MaxSurf Introduction:
MaxSurf allows the user to construct numerical models of hullforms using NURBS surfaces. The NURBS representation can be used within MaxSurf to generate lines drawings and determine hydrostatics. Within the MaxSurf suite of software the NURBS representation can be used to develop resistance and powering predictions (HullSpeed), develop seakeeping estimates (SeaKeeper), investigate damaged stability (HydroMax), or perform structural analysis (WorkShop). Finally, it can be exported to programs like Esprit, Ideas, or Rhino to create the construction/milling files required to produce your ship on the shops NC mills.
When starting MaxSurf, initially 10 windows pop up. . The Plan, Profile, Body Plan and Perspective windows are used to visualize the NURBS surface. The other windows display calculations and enable the user to enter or edit text directly when constructing or modifying the NURBS surface. All MaxSurf windows can be manipulated (minimized, maximized, etc) like any other Windows display.
Surfaces are defined within MaxSurf by the position of a set of control points that collectively form a control point net. The shape of a surface cannot be directly manipulated, rather it is the movement of the control points (i.e. the intersections of rows and columns in the control point net) that allows you to modify a surface into a desired shape. Essentially it is if you are changing the shape of a balloon by gluing rubber bands to it and stretching the rubber band to each control point. Corners of a surface are defined exactly by the position of the corresponding corner of the net and are represented as pink squares. Edges are defined only by the control points on the corresponding edge of the net, while the internal points of the surface may be influenced by many or all of the control points in the net. Internal and edge control are represented as grey squares.
Multiple surfaces can be used to create hulls panels, transoms, decks, keels, etc. which in turn can be joined or intersected with each other within MaxSurf to create a single hull model.
Procedure for creating a hull in MaxSurf
The following steps are required to create a hullfrom from scratch in MaxSurf. Instructions written in “SmallCaps” refer to pull-down menu options within the MaxSurf. Many of these options have shortcut buttons on the default toolbars that can be used as you become more proficient.
Use File/New Design to initiate a new design Use Data/Units to set the desired unit (metric, imperial, etc.) Create a surface using Surfaces/Add Surface/Cylinder Open the Assembly window using View/Assembly Within the Assembly window rename the surface Stretch the surface to its approximate final size using Surfaces/Size Surfaces Before progressing much farther the coordinate system within MaxSurf should be initialized.
Data/Frame of Reference is used to set the design waterline height relative to the baseline and the location of the forward and aft perpendiculars. Click the Find Baseline button, then enter the appropriate waterline height in the DWL box. Next click the Set to DWL box. Note that distances measured aft of the zero point (currently amidships) are assigned negative X coordinates. The reference zero typically corresponds to the intersection of the forward perpendicular and the baseline. These values are set using Data/Zero Point. Also uncheck the Locked Zero Point button. ß Important Use Data/Grid Space/Add Sections to enter station definition. Either use the space command to evenly space the sections, or individually enter the longitudinal locations in the section table. Note: the section spacing should correspond to any offset data that you may later import. The shape of the desired hullform is determined by the shape of the control point net. The simpler you keep the net, the easier the surface is to manipulate and fair. In general, the net columns should be kept vertical and be concentrated more in the ends than in the middle. As a first cut 5 columns should be adequate to define a simple ship shape without a bulbous bow or faired in skeg. Net rows should follow the diagonals if the hull is round bilged or follow the chine lines if the hull is hard chined. If the hullform is a typical monohull, the forward most column and the bottom row will be at Y=0 to insure that the port and starboard sides meet at the bow and that there are no holes in the bottom. Two simple nets are shown below:
Hard Chined Hull Round Bilged Hull
If the hull has no transverse curvature, the net can be simplified by reducing the order of the surface to linear (2nd order) in the transverse direction, requiring only