    I was confused after my initial question because I was thinking in plan view but the answers came in elevation. Let's try again.
     

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  12. Perm Stress
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    Perm Stress Senior Member

    As this is backstayless rig, forestay tension is provided by two principal sources:

    1) tension in shrouds, (almost exclusively windward shroud when sailing); level of influence is adjusted trough the length of spreaders -matter of fine tuning and hands-on experience.
    2) leach tension in mainsail (only the very leech of main should be considered as a source of tension); level of influence is dependent on stiffness of mast as a whole and stiffness of upper mast.

    As for #1, good source of advice could be experienced 420 or 470 dingy racers -these boats have similar rigging in broadly general terms.

    As for #2, please see attached sketch. If you go this way, it would be beneficial to make sweep angle of diagonal spreaders adjustable, so you could tune the stiffness of rig to your requirements and preferences.

    It should be remembered, tough, that jib, principal sail for windward performance, should be in good condition, and very importantly, its LUFF CURVE MATCHED TO THE SAG OF STAY. If stay is soft and sag a lot, luff of sail should be hollowed accordingly. I mean, if your present jib is stretched out, or designed for less forestay sag as could be maintained on your rig, fiddling with shrouds could be not so useful.

    Windward performance has a few more key components (besides properly tuned rig and good quality sails):
    a) stability: more meat on the rail, more windward performance
    b) foils: enough area, proper profiles, clean surfaces

    All this advice do not take in to account any class rules.
     

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  13. catsketcher
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    catsketcher Senior Member

    Listen to Perm Stress

    This dinghy like rig is right up my alley. Hobie 16 cats are the extreme embodiment of the mainsheet gives forestay tension idea. The rigs flop around but pulling the sheet on pulls the forestay tight.

    I would not be surprised if the deck flexes if there is no compression strut. It would be easy to stick a pole under the mast which is just not touching the deck head. Go sailing and see if it touches.

    The easiest way to keep forestay tension is to keep the mainsheet on tight and use traveller to ease main in heavy winds. Dinghies either do this or use vang sheeting which does basically the same thing. DON"T ease the sheet or the jib gets fuller and you power up again.

    I don't get spreader length to make forestays tighter. In fact a rig like this gets a tiny bit shorter if you bend the mast so the forestay will sag more with a bent stick. So you have to get it tight again with sheet tension.

    Here is a tip. Use the mainsheet to trim until you are fully powered up and then pull the sheet on until the leech telltale at the top batten is streaming half the time and flicking to the lee side half the time. Then if the wind comes in more - drop traveler - not sheet

    cheers

    Phil
     
  14. SuperPiper
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    SuperPiper Men With Little Boats . .

    Catsketcher, thanks for the comments. I completely overlooked the impact of sheet and vang tensions.

    These little boats originally came with only a 2-part vang. Some skippers have increased theirs to 6:1 vangs. Maybe we should think like dinghy sailors and consider 16:1 or 32:1. The traveler is on the transom and is only about a half meter long. It is typically equipped with travel stops rather than adjusting tackle.

    On Saturday, I'll pay closer attention to the running rigging and leave the standing rigging alone for now.

    Thanks for your help.
     

  15. Perm Stress
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    Perm Stress Senior Member

    Completely agree with trim advice.

    Please report on your Saturday's findings! :)
     
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