Some thoughts about Twin Keels (and an implied cry for help)

Some thoughts about Twin Keels (and an implied cry for help)

AvatarJohn R. Coil


I’ve been reading about twin keels — my interest stemming from their reputed benefits for moderating rocking motion given a family full of non-sailors (so many relations, so little railing… :whistle:).

Recently I actually applied some of the design rations in Sponberg’s The Design Ratios to some of the hull shapes I’d been toying with. One in particular, Motion Comfort Ratio, grabbed my attention because of how it dealt with a boat’s beam and what was said about why.

For anyone unfamiliar with MCR, this was devised by Ted Brewer and included in what he considered a tongue-in-cheek article in September 1990 Cruising World. Even so that Mr.Brewer had actually applied himself in coming up with MCR was attested to by Sponberg and it apparently “does provide a reasonable comparison”.

MCR = Displacement / (0.65 * (0.7LWL + 0.3LOA)*BEAM^1.333)

… where Displacement is in pounds, “0.65” is a generic waterplane coefficient, and the rest are self explanatory … or are they?

“BEAM” and not “BWL” makes me wonder if the equation doesn’t actually call for overall beam that just the beam at waterline because of how Brewer describes it in his book, whichj Sponberg quotes:

Beam enters into it also as wider beam will generate a faster reaction, particularly in beam seas. In effect, the comfort ratio measures the displacement of the vessel against its waterline area, adds a factor for beam, and thus is a means to compare motion comfort for boats of various sizes and types.

At one level I suppose it really doesn’t matter which is actually used because this got me thinking about why having a lot of extra width at the deck level (never mind situations like this: ) could “generate a faster reaction, particularly in beam seas” and from there I passed on to thinking about the reverse being true for twin keels.