center of effort of a gaff sail

I am looking for general rule of thumb info on finding the center of effort of a gaff sail. Principles of Yacht design (Larsson & Eliasson) states that for a sloop rig the geometric center is used. Would it be safe to assume this same rule for a gaff sail???

 

The centre of geometric area will be as accurate for a gaff sail as it is for a triangular sail.

 

and a gaff sail has four sides.

 

The center is obtained by dividing the sail into two triangles, finding the centers of those and then plotting the ratio of one to the other along a line that runs from the centers.
Jim Michalak has an article in his newsletter that explains it. Here is the address:
http://homepages.apci.net/~michalak/1aug06.htm

 

Stick a pin into a scale cutout of the sail and pin it to a wall. Give it a spin. If it stops at the same point each time you missed, try again, it is pretty easy to nail down the center. When it can spin without stopping in the same location, that is the center. After the math was performed during NASA's early years, this routine was performed on each piece needing a center. If the math did not match the pin hole, the pin hole was what was used as the true center. The cool thing about gravity is it always pulls in the same direction and the heaviest side will always be down, unless it is balanced.

 

The question is not how to find the geometric center of a gaff sail.

The question is if the CE of a gaff sail is at the geometric center.

 

Rule of Thumbs

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Wrong! To utilize the Rule of Thumbs such as published in Larsson and Eliasson it most certainly is important to find the geo center and use it as specified .
These rules have stood the test of time and work well in most design applications.
Should you wish to study aerodynamics and wade up to your neck in conflicting theories then abandon the Rule of Thumb. If you want to design a well balanced boat in a reasonable time don't.

 

Here! Here!

 

????
The first post quite clearly asked if the geometric centre of a gaff sail could be used for "rule of thumb" centre of effort calculations. EAP did not ask how to find the geometric centre, only if it was as valid for the rule of thumb as the geometric centre of a sloop.

The answer is yes, of course it can. The geometric centre of a gaff rig is every bit as valid as the geometric centre of a sloop rig.

What is "wrong"?

No one said that the rules of thumb don't work, no one said that the Centre of Effort was not at the geometric centre. Why try to confuse the issue?

I didn't say that it wasn't important to find the geometric centre. Just what in my post was wrong?

Sheesh ... lighten up and read.

 

Sorry. Didn't mean to get personal.

Just supporting the idea that a sail' Center of Effort is ruinously difficult to pin down.

That is because there is such a calydiscope of influences ranging from the sail's foil shape, to whether or not choppy out that day, to wind gradients, to other sails on the boat, all the way to the hull's Center of Latteral Resistence, which has it's own laundry list of factors.

When I was designing my 'final project boat' at the Landing School, I asked the co-instructor how much lead I should give it. He seriously started avoiding me after that. I was quite insistent on getting it absolutely right and wanted concrete, concise, rules and/or formulas.

The fact that he could be in this game for over forty years and still not have them, I was not then willing to accept. He had to be, I reasoned, holding out on me.

Now. After a few years of hanging out at this sight, I can see how much of a guessing game boat design can become.

It appears that a lot of formulas, rules of thumb, and asumptions are used because they usually work, But you can take that warrantee and whipe the bottom of your shoe off with it.

Bob

 

Rules of Thumb

The problem with rules of thumb is that it can take experience to apply them well. How do you get experience? By making mistakes....
Of course, you can also make an accurate study of some existing boats and sail them to back up your understanding of how to apply the "rule of thumb".

 

The 'theories' relating the positions of CE and CLR (lead) are beautifully clear and concise and nicely fill a chapter in any yacht design text book. Everyone can get their heads round it and its blinding logic is both powerful and all persuasive.

However there are very real problems. Not only are the values of 'lead' wide ranging and controversial, but there is also a large body of real world experience and experimental data that, at best, doesn't comply with the theory and may even contradict it.

Case 1: Having raced a small keel boat one-design (National Sonata) for many years, I was always amazed at how well they sailed under mainsail alone. No one every bothers to put up their genoas until after the 5 minute gun, because they are so handy and manoeuvrable under mainsail alone, irrespective of the wind strength. There was also no change to the balance when the genoa was hoisted. They would even sail well to windward under genoas alone and without any sign of lee helm despite a change from 'lead' to 'lag' in the relationship of CE and CLR. I have noticed the same thing on J92s and other boats.

Case 2: There are some boats that have terrible weather helm and nothing you can do will cure it. I have seen people add bowsprits, add skegs, recut sails, move mast position, etc, all to no avail.

Case 3: I built a large model yacht with internal ballast but with a high aspect vertical keel that could be slid backwards and forwards over about 25 percent of the LWL. It sailed perfectly in all positions with no detectable change in helm input.

Case 4: In the late seventies I crewed an Admiral's Cup 2 tonner (44ft) that was a complete pig. It had terrible weather helm and would broach at the sight of a gust. No one ever tried to go through our weather as it was just too dangerous. Eventually the keel was brought back 6 inches which transformed its handling, but way out of proportion to the change in lead.

I think it's no wonder that the Instructor at the Landings School was so diffident about answering your question. I am becoming more inclined to believe that a boat's balance has as much to do with 'lead' as it does the colour of its hull.

That said, there is obviously a very complex relationship that includes the drive from the sail, the forces generated by the foils, but most importantly I believe, the dynamics of the hull. In lieu of the real answer we cling to the comfort of the accepted lead theories, because if we produce a real dog, we can always say "well it's not my fault - I gave it 5 percent of lead".

 

Rules of thumb for lead work because the hulls they were based upon have the same basic shape and stability charistics when heeled.

The true answer is that as long as the moment couple between driving force and laterial force is less than the yaw restoring moment of the hull at the equlibrium (heel and lee angle) attitude, the hull will track true. Now, getting the couple correct and calculating the yaw stability indices can be problimatic, especially in a real seaway.

You can design a hull that will track like a train and be totally insensitive to lead, but most likely it will be slower than a similiar hull thaty has marginal yaw stability. Conversely, you couild have picked the lead perfectly, but if the hull has negative yaw stability, it will be all over the place and go very slow also.

Boat design is very conserative and incrementally evolutionary because it is very difficult and expensive to accurately determine the optimum lead and yaw stability. Copying hull/rig combination that work is the tride and true way to go.

 

CE is estimated to be the geometric centre of the sail plan when it is perfectly parallel to the centreline of the boat.

CR is estimated to be the geometric centre of the hull plan below the water line.

Lead is somewhat of a CORRECTION FACTOR between these 2 estimates and the real handling characteristics of a boat in typical sail trim and conditions.

We are discussing how much CORRECTION FACTOR to apply . . .

 

Gosh I hope not!

That subject has been beaten to death more than once here.

All EAP wanted to know is if the geometric centre od a gaff sail could be used the same way it is for a sloop ...