Recently there has been much debate about Water Ballast versus Traditional Keel (Lead) Ballast on my favorite Web Bulletin Board (www.trailorsailor.com). There seems to be two camps on the issue; those who feel Water Ballast cannot take the place of traditional lead keels, and those who feel Water Ballast is a good alternative for trailer sailors. Of course, I'm somewhat in the middle on this issue, as I feel that Water Ballast is a good alternative for trailer sailors, but what you get is a boat with different sailing characteristics.
Much discussion has clouded the 'science' of the matter, so I thought that a take-off on my "Ballast and Heeling" page to analyze the mechanics of Water Ballast might clear things up for anyone confused by the debate. Water Ballast, like any aspect of a sailboat, is a compromise as you can see be the analysis below.
If
my Catalina 22 were water ballast (rather than
swing keel), how would it perform??? I can use the diagrams from my Ballast & Heeling analysis and modify them to represent a boat with water ballast. Assuming that if the water ballast can deliver the same 'righting torque', then the boat should have similar heeling characteristics to the swing keel. To the left is a diagram of my boat as it might look, if it were water ballasted. I can figure out how much water ballast would be needed to give the same righting torque as the swing keel. |
If I take 45 degrees of heel,
and figure the righting torque. In my original figures
from the "Ballast & Heeling" page. . . The math (swing keel): for CG1: (1.75 ft x 1700 lbs) = 2975 ft-lbs for CG2: (4.5 ft x 550 lbs) = 2475 ft-lbs Total torque = (2475 + 2975) = 5450 ft-lbs Force on the Mast: (5450 ft/lbs / 13.5 ft) = 404 lbs (wow!) In order to maintain the same righting torque, the water ballast would have to generate 2475 ft-lbs of torque. for CG2(WB): = 2475 ft-lbs / 3 ft = 825 lbs. Therefore the water ballast should be 825 lbs., whereas the traditional ballast was only 550 lbs. |
At
the ultimate stage of heeling, my previous
figures were . . . More Math (swing keel): for CG1: ( 0 ft x (1700 - 80) lbs) = 0 ft-lbs (!!!) for CG2: (4 ft X 550 lbs) = 2200 ft-lbs (Total Torque) Similarly - I can figure how much water ballast would be needed to generate 2200 ft-lbs at 90 degrees. for CG2(WB):=2200 ft-lbs/2 ft=1100 lbs. Therefore, about 1,000 lbs. of ballast would be needed (average of 825 + 1100) to give similar righting characteristics to my Catalina 22, if it were a water ballast boat. |
Given that a water ballast Catalina 22 is only hypothetical, it may be better to compare an actual sailboat that has product lineage in both traditional and water ballast designs. The next size boat up in the Catalina line, the Catalina 25, has a new version (the Catalina 250) offered in water ballast. The dimensions for these boats are substantially the same that I used in the previous analyses; I can just apply the relevant weight and ballast numbers.
Comparing
the C25 and C250 If I take the figures from the 45 degree analysis and apply them using the weight and ballast of the C25 and C250 respectively, then I can determine the difference in the righting torque for these two boats. The C25 (swing keel) has a total weight of about 4,150 lbs - 1,500 lbs of which is the keel. The boat alone weighs 2,650 lbs. The C25: for CG1: (1.75 ft x 2650 lbs) = 4638 ft-lbs for CG2: (4.5 ft x 1500 lbs) = 6750 ft-lbs 11,388 ft-lbs Total for the C25 (wow!). The C250 (Water Ballast) has a total (unfilled) weight of 2,400 lbs and holds 1,200 lbs of water ballast. The total sailing weight is 3,600 lbs. The C250: for CG1: (1.75 ft x 2400 lbs) = 4200 ft-lbs for CG2: (2.0 ft x 1200 lbs) = 2400 ft-lbs 6,600 ft-lbs Total for the C250. Obviously, these two boats have very different righting torques (11,388 vs. 6,600), and could be assumed to have different sailing characteristics. If the boats were to have about the same righting torque from the ballast (i.e. increasing that of the C250), I can figure out how much water ballast would be needed in the C250. for CG2 (C250): = 6750 ft-lbs / 2ft = 3,375 lbs. Water Ballast |
The analysis indicates that the C250 would need 3,375 lbs. of water ballast in order to give similar righting characteristics to its predecessor, the C25. However, the C250 only has 1,200 lbs of ballast. This would indicate that the C250 would be more 'tender' than the C25. (Can you imagine if the C250 had 3,375 lbs of ballast - the total displacement would be about 5,775 lbs !!!)
Summary
From the manufacturer's standpoint, and considering the other factors of sailing performance, it is not practical to put 3,375 lbs of water ballast in the C250. This is typically the design constraint that causes Water Ballast boats to take on different sailing characteristics than their traditionally ballasted counter-parts.
The manufacturers of water ballast boats are hard pressed from a design standpoint to put enough water ballast in to equate to their traditional ballasted counter parts. This is why water ballast sailboats typically take on more 'tender' sailing characteristics than the traditional ballasted boats. Keep in mind that the manufacturer can modify other aspects of the boat (like hull shape) to make up for some of the difference in ballast.