Every boat has one thing in common: a hull, which is the hollow, lowermost portion of a vessel that floats partially submerged and supports the rest of the boat. Hulls are designed to provide specific handling and performance characteristics, which, of course, dictate the boat’s purpose.
Whether your reason for taking a voyage involves a day cruise, sailing, fishing, water skiing, diving, extended cruising, or racing, understanding hull design, and performance characteristics and selecting a boat that best matches your needs will enhance your enjoyment while underway.
In general, hull designs are in one of three categories: displacement, planing, and semi-displacement. Each is described briefly below, with advantages and disadvantages.
Displacement hulls typically have a rounded bottom with a tear-drop shape running from bow to stern. Displacement hulls displace or move, an amount of water equal to the boat’s weight so that the boat can move forward through the water. This type of hull is very efficient at design speeds, getting the most use out of a gallon of fuel, but the hull design restricts speed. For example, a 64-foot cruising trawler can realistically achieve a top speed of just over 10 knots. At slow speeds, a displacement hull boat easily pushes aside the water it displaces, but as speed increases, the engine must work harder to move the displaced water, thereby reducing speed and increasing fuel consumption.
Planing hulls are designed to ride on top of the water, regardless of boat weight and feature V-shaped hulls that enable the boat to rise above the water, or on the plane (essentially keeping the hull above the water). Less power is needed to attain high speeds and compared to displacement or semi-displacement hulls, planing hulls are far less comfortable in rough seas or weather.
Semi-displacement hulls are hybrids, with a design somewhere between displacement and planing hulls. This type basically combines the stability of a full displacement hull and the maneuverability and speed characteristics of a planing hull. Their design, propulsion plant, and weight, however, do not provide enough hydrodynamic lift to achieve full planing.
Federal statutes require that boats of a certain length and powered with an inboard, outboard, or stern drive engine display a permanent, manufacturer-installed capacity plate that defines safe load limits. Small sailboats, canoes, kayaks, and inflatable boats are exempt from this statute, but may have similar information on board to help guide the user.
Loading and capacity relate to the weight of people, fuel, and gear that can be safely carried on a boat. A boat’s safe load depends on several characteristics, including hull volume and dimensions, engine weight, and, for boats with outboard motors, how is it mounted. Capacity plates are generally affixed inside of the boat on the transom or near the helm station. The number of seats on a boat does not indicate the number of people a boat can safely carry.
Capacity plates should never be removed or altered. Many states have regulations that prohibit carrying people and gear in excess of capacity limits, as well as laws that prohibit the installation of a motor that exceeds the indicated horsepower limit.
The limits on capacity plates apply only to optimum weather and sea conditions. It is good practice to ensure that the boat’s load (passengers and gear) is distributed evenly and to avoid abrupt shifts in weight distribution. It is important to adjust the weight distribution should weather conditions deteriorate. Also, remember that people represent a live load on board. When they move about, the movement can adversely affect the boat’s balance and performance. The best time to shift human or other weight is when the boat is stopped or at a slow or idle speed.
Though a capacity plate specifically indicates how many persons may be safely carried, it typically does not identify or quantify the weight of removable gear, like clothing, or consumable items, like water or fuel. If the boat is carrying fewer passengers but a heavy load of consumables, it is good practice to determine the weight of the items. For example, use standard weights for water (eight pounds per gallon) and fuel (six pounds per gallon) and multiply by the amounts of water and fuel on board to determine if the load is within capacities.
For vessels that are not required to have a capacity plate, use the formula below to calculate the number of persons that the vessel can carry safely.
Number of People = Vessel Length (in Feet) x Vessel Width (in Feet) ÷ 15
Understanding hull design and performance characteristics and selecting a boat that best matches your needs will enhance your enjoyment while out on the water, whether for a reflective day of fishing or an exciting day of tubing.