For many prospective powerboat owners purchasing a new boat requires a visit to a nearby boat show. At an “in water” show, boats are eye catching at the dock and creature comforts down below are considered its top selling points. One can easily buy a boat on the spot without a haulout and never once seeing its hull. However, it’s the hull shape of a powerboat that will determine the boat’s performance when the throttle is pushed forward.
The first decision to be made before buying a boat is how it will be used: weekend cruising, fishing, water sports, go fast and passage making are some popular options. The final choice will dictate the best hull shape for a monohull.
Hull shapes can be visualized by the body plan drawn by a naval architect. A body plan is a cross section drawing that depicts the stern and the bow sections of a vessel in one composite drawing. By convention, the sections forward of amidships are drawn on the right side, and the sections aft of amidships are drawn on the left side.
What is a Planing Hull?
The ability of a boat to skim fast across the surface of the water is an apt description of a typical planning hull. Above a certain speed, hydrodynamic forces lift the forward portion of the hull out of the water. V-shaped hulls, whether shallow or deep, combined with a flatter underwater section aft are signatures of planing hulls. Lifting strakes help ease the boat onto the plane and hard chines assist in directional stability, particularly in high speed turns.
Types of boats that utilize planing hulls include:
- RIBs (Rigid Inflatable Boats),
- waterski boats,
- center consoles,
- express cruisers,
- picnic boats and
- sport fisherman.
Small flat bottom planing boats such as skiffs are excellent for fishing in protected waters.
What is a Displacement Hull
In cross section, this rounded hull exhibits the opposite characteristics of a planing hull. A soft turn of the bilge and a full shape from bow to stern keep the boat buoyant for its full length while underway. Rather than lifting above the water, it pushes water out of the way when in motion. The hull speed of a slow moving displacement vessel is a function of its load waterline (LWL). Specifically, 1.34 times the square root of LWL. That means a boat with a 36ft LWL has a maximum hull speed of eight knots.
The rounded hull shape also makes a displacement hull seaworthy, but inclined to roll in beam seas.
Recreational trawlers are prime examples of displacement hulls.
What is a Semi-Displacement Hull
Marrying the bow section of a displacement hull to the stern section of a planning hull gives a rough approximation of a semi-displacement hull. The design goal of a semi-displacement hull is to keep the boat’s weight supported by buoyancy at slower speeds but allow it to exceed the hull speed of a similarly sized displacement vessel. A flatter stern section provides the needed lift to increase the boat’s top speed by raising the forward section higher in the water. Typical top speeds are 10 to 20 knots. However to gain this moderate increase in speed, fuel consumption increases dramatically. The more rounded hull shape forward also allows for greater capacity of tankage and accommodations, but with an associated weight increase.
Classic wooden motor yachts, such as Trumpy houseboats utilized this hull shape. Today, there are a wide range of popular yachts in the 40 to 70ft range that use semi-displacement hulls. Even large luxury yachts, in the 130 to 160ft range, use this design to reach speeds between 25 and 30 knots.
What is a Cathedral Hull
Although reminiscent of a trimaran, the cathedral hull is a type of monohull. The forward section has a V-shaped center hull flanked by sponsons that extend toward the bow. Unlike a trimaran, there is little or no space between the hull and sponsons. The V-shape flattens out toward the stern to produce a scow shape allowing the boat to plane easily. The Boston Whaler popularized this design 50 years ago. While this open deck design is very stable and has a good carrying capacity, it offers a hard ride in choppy seas. Today, cathedral hulls remain popular. They are perfect for fishing, water sports and day cruising.
Purchasing a powerboat is a series of compromises, but selecting the proper hull shape is the foundation on which the rest of the boat is built.