Conference unifix::sailing

Do you mean the boat momentarily make more leeway, or that it heels
more?

Do you mean the waves generated as wake (which stream out from the other
boat in a V-shapped pattern), or the foamy, disturbed area in the center
of the path (more pronounced with a power boat)?

For waves in a wake, the effect is similar to any other waves. The pitching
or rolling sucks up enery from your boat, and the circular velocities in
the wave affect the efficiency of your keel's foil action, which effectively
results in more leeway.

If the turbulent area in a wake, the reason is most likely that your keel
loses most of its lift in severe turbulence, which will tend to seriously
increase your leeway (the keel's lift is a significant factor in reducing
leeway).

Ross Faneuf

    The boat makes more leeway when I cross the foamy disturbed
    area in the center of the path.
    
    I suppose I'm curious how the keel produces lift since a keel
    isn't curved like a wing or a sail.  I'm also wonder how the
    turbulance spoils it.
    
    Martin

    
    	Unless somethin' funny happened, your keel should look a *lot* like
    a wing- more properly, 2 wings glued together. More properly yet, if
    it's a fin keel, should look like a fin on a missle! (NACA fans, take
    note!) Lift on a keel is when you can get a pressure differential
    between the sides. The oft-talked about lift from current can be
    imagined if you are tacking, say, up a river, with head current. 
    
    	Now a full-keeled boat may not look as much like that missle fin,
    but the general streched teardrop shape should be there... And the full
    keel will track better, being longer along the keelson, but not point
    as close to the wind.
    
    	What sort is yours?
    
    	All bets off with catboats!! hahaha

    	Even it if is flat plate with no airfoil shape it will still
    generate lift.   Modern nicely shaped foils are designed to have given
    lift properties and stall angles and thus can be tweaked for their
    purpose.  What is finally chosen is one of the great trade offs in
    boat design.  In classes that allow changes in the fin shape, and
    rudder shape you will see a wide diversity in foil shapes and sections. 
    Small changes can mean big gains or losses out on the race course.

    	Another thing to consider that while passing through disturbed
    water the angle of attack of the foil changes and thus lift changes. 
    As you work through a wake or wave boat speed changes and this also
    factors into the lift. This is what you are most likely seeing when
    passing through a wake.  When sailing up the transom of a lead boat you
    have two factors disturbed air flow, the disturbed water flow, and
    finally as a result of the others a change in boat speed.  This
    all combines to change the lift.

    	It is all of these factors plus scaling factors that used to make
    tank test data from sail boats largely useless in an absolute sense. 
    Anybody remember the 12m mariner one of the all time dogs in Americas
    Cup history.  This boat was a tank test demon and looked like a big
    winner.  Computer modeling, and tank testing can now fairly accurately
    correct for all of this and get better results.


    Forrest

Most keels are NACA foil sections (originally developed
as airfoils). They are all symmetric sections (same on
each side). Some aircraft wings are or have been
symmetric airfoils as well (I'm sure the WW2 B-17 was,
for instance). Symmetric section develop lift, in spite
of their symmetry, because of their angle of attack
(the angle the chord of the foil makes with the direction
of the fluid flow). It's not apparent in a boat, but
it's none the less true that your boats heading with
respect to its direction of motion through the water
will develop an angle of attack on the keel, thus
generating lift.

Not assymetric airfoils all have an angle of attack, as
well, and for angles well below stall the lift is a
linear function of angle.

The amount of curvature in a foil is mostly affected
by the speed of the fluid over it (in dimensionless
numbers). The lower the speed, the more the curvature;
that's one of the reasons hang gliders and sails have
a lot of curvature - they're operating at low flow
speeds. High speed aircraft have foils that look more
and more like thin flat blades (although supersonic
effects dominate; I don't know much about that, and
it doesn't seem to matter a whole lot to my boat).

Ross Faneuf

    	The flat plate diversion was because I suspect that Martin is doing
    his sailing at Community Sailing in Boston.  Their primary boat is a
    Cape Cod Mercury with truly flat questionably located centerboard. 
    The boat tend to exhibit lee helm under many conditions.

    	This original S & S design had a moderately deep fixed keel design. 
    The builder later created a center board version.  They use a hunk of
    black iron with square leading and trailing edges.  It looks like they
    guessed at the mounting point and a travel limit point.  Their default
    points are not all that great.  I have sailed box stock boats and ones
    with the board limit increased to give it a slight weather helm.  The
    boats performance and pointing ability is like night and day.

