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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 05-09-2003
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Factors influencing boat balance

The current discussion of mast rake and its effects on yaw balance (weather vs. lee helm) leads me to a question.

Take an elevation (side) view of a boat and mark the centers of effort (sail plan) and center of lateral resistance (keel and underbody) - assuming you know where these are for the sake of this question. If considering only the component of force perpendicular to the centerline plane of the boat, you can see how the boat should balance when the applicable force vectors are applied.

But the resultant force from the sails is not usually perpendicular to that centerline plane. Most times there is a foward component, as well, especially if your sails are in good shape and trimmed properly. For example, when you are close-hauled and allowing the boat to heel quite a bit, the forward force components of your sail plan are now acting at a fair distance from the "pivot point" (which I assume is somewhere near the CG of the boat).

My questions: Isn''t that behavior a greater cause of weather helm than the mast rake? And isn''t that one of the reasons not to sail with too much heel?

I''m sure people have written treatises on this subject; I have just not read any of them. Any comments?
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Old 05-09-2003
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Factors influencing boat balance

Duanelifesling:

You desperately need to buy that boat and start working on her right away, apparently you have far too much time on your hands if you can devote so much gray matter to this mind numbing subject. Come to Chicago this weekend, I''ll take you sailing, you bring the refreshments, I''ll supply the fiberglass.

Does this answer your question?
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Factors influencing boat balance

Denr,

That''s a might generous offer there, but I will have to decline for logistical reasons.

As to the physics of sailing being a "mind-numbing subject," I sure don''t consider it that way. You''ve most certainly got a lot more sailing knowledge already stored away in your skull than I do, and you''re not numb (I hope).

Once I get the boat (counting down about 15 months), I''ll have lots to keep me occupied, don''t worry. Have you ever viewed the Cruising World BB? Talk about people with too much time on their hands. It''s a regular Kaffee Klatsch with folks arguing over any and everything they can think of. At least SailNet''s BB is civilised most of the time.

Enjoy your sail this weekend, Denr.

Duane
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Old 05-09-2003
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Factors influencing boat balance

There are many variables that must be answered that contribute to an answer to that question. for example: What color is the bottom painted, what shape is the bottom in, what color hair does the blonde on deck really have, what shape is her bottom. I digress. If you look at the change in the shape that the boat presents to the water when healed, it is not to hard to figure out where weather helm comes from. It will be fatter on one side (unlike the blonde).

Paul
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Old 05-09-2003
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Factors influencing boat balance

Dear Duane (Mesmer)Ising,

You''ve received two good (???) anwers so far, well, sort of. I know you are oriented towards the engineering/physics world -- which you really need to try to overcome when dealing with the "physics" of sailing. I''m trained in the liberal arts myself, meaning I know a whole lot about nothing in particular. I did take freshman physics to satisfy my science requirement back when, but I must''ve missed the week we studied the physics of sailing. Nevertheless, here is my explanation for what makes a sailboat go.

It''s magic, pure and simple. Somehow the wind blows on the sails, and the boat knows to go with the pointy end first. Yes, the boat does heel over some in the process, and agreed you don''t want too much of that. Excessive heeling spills my Mt. Gay Rum and Tonic for one thing.

Read all the treatises you want, but they only obscure what it''s all about. Yes, it all can be broken down into formulas and theories. But does that really matter?

My advice is this: don''t over-analyze it. Go with it. Be with it. Become the boat. Let your mind go and it will come into focus.
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Factors influencing boat balance

Good point, Paul, about the underbody contribution to weather helm when heeled. I assume broad beamed boats exhibit more of this.

I think your analysis of the blonde on deck deserves far more attention, however. ;-)

Duane
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Factors influencing boat balance

DuaneIsing, don''t let Denr talk you out of thinking about such things. I''m not completely clear about your theory, but will volunteer what think I know, and maybe someone else will help.

