Aftmast rigs???

Discussion in 'Sailboats' started by jdardozzi, May 28, 2002.

  1. brian eiland
    Joined: Jun 2002
    Posts: 4,955
    Likes: 181, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    No 2 idea for Phil

    Go back to my posting #479, on this subject thread and follow those links.
    http://www.boatdesign.net/forums/sailboats/aftmast-rigs-623-32.html#post488692

    This vessel Catbird Suite makes for a very interesting study that should fall right in line with your desires.
    1) very lt-wind performace,
    2) 'heeling' sail.
    3) all furling rig

    Read some of these discussions, and including those of the owner/developer of this rig.
    http://www.boatdesign.net/forums/boat-design/wishbone-sailing-rig-1999-12.html#post485560

    ...and keep in mind it would't have to be an A-frame mast.

    http://thecoastalpassage.com/xit.html
     
  2. brian eiland
    Joined: Jun 2002
    Posts: 4,955
    Likes: 181, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Attached Files:

  3. pbmaise
    Joined: Jul 2010
    Posts: 115
    Likes: 5, Points: 18, Legacy Rep: 80
    Location: Cebu the Philippines

    pbmaise Senior Member

    Rethinking the forestay, halyard and backstay

    Thanks for all the suggestions coming in. I am excited about getting something going again, and sure everything I think up has been thought up other before.

    This week I spent a bit of time getting the boat ready to move (new LED running lights are way bright!) and rethinking this business about the halyard.

    Yesterday I was working out compression owing to the halyard, forestay, and backstay, when I realized something I'm sure others have thought about before.

    Getting that halyard with all that tension on it back down to ground level and around a winch really effects overall mast compression. As I posted before I've already took the step of making sure the halyard came down to a block on the deck and then turned to a deck mounted winch. This eliminated the upward compression on the first design. However, it didn't eliminate the downward compression.

    That is when calculating compression for these three lines that I realized that I can eliminate one of the completely.

    If I begin the halyard from a deck winch over to a turn block mounted on the rear backstay chainplate, it can then go to the mast head and serve three duties. The halyard line is the backstay, goes up and over the mast head, and then connects to the bolt rope in the sail. That bolt rope in the sail can be pulled with a 2 to 1 purchase by adding two blocks at the mast head.

    The net result I am calculating is that for 1000 of tension on the mast, the halyard after passing through the 2 to 1 purchase comes out with 500 lbs of tension that now runs behind the mast and down to the backstay chain plate.

    Total compression on the mast is almost 1/3 the typical arrangement of separate forestay, backstay, and halyard pulled from a winch mounted on the mast.

    Sounds good so far. Issue number one is how to configure the mast head with two blocks and one rope that passes over the top of the mast.

    Harken makes a high load snatch block that would handle the loads.
    http://www.shopsoundboatworks.com/ha12thisnbl.html?cmp=pricegrabber&kw=ha12thisnbl

    Anyone here of an arrangement like this?
     
  4. brian eiland
    Joined: Jun 2002
    Posts: 4,955
    Likes: 181, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Ketch Rigged Vessels,...Deketchification of America

    I just happened across this great new word, 'Deketchification' :cool:
    What a great word. :D

    And of course what drew me to investigate was my long term interest in ketch rigged sailing vessels.

    [​IMG]
     
  5. Guillermo
    Joined: Mar 2005
    Posts: 3,644
    Likes: 185, Points: 63, Legacy Rep: 2247
    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    Totally agree with Eric! :)
    Ketchs are a very convenient rig for both shorthanded globetrotters & coastalpotters.

    I love the word 'Deketchification' too! :D
     
  6. sharpii2
    Joined: May 2004
    Posts: 1,954
    Likes: 137, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Aft Mast Cutter

    Here's an idea I've been playing with.

    What happens if you take a cutter and move the mast wa-a-a-y back.

    In the attached drawing, I've moved it about as far aft as I think is possible, without aft facing spreaders.

    I've further simplified the rig by moving the headboard of the staysail (which acts as the mainsail) all the way to the top of the mast.

    Though this is not entirely aerodynamically kosher, it does get rid of another set of spreaders and those nasty running back stays.

    To somewhat compensate for this aerodynamic sin, I've moved the tack, of the main (staysail), aft to halfway between the mast and the forestay.

    Arguably, the top 25% of each staysail is going to interfere with that of the other, producing little actual drive. But that accounts for only about 1/16 of the area of each sail. A worthy sacrifice, in the interest of simplicity and durability. For a cruising boat, I think this is a good trade.

    In blowing conditions, the jib and mizzen are struck, leaving the the main (staysail) to drive the boat. This is also a good arrangement for anchoring.

    In survival conditions. The main is struck and the mizzen and a very small jibe are raised. The small jib acts primarily to balance the the far aft mizzen. This relieves quite a bit of mast compression and provides, I hope, adequate drive to stay off the lee rocks.

    In order for this to work, the ribbon mizzen must be able to provide at least decent drive.

    Running it up a track behind an oval or elliptical section mast will not do.

    The mizzen, drawn, only attaches to the mast with one or two hoops, the top of which can go no higher than about one foot below the spreaders. Besides the hoop(s), the main runs up a jack line, behind the mast, so less load is put on its halyard to keep the luff reasonably straight.

    The sag of the jack line allows the luff of the mizzen to get closer to the lee side of the mast, which, hopefully, restores at least most of its drive.

    The mast is round in section for this reason.

