Boat Hull

A boats hull shape is important for a number of reason, safety, sea kindliness, load carrying capacity, speed and efficiency in a particular speed range and operating conditions.

Hulls can be classified/categorized into three major types based on intended boat speed range per hull length:

• Planning hull
• Semi-Displacement hull
• Displacement hull

Boat hulls

Flat Bottom
Although this hull type is stable in calm weather it is, because of its flatter bow, a comparatively rougher ride. This is typically a planing boat that rides on top of rather than through the water. Due to issues with maneuverability and the roughness of the ride these hulls are limited to the amount of horse power applied. This hull type is typically used on calm waters.

Round bottom
This hull is easier to maneuver than the flat hull and moves efficiently though the water at low speeds. These are displacement hulls, which are limited in speed due to the bow wave they create while pushing though the water as opposed to a planning hull that climbs on top of the water. This hull is usually used for ships, canoes/kayaks, sailboats etc.

Deep-V
Most power boats have this hull shape. You can think of this hull as a hybrid of the flat and round bottom hulls. This is generally associated with a semi-Displacement vessel that can, at low speeds push though the water relatively efficiently like a round bottom hull but it can also plain to a certain extent, but not at speeds of a flat bottom hull. The V allows the boat to cut though the water to minimize slapping and provide some grip in turns. The angle of the V is called the dead rise and tends flattens out toward the stern where the angle of the boats run, the V at the stern, will be a big factor in determining the boats planing capabilities.

Cathedral hull

Used for power boats, you can think of this as multiple v hulls on one boat hull. This design provides more stability, better handling and some claim more lift, due to the air pockets between the mini-hulls.

Tunnel hull
Tunnel hulls or catamaran tunnel hull boats are the go fast boats. This hull design is used on the supper fast racers and now even on some fast dinghy’s. This hull has a sharp inner corner or a 90 degree inside hull but is rounded to the outside. This shape provides exceptional grip in tight turns and allows for fast planing. The hull being a catamaran design also allows for good stability and the ability to carry large motors.

Pontoon
This is a very simple hull design to allow for a flat deck to lounge around on. It’s like a catamaran but think of it as a deck with a line of drums tied to either side. This hull shape is not intended to plan and is not the most efficient but it’s great for parties on calm water.

Multihull
This term is generally associated with catamaran and trimaran sail boats which is simply a boat with two or three of round bottom hulls respectively, placed apart for additional stability. This additional stability gives multihulls a significant advantage in that, unlike displacement monohulls (single round bottom), they do not have to be weighed down at the keel to hold them upright. This makes them lighter and faster.

Boat hull design

Boat hulls are designed by Naval Architects using sophisticated software and are designed with a particular speed range, conditions and requirements in mind. For example a high speed catamaran ferry’s hull, due to its size and operating speed will look very different to a catamaran sail boat. Designing a boats hull is an iterative process. A boat designer will use scantlings rules. Scantlings are the dimensions of all parts that go into building a boat for example frames, stringers, girders, etc. The scantling rules are currently set by Lloyds and American Bureau of Ships for fiberglass boats.

Boat hull Materials

Hulls are made with a number of materials and combinations of materials, the major types are:

• fiberglass hull
• Aluminum hull
• Steel hull
• Composite hull

The size of the hull and the money available for the project are major factors in determining what material and/or method to use.

Boat Hull Terminology - Nomenclature

• Hull Length Overall (Loa) - This is not necessarily the length the dock master will charge you for but is the full length of the vessel from forward most to aft most points excluding fittings and spars like a bowsprit, rudders, swim platform etc.
• Hull Length at water line (Lwl) - Also referred to as DWL it is exactly what it implies, the length of the hull at the waterline. Now you may be thinking that the waterline will probably change if you load the boat and you'd be right but the waterline in this case is the waterline where the designer intended it to be not where it is once you have loaded the mother in law. .
• Hull Beam (B or Bmax) - The maximum beam of the hull, like Loa it's the entire hull that's considered.
• Hull Beam at waterline (Bwl) - The same as Lwl but for the beam (width).
• Hull Displacement - This could refer to how much water a hull displaces (volume) when you put it in the water or the weight (mass) of the yacht. Again this can change once you have loaded half your house into the boat, typically there is displacement that is dry weight quoted by the designer/builder and the payload. For speed calculations using displacement use the maximum displacement.
• Boats Bow - The forward most part of the hull. To remember this I thought of when the bow goes over a large wave and it dips down it's bowing down to the ocean.
• Stem or Prow- The forward part of the bow usually at centerline, not used much these days as it came from wooden boat building. Prow is the bow above the waterline.
• Keel - A structural or hydrodynamic appendage at usually at the bottom of the hull. A ships hull is built around a large beam or steel plate which is the structural keel, the backbone if you will. In a monohull sailboat a keel serves two purposes keeping the boat upright (provide ballast) and as an underwater wing for resisting side motion or converting to forward motion. On multihull sailboats the keel's then to be small as it is not neceassary to hang a large lead weight off the bottom to keep it upright.
• Hull Transom - The surface that forms the stern (back) of the vessel.
• Hull Chine - A sharp (or sharper) angle on the hull, this is more pronounced in planing hulls, power boats, where a hard chine is a sharp angle and a soft chine a smoother angle. A barge has a hard chine as it has vertical sides. With planing boats it's necessary to have a sharper angle due to the flatter bottom needed for planing surface area.
• ThruHull - A place where the entire ocean is waiting to get into the boat. Seriously, it's a hole in the hull, either above or below the waterline. If it's below it'll have a seacock (valve) hooked up by pipes to whatever is needs salt water like for example the head (toilet).
• Hull Draft (T) - The vertical distance from the waterline to the bottom of the hull. You don't want to go in shallower water than the draft.
• Depth (D) - From the upperdeck at side measured verticle to the bottom of the hull, or Dc to the top of the keel.
• Freeboard (FB) - The difference between depth and draft. i.e. the height of the sides above the waterline.
• Midships - The midpoint of the Lwl.

 

 

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