Mast aft rig

Discussion in 'Sailboats' started by Moss, Apr 7, 2006.

  1. Moss
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    Moss Junior Member

    Anybody out there with experience of mast-aft rigs, such as the single-masted ketch design of Running Tid Yachts? (See http://www.runningtideyachts.com/sail/)

    I'm keen to hear how they sail compared to other rig types.:rolleyes:
     
  2. Brent B
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    Brent B Junior Member

    My only mast-aft experience was to design and build one for a Junkyard Boats competition. It used a 35 sq ft jib from a Jet-14 as its only sail (required). Hull was 8 ft long, 10 deg constant deadrise, modest rocker. We built it in one day, won all 3 races with it the next day, each with a different skipper.

    Performance was quite surprising. It jumped up on a plane a few times in puffs while reaching. This with a 180 lb adult on board and only 35 sq ft of sail.

    I keep thinking about designing a properly built 14 ft boat with this sort of rig. What size boat do you have in mind?

    Brent
    Benson Sails
     
  3. DanishBagger
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    DanishBagger Never Again

    Not being a naval architect, I'm not sure I'm right, but that backwards-pointing spreader, and with the tilt of the mast, doesn't that put extreme bending forces on the mast just where the two are connected?

    Also, the bloke states, that there is less windage, but how can that be, there's still three sails, still a mast, there's even a boom (although windsurfer-like)?

    One thing, though, I really like the looks of it, strange as that may seem.
     
  4. SeaSpark
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    SeaSpark -

    Bending moment

    The bending moment can partially be reduced by the back stay, but this will put great compression load to the mast.

    Anyway, the profile in the upper part of the will have to be very heavy compared to a conventional rig, the spreaders will also have to be heavy becouse of the luff tension of the mizzen.

    All this weight at this height can not be good for performance.
     
  5. Moss
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    Moss Junior Member

    Brent B - It's a 20ft ply-on-frame dory. Still finalising the keel configuration, but leaning towards asymmetrical twin keels so I quite like the low aspect ratio of a mast-aft rig. How efficiently were you able to point?

    Bagger - I'm not interested in the three-sail plan, just the efficiency of a genoa on its own, with the mast set well aft to resolve any CE problems. Kind of like a catboat without the mast getting in the way or the air flow.
     
  6. SeaSpark
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    SeaSpark -

    Lattice an A-mast

    You could also look into the lattice and A-mast options, they have some of the advantages you are looking for.

    (lattice-mast picture is from MikeJohns gallery)
     

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  7. Brent B
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    Brent B Junior Member

    "Brent B - It's a 20ft ply-on-frame dory. Still finalising the keel configuration, but leaning towards asymmetrical twin keels so I quite like the low aspect ratio of a mast-aft rig. How efficiently were you able to point?"

    Our tiny little mast-aft boat would tack through 90 deg. Overall the performance was quite amazing. Four different skippers sailed it.

    One disadvantage is that the sail, if boomless, is not self-tacking like a conventional mainsail.

    Your effective J would be about 15 ft on a 20 ft boat. Consider a single sail of 200 sq ft. It could have a foot of 15', and a mast height of 27 ft.

    The keel location is critical to helm balance. On our boat I made a long daggerboard slot so the board could moved fore or aft to balance the helm. Interestingly, my original placement was correct, and we never adjusted the board position.

    I assume you would use roller furling. Remember that this does not work well for roller reefing because the reefed sail shape is way too full. Installing a banana shaped foam layer along the luff helps a lot to flatten the partially rolled sail, making it more effective upwind.

    Brent
    Benson Sails
     
  8. Moss
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    Moss Junior Member

    Thanks Brent, that's given me some food for thought.

    SeaSpark - That A-mast looks like it's stepped a little forward of amidships. What shape sail/sails does it take, and how are they rigged?
     
  9. SeaSpark
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    SeaSpark -

    A-Mast

    I made the picture very quickly and did not think to much about mast positon.

    A have seen one with a normal sloop rig, the mainsail on a vertical wire from the top of the mast to deck level. Other rig types are possible ofcourse.