    	The builder also used the flat plate model for the rudder with a
    slight rounding of the leading and trailing edge.  I have used friends 
    foil shaped rudder (forget what section) it is a huge improvement over
    the stock blade.   But that is another story,  it illustrates how with
    a moderate investment in $ and time a boat that performs like a dog can
    be made much better.

    	I have never sailed one of the original S & S full keel models but
    folks who have say it is a nice boat to sail.  I have been told that
    Olin Stephens claims that it was one of his favorite designs.

    Forrest

    Right you are Forrest.                                             

    I have sailed both the keel and centerboard versions of the Mercury
    and your comments are right on the money.

    I like the keel version much better, but it and the Typhoons exhibit
    similar behavior when passing through a wake, so I became curious
    about the mechanics.

    Thanks for the information.

    Martin

    I'm interested in the effect of the cenre board stalling in a
    turbulent wake. Would the boat side slip quickly? It might explain why
    I capsized to wndward whilst saling up the transom of another dingy in
    a race. One second we were hiked out close the lee side of the other
    boat's transome, the next on our backs in the water! We had his dirty
    wind as well. Any thoughts? 

This is ALL guessing (but I love it ...)

If your capsize was due to hydrodynamic effects, it suggests your
boat suddenly acquired a lot more righting moment. Now it may be
the effect of your center board is mostly to increase lateral area,
and thus resist leeway. It could be its actually producing negative
lift normally (but desirable righting moment anyway due to its
weight). The local hydrodynamics of the situation may have suddenly
changed the angle of attack to produce positive instead of negative
lift on the board, and over you go. (I'm not sure I believe this).

How about the possibility that the dirty air from the other boat
suddenly decreased the heeling moment produced by your sail, and
you went over for that reason (I'd find this easier to believe, frankly).

Ross Faneuf

    
    
    	I would bet on the loss of healing moment due to dirty air flow
    over the sails.  I find it almost impossible to believe that any loss
    of lift from the board would account for this.  Sailing into dirty air
    is the equivalent of having a puff you were hiking hard for die out way
    before expected.
    
    
    Forrest

    re.8
    
       If you were hiked out and then on you backs, I take it you did not
    flip, but instead rolled the boat on top of yourself. If so,this is
    because of the dirty air and not the water. I almost got crushed by
    the Mt Washington, a huge ferry, one time from flying a hull (ie
    trapped) into it's windshadow (wind was from behind her). The boat
    dropped like a rock back into the water and if we had those oxygen
    masks on board like airplanes, they would have popped out!
    
      If you did flip (ie you flew though the air and landed on what used
    to be the leeward side of the boat, it sounds like you caught a fluke
    gust and were so close to the other boat that the shadow did not come
    into play. Hard to believe unless the guy in front of you is REALLY
    smooth on letting his sails out as the gust hit. And you should have
    seen it coming across the water at you.
    
       I find really turbulent water (ie lots of air in it) affects the
    control on my catamaran. Basically it slips to leeward more and the 
    steering gets "mushy". This is due to the fact air is more compressable 
    and there is more give. It takes a powerboat wake or a ride through 
    breaking waves to do this though.
    
    
      john
                                           

    While we're designing boats on the fly, consider that the
    aspect ratio of most centerboards and many keels is really quite
    low.  This is obviously because of the draft limits imposed by
    the various measurement rules and also because you want to be
    able to sail in shallow water, but ignoring those, you could
    make a pretty efficient centerboard if it were, say 3 inches
    long and 10 feet deep!  A Laser centerboard is about 1x2.5 feet,
    so both would have the same area (thus the same surface
    drag), but you'd get quite a bit more lift from the long one...
    
    Doug.

    the issue is structural. It is going to break unless you have some
    really neat technology.
    
    Hobie just released their newest boat, the Miracle. It is 20 ft long, 8
    ft wide, and has a set of centerboards 1.5 feet wide and 4 ft long. The
    operating instructions tell you to pull the boards at least half way on
    broad reaches, else with the speed this puppy generates, you will snap
    the centerboards (besides losing speed).
    
    by the way, this catamaran points like a monohull.

    What determines how far inboard you may sheet the boom when close
    hauled?  At Community Sailing in Worcester the instructions are to
    position the boom over the aft corner of the Daysailors they use.
    However, I've seen lots of pictures of other dinghys and keelboats
    with the boom sheeted in almost to the center of the boat.  Is this
    something about the Daysailor design (like: too small a centerboard)
    or is it to keep novices (like me) from pinching so much that they
    end up going backwards (or sideways)?
    
    Doug.
    

