Join Date: Feb 2000
Location: Annapolis, Md
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MacGregor 26 vs. ?
There are three ways that boats develop stability; form (also called initial stability), ballast, and dynamic. Of the three, dynamic has the least relevance to displacement boats as they rarely have enough speed to create a useful amount of dynamic stability but with the large outboards on the Mac 26sx they depend heavily on Dynamic stability when at speed.
As the name implies form stability derives from the shape of the hull of the boat. A shallower and wider hull generates more stability than a deeper narrower hull. Visualize a piece of wood lying in the water. On the flat it has a lot of form stability. On edge it has next to none. This is because stability in any boat comes from the distance (lever arm)between the center of gravity (the balance point for all of the weight in the boat including those things that are part of the boat and those things that can be moved around) and the center of bouyancy (the single point that is the center of all of the volumes under water). In a wider, shallower boat the center of bouyancy moves more quickly toward the low side for small increases in heel angle and so the lever arm grows quickly.
Form stability has several problems. First of all at large angles of heel, approaching 90 degrees form stability drops off dramatically. This is the point where it can be needed most. Second form stability tends to give the boat a quicker motion. This is less comfortable for the crew in rough conditions and can be more tiring. Lastly, in the extreme conditions of a blue water passage, a boat that depends on large amounts of form stability also tends to be more stable in an inverted position.
Another performance related disadvantage to form stability is that boats with lots of form stability tend to also have a lot of wetted surface which quickly translates to lots of drag. This means that it takes more force to drive the boat through the water and makes it harder to beat to weather.
Ballast stability has mostly positives associated with it. All other things being equal, the deeper and heavier the ballast the more stability a boat will have. Ballast that is heavier and deeper also gives the boat a slower, more comfortable motion. Of course like most things in yacht design, there are some tradeoffs in this area as well. If the ballast occurs at the end of a deep keel, the boat cannot get into as shallow water. As you start to shorten the keel length you substantially give up performance for the same stability because you either end up with a low aspect ratio foil or you end up with a big bulb or wings also increasing drag. No matter what the ads say, nothing performs like a deep fin keel. Of course deep fins have their own compromises but that is not the point of this.
Then there is the issue of low density ballast. By definition, the higher the density of the ballast the smaller a volume is needed to get a certain weight. This means that for a given keel volume a higher density ballast will have a lower center of gravity, or for a given stability the keel can have a smaller volume. In either case the boat with the higher density ballast will have the better performance. That is why performance oriented and serious cruising boats usually have cast lead ballast. Other building techniques (cast iron, or encasolated metal ingots or scrap metal have far less stability and performance)
Then there is this whole water ballast business. There are two types of water ballast; movable and what I will call fixed position water ballast. Moveable water ballast is the type of water ballast used by the Volvo round the world ocean racers and consists of tanks located on either side of the boat so that water is shifted from side to side every time the boat is tacked or jibed. This form of water ballast is typically used in conjunction with a fully ballasted boat. I personally would love to see this is the type of ballasting system improved and incorporated in production boats.
The second type has a tank (or tanks) in the bilge and uses water in conjunction with some small amount of higher density ballast. The issue with this type of water ballast is the same as with all forms of low-density ballast. If you compare water to lead, water is approximately one tenth the density of lead. That means you need ten times more volume of water to equal the weight of lead. This means that you will end up with some combination of either:
- The water being higher in the boat resulting in a higher center of gravity and less stability than the lead,
- More water ballast to overcome the higher center of gravity meaning a heavier boat (Remember weight, in and of itself, does nothing positive for a boat and does have a lot of negatives.),
- Appendages that are shaped to hold water rather than to be efficient as sailing foils,
- More dependence on form stability which means a less comfortable motion and a poorer ultimate stability,
- Less interior storage or no sump for bilge water to sit in,
The bottom line a well-designed fixed position water ballasted boat will always be an inferior sailer when compared to a properly designed fin keelboat. As in all things in sailing there are trade-offs. In my book, even if water ballast reduces towing weight (which is questionable since the retractable bulb keel boats do not have to weigh that much more than a dry water ballast boat), I really think its too much of a compromise in performance and safety for my taste. There are people who are perfectly comfortable with water ballast, but having been aboard a wide variety of boats from 20 to 41 feet that have been knocked down to an angle close to 90 degrees,I see water ballast as too much of a risk for my taste.