Hull Form Symmetry

The Pure Full Displacement hull is differentiated from other hulls based upon two form fundamentals: longitudinal symmetry and displacement to length ratio (D/L). ┬áLongitudinal symmetry refers to the degree to which the stern shape matches the bow shape. The image of a barge quickly enters the mind. A barge is highly symmetrical and seaworthy, but does not have ideal longitudinal symmetry and therefore is not very sea kindly. A simple way to describe what is desired is that a hull with good symmetry will have V-shaped sections and sharp waterlines at both ends. The Kadey-Krogen is on the left. Which entry form do you think moves easiest through the water? Which form do you think has a softer ride?hullform-symmetry-1-1




So why do you want a vessel with a symmetrical hull form? There are four compelling reasons. First, the fine entry has superior wave cutting ability making it more efficient compared to blunt, stout-looking forms. The fine entry also yields a softer ride, which means less pounding in head seas.Second, symmetrical forms track better in a following sea. The V-shaped sections aft slice following seas rather than surfing them, making for a safer and more comfortable ride.

Third, the aft V-shaped sections also offer less resistance and drag than the broad waterlines of asymmetrical hulls. This results in better fuel economy.

hullform-symmetry-2And fourth, symmetrical forms roll less. Yes, contrary to popular belief, an asymmetrical hull with an immersed transom and/or relatively flat aft sections will actually tend to roll more than a symmetrical hull in a following or beam sea. Why? Simple physics. The leverage (upward force) that wave action has on those flatter sections is greater than on a hull that is more rounded and/or tapered. The upward force on one side creates a downward force on the other side and voila, you have roll. It is this roll (the downward pressure on the opposite side) that will then cause the vessel to veer off course (yaw). Those of you that snow ski or water ski will understand exactly what I mean. The appropriately symmetrical hull form is much more sea kindly than one that is not. Vessels roll and heave as though they are on springs, with less symmetrical forms having stiffer springs (lots of force per immersion) whereas the Kadey-Krogen transom effectively rides on softer springs. Comfort-wise, you can think of operating a wide-transom boat in rough seas as trying to drive a sports car through deep potholes.