Fat head Bermuda Sail Design rules

Discussion in 'Sailboats' started by sharpii2, Nov 4, 2016.

  1. sharpii2
    Joined: May 2004
    Posts: 1,948
    Likes: 135, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Questions About The Fat Head Bermuda Sail

    The Fat Head Bermuda Sail has been brought up in several threads. Recently, I’ve become curious as to:
    1.)when it was introduced.
    2.) if it can be successfully made out of ordinary Dacron, and
    3.)the general rules of it’s design.

    In my attachment I have attempted to draw a version of this sail three times.

    All three drawings share these three rules:
    A.) the Luff is twice as long as the Foot,
    B.) the depth of the Roach is 10% of the Foot length, and
    C.) the Short Battens project twice as far into the Sail triangle as they do into the Roach itself.

    They only differ in the length of the Head, which is measured in proportion to the Boom. 40% seems to be a bit too generous.

    From my way of thinking, the diagonal Batten, which holds the aft corner of the Head up, must be longer and stiffer, in proportion to the length of the Head. If it is too long, it will have to be too stiff to let the sail form a good sectional curve. If it is to short the turbulent flow from the mast head will never reconnect.

    25% seems a bit too stingy. The Head may be too short for the turbulent flow from the mast to reconnect.

    1/3 seems to be a good balance between the two.

    Any comments?

    I have gone ahead and calculated the Sail Areas of all three drawings. Even the one with the shortest Head, along with its Roach, renders a almost 25% increase in SA, over a plain triangle with the same Luff, Foot, and Roach. This is indeed substantial.
     

    Attached Files:

  2. CT249
    Joined: May 2003
    Posts: 1,249
    Likes: 86, Points: 48, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    "Fathead" seems to be a windsurfing term from the early 1980s to describe a large high roach. However, fatheads were not squaretops; the head was not horizontal but sloped, and the peak was rounded not square. There's a pic of a classic fathead here.
    [​IMG]

    However, roachy sails were common in 19th century canoes. Many Australian and NZ dinghies had big roaches with lots of area up top in the '50s and '60s. Moths tried extended masthead cranes to hold out their heads. National 12s and Merlin Rockets had an extended roach from about 1960, I think.

    The squaretop as we know it ie with a horizontal top and sharp angle,has been claimed by Sailing World and others to be have been invented (or at least largely proven in world class competition) by the Goodall brothers, best known for Taipan, Viper and Capricorn high performance cats. That was around the mid/late '80s, I think, perhaps when Allen Goodall won the A Class worlds in '84. Those squaretops were smaller than current ones because of the stretch in the material, I think. The exaggerated fathead seems to have come into dinghies in the 12 Foot Skiffs a few years afterwards.

    They have been made out of dacron but IMHO they don't really suit dacron or deep-drafted sails very well, because if there is too much draft and stretch the head blows open too much in a breeze and slams shut too much in the lulls. It therefore requires lots of mainsheet, vang and traveller adjustment, which is actually quite rewarding if you get it right. A good modern squaretop is largely automatic, but I think there are no real rules that are not earned by experience and guarded by those who earned it.

    ggggGgguest, Richard and Rob have probably done work in developing them, I just use what comes with the boat.
     
  3. gggGuest
    Joined: Feb 2005
    Posts: 828
    Likes: 24, Points: 18, Legacy Rep: 76
    Location: UK

    gggGuest ...

    Reckon that's about right Chris.

    Increasing roach at the top of the sail has been a general trend for decades really.
    Most of the development classes do their best to measure total sail area, so what's added at the top has to be removed from somewhere else, unless, like the true skiffs, they don't measure sail area at all.

    The Merlin sail, though, was primarily an artifact of inadequate sail measurement rules.

    The NZ twelve footers certainly influenced me when I had an earlyish pseudo horizontal top sail on a singlehander about 1998, (90 degree to mast tip but not parallel to water due to bend and rake), but obviously dinghy folk were also looking very hard at sailboard rigs and even ultralight aircraft. I was always a follower though, not (b)leading edge.

    I tend to agree that there's a point at which the loads and behaviour required get a bit much for dacron sails. I also don't think I'd care to mix big heads and short battens. Part of the point, to my mind, is that the top should twist off while the mid leech stands up, but if you only have short battens then the mid leech isn't supported and the whole sail flops off, which isn't much good. Mast bend and sail have to be matched very well too, won't get much of a result from just chucking a big square top on an existing rig. Takes someone smarter and more knowledgeable than me to do the design though.
     
  4. sharpii2
    Joined: May 2004
    Posts: 1,948
    Likes: 135, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Thank you for your comments, ct and gg.

