Glossary
Every term the app uses that is not everyday English. Written for someone who knows boats but not naval architecture.
The shape of the hull
| Beam | Width. Beam overall is the widest point at the gunwale; waterline beam (Bwl) is the width where the boat meets the water, and on a V-hull it is always less. Stability and resistance care about the waterline figure. |
| Chine | The corner where the bottom meets the topsides. On a planing boat it is a sharp edge — a hard chine — because water needs to separate cleanly there instead of curling round. A displacement hull rounds the same corner instead; see turn of bilge. |
| Turn of bilge | The rounded corner between bottom and topsides on a displacement hull — the same place a planing boat puts a chine, but curved. A tight turn is nearly a corner; a slack one is a gentle arc. |
| Rise of floor | The slight V at the keel of an otherwise flat-bottomed displacement hull. The displacement equivalent of deadrise, but usually only a few degrees. |
| Hull speed | The speed at which a displacement hull's own bow wave is as long as its waterline, at a length Froude number of about 0.40. Past it, drag climbs very steeply for very little extra speed — which is why a displacement boat's length sets its pace. |
| Holtrop-Mennen | The standard empirical method for a displacement hull's calm-water resistance, regressed from towing-tank data. Hull's displacement-mode answer, and it warns when your hull sits outside the range it was fitted on. |
| Prismatic coefficient (Cp) | How full the ends are compared with the biggest section: volume / (largest section area × waterline length). Around 0.55–0.65 suits a motor launch. Low means fine ends, high means the fullness carries a long way fore and aft. |
| Deadrise | The V-angle of the bottom, measured up from horizontal. 0° is flat; 20°+ is a deep V. Nearly every planing boat is finer at the bow than at the transom. |
| Flare | Topsides that lean outward as they rise. Throws spray down and away, and adds buoyancy as the boat is pressed down. The opposite is tumblehome. |
| Freeboard | Height from the waterline to the gunwale. How dry the boat rides. |
| Gunwale | The top edge of the hull side. Pronounced "gunnel". |
| Keel | The centreline of the bottom, running bow to stern. In Hull it is the first of the seven longitudinal lines, held at y = 0. |
| LOA | Length overall — bow to stern, the number you would quote. |
| LWL | Waterline length — the length actually in the water. Always less than LOA, and the one that matters for speed. |
| Sheer | The line of the gunwale seen from the side, and in Hull the fifth longitudinal line — the deck edge. |
| Station | A transverse slice through the hull, like a frame. Hull stores a boat as a series of stations, each carrying the same set of points. |
| Stem | The leading edge of the bow. Raked leans forward; plumb stands vertical. |
| Transom | The flat stern. Rake is how far it leans back at the top. |
| Tumblehome | Topsides that lean inward as they rise. A classic look; slightly less reserve buoyancy than flare. |
| Entry half-angle | How sharply the bow parts the water at the waterline. Small is fine and slicing; large is blunt and buoyant. |
How it floats
| Displacement | The weight of water the hull pushes aside — which equals the weight of the boat. Used both as a volume (m³) and as a mass (kg). |
| Draught (draft) | How deep the hull goes below the waterline. |
| DWL | Design waterline — the level the boat floats at with its intended load. Not drawn; solved for. |
| Trim | How far off level the boat sits fore-and-aft, in degrees. Bow-up or bow-down. |
| Reserve buoyancy | The hull volume above the waterline. What you have in hand before the boat is swamped. |
| Waterplane area | The area of the shape the boat cuts out of the water's surface. Governs how much deeper it floats per kilo added. |
| Wetted surface | Hull area in contact with water. Friction drag is directly proportional to it. |
Stability
| GM | Metacentric height — the distance from the centre of gravity to the metacentre. The headline measure of initial stability. Compared against the waterline beam (GM/B) rather than in bare metres, because GM grows with beam on its own. |
| GM_L | The same in the fore-and-aft direction. Always far larger; boats resist pitching much more than rolling. |
| Metacentre (M) | The point the boat effectively pivots about when it heels a little. Above the centre of gravity means stable. |
| KB, BM, KG | Heights above the keel: of the centre of buoyancy, of the metacentre above that, and of the centre of gravity. GM = KB + BM − KG. |
| Stiff / tender | A stiff boat has high GM: resists heeling, snaps upright, rolls uncomfortably fast. A tender boat has low GM: leans easily, rolls slowly, and taken too far is unsafe. |
| MCT | Moment to change trim — the twisting effort needed to alter trim by one centimetre. How fussy the boat is about where you put weight. |
| VCG | Vertical centre of gravity. Lower is better; it is why heavy things go low. |
Centres
| LCB | Longitudinal centre of buoyancy — the middle of the underwater volume. Buoyancy pushes up here. |
| LCG | Longitudinal centre of gravity — the middle of everything the boat weighs. Weight pulls down here. If LCG and LCB are not vertically in line, the boat trims until they are. |
| LCF | Longitudinal centre of flotation — the middle of the waterplane, and the point the boat pivots about when trimming. |
| LCP | Longitudinal centre of pressure — where hydrodynamic lift acts when planing. Aligning it with LCG is the key to a boat that runs at the trim you designed. |
| LCLP | Longitudinal centre of lateral plane — the middle of the underwater side area. Affects tracking and turning. |
Form coefficients
Dimensionless shape descriptions, so hulls of different sizes can be compared.
