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Slogging to Windward
by Chuck Merrell
sloggingtowindward@hotmail.com 

March 2000
BALLAST: BORING OR BEAUTIFUL?

When I decided to write this column, it didn’t seem like it was going to turn into a monthly article about boat design. The concept was that I’d pick a subject and try to inject a bit of humor, controversy and sacred ox goring into the mix-hopefully strong on entertainment, less so on instruction and information.

I haven’t lost sight of that goal. Those columns will come as time goes on, but based on what I see in the way of misunderstanding* and misinformation in various boat interest venues makes it seem like a good thing to take a shot at clearing up a couple of these muddled areas.

*For example, I recently saw a posting on one of the news groups by some guy whose email address was near enough: cliffthesquirrel@yahoo.com. It seems that he wanted to instruct fellow list members that by adding more wood on the bottom of the boat, the vessel will float higher (because wood floats, ya know!), and make the boat stiffer because more wood equals more weight. Nice one Cliff, but weight is weight and a pound of feathers weighs the same as a pound of lead, the only difference being the volume.

On occasion here in the boat yard, Wombat folk like Cliff show up with their preconceived and immoveable ideas. However, at the end of the day, with that quality of thinking, level of understanding and resistance to education, it’s no wonder their projects wind up looking like the leftovers of Hannibal the Cannibal’s breakfast.

 

It seems that most folks who are interested in learning and understanding (rather than making their own answers from the whole cloth of imagination) often get a little snowed when trying to understand hull stiffness and stability and how the added influence of ballast plays into the picture. It’s no wonder, because most texts on the subject start out with such sentences as: The static stability of a vessel is the moment of the couple formed by the weight and buoyancy.

Wow!

 

All I wanted to know was: What keeps a sailboat or sailing ship from getting knocked over in the water when the wind blows from the side and hits the sails, causing it to heel?

The simple non-technical answer is: It’s the shape and width and weight of the hull as it’s designed that resists initial heeling. When the boat does heel over (as the wind blows harder or waves try to capsize the vessel), the resistance of the hull to the external force gets stronger as the hull tilts to leeward-up to a point.

 

Ok, is there any way to make the hull even more resistant to being blown over, or getting turned over by wave action? Also how can a hull be designed to come back from a knock down or even a roll over?

You can do one of two things, either you can increase the beam of the hull so that it has a wider base which will make it more difficult to turn over, or if you don’t want to do that you can lower the center of gravity of the hull, which will make it even more resistant to heeling or capsizing.

 

What’s the center of gravity?

The CENTER OF GRAVITY for any three-dimensional body or system is that point where the body's weight, or mass, may be considered to be located. The center of gravity of a uniform sphere (baseball), for example, is the center of the sphere. The center of gravity of an irregular object, however, sometimes even may be located outside the object itself. When an object is in free motion, the center of gravity describes a smooth curve around which the rest of the object, or system, may rotate in a complex manner.

If the irregularly shaped object is a boat hull, the Center of Gravity is calculated using a mathematical formula that compares the weight of the material making up the hull above and below the waterline. Usually the CG locates at a point slightly below the designed waterline in the case of ballasted hulls, and about the same level as the LWL in high-sided powerboats and un-ballasted hulls.

 

Fine, so how do I lower the center of gravity and in so doing make the boat more resistant to heeling forces?

OK. The answer is either to lighten the topsides to reduce weight above the LWL, or add weight (ballast) inside the boat below the waterline; or both-BUT-past a certain point, adding more weight does no good and can do a lot of harm. You can only add the amount of weight/ballast that will lower the boat in the water to the optimum point you designed into the hull. More ballast than is called for, usually screws up the whole system to a larger degree but adding weight to the topsides and above is worse. First time builders and guys who believe that heavier is better overbuild in the name of safety, which tends to make the boat unsafe. I can tell you stories about overbuilt boats which have resulted in useless boats and losses in the hundreds of thousands of dollars for the "I think therefore it is guys".

Keep in mind that the displacement weight of the boat in the water should not total more than a combination of the weight of the structure plus the weight of the ballast, plus the weight of the crew and cargo as projected and planned in the design phase. The drawing below uses TESTBENCH, the hull I drew to illustrate the January Column and shows that hull with and without ballast. All other weights, structure and load are assumed to be identical and correct.

tblg.gif (23820 bytes)

You’ll notice that the physical effect on both figures 1 and 2 is about the same. The amount of heel for both is nine degrees. The weight pressing down on the rail in order to either achieve or neutralize the nine-degree heel is about the same (within twenty pounds or so). The righting moment (in foot pounds) generated by the heeling action is close enough to compare. The main difference of the two is that the righting arm generated by the ballasted hull is almost twice as strong as the arm for the un-ballasted hull. In other words, the ballasted hull is stiffer with a stronger arm (by almost double) than the hull that depends solely on form stability (width, overall weight and hull form) to stay on its feet and not fall over in the water when under attack by the forces of wind and wave.

BUT, and here’s the BUT, the bad news is, that’s as much stiffness as you’ll get from adding ballast inside the hull.