As best I can determine, the relationship between the center of effort of the sails and the center of lateral resistance of the keel and underbody, change constantly as the boat bobs up and down the waves. When you adjust the rake of the mast in order to reduce weather helm, you are changing the relationship between the CE and CLR *on average.* Any variations in that relationship that result from the movement of the boat over the waves are only momentary in their effect, and relatively insignificant. You adjust the rake of the mast in order to reduce the *average* amount of drag caused by excessive weather helm.

I''m not sure whether heeling changes the relationship between the CE and the CLR (and maybe someone else has some thoughts on that question), but even if it does, I doubt that the changed relationship is a significant cause of excessive weather helm or drag. IMHO, when the boat heels, the increased drag is caused primarily by the change in the shape of the underwater surfaces of the boat, and not by the changed relationship between the CE and CLR.

The CE represents the center of *all* the force vectors acting upon the sail, including any forward component. Therefore, the fact that there is a forward component is taken into account, and doesn''t change the relationship between the CE and CLR.
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Old 05-09-2003
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Factors influencing boat balance

Geeeez Loueeeez, You ask a serious question and you get Looney Toons.

To start with, there are two ways to think about balance. The first looks at all of the component forces acting on a boat and the second is the method used in designing a boat. At any given time there is the real instantaneous balance that is the resolution of all forces acting on the boat. These include the solid-state forces of the sails and underbody plus those non-solid state forces such as induced by wave action and changes in windspeed and direction. If you resolve all of the forces of drive and side force with all of the forces of drag, heeling, leeway, steering moments, lift, and righting moment from the hull and keel, and asymmetry of rig and hull when heeled over, AND if the boat is going in a straight line these forces are in balance.

Of course there is such a wide range of variables to this dynamic balance that it is next to impossible to actually calculate the actual forces at work to sufficient detail to accurately predict the balance of any particular boat at any specific instant.

In sailing a boat we are constantly altering the balance of the boat as we trim or ease the sails, or power the sails up or down. We alter balance by controlling heel, altering fore and aft trim (bow down more weather helm- stern down less). In a general sense beamier and more flat bottomed boats do tend to get more assymetrical as they heel than narrower or rounder bottomed boats. To a great extent modern hull modeling has greatly reduced this heel induced imbalance over earlier designs for beamier boats.


When a boat is actually being designed, balance is calculated on a static basis and adjusted empirically. In other words the center of lateral plane is calculated from the static profile of the hull and keel. The center of effort is calculated from the static profile of the sail plan without adjustments for roach or genoas or spinnakers and the two centers are then checked for alignment. On a design that is actually in balance, the center of effort viewed statically is generally forward of the center of lateral resistance. This distance is called the ''Lead'' (pronounced like to ''lead'' a horse rather than lead like the metal). The amount of lead that a designer gives a boat comes from that designers experience in designing similar types of boats. The lead does not always work out right. In most cases the error results in weather helm, which is better than the other choice a lee helm. One classic case of the lead being worked out wrong is the J-24, which to this day typically has a lee helm.

Jeff

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Old 05-10-2003
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Factors influencing boat balance

Well, thanks to all for your replies.

As to Jeff''s answer, I understand that there are very many contributing factors to stability and their individual contributions are tough to calculate. Thanks for all the additional insight.

For those of you worried that I over analyze, you could be right, but that''s what makes me, "me." It doesn''t detract from my enjoyment; it enhances it. [Of course, I don''t apply that to "everything" ;-) ]

Talking and reading about sailing are the next best things to doing it, IMHO. If we were sitting around having a few beers (or whatever), we could have discussions on all sorts of subjects and the dialogue would flow nicely. Since we''re limited to this exchange, it''s tough to do.

Fair winds, all.

Duane
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Old 05-14-2003
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Factors influencing boat balance

No, Jeff. Not a loony toon. Just a guy who oes not have to take 400 words to prove I am so smart. And I can do it with humor!!!

Look closley at the shape of late 70s early 80s offshore designs and you will see treir shape does not change so much as they heel. They actually remind me of the shape of a minnow that is bulbous in the middle. That look is by design, not accident.

Now Jeff I am sure you can expand upon my observation ad nauseum.
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