The theoretical maximum speed that a displacement hull can move efficiently through the water is determined by it's waterline length and displacement. It may be unable to reach this speed if the boat is underpowered or heavily loaded, though it may exceed this speed given enough power. Read more.
Classic hull speed formula:
Hull Speed = 1.34 x √LWLA more accurate formula devised by Dave Gerr in The Propeller Handbook replaces the Speed/Length ratio constant of 1.34 with a calculation based on the Displacement/Length ratio.
Max Speed/Length ratio = 8.26 ÷ Displacement/Length ratio.311
Hull Speed = Max Speed/Length ratio x √LWL
A measure of the power of the sails relative to the weight of the boat. The higher the number, the higher the performance, but the harder the boat will be to handle. This ratio is a "non-dimensional" value that facilitates comparisons between boats of different types and sizes. Read more.
SA/D = SA ÷ (D ÷ 64)2/3
A measure of the stability of a boat's hull that suggests how well a monohull will stand up to its sails. The ballast displacement ratio indicates how much of the weight of a boat is placed for maximum stability against capsizing and is an indicator of stiffness and resistance to capsize.
Ballast / Displacement * 100
A measure of the weight of the boat relative to it's length at the waterline. The higher a boat’s D/L ratio, the more easily it will carry a load and the more comfortable its motion will be. The lower a boat's ratio is, the less power it takes to drive the boat to its nominal hull speed or beyond. Read more.
D/L = (D ÷ 2240) ÷ (0.01 x LWL)³
This ratio assess how quickly and abruptly a boat’s hull reacts to waves in a significant seaway, these being the elements of a boat’s motion most likely to cause seasickness. Read more.
Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam1.33)
This formula attempts to indicate whether a given boat might be too wide and light to readily right itself after being overturned in extreme conditions. Read more.
CSV = Beam ÷ ³√(D / 64)
If I had it to do over again I’d change a few things, hence FRANCES II (Frances the second). The most significant improvement would be to replace the undulating profile shoal draft keel with a much more modern, deeper and therefore more effective one. With a slightly deeper keel and the “full flow aperture” I developed on the later of my offshore oriented custom designs, FRANCES II will stand up to a whole lot more sail and point closer to the wind. The flush deck which offered sitting headroom only will give way to a nice looking but smallish deckhouse with full headroom for a person up to 6 feet tall.
I adored my little FRANCES. She was beautiful, well mannered, stable, and just plain fun. Narrow of beam, her sleek lines cut through resistance like a dreadnaught. Point FRANCES toward a destination and she was unstoppable. Her timeless virtues seem to have disappeared from the modern world. They just don’t build ’em like FRANCES anymore.
But you could, and if you do it should be the slightly longer, wider, heavier, deeper, and more powerfully canvassed version I perfected in my retirement, FRANCES II.
Besides being slightly larger in all dimensions, the big change is the deeper, shorter and much more effective keel. The rudder is fully balanced to reduce helm forces. With this keel the new FRANCES is a much better performer.
FRANCES II embodies all the improvements I could think up in 39 years. She carries a taller rig with a masthead genoa. Her house provides full headroom (if you’re not too tall) and has a neat extension aft of the mast for storing deck gear. The keel is a little deeper, much shorter fore and aft, and has a narrower trailing edge than the original one making it far more efficient. The rudder is deep and partially balanced, so helm forces are minimal. The ballast ends up deeper than the original version, making FRANCES II one of the stiffest designs for her size my studio ever designed!
The arrangement has full headroom where you need it (6 feet on centerline).
The original FRANCES is an imperfect, classic design. She’s comparable in many ways to an MGTD or a J3 Cub. They spewed oil and weren’t very fast by today’s standards, but they were so cute and stylish that owners have enjoyed every minute of their use for decades. With nearly 40 years to think about an upgrade, FRANCES II approaches perfection!
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