The good news is that for this hull, ballast in comparison with the overall weight (displacement) or defined as the Ballast to Displacement ratio (B/D) works out to be about 37%, which is nearly exactly in the desired profile of a cruising hull. Even better is the fact that if you equip the boat with an average (for cruising boats) sail plan, in this case a working rig of 165 square feet gives a SA/D of 16.1 The boat will perform very well in average conditions. Average Conditions defined as operating in winds of up to about 15-18 knots under basic sail plan. Below, add more sail. Above, reduce sail. In other words, in operation the boat will be in balance in normal conditions and when properly managed in a range of conditions too. Cool, huh?

 

Great, but what do I need to do if I want to push the boat harder than the above profile will allow? How can I make the boat stand up and operate safely with much greater quantities of sail? Or, come to that, how can I build in a reserve of stiffness for better safety even if it’s at the expense of comfort?

No problem. Boat designers have learned that if you mount the ballast outside the hull, the boat becomes stiffer because you have (using the weight of the inside ballast, but now on the outside) created a lever that increases the stiffness and sail carrying ability. In other words, by lowering the ballast and putting some or all of it outside the hull envelope, or even concentrating it in the form of a bulb on a long appendage, you lower the center of gravity somewhat, depending on how far you place the weight from the hull, but most important you do greatly increase stiffness (torque) by moving the ballast away from the hull.

By the way, levers are used a lot to transform a small turning effort into a large twisting force. A winch handle is a good example; so is a pipe wrench-gee, so is a long fin and bulb keel.

What a great solution. You don’t compromise the intended design of the hull by trying to add more ballast inside to get stiffness, and you can extend the fin with bulb fairly long distances and have micrometer control over the stiffness desired and the forces expected. As a practical matter, that’s exactly the way America’s Cup match racers are designed. They are balanced to allow Sail Area to Displacement Ratios of around 25 and are allowed to race in a range of wind speeds up to about twenty-five knots maximum. Even more astounding are the Vendee’ Globe Round the World "Open 60" Racing Boats as designed by Jean-Marie Finot and others. The compromise, of course, is you lose the shoal draft ability, but in my opinion that may not be terribly undesirable. In fact, shoal draft is quite overrated except in areas where you have no other design choices, and I’ll amplify on that thought in another column sometime. I have some fairly well justified reservations about flat bottomed, keel-less, high-sided, water-ballasted, sailboats, most of which is based on lots of practical hands on experience. So maybe I’m just the "Cookie" of choice to write about the pros and cons of such craft.

Take a look here at the following Table of Particulars of "Group 4", Britisher Mike Golding’s Finot boat. (Mike just finished the VG today, February 28, 2001 in seventh place with the time of 110 days 16 hours-16 days short of the winner, but still incredibly fast and a time which would have won top honors in past VG races).

Characteristics of “Group 4” a Groupe’ Finot Open 60 Round the World Racer

Length 59.97 Ft.
Beam 18.37
Draft 14.76 Ft.
Displacement 19847 Lbs.
Mast (above Deck) 83.66 Ft.
Working Sail Area 3014 Sq. Ft.
Main Sail Area 1798 Sq. Ft.
Genoa Area 1104 Sq. Ft.
Spinnaker 3229 to 3767 Sq. Ft.
Hull Material Carbon/Nomex 120
Ballast (Est.) 10,000 Lbs.
Ballast/Displacement Ratio 50%
Displacement/Length Ratio .53!!
Sail Area/Displacement Ratio 65.78!!

 

At 65.78, the SA/D is four times larger than what a designer would put on an average cruising boat, and that’s the reason for that fifteen-foot draft fin keel which carries a 10,000-pound blob of lead on the end. I’m not going to comment on the design other than to point out the efficacy of the righting arm principle. With that much sail area, the forces on the boat, rig, and spars are enormous and a corresponding righting moment device (in this case is a high B/D and a long righting arm) is essential. Moreover, Finot has an additional way of getting more out of this deep fin and bulb per this diagram.

swingfroup4.gif (17482 bytes)

What you’re looking at is a mechanical, moveable keel assembly powered by two powerful hydraulic rams which can be activated to easily move that 10,000 pound bulb keel in line with the weather rail (above the bulb) which further enhances not only the righting arm effect, but acts much like adding weight directly to the weather rail just like when on a beat, the whole racing crew moves to the weather rail, or the helmsman hikes out on a trapeze in dinghy racing.

For those of you classic design purists that hate high-tech stuff like this, I might point out that you should know the original idea for the controllable swing keel originated, or at least was formalized by the Herreshoff family nearly a hundred years ago.

So, in the words of Ricky Ricardo, "I jus ‘splained all about ballast and how it works to you, Loosie!" And did it hopefully without confusing the issue with mathematics, formulas and obscure wording.

In the meantime, you might want go study the large, comprehensive and well-done Finot Web Site. There’s a lot of interest to look at there especially for the serious student of yacht design. The site will provide you with plenty to think about including a very interesting houseboat that Finot designed for a couple in Japan (Great Pictures). Their URL is: http://www.finot.com/general/accueil_ang.htm also, if you get the chance read the recent beautiful and instructive article about Finot in Professional Boatbuilder Magazine. Shipwreck survivor and my former instructor Steve Callahan wrote it.

I don’t think that was a boring column, do you?

 

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