Conference unifix::sailing

    The reason you haven't seen much discussion is that is isn't usually an
    issue.  The expectation is that you put the main to the top of the mast
    (within measurement rule restrictions).  The floating gooseneck is
    probably meant to allow you to put tension along the luff to change its
    shape for various windspeeds.  
    
    How big a boat are we talking?  What kind?
    
    Dave                                                       

    
    re: how big/what kind?
    
        Catalina 25, lwl 22'10'', if I remember right.
    
        You're right about using the floating gooseneck as a downhaul,
        that's how it's used.  There's about 18 inches of adjustability
        though.  I can't put enough tension on the halyard to strech
        the luff 18 inches, even if that was desireable, so the extra
        length can be used for adjusting the entire main up or down
        by, say, 12 inches.
    
    /jim
    

    
    
    If you look at the one of the versions of America's Cup yacth INTREPID
    (sp?) you will see the boom set almost at deck level.  This was done on
    purpose to cause the hull to act as an end plate.  This much more
    efficient than when the boom was at a conventional height.  The problem
    was that the crew was at peril of being hit by the boom.  To work
    around the safety issue they cut many holes in the deck and had the
    crew working below deck most of the time.  The lowering of the boom to
    deck level was banned in later America's Cup races.
    
    If you look at the formular 40's raced in Europe they carry the bottom
    of the sail as close to the deck as possible.  Up to a point lower is
    better but you have to trade off safety, and what the class rules
    allow.
    
    
    Forrest

    Ah, a Catalina 25.  If you are racing it you don't have any choice but
    to raise your sail to the black band on your mast.  For day sailing I
    suppose you could lower it a bit.  The advantage of the floating
    gooseneck is that you don't have to tension the halyard while it is
    under load.  That is how we break our main halyard so often.
    
    My brother's boat is an older Carter designed 3/4 tonner, 34 feet long.
    The boom sheets in so low that you have to have the hatch open when
    hard on the wind to let the boom droop into it.  I had my thumb caught
    between the boom and the companionway once when the traveller slipped. 
    Broke my thumb pretty good.
    
    Jibing that deck sweeper is the scariest thing.  I guess I like a boom
    to be high enough to be safe, but not too high to look stupid.  How's
    that for a lot of help?
    
    Dave

    From my windsurfing experience, closing the gap (the gap between the
    foot of the sail and the board), gives you a dramatic increase in speed. 
    I don't know how this would apply to big boats but it sounds like the
    effect would be the same.
    
    Then there's the old Ted Hood quote I remenber, "When raising the main
    pull it all the way to the top, and then three inches more." 
    
    Don

    On my COM-PAC 19 (definitely not a racing boat) there is a black band
    on the mast. I finally figured out why. If the gooseneck is above the
    black band, my topmost batten hits/rubs against the backstay.
    Fortunately, I discovered this before any real damage to the stitching 
    was done. 
    
    I have since gone over all of the stitching on my sails - especially
    the ends of the threads with a bottle of fray check - which my wife had
    gotten from a sewing supply store.
    
    Bill
    

    Hi Jim:
    
    Have you checked to see if your sail is the normal size for your boat? 
    Normal sailmaking practice is to have the luff just as long as the
    distance between the black bands with the top batten barely clearing
    the backstay.  The main sounds a bit undersized.  But if it works for
    you, enjoy it.  It's nice having a bit more height under the boom.
    
    - gene

    
    re: Have you checked to see if your sail is the normal size for
        your boat?
    
    it's the size specified on the drawings, but it does look somewhat
    undersized to me.  it sails well in combination with a 130% genoa,
    but it is unbearably underpowered without it.  
    
    there aren't any black bands on my mast, but the drawings show the
    main hoisted right to the sheave at the top of the mast, as you'd
    expect.  this gives ample head room... more than enough, I'd say.
    
    on any course lower than a close reach I don't think there's any
    advantage to lowering the main, since the boom is outboard anyway
    and lowering it won't reduce any induced drag.  at or above a close
    reach i'm still not sure.
    
    trim the boom to the verge of a thud?  ;)
    
       /jim
    

    Jim, Jim, Jim,
    
    Raise the main ALL the way up.  If you don' have a 
    band, stop the hoist an inch shy of fouling the headboard
    in the backstay.  Then take Ted Hood's 3 inches off the
    downhaul.  I do not beleive you could quantify the performance 
    advantage of lowering the main's profile.
    
    If stability becomes an issue, use the reef 
    points.  It's that or wait until you give someone a good 
    crack on the noggin.  Then it'll be "Sink the &*#!!%^ thing!"

    How are you coming on the simulator?
    ;)    

    
    btw, the specs are as follows:
    
    	I = 29'   (mast)
        P = 24.7' (luff)
        E = 9.6'  (foot)
    
    so there's about 4.3' of space between the top of the deck and the
    boom.  Seated in the cockpit, about 1/2 of that is required for
    normal seated headroom (without changing posture), leaving about
    25" for adjustment, or let's say 22" after Ted Hood gets his way.
    
    i guess the racers out there agree that higher is better, though
    it's interesting that the sailboard aerodynamics were improved,
    even without the reduced hydrodynamic drag that I'd expect from
    a keel boat that is heeled less.  presumably the sailboard is
    sailed upright independent of the mainsail height.  I'm not sure
    what effect the lowered main would have on the genoa, though.
    
    thanks for all your comments!
    
