Design Factors to Step The Main Mast.

Discussion in 'Sailboats' started by seasaltchap, Jun 23, 2007.

  1. seasaltchap
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    seasaltchap Junior Member

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    What design factors are taken into account to know where the step the main mast along the line of the keel?


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  2. messabout
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    messabout Senior Member

    Classic question Seasalt. Bar room fights are started over such things. The answer is a bit more complicated than we'd like. The boat has a feature called CLR (center of lateral rsistance). Use you imagination for this clumsy explanation....Put the boat in the water with whatever load is normal, then push the boat sideways. It depends on where you push (or pull). You can experiment to find the push point that will cause the boat to go sideways without tendency to turn. You have now located the static CLR. There are graphic ways to determine this location but let's keep this simple.

    The next thing to worry over is the center of lateral pressure caused by the sail or sails when underway. That is usually called the center of effort CE. This can be done graphicly also. The trick is to place the CE somewhere near the CLR when the boat is in motion. In most cases the CE will be located somewhat aft of the CLR by a percentage of the static waterline. The percentage differs for different boat types and sail rigs. This is called lead (as in leed.)

    Here's where the fun begins. The CLR will move fore or aft when the boat is underway, especially when it is heeled. If that is not enough complication then one must consider that the CE of the sails will move also. When the main is sheeted out the CE moves forward by some amount that is a function of the sail angle with respect to the centerline of the boat. Say when reaching or running. There is more. When the sail is sheeted out the pressure on the sail center of effort, which is now outside the boat, causes a moment arm...think of it as a lever. This will tend to turn the boat upwind. That is why we put some lead in the layout. Otherwise the rudder will have to work very hard to make the boat go where you hoped it would go. If you have not put in enough lead the boat will try to turn downwind when you are close hauled. That is lee helm which is a real bad idea that should be avoided. So far you do not have a definite answer to your question. That's because the correct location is a moving target. Competant boat designers select mast location by virtue of experience along with an understanding of the forces involved.

    What most of us do is to place the CE about 8 or 10 percent of the static waterline length, aft of the CLR and hope for the best. After that we start figuring a way to move the mast to the location we wish we'd have selected in the first place.

    Tell us more out your boat and rig and we'll be able to get a little closer to the right answer.
     
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  3. SeaSpark
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    SeaSpark -

    clumsy explanation

    Think the "clumsy explanation" Messabout offered was an excellent one.
     
  4. seasaltchap
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    seasaltchap Junior Member

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    Thank you Messabout, I agree Seaspark; it is indeed an elegant reply - fully understandable.

    I do not have a design, but I would like to design & build a wooden sailboat. Many years ago I built a Wayfarer from a Small Craft kit, and helped build a YW's E J Van Der Stat Buccaner from plans. I am thinking in terms of something about 10ft, gunter rigged to tow. I am fairly well fixed with woodworking equipment.

    The questions that follow are:

    1) can a centerboard's shape and position help to make fine adjustments to the position of the mast?

    2) can internal ballast help?

    Regards


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  5. SeaSpark
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    SeaSpark -

    Mast trim

    Changing the centerboard shape is hard as most have a hydrodynamic profile. Internal ballast can help but will add weight.

    Most common is to put the mast on a track that gives the possibility to move it by one or two inches. When the mast is not deck stepped you can have wedges in the mast tube (correct English definition?) to change its angle and adjust the fore/aft stays.

    Movable leeboards at the sides of a boat are very effective for changing the CLR.
     
  6. Omeron
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    Omeron Senior Member

    messabout, shouldnt the CE be placed some percentage FORWARD of the
    CLR? rather than aft of the CLR as you propose.
    I think the definition of lead is the % distance of CE being ahead of CLR,
    measured in proportion of DWL.
    If CE is placed aft of CLR, you would have a weather helm to begin with,
    when the boat heels, CLR would move even further ahead, and you would have excessive weather helm.
     
  7. seasaltchap
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    seasaltchap Junior Member

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    Tracks for adjusting the step of a mast on racing dinghys are common - but not on a pram-dinghy.

    Tuning has always involved putting a "rake" on the mast, by adjusting the stays; indeed stays on a racing yacht often also have a track for quick adjustment.

    Where "Changing the centerboard shape is hard as most have a hydrodynamic profile", for all practical purposes it is not possible to have it immediately under the step of the mast. How critical is the position of a centreboard/dagger board to the CLR viz. as a fine adjustment?