    Probably the top 1/3rd of the mizzen will provide no drive at all, but that accounts for about 11% of its area. What it doesn't provide in drive, it may well make up for in mitigating at least some of the drag of the top 25% of the mast

    The mizzen boom has a cable, bar, or chain for its vang, so its vang is not readily adjustable. It may have a turn buckle. I feel a vang of some kind is absolutely essential, for a Bermuda sail that is this tall and narrow.

    For sailing down wind, the mizzen would be struck, and the boat would be sailed in a series of down wind tacks.

    The Horizontal Center of Area (HCA) of the two jibs will be far enough forward (I hope) to prevent the boat from jibing, and to put less work on the auto pilot or vane.

    For safety reasons, there would be two back stays, each with sufficient strength to hold the rig up. They would be set maybe a foot apart, with just one tensioned to the tune of the rig.

    The vang would be strong enough to break the boom, before failing itself.

    Most likely, none of these three sails will have any reef points at all. There would be no roller furling either. Down hauls would be used for the two jibs, so they can be struck from the safety of the cockpit.

    This rig, with its perceived virtues and inefficiencies, should be quite instructive on how good racing rigs and good cruising ones may be quite different from one another.

    NOTE:

    The Boom length is actually 6.0 ft, not the 4.0 ft indicated on the drawing.
     

    Attached Files:

    Last edited: May 1, 2013
  7. upchurchmr
    Joined: Feb 2011
    Posts: 3,054
    Likes: 150, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Sharpii2,

    I think you have recreated the entire premise of this thread.

    I think I saw this on a Prout Catamaran in the early 70's. I might still have the advertising copy.
     
  8. T0x1c
    Joined: May 2013
    Posts: 78
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Luxembourg

    T0x1c Junior Member

    Powerhk40

    Replace the boom with a wishbone, and this is the rig of the HK40.
     

    Attached Files:

    Last edited: May 3, 2013
  9. sharpii2
    Joined: May 2004
    Posts: 1,954
    Likes: 137, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    I look at your attachment and see very poor purchase for the back stays.

    Has this boat actually been built?

    My guess is the mast compression will be quite massive and the fore stays might sag a bit. Even so, when you're not fighting for that last few degrees of pointing ability, it may not matter all that much.
     
  10. T0x1c
    Joined: May 2013
    Posts: 78
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Luxembourg

    T0x1c Junior Member

    Yes, the boat is sailing, see www.powerandsailing dot com/boats-catamarans/catamaran-power-sailing-HK40.html

    This is a motorsailor with high structures, I don't think they mind about pointing ability. The mizen sail seems to be there just for stability.
     
  11. brian eiland
    Joined: Jun 2002
    Posts: 4,955
    Likes: 181, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Actually there are 2, maybe 3 of them now. If you look back thru this thread you will find pics for the first 2. You will also find a few comments from the builder.

    I'll post a few more pics when I get a chance.

    I had some reservations about the actual staying arrangement, and the lack of a upper jumper stay as I had configured the design originally. I voiced those concerns to the builder, but neither he nor I did that final configuration.

    But to my knowledge neither of the first 2 vessels have experienced any problems with the rig.
     
  12. brian eiland
    Joined: Jun 2002
    Posts: 4,955
    Likes: 181, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

  13. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    Is it possible to use a lever arrangement on the masthead so that it kind of looks like a T. The backstay would join to the longer part of the lever and the shorter end of the lever would join to the forestay. That would increase forestay tension for the same backstay tension plus give the ability to run squaretop sails? The T top would have to pivot, which would allow the leaver effect to not bend the mast tip. I guess this arrangement would have all sorts of other negative ramifications?

    I think Doug done something similar, but mainly so he could use squaretop sails.

    [​IMG]
     
  14. sharpii2
    Joined: May 2004
    Posts: 1,954
    Likes: 137, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Entirely.

    Most, if not all mast head rigs, have what is called a 'mast crane'. It is set up pretty much the way you describe, except in much smaller scale. The purpose is to cause a bit of fore and aft mast bend, so the mast acts a little more like a spring than a compression post.

    I once drew an exaggerated one, because I wanted outside halyards and I wanted two back ups. I nick named this thing 'the roller skate'. It would have had four four inch sheaves.

    Yes. You can do it, but at a price.

    The price is this:

    A compression post needs far less material than a beam under the same load, made out of the same material.

    There are, of course, ways around this.

    You could make a tubular structure that resembles a bike frame, so all the loads on its members are either tension or compression.

    You could make it out of a material that has much greater strength per weight.

    You could put up with more fore stay sag and hope the better aerodynamics of the square top jib make up for it.

    Or you could do a combination of any of the three.

    One thing for sure. It's going to cost more, and need trickier engineering, especially if the size involved is greater than a dinghy.
     

  15. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    I propose to use the mast crane as a lever so it will not bend the mast. The idea is to reduce forestay sag by using leverage! This will be a complexity of engineering but nothing more complicated than calculating other rigging stresses.

    My perceived pros which may not be correct are:

    The more the wind blows the tighter the forestay gets!
    Smaller mast section can be used since huge backstay loads will be reduced to get a tight forestay. (saves money, windage and weight of mast)
    Shorter mast can be used for same sail area while also getting a more efficient square top sail shape (saves money, windage and weight of mast)
    Less compression loads mean reduced weight and cost of the hull structure (saves weight and cost again)

    Cons.

    Lever crane adds weight cost and windage, but the mast has lost weight cost and windage and hull has lost weight and cost due to reduced backstay loads.

    I am sure the list will grow larger when others see this. There is probably some obvious flaw I have overlooked rendering the whole idea unworkable :)



    I made a quick sketch not to scale or anything just to show what I mean and to see if it would even work like I imagine?

    [​IMG]
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.