    	The instructions given to new sailors are intended to be a guide
    line.  At least that is how we treat them at the community program in
    Lowell.  This is a nice simple place to tell folks to trim to that
    works fairly well.  Later in the more advanced courses we go into more
    details and theory.  

    	The correct position varies with a large number of factors.  Sail
    shape, wind speed, boat speed, hull shape, board shape etc.  If the
    main has a tell tale on the top batten the general goal is trim so that
    it is tending to break to leeward.  For example this may be wrong if
    you have an awful weather helm deal with you may need to ease the sail
    out to balance the boat no matter what the tell tale is saying.


    Forrest

RE:.14 by STARCH::HAGERMAN "Flames to /dev/null"

> What determines how far inboard you may sheet the boom when close hauled?  

The simple answer is:  sheet to the point that makes the boat go fastest.

The Daysailor's rig has the main sheet making a triangle to both corners of 
the stern, so if you wanted to sheet the boom in to near the center of the 
boat you would have to put a lot of tension on the leach of the mainsail.  
This would cause the leach to "hook",  and this "hook" would generate a lot
of air drag,  which would slow you down,  and/or increase heel.

To get good sail shape you can't put too much tension on the mainsheet.

The easy answer (boom over the corner of the stern) is pretty close for a stiff
breeze.  It's not bad for a good breeze.  It's pretty bad for light air.


> I've seen lots of pictures of other dinghys and keelboats with the boom 
> sheeted in almost to the center of the boat.  Is this something about the 
> Daysailor design ... ?

Yes.  Many boats have a sheeting scheme that allows for separate control of
leach tension and boom position.  The Daysailor does not have such a scheme.


Phil

>================================================================================
>Note 1907.14                Sailing Theory Questions.                   14 of 15
>STARCH::HAGERMAN "Flames to /dev/null"               11 lines  21-SEP-1992 10:55
>--------------------------------------------------------------------------------
>                             -< sheet in how far? >-
>
>    What determines how far inboard you may sheet the boom when close
>    hauled?  At Community Sailing in Worcester the instructions are to

Doug,

	As above it depends upon a lot of things.

	I have been learning for the last thirty years and still have rows of 
tell-tales across my job and main. (main is 20 sq m and  has three rows of five)

	First I set the jib.

	Then I sheet in 'til the bottom row just stream nicely, then haul down 
(or pay out) the kickin-strap so that all of the rows above stream as well. 
That has the sails set approximately. Then I set repeat the whole lot over and 
over again.

	In light wind very little kicker tension is required as the main needs
to have a twist. This can meen setting the traveller to windward to reduce the 
downward force on the boom.

	My usual fault is to over-sheet. So I also keep a close eye on the log
to see if the speed drops when the wind doesn't.


	Regards

		Gregg

    Here's some rules of thumb for a Main and 150% genoa that work
    well on most boats in moderate wind:
    
    Genny:
    1. Trim the sheet so the leech is 2"-3" off the spreader.
    
    2. Set the lead on the jib so the luff breaks at the same time
       up high and down low. An imaginary batten halfway up the jib leech
       should point straight aft.
    
    3. Set the jib halyard/cunningham tension so the center of max draft 
       (belly) is 30-40% back from the headstay. You have to lay down on 
       the foredeck to check this one.
    
    Main:
    1. Tighten the outhaul until you see horizontal wrinkles just above the
       boom. In other words, the foot panel should be closed. 
    
    2. Tighten the mainsheet until the top batten is parallel to the boom.
    
    3. Set the main halyard/cunningham tension so the max draft is 50% back
       from the mast.  
    
    4. Pull the boom up to centerline with the traveler. A piece of tape 
       on the stern pulpit center helps you eyeball down the boom. It's very 
       easy to be 1" high or low without a reference.
    
    
    I've seen alot of cruisers with terrible sail trim having trouble 
    beating against a strong tide or trying to get through a narrow inlet. 
    These simple rules just take a minute to follow and can make your boat
    sail up to it's potential.
                              

    
    
    don't forget about using a barberhaul on the genoa sheet.
    If you find your sail telltails are flying well at the bottom
    of the sail but not so well at the top, put a snatch block with
    a line attached on the sheet between the clew and the track. Run
    the line down to a block connected to the toe rail. by hauling this
    barberhaul in , the sheet will pull down and the clew will follow.
    This will tighten the sail and change the upper section angle to the
    wind, allowing the sail to be adjusted for wind differences.
    
    Also.. in light air loosen the backstay and in heaver air tighten
    the backstay. this will change the overall sail trim a lot. I have
    found a knot to a knot and a half by simply tuning the rigging while
    undersail... 
    
    							georgia