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  8. PAR
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    PAR Yacht Designer/Builder

    You'd have to change the "shape" (plan form modification) of the board pretty significantly to move the CLP around much. It's not uncommon to have a pivot pin location adjustable and a extra long case. It's also not unusual to design an extra long board and cut off the end until reasonable balance is achieved. This is a pretty "Mickey Mouse" way of doing things and if you are that far off, your other "calculations" should be questioned.

    This said, mast rake on keel stepped sticks can be altered at the partners (correct term SeaSpark) with wedges and is a well proven method to add or subtract a few degrees. Deck stepped poles typically have a hinge so they can be lowered, which also provides the option of wedging to affect rake.

    Omeron, you are correct and caught Messabout in a "mis-speak", though there are many examples of reversed lead in sailing vessels, it's not the norm in craft of typical proportions and rigs. The original yacht "America" which won that ugly cup off the British in the mid 1800's and began the cup races that bear its name, had a reversed lead, for example. Typically, leads have the CE forward of the CLP. I use 7% as a starting point, working up for high aspect Bermudian racers and down on divided rigs and gaffers. It's rare for me to go below 5%, but I've also seen success with 17%. Hull shape, entry, appendages, aft buttocks, rig type and configuration and a few other factors including intuition, experience and good old fashioned guess work, all play substantial roles in balancing the rig over a given hull. Then of course, you sweat come launch day and make "expected" adjustments.
     
  9. Omeron
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    Omeron Senior Member

    PAR, thanks for the confirmation.
    I would like to take this opportunity to ask a question.
    As you explain,very detailed calculations are made, to find the correct balance. And all of those are made, assuming that both the main and genoa
    are working together. Which they do most of the time.
    But sometimes, we choose to sail with the genoa only.
    In strongish weather, and when i am lazy to get the main up, i had
    quite satisfying sailing experience on numerous boats,with just the genoa.
    Now, when we think how much forward the CE becomes, when you are sailing without the main, and what a huge lead you would have, you should develop
    an embarrasing lee helm, or quite simply not be able to steer the boat.
    Yet in practice, once the boat gets out of hesitation, and picks up speed,
    it gets into the rail and drives to windward even with some weather helm.
    In fact in modern sloop yachts, i have yet to experience lee helm,no matter
    how much forward CE is in relation to CLR.
    How do you explain this, and why are we overly concerned that the design
    lead might be a few percent more than what the theory is telling us.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Simply put, carrying just a genoa, which typically has a generous overlap, particularly with building winds, places the CE about where you want it. In heavier air, you want the CE to move forward some to ease the helm. The genoa only does just that and because of the substantial overlap, the CE is much farther aft then it's usual location in the fore triangle, carrying typical headsail(s), which is why you can have a reasonable balance with just a genoa. This is an over simplification, there are other factors, such as apparent wind angles, appendages, hull form, etc.
     
  11. messabout
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    messabout Senior Member

    Omeron and Par;
    You are observant and correct. CE, in most cases will be positioned forward of the CLR not aft as I mistakenly wrote. My only excuse is that I can type faster than I can think. (actually I do not type very fast either)

    There are odd ball boats that use negative lead but that is probably not the case for the dinghy that Seasalt is building. Certain inland lakes scows do indeed use negative lead but that is because the CLR moves quite far aft when the boat is heeled. Boats with an odd combination of aft rocker, run angle and transom width can have large aft movement of CLR . In that case negative lead may be appropriate. But I say again, conventional boats need Positive lead. Positive lead is to be construed as CE leading CLR.

    My original post hinted that disagreements on mast position have been known to cause bar room fights. Having no wish to fight, I will offer to buy you two a drink instead.

    Cheers
     
  12. seasaltchap
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    seasaltchap Junior Member

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    I tend to work to the old addage, that if the curve is pleasing, then it is right.

    Most prams take a sail, it is just a matter of where?

    Is there a good book available in the US for a starter in dinghy design?

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  13. PAR
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    PAR Yacht Designer/Builder

    No kidding Messabout, I've been involved in a few of those debates. Everyone has an opinion, yet few can qualify the percentage they suggest, with a design of theirs that balanced perfectly after launch. Hell, I've moved the mast a few feet on a couple, to get things right. I'll take the beer though, preferably cold, but I'm not that picky when it saves a fight.
     
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