Nautical dictionary: the leapfrog. What it is and why you need to recognize it

What on earth is the “little horse“? One of our recent article on nautical terminology created a heated debate about this term. Here we are, then, to shed some light and also to reason about why this (admittedly somewhat esoteric) topic arouses so much interest.

English: Sheerline

The best definition, I think, may be this, the same as that used in shipbuilding plans: “the curve obtained from the projection perpendicular to the high limit of the broadside (in nautical terms the head of the bulwark) on the plane of symmetry.” The latter also called “diametral,” is the imaginary plane that divides the ship vertically into two equal and symmetrical parts along its entire length.
Basically, it is the top line of the broadside as seen from the side.
In English for leapfrog, the term “Sheerline” is used (which, wanting to translate somewhat romantically, means “the absolute line”)

The misunderstanding and doubt about the term may arise from the fact that, originally, when speaking of traditional wooden construction, “leapfrog” was synonymous with “insellation,” that is, the curvature of the center of the deck in the longitudinal direction, relative to the bow and stern.

The hull-deck junction

The difference is that in modern construction (fiberglass, composite, or even steel/aluminum) the deck profile is often, but not always, integrated into the hull extension at the broadside (visually, but often structurally as well).

What we see when looking sideways at most powerboats is not the hull/ deck junction (called the “line of intersection“) but, indeed, the leapfrog, the top edge of the broadside as it runs from bow to stern. The two lines sometimes coincide, but not always.

On sailboats, where there is almost always no such extension of the hull beyond the junction of the deck (in nautical terms “impavesata,” vulgarizing: the “bulwark”) but rather a succession of stanchions and draperies, the leapfrog, in its original definition is still quite evident and coincides with the insellation of the deck itself.

Now, why all this talk about the little horse, and why has it sparked so much discussion on the web and social media? Because it is such an important “line”; it is the first to define the aesthetics, the designs, of a boat. Along with the starboard bow and the stern (other terms derived from the ancient world of wooden boats) it is the one that immediately conveys to us the “philosophy” of the design (traditional, working, racing, fishing… and so on). It is no coincidence that in imagining, thinking about boats, it is from here that designers start to “draw lines,” to create the concept of the boat to come.

Speaking of the lines that define a boat’s profile and identity, the waterline and nearby“beauty lines” should also be mentioned, the latter purely decorative elements that characterize either an owner’s individual boat, or a range in mass production.

Let us conclude, just to be nitpicky, by recounting that the term “leapfrog” once also meant the metal auxiliary hand bilge pump . In English they were called donkey-pump, in both cases with obvious reference to the necks of animals (in the “land-based” world, on the other hand, donkey pumps are the classic oil-extraction pome).

By Luca Sordelli – Lecturer in Contemporary Nautical History – Yacht Design Course IED Turin