| Cb — block | How much of the box around the underwater hull is actually hull. Low means fine and wedge-like. |
| Cp — prismatic | How evenly volume is spread along the length. Low means volume concentrated amidships with fine ends. |
| Cm — midship | How full the largest section is. A deep V is low; a flat bottom approaches 1. |
| Cw — waterplane | How full the waterline shape is. Higher means more stability and more resistance to sinking under load. |
Going fast
| Planing | Going fast enough that water flowing under the bottom lifts the boat, rather than buoyancy alone holding it up. Wetted area shrinks and drag stops climbing the way it did. |
| Displacement mode | The slow regime: the boat pushes water aside and its speed is limited by the wave it makes. |
| Hump | The drag peak at the transition between the two. The most power the boat will ever need per knot; an underpowered boat gets stuck there, bow high, going nowhere. |
| Porpoising | A pitch instability at speed: the boat starts bouncing rhythmically, bow up and down, and it gets worse rather than damping out. Trim too far bow-up is the usual cause. |
| Porpoising margin | How many degrees of trim you have in hand below the angle where porpoising starts. Positive is safe. |
| Savitsky method | The standard engineering method for predicting how a planing hull behaves — trim, lift, wetted length and drag. It is what Hull's planing analysis is built on, and it is only valid for planing hulls in the planing regime. |
| Cv | Speed coefficient — speed made dimensionless against beam. "How fast is this, for a boat this size." |
| λ (lambda) | Wetted-length ratio: mean wetted length over beam. Above about 4 the planing model stops being valid. |
| τ (tau) | Running trim angle while planing. Usually best around 3–5°. |
| EHP | Effective horsepower — drag × speed. The power to move the hull, not engine power; a propeller and drivetrain are only 50–65% efficient. |
| Appendage | Anything hanging in the water that is not the hull itself — shaft, bracket, rudder, skeg, bilge keel. Each adds drag out of proportion to its size, because it sits in disturbed flow. |
| Form factor (1+k₂) | How much more drag an appendage makes than a flat plate of the same wetted area. A rudder tucked behind a skeg is about 1.75; a strut standing clear in open water is 3. Holtrop tabulated them. |
| Froude number (Fn) | Speed made dimensionless against length. The standard way of asking which speed regime a hull is in. |
| Spray root | Where the oncoming water first strikes the bottom and is thrown sideways as spray. It carries real drag. |
| Seakeeping | How the boat behaves in waves rather than flat water — chiefly how hard it slams, measured as vertical acceleration in g. |
| k_yy | Pitch gyradius — how spread out the weight is fore-and-aft. A boat with weight in the ends pitches more sluggishly and slams differently. Hull measures it from the boat’s own structure as well as the weights you place, counting the fact that the shell and deck are spread the whole length of the hull rather than sitting in one spot. |
Working in Hull
| Fairing | Smoothing a line so it has no lumps or flat spots. Traditionally done by springing a batten along the hull and letting it find its own curve; Hull does the mathematical equivalent. |
| Hard kink | A point where the surface is allowed to break rather than curve smoothly through — a chine, for instance. Hull has separate kink flags for the cross-section and for the fore-and-aft line. |
| Pin | A lock on one axis of one point. Neither dragging, the optimiser, nor fairing will move it. |
| Range | A minimum and maximum on one axis of one point — a band the optimiser may work inside. |
| Constraint | A rule tying one point to another in the same station, such as "the chine must sit 50–100 mm outboard of the point below it". |
| Longitudinal line | One of the lines running bow to stern through the same point index of every station. Seven come as standard; you can add your own. |
| Control polygon | The raw grid of points the optimiser sees, before it is smoothed into the surface you normally look at. |
| Pareto front | When two goals conflict, the set of designs where you cannot improve one without giving up the other. The knees are where the trade rate changes most sharply — usually the sensible compromises. |
Building it
| Offsets | A table of coordinates describing the hull at chosen positions along its length. The traditional way of transmitting a design to a builder. |
| Lofting | Drawing the hull out full size from a table of offsets, fairing the lines as you go, so you can make patterns from them. |
| Rabbet | The groove cut along the keel or stem that the planking or plating lands in. |
| Bearding line | Where the inner face of the planking meets the backbone timber — the other edge of the rabbet. |
| Bevel | The angle an edge is cut at, away from square. A transom's edges are bevelled all the way round so the planking lands flat against them; the angle changes at every point because the hull's shape does. Hull reports it per point in the table of offsets, positive for an undercut edge. |
| Backbone | The keel and the stem together — the centreline structure the planking lands on. Hull weighs it from the siding and molding you set in Parameters → Construction, so it shows up in the Weights panel as its own row. |
| Siding | The width of a piece of backbone timber, seen from above. |
| Molding | The depth of that same piece, seen from the side. |
| Developed panel | A curved panel unrolled flat, so it can be cut from sheet material and bent into place. Hull emits one for the transom whenever it is raked or has a thickness. |
| Scantlings | The dimensions of a boat's structural members — the sizes of the timber, the thickness of the plating. |
| Sole | The floor of a cockpit. |
| Coaming | The raised lip around a cockpit that keeps water out. |