    From what you two have said, I get:

    1.) the sail that I drew was a 'squaretop', which is a sub-class of Fat Heads,
    2.) that this type came about some time in the 1980's,
    3.) that this sail type should have full length battens, so the lower part of the leach will stand while the top part can sag off in gusts,
    4.) that the sail material should be something stronger and more dimensionally stable than than ordinary Dacron, and
    5.) it may take a lot of Vang Traveller, and sheet adjustments to get the best out of it, if it is made out of Dacron.

    It seems to me that the key component of this type of sail is the diagonal batten at the top, which for convenience, I will call the 'sprit batten', as it does act somewhat like a sprit on a sprit sail. It not only holds th top out, but also holds the leach taught. Once the top sags off, I would think that this batten would be put under greater compression, causing it to bend more.

    This, in my mind, would cause it to do two things:
    1.) allow at least the top of the leach to sag off, down wind, which is good, and
    2.) cause the head of the sail to cup somewhat, making it fuller, (I suppose that's what you mean when you say 'opening up') which I don't think is such a good thing. This could cause more drag than lift at the top of the sail, which I would think is a bad thing.

    I imagine that the longer the Head is and the shorter the Sprit Batten is, in relation to the Head, the more this is true. So my guess is the Head length should be a relatively small portion of the Foot length of the sail, no more than a third, and the Sprit Batten should be about 50% longer than the Head, so it can angle down from the Head at at least a 45 Degree angle, if not a little more. This way the increased sectional curve of the top of the sail ends up ein no more than about 70% of the bow of the Sprit Batten.
     
  5. gggGuest
    Joined: Feb 2005
    Posts: 828
    Likes: 24, Points: 18, Legacy Rep: 76
    Location: UK

    gggGuest ...

    The top batten isn't that special or extreme angled I don't think, and typically at a very much shallower angle than you've drawn. The next batten down would be angled up some too. Have a good web search session for 18ft skiff and 12ft skiff rigs to get an idea.

    You absolutely need to consider mast bend in the equation too. Typically the top of the rig will get a fair bit of shape from luff round, so if the downhaul is hammered on in stronger weather the top mast will bend considerably, the luff round will be used up and the top of the sail go very flat. Typically the kicking strap/vang will be used primarily to control twist, and the downhaul/luff tension mast bend and thus sail flatness. But they interact greatly of course. Vang/kicker loads can be huge with these rigs.
     
  6. JRD
    Joined: May 2010
    Posts: 224
    Likes: 17, Points: 18, Legacy Rep: 192
    Location: New Zealand

    JRD Senior Member

    A couple of observations and experiences based on my own square top main sail. This was re-cut from an older sail so not an ideal starting point but it does work.
    The sail maker referred to the top batten as gaff batten so your thoughts are not far off the mark Sharpii. He also said the head chord was limited to 40% of the foot length (I don't know the basis of this comment). I think this is very much the extreme end of the spectrum and nowhere near the configuration you are considering.

    In this case the top batten takes a lot of load, a conventional batten did not work well for the reasons you suggested and i changed to a tubular carbon fibre batten which is very stiff.
    You will see the extra reinforcing that needed to be added to get the shape to behave as the cloth was distorting even with a heavily reinforced film sail. At this ratio I am certain that dacron cloth of the weight suitable for the sail size would not cope with the loadings.

    The developments in 12s, 14s and 18s with square head sails have been built around very stiff HM carbon masts. Especially when flattened with heavy down haul tension, the head falls off and flattens nicely in gusts. A flexible mast would make it impossible to obtain enough leech tension to point well.

    Overall I like it, but its horses for courses. I'm sure I wouldn't want this rig for leisurely cruising. As said by gg the loads are substantial. I have a 24-1 vang and a 4-1 downhaul to control this sail which puts a lot of load on the hull, boom and gooseneck fitting.
     

    Attached Files:

  7. brian eiland
    Joined: Jun 2002
    Posts: 4,948
    Likes: 179, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Interesting observations, particularly that last one. And yet I see a lot of cruising boat folks trying to adopt this latest 'fad' ?

    Do they have any ideas as to the extra difficulties those top battens often add to the handling of those sails??
     
  8. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    Many barge like production condomarans are fitted stock with square top mains these days. A good idea, as anything to help these things move under sail is a good thing. The loads these sails must take is huge, given the weight of the boats. Material? Dacron from what I can tell.

    I fitted a squaretop to my last boat. It was not really harder to control at all. I loved it. Cats like this don't have vangs anyway. The traveller did have a bit more tension for sure.
     
