The effects of water-ballast on sailboat stability

There is much talk about the usefulness of water ballast in sailboat.  Some argue that the ballast is only useful when raised above the waterline, others that it always increases stability.  There are obvious advantages to water ballast:  It can be filled and emptied at will in order to optimize trim, stability, reduce trailering weight and increase comfort at sea.  There are clear drawbacks as well:  Water not being very heavy, takes up precious room on board, over 11 times more than lead.  The tanks increase construction and maintenance costs.  Then there is the issue of environmental responsibility: Water ballasts have the potential to spread invasive species like the infamous zebra mussel.

Are water tanks worth it or not?  The answer of course is that it depends on your needs:  Different systems have different utility and drawbacks. Let’s dig into it to clarify the situation.

Below are 4 sailboats with identical canoe bodies (click image to enlarge):

Bedard-Yacht-Design-Water-Ballast-study

Boat A is a keel boat with water ballast in the keel.  Her ballast-tank is full and accordingly, she displaces 13694lbs.

Boat B is similar: She has a keel with water-ballast tank but it is empty.  As a result, she floats higher by about 4”.

Boat C is a centerboard boat.  She has the exact same canoe body as A & B but in order to float on the same DWL, she displaces only 9887lbs.

Boat D is the same as C but we’re going to include Port&Starboard water-ballast tanks, only the windward one is filled.  Accordingly, she displaces a bit more: 10546lbs.

Righting moments (lb-ft) at varying angles of heel
Angle of heel Boat A Boat B Boat C Boat D
10° 8998.6 4792.9 8997.5 10979.7
20° 16697.3 7974.1 16695.1 19035.4
30° 22087 9394.3 22083.7 24782.3
40° 25733.7 11540.9 25024.9 27709.7
50° 27817.3 14827.9 24987.5 27354.9
60° 29225.4 17792.4 23069.6 25047.6
70° 30550.2 19698 19925.1 21455.2
80° 30254 15903.1 15903.1 16945.9
90° 24613 11256.3 11256.3 11788.2

Looking at them individually in more details:

Boat A is the heaviest boat of the four.  Looking at her stability numbers we see that for small angles of heel, up to 40°, there is no advantage to the extra 3500lbs you’re pushing around in the water, no extra stability compared to C or D.  Not only that but in order to push that weight, you’ll need more power: i.e. sail area.  So A is going to cost more to build and maintain, not only because of the ballast tank system but also because of the taller rig. She will be slower in light air because of the added wetted surface of the keel, compared to a centerboard boat.  Now for the good sides: safety and comfort.  The stability numbers here again show a clear picture.  At 90° of heel, she has more than double the Righting Moment of any of the 3 other boats.  That means once the mast goes in the water, this is the boat you want to be in, plain and simple.  This added displacement has the bonus of giving you a lot of comfort too:  Most folks go sailing to relax and enjoy themselves, not to go fast (for which they have an automobile).  In this respect, Boat A will provide a solid, safe and stable platform with a comparatively smoother ride than the centerboard boats.  Achieving this while being able to tow her behind aforementioned automobile is a strong case for Boat A.

Boat B is a boat designed to have ballast in the keel but without it.  Because of that she is floating high on her lines and suffers poor stability.  Indeed, she has less initial stability than the centerboard boats and no more ultimate stability either.  And still paying the wetted surface penalty in light air.

Boat C has no keel at all.  We can see from the righting moments figures that she has good initial stability; as good as Boat A in fact.  That is until 40° though, right about the angle at which Boat A shows her keel above the waterline (at which point the density difference between water and air makes it really effective).  Under most circumstances though, she will sail under 40° of heel.  Even past that point, she will still right herself, but not as fast.  The key point here is that you save 3500lbs and some wetted surface.  She will be noticeably faster than either boat A or B.  She will tow easily, due to its light weight and even more so because she doesn’t sit on a keel.

Which brings us to Boat D.  Here we have a boat that is light, with the best initial stability of the 4 designs, better ultimate stability than C (capsize recovery) and all that for a mere 650lbs displacement penalty and no extra wetted surface from a keel to slow you down in light air.  Now we’re onto something.  Indeed with her righting moment at 10° of near 11000lb-ft, she will be stiff and fast.  Should you get into a blow, fill both ballast tanks to increase comfort and ultimate stability via an added 650lbs of water ballast!  A few drawbacks are increased construction & maintenance costs related to the ballast tanks as well as a slight list to windward at rest (if the tanks are not emptied).

In conclusion, different water ballast arrangements suit different needs and what’s best for one person might not be best for another.  It is most important to assess your own sailing aspirations and habits, as well as the sailing conditions in your area:  The best boat is the one that will fit those needs more closely.  That being said, I would tend to learn towards including water ballast in a design that needs to be light, either for towing or for endurance events such as the Everglades Challenge or the Texas 200.  I would probably build port&starboard tanks to increase safety and sailing ability options and try to put them as far away outboard and as low as practical, to optimize the weight of the ballast.

* Note that these sailboat designs are not intended to be realistic, they are provided for comparison purposes only, as well as simplified and optimized to emphasize the discussion.  For instance, modern sailboat’s vertical centers of gravity tend to be higher than depicted here and one cannot tow a 6-ton boat behind one’s car…

 

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