       /jim

    No, I don't think the racers think that.
    If you lowered the boom, we'd tell you
    to bend on a bigger mainsail!  
    
    In a cruising boat, it's safest to have
    standing headroom beneath the boom -- when 
    standing on the cockpit sole, that is.
    
    Later

    I did a little searching around to try to estimate the effects of
    lowering the main:  according to "Sailing Theory and Practice",
    the hydrodynamic drag difference is essentially nil if the boat
    is operating below 20 degrees of heel with the main raised, so if
    there is any improvement at all it is likely to be due entirely
    to aerodynamics.                  
    
    As far as end effects around the boom are concerned, a test was
    conducted with an end plate attached to the leeward side of the
    foot to prevent air leakage, which resulted in 15-20% more driving
    force.  In this case they did not have to lower the sail at all,
    so I'd expect to see less than 15% more driving force in my case.
    Then again, even half this much would be a major improvement.
    
    In "The science of yachts wind & water", low booms are specifically
    mentioned with respect to reducing end effects (induced drag), but
    according to the author the benefits are minor (no quantitative
    data presented).
    
       /Jim
                                                                 

    
    In "Aero-Hydrodynamics of Sailing," C. Marchaj has the following
    to say about setting mainsail height: (pg 555)
    
    	"Influence of rig height above sea level
    
    	in this investigation, the ... only change made on the rig
    	was to reduce its height above the wind tunnel floor, which
    	simulated the flat sea surface, by 40 per cent, i.e. from 10
    	in to 6 in.  The variations of Lift/Drag ratio with
    	incidence angle for the two cases is presented ...
    
    	It can be seen that by reducing the gap between the sail 
    	foot and the tunnel floor or eventually the deck of the
    	hull, the maximum L/D ratio increases some 10 per cent from
    	5.0 to 5.55.  This quite measurable effect may be attributed
    	to the diminution of the end-losses due to the trailing 
    	vortex shed underneath the boom.  In general, except in 
    	very light winds where strong wind gradient cannot be ignored,
    	the lower the rig is mounted in the boat the better..."
    
    /Jim
    

    And that is why the top sailboards lower the boom of the sail until
    the foot of the sail is just skimming the water when they are trying
    to travel at top speed...
    
    Something about 'closing the slot' and stopping air escaping underneath
    the sail, but forcing it to flow along the sail, hence giving you
    more power

RE: .14, 

>>>    And that is why the top sailboards lower the boom of the sail until
>>>    the foot of the sail is just skimming the water when they are trying
>>>    to travel at top speed...
    
Properly trimmed for speed, the sail is sheeted aft and in, so the foot of 
the sail closes the gap between it and the top of the board itself.  This 
necessarily brings the center of effort (CE) also quite far aft, so you
raise your centerboard, step back into the straps, and let the skeg fin be
the main source of lateral resistance (CLR).  Sailboards are similar to 
performance multihulls in that given a breeze, you stay sheeted in and 
build apparent wind from forward.  One never sails at speed with the clew
out more than a few inches over the tail of the board. 

The average monohull benefits from exploiting this end-plate effect 
over a much narrower range of wind directions, mostly beating.

J.

My two cents worth:

Here are some of the factors that go into setting the position of the
sail on the mast.

1. Wind gradient factor. The true wind speed increases as you go aloft.
I doubt it makes much difference in a sailboard, since they are so short, 
but in a sailboat with a much taller mast, it is quite noticable, especially
in light airs. The increase in wind velocity causes tha angle of apparent
wind to be about 7-8% greater at the head of the mast than at the foot.
This translates to more forward force higher up.

2. Points of maximum draft. These are the points up the sail where the 
sail is fullest. It also affects the amounts of forward and heeling force.
The points of maximum draft move increasingly aft as you go from the head to
the foot. Genoas typically have points of maximum draft set about
30% aft on average, although there is quite a bit of twist at the head with
the point of maximum draft far forward there. Mains run 50% on average, 
maybe 55% at the foot and 45% at the head. Having the point of maximum
draft forward at the head works with the wind gradient factor to take
advantage of the greater apparent wind angle.


3. Height and overlap of the headsail. When the headsail is up, it is
better to have the mainsail high with the upper portion of the main
fuller. The foot of the main is usually pretty flat, and will contribute
a lot to heeling or sideways force. However, with the genoa up, it gets
the lion's share of the wind velocity down low, and the lower part of
the main plays a lesser role. You just want to keep the slot open between
the genoa and the main and have a smooth flow over the lower main. Without
the Genoa, moving the main up may contribute more to heeling force if
the main was designed flat at its bottom.

3. Shape of the main. A very triangular sail (as in older wooden boats) 
wouldn't do to well if hoisted up high, mainly because of the action of
greater apparent wind angle against the lower part of the sail causing
excessive heeling force. However a tall thin sail almost like an airplane
wing, but with twist, should work very well, especially in light airs. 
When 2 sails are up the whole picture changes. With the head sail up, the 
angle of apparent wind on the main becomes less, thus you can get away with 
hoisting a  more triangular sail a bit higher. I guess I would still opt
for the tall main even with the genoa, particularly in the New England light
airs.


Conclusion: I'll bet setting it low without the genoa and high with the
genoa would work out best, unless the main is very full down low, in which
case without the genoa setting it high in light airs and low in heavy airs 
might be better. If it was designed to work with a genoa, it probably isn't 
very full down low. So much for theory, now let's go try it!