    Could not find much information on the internet on this rig type, problem is that "a mast" is a very common word.
     
  10. windward
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    windward New Member

    Advantages of the mast-aft rig

    I've been interested in this design for a while. Here are some sites for anyone wishing to look into it further.

    http://boatdesign.net/articles/mast-aft-sailing-rig/
    http://www.runningtideyachts.com/monohull/
    http://www.barefootsworld.net/windwalker/aftmastsailingcalc.html

    I believe that sometime in the next few years someone is going to show up at a big-name race with a boat with a mast-aft rig, probably with an aluminum hull and carbon-fiber mast. At that point we’ll start seeing them everywhere, especially if they perform well. We've seen more radical developments than this in the last few decades.

    Of course, the real advantages of the design aren’t so apparent against spare-no-expense racing yachts, with their rotating, aerodynamic, hydraulically-tuned masts and olympic athlete crews. For more modest boats, however, the benefits are more readily apparent.

    The mast-aft rig designs I’ve found online are monohulls or cats with the mast placed just in front of the cockpit, at about 2/3 of the LWL, canted forward 10 deg. and hinged to fold down for clearing bridges. They use anywhere from 2 to 5 sails. Typically a masthead genoa is used with a standard jib, sometimes with a fore staysail and/or mizzen.

    Anyone taken aback by the idea of a canted mast should take a look at some pictures of clipper ships like the Flying Cloud - now there's some canted masts!

    Advantages of the mast-aft rig
    (cribbed from one of the sites above)
    1) A rig that – is more aerodynamically efficient, enabling more speed with the same sail area and using a shorter mast.
    2) – delivers an aerodynamically clean, parallel leading edge for all the sails.
    3) – allows the whole sail plan to be roller-furled away.
    4) – allows the furling of the sails without turning to weather.
    5) – divides up the total sail area into smaller manageable sizes.
    6) – maintains its balance center (CE) with different sail combinations.
    7) – produces less heeling moments due to a lower center of effort.
    8) – can be operated single-handed without leaving the cockpit. All the sails self-tack. The genoa and jib need to be tacked over past the forestays, which can be done quickly with roller furling from the cockpit.
    9) – has the leech of all the sails operating clear of the mast, spreaders and shrouds, allowing sheeting for optimal performance of each sail, and eliminating spreader wear.
    10) – simplifies rigging by eliminating mast track, slides, and mainsail boom with all its rigging and hardware, and the lubber's bane - an accidental gybe.
     
  11. SeaSpark
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    SeaSpark -

  12. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Windward,
    You are going to have to defend those assertions.
    Nos 1, 2, 3, - naah, all of it!
    Even No.10 is suspect, especially the gybing part. I would imagine the accidental gybe with a socking great genoa out there would be just as bad, being wrong-sided and all.
    Steve "not a believer"
     
  13. SeaSpark
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    SeaSpark -

    5,6,7, 8

    5,6,7,
    Schooner rig has same advantage.

    8,
    Did you ever try this yourself? If so you must have noticed it takes quite some effort.



    My conclusion: If you are looking for a rig with all the advantages you mentioned consider a schooner with free-standing rotating wing masts.
     
  14. AleX`G
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    AleX`G Junior Member

    Model Aft Mast Boat

    I am going to be entering a school model boat race next year. We are gathering ideas and have decided to give the aft mast rig a go. We will be building other rig types like standard bermuda and perhaps a lateen rig with simmilar hulls.
    Might show some of the advantages.
    Then again alot of things are different scaled sown so much.
    It certainly looks different though. Can this rig sail closer to the wind than a standard bermuda with a jib/genoa.

    Any advice is greatly appreciated on any subject relating to our project.
     

  15. DanishBagger
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    DanishBagger Never Again

    I really don't have any doubts that the aft rig would work on a model. However, when making things life-size, "weird" stuff happens ...

    _Not_ being a designer, nor a sailmaker, but I doubt that this will point any higher.
     
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