  9. sharpii2
    Joined: May 2004
    Posts: 1,948
    Likes: 135, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    It appears that the head of the sail can be much longer than I thought.

    From what I have read from the posts here, I have come up with new design rules:

    1.) Foot equals half the luff or less,
    2.) Head can be half the foot, if not more,
    3.) Sail must have full length battens,
    4.) ''Gaff Batten" must have a pitch of less than 45 degrees from horizontal, and
    5.) at least one batten below it must also be pitched up, but at a lesser angle.

    Thank you all for all your impute.

    I might someday be tempted to try to make one of these out of very cheap materials, just to see if it would work. My reasoning is that the Battens and their associate re-enforcement add so much strength that the sail material may be less relevant. The only catch seems to be that the sail material is best as no-woven, or with vertical seams.

    I have thought of the idea of making a gaff rig of roughly the same proportions, with the Gaff and Boom designed to bend, to flatten the sail.

    Just reliving some Peak tension should allow the Head to sag off when necessary.

    It would be an interesting experiment to make this and the Flat Head, with roughly the same profile shape out of the same cheap materials to see which one does better.
     
  10. gilberj
    Joined: Oct 2010
    Posts: 72
    Likes: 4, Points: 8, Legacy Rep: 57
    Location: 034

    gilberj Junior Member

    I'd say these fat/square head sails are at least sort of descended from LF Herresoff's short gaffs.
    The short gaffs were I think intended to have some of the advantages of the gaff sail, and the advantages of the Bermudian sail.
    The advantages of the gaff is there is a chunk of wood at the top of the sail to help it come down... I have had sails stuck up just often enough to value any help there is. The short gaffs also allow a lower CE of the sail... more drive with less healing moment.
    The advantages of the Bermudan sail in this context means the stays and shrouds can be attached where the work best. The sail and gaff run on a track so the lower shrouds are not limited to just above the gaff jaws.
    So you can have a lighter mast, stayed efficiently. The sail can be much more high aspect ratio than a traditional gaffer.
    There is less twist in the sail than you would expect from a gaff sail, more similar to a Bermudan sail.
    I do not think LFH's short gaffs would out perform a modern fat head sail, even if it was rigged with all the go fast strings and vangs of a modern race boat. It is a cruising sail. LFH drew short gaff rigs for a bunch of his later cruising designs.
    If I was working on a hot racing boat, I'd use fat/square head sail(s)....no question.
    I have sailed two of LFH'S short gaff cruising boats and now believe it is a very strong option for a cruising boat. The advantages of both gaffs and bermudan sails, and it's not like every other 'white boat' to boot.
     
  11. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    I think the evolution is clear. There was no gradual evolution from short gaff sails even though they are similar. Roaches got bigger and bigger until they went to square top. The evolutionary paths led to something similar but it was not the same path.
     
  12. gilberj
    Joined: Oct 2010
    Posts: 72
    Likes: 4, Points: 8, Legacy Rep: 57
    Location: 034

    gilberj Junior Member

    I don't disagree with you. The point is a pretty darn similar solution, but in truth since Herreshoff first started playing with the concept in the 1930's with, I think it was the R boat Live Yankee, I think his contribution needs to be noted. A lot of folks look at his short gaffs with faint amusement, but some who have sailed with them are impressed. I am sure that the designers who really got the fat head sails first were familiar with has work.
    I do not see a direct lineage, but this is only one of the innovations LFH invented.
     
  13. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    If trying to find a pic of a modern classic Nigen Irens design (I think) that had short gaffs instead of a regular square top. I can only find pics of one but it has a conventional square top. Does anyone know the boat I might be talking about?

    [​IMG]

    One of my favorite monos. Love the design.
     
  14. gilberj
    Joined: Oct 2010
    Posts: 72
    Likes: 4, Points: 8, Legacy Rep: 57
    Location: 034

    gilberj Junior Member

    I tried to post a photo....no luck with my phone...
     

  15. CT249
    Joined: May 2003
    Posts: 1,249
    Likes: 86, Points: 48, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    I'm not sure that LFH had any real influence on small high-performance craft like the ones where the modern squaretop was evolved. I've never read or heard of any of the designers of such craft referring to him as an influence at all. Nat, yes; the Bethwaites and Ben Lexcen were fans. Francis, no.

    The roachy main was in use as early as canoes of the 1890s, when Francis was still in short pants. Guys like Clarence Farrar, Manfred Curry and Uffa Fox were well aware of the use of long battens and roaches, and all the info I can find indicates that the squaretop was a development of that line of normal roachy bermudan mains, carried out by people like Andre Lefebre and Greg Goodall.
     
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.