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Fair winds!

Dave and Anke
triloboats swirly gmail daughter com

Wednesday, January 11, 2012

Where Ultra-Shoal, Square Boats Get Their Stability

Rick Bedard knock-down testing his Michalak JEWELBOX JR

A ballast keel is necessary for a boat's stability. TRUE or FALSE?

If you answered TRUE, you lose the tickets for two to the Bahamas (as if there were any to lose). But it's understandable. The term ballast keel is itself misleading, making it a bit of a trick question.

In fact, a keel's primary function is lateral resistance (LR); resistance to lateral (sideways) motion while allowing it to slide forward with relative ease. On the wind, it helps counter leeway (side slippage) and thereby enhance progress to windward. 

Plenty of other gizmos can do the same thing. All LR devices help, when sailing into the wind; all of them hinder (drag) when sailing off the wind. Therefore, ones which may be raised (pulled clear of the water) have the advantage off the wind. They may or may not be as effective as fixed keels on the wind (other factors involved).

But these gizmos aren't likely to be as packed with ballast as a keel. We've got to raise them, after all, and raising ballast is a lot of work.

So, in shoal boats - and I mean ultra-shoal boats - ballast is usually fixed low, inside the hull or as plate outside.

Ballast Stability

Remember Weebles? Here's a sea-going specimen...
Weebles wobble but they don't fall down!

They tell you at a glance everything you need to know about ballast stability in shoal boats. 

No keel necessary. When afloat, a Weeble's or boat's center of buoyancy (CB) - the midpoint of all the 'floaty forces' - acts as a fulcrum. Ballast fixed well below the CB levers it back to upright.

Boats with ballast keels providing a longer lever require proportionally less ballast for the same righting moment.

Shoal boats, with a short lever, require proportionally more ballast for the same righting moment.

There's more to it, but that's the gist. Either way, you can achieve the same ballast stability whether the boat is deep or shoal. 

Form Stability

Let's compare a floating wine bottle to a floating milk carton.

The wine bottle's form lets it roll easily. Without ballast, it's just as happy on its side, upside down or any point whatsoever. It and round bilged boats have low form stability.

The milk carton, even without ballast, resists rolling, up to a point-of-no-return whereupon it settles happily onto a new face. It too, is just as happy on its side or upside down, but doesn't care for points in between. It and square boats have high form stability.

Another contributor to form stability is length. The longer a boat, all things being equal, the higher its form stability. Picture a catamaran. Longer amas (hulls) have more buoyancy than shorter ones, making the whole harder to rock. Same thing happens with a monohull... lengthening it is adding more buoyancy along the chines with every added foot.

A boat with low form stability needs proportionally more ballast to achieve the same overall stability. They knock-down easily, but they also recover easily.

A boat with high form stability needs proportionally less ballast to achieve the same overall stability. They resist knock-down, but they also resist righting.

Reserve Buoyancy

Reserve buoyancy is form stability that is above the upright waterline. When the boat heels, plunges, is overtaken by a wave, knocks down or capsizes, some portion of this reserve is immersed and comes into play.

Aspect ratio is that of height to width. We're interested in the sectional aspect ratio, that seen when looking from one end of a boat or the other.

A plank has low aspect ratio and not much reserve buoyancy. A plank - especially a ballasted one - is very hard to flip over, but once over likes to stay that way. It takes almost as much force to right it as it did to flip it.

A beam has high aspect ratio and lots of reserve buoyancy. A ballasted beam is still difficult to flip... even harder than the plank. But it floats its ballast much higher, once over. It doesn't take much to get that ballast to the tipping point, whereupon it 'avalanches' down, levering the beam back to its feet.

Likewise, hulls with high aspect ratios - that is, tall for their beam - have lots of reserve buoyancy, which resists heeling, knock-down and capsize and is relatively unstable when inverted. Unflooded deck structures, such as trunk cabins, further destabilize a boat when upside-down.

Side benefits include increased headroom, interior volume and wall surface area. Downsides include increased windage and higher center of gravity.


Shoal, square boats get their initial stability from moderate ballast and high form stability.

Reserve buoyancy from high sectional aspect ratios (high sided hulls relative to their beam) resist heeling, knock-down and capsize, and, in the latter case, contribute to the instability of the upside-down hull.

Knock-down testing our unballasted T16x4... Anke has to lean out to keep it from righting.

1 comment:

  1. For a deeper dive, here's a link to a US Coast Guard treatise on stability, specifically oriented for fishing vessels, but with lots of good info for sailors.

    Dave Z