A-frame vs. mast on a cruising cat

Discussion in 'Multihulls' started by Becaris, Aug 17, 2010.

  1. Becaris
    Joined: Aug 2008
    Posts: 96
    Likes: 3, Points: 0, Legacy Rep: 46
    Location: Los Angeles

    Becaris Junior Member

    I was looking at some cats on the net and ran across the SMG 50 plus (here is a link to it):


    This yacht doesn't use a boom for a main sail, instead it drops a second self furling sail (similar to the headsail).

    I'm just curious to hear what advantages and disadvantages one might expect if you didn't have a boom rigged main? Or, any other comments on an A-Frame rig like this, pro or con.
  2. catsketcher
    Joined: Mar 2006
    Posts: 1,315
    Likes: 165, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    There is a reason for a mast

    Engineers will tell you about a phenomena called Euler buckling. It has to do with the second moment of area of the mast section. The second moment goes up by the fourth power. Basically this means it is always heavier to use two masts to carry a compression load than one. So an A frame mast will be heavier than a single mast.

    I never get the thing about clean airflow over the "genoa/mainsail" that proponents of main less rigs propound. The fastest boats in the world have mainsails not jibs. A mainsail can be tuned with bend to cater for different conditions and can even have built in gust response. You can't get gust response from a jib.

    Then no boom - you need a boom on a broad reach at least. Upwind I would bet that the main is way too full down low as well.

    Interesting idea but I would stay away. My personal take is for a fractional fully battened main rig with a wishbone boom and staysail with a screecher on a furler for close downwind work plus a SYMMETRICAL kite for deep downwind running .


    Phil Thompson
  3. captainsideburn
    Joined: Mar 2009
    Posts: 88
    Likes: 2, Points: 0, Legacy Rep: 23
    Location: Tasmania

    captainsideburn Junior Member

    but their little tacking proa/outrigger is very cool
  4. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 148, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    The idea is to improve sail performance by not having several feet of the area behind the mast useless for creating lift (airflow detatched and messed up). So the sail can be smaller and lighter than with a conventional mast of the same performance.

    Racing sailboats have very costly shaped and pivoting masts so the mast does not blank off the sail as much (and with a fully contoured mast that is foil shaped, perhaps no lost sail area at all). It is less costly for the home builder to make an A-frame than contoured carbon graphite mast.

    For recreational sailing I vote for KISS, keep it simple stupid. Complex rigs just become more work to rig, trim and maintain. Unless you want to race it, there is not a lot of benefit to complicated rigs.
  5. ThomD
    Joined: Mar 2009
    Posts: 552
    Likes: 20, Points: 18, Legacy Rep: 111
    Location: TO

    ThomD Senior Member

    Complication in rigs has a number of meanings: Stuff that makes no sense because class rules have made it so; Stuff that makes a lot of sense, but doesn't exist so you have to invent it, prototype it, build it, yourself. In the real world the sweat spot is normally between the two. "Bad" ideas but already available second hand or off the shelf.
  6. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 295, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Rotating masts need not be made of carbon, nor more expensive than a homebuilt fixed mast. Gold Coast catamarans have done quite well with wooden rotating masts in charter service.

    The sail area behind a mast is not as useless as it's being made out to be.

    But even if it is, it performs a function - along with the jib - that is very important: reducing the drag of the mast. At the very least, the mainsail acts as a splitter plate that can significantly reduce the drag by preventing alternating vortices in the wake. The favorable pressure gradient leading to the slot results in reattachment of the boundary layer to the lee side of the mainsail, so there's not a big separated wake from the mast of a sloop rig.

    The fact that the mast of a sloop is on the windward side of the jib means it sees an apparent wind speed that is significantly less than the freestream. Just stand next to the windward side of the jib when sailing to windward, and then go around to the leeward side and feel the difference in the local wind speeds. Since parasite drag scales with the square of the wind speed, it only takes a 30% reduction in the speed of the wind hitting the mast to cut the parasite drag of the mast in half.

    You get none of these benefits with an A-frame mast. The mast is completely exposed. There's no splitter to prevent a Karman vortex wake. The influence of the circulation around the sails will be much weaker on the windward side, and increase the drag of the leg on the leeward side. You'd have to run the numbers to see which mast has less frontal area, but since most pictures of A frame rigs I've seen have additional struts, I'm betting the A-frame has more frontal area presented to the apparent wind.

    I think there are a lot of virtues to the A frame mast, such as the ability to raise and lower it stably (still need a gin pole, though), and the use of roller furling sails for easy sail handling. But I've not yet seen a convincing case made for improved efficiency over a conventional sloop rig.
  7. Becaris
    Joined: Aug 2008
    Posts: 96
    Likes: 3, Points: 0, Legacy Rep: 46
    Location: Los Angeles

    Becaris Junior Member

    I'm not so sure that the A-frame rig should be discarded so quickly. One of you was concerned about performance, another about keeping it simple. Two self furling sails IS simple, nothing new or complicated and can easily be handled by a single person. As for performance, check out this link about how that smg50 did:

  8. Corley
    Joined: Oct 2009
    Posts: 3,777
    Likes: 194, Points: 63, Legacy Rep: 826
    Location: Melbourne, Australia

    Corley epoxy coated

    John Hitch's Xit uses an interesting rig it seems to be geared more towards ease of management than performance though

    Attached Files:

  9. sandy daugherty
    Joined: Jun 2008
    Posts: 132
    Likes: 4, Points: 0, Legacy Rep: 52
    Location: Annapolis, MD

    sandy daugherty Senior Member

    I wonder if the karman vortex [at 25 knots in stead of 100] can be minimized by an symetrical foil section for the mast.

    Where should the shrouds lead on an "A" frame mast? If it furls on a roller, and uses vertical battens, what are the consequences of carrying a bit of roach in terms of rubbing on the shrouds.

    There are some appealing aspects; reefing on any point of sail with one line, better wing-and-wing control using wide travellers, and balancing the rig with a moveable tack. Both sails could have booms. You could even go with gaff heads and horizontal reefing to maximize sail area, at some cost in complexity.

    Clearly this arrangement would come behind wing sails and sloop rigs in terms of efficiency, but people still sail ketches for reasons very similar to the arguement for "A" frames.

    I do agree that the arguments about the mast compromising lift are much inflated.

  10. Spiv
    Joined: Jun 2007
    Posts: 221
    Likes: 16, Points: 28, Legacy Rep: 207
    Location: The Big Wide Blue Brother

    Spiv Ancient Mariner

    1 person likes this.
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.