Basic flotation: A flotation system that will keep a boat carrying its maximum load from sinking when swamped, assuming the occupants of the boat have left it and are in the water clinging to it. With basic flotation the swamped boat may float at any attitude" i.e. Bow just poking out of the water.
Level flotation: A flotation system that will keep a boat carrying its maximum load from sinking when swamped, assuming the occupants remain within the boat and supported by the flotation system. The flotation system must be such that it will keep the swamped boat floating level, and prevent it from capsizing in calm water. Level flotation does not provide a self-righting capacity.
Sponsons: The (pontoons / tubes) that run along both sides of the boat that give it Buoyancy, making the boat highly resistant to capsize.
Free surface effect: E.G. When the water in a swamped boat moves freely about the floor of a sealed deck, it causes a transference of weight which can badly effect the stability of a boat, that under some circumstances will cause the boat to capsize, particularly in heavy seas. Having a sealed deck creates a void of trapped air under the floor (with or without foam) that prevents the boat from sinking, but causes the weight of the water above the void, to have a much higher centre of gravity than if it were in the bottom of the boat. When the "swamped" water sloshes to one side either by personnel moving about, or heavy seas, the weight of the water on that side, combined with the buoyancy of the void, is often enough to cause a boat to roll over. Have Pontoons (sponsons ) of trapped air on each side of the boat greatly reduces the chance of capsize.
rBB: (Rigid Buoyancy Boat) A boat where the sponsons ( pontoons / tubes) are made of a rigid material that trap air, thus giving a high level of built in Buoyancy on the side of the boat on or above the waterline.
RIB: (Rigid Inflatable Boat) A boat where the sponsons( pontoons / tubes) are made of flexible material inflated by air, that gives a high level of built in Buoyancy running down each side of the boat on or above the water line.
Buoyancy: Flexible sponsons (pontoons / tubes) inflated with air for rigidity. Ridged sponsons trap air. (1Cubic Meter of trapped air will support 1 Tonne)
Positive Buoyancy Boat: A boat which displaces a volume of water measured by weight greater than the weight of the boat.
Self Bailers: Used in place of Scuppers. Usually a tube made of water proof material folded and secured above water line and released when necessary.
Self Draining Deck: Where the deck is above water line and the scuppers drain the deck.
Self Bailing Deck: Where the deck is below water line ; it is necessary to have the boat underway to extract the last of the water from the bottom of the boat. (Many RIBs & rBBs have decks at or below water line )
Hull: The area of the boat between the keel and the chine.
Chines: ( Occur at the transition point where the Hull meets the Sponsons (or the sides of the boat) The width of the chine gives stability to the boat.
Horizontal Chine: Area at the bottom of the sponsons that deflects the spray from continuing up the side of the boat, usually the wider the chine the more stable the boat
Reverse chine: The area at the bottom of the sponsons that deflects the spray back down to the waters surface. Arguably better than a horizontal chine, the theory being that they trap air and water giving additional lift and a softer ride (the faster the boat speed, the more effectively the chines work)
Hogging: Describes the effect on the performance of a boat, caused by a hull that is concaved, running from Bow to Stern. Symptoms can include the boat "running on" (surfing) after throttling off, a Boat that is slow and sluggish to get up on the plane, particularly prevalent in Fibreglass boats that have sat on trailers for long periods. A concave as little as 10mm, that won't be seen by the average layman, can have a dramatic effect on performance. This can often explain why owners wonder why their boats don't perform as well as they used to .
Fair Hull: A hull that is True & Flat . A "Fair Hull" is testament to a good boat builder. It is such a Hull that determines the performance of a boat. As Aluminium distorts when welded, it requires a good design and skilled personnel to ensure that the correct procedures are utilized to achieve a Fair Hull during manufacture.
Fibreglass Boats are generally made in a mould, which ensures uniformity, however distortion can occur during the curing process, which may impact on performance.
Dead rise: The angle of the Hull usually measured at the Stern (back). A low dead rise would be under 10 degrees a moderate dead rise would be around 15 degrees a steep dead rise would be around 20 + degrees. In conventional boats the flatter (lower) the dead rise the more stable the boat. Usually as the dead rise increases the boat becomes less stable at rest. Generally the steeper the dead rise the smoother the ride in rougher conditions. Thus a boat design is usually a compromise of these factors.
A common dead rise for Aluminium boats is around 14 to 16º that gives stability at rest... for Fibreglass its around 20º
Why don't Aluminium boats have a similar dead rise to Fibreglass boats.......Its to do with weight and stability. For a boat to be stable at rest it must have it's chines in contact with the water. The greater the dead rise the higher the boat floats in the water, Fibreglass boats are heavy therefore they can afford much greater dead rise before the chines begin to lose contact. Conventional Aluminium boats being light, require much less dead rise before the chines start to lose contact.
Water Ballasting: Adding extra weight to the boat at rest such as water ballasting is an accepted practice which enables a designer to increase the dead rise while maintaining stability.
Boat width: Generally speaking the wider the boat for a given dead rise the harder the ride. e.g. A 2.2 meter wide boat with a 14º dead rise will have a harder ride than a 1.900 meter wide boat with a 14º dead rise.
Variable dead rise: Where the Dead rise at the Stern (back) continues to become steeper as it moves to the Bow (front). A boat with variable dead rise is generally less stable than one with constant dead rise.
Stability: The ability for a boat to remain stable at rest (important when fishing/ diving) An inherent attribute in most rBBs and RIBs.
Stern: Back
Bow: Front
Chine walking: When a boat oscillates from chine to chine while underway, usually at speed, can be caused by poor design, Insufficient ballast, poor weight distribution within the boat, or a boat that is overpowered.
Frames: Gives the Hull plates strength and rigidity.
Floating Frames: Frames that are not attached to the hull plates, used in some production boats as an expedient productivity measure, not generally accepted as good boat building practice, not acceptable in surveyed boats.
Stringers: Run from stern to bow, gives the hull plates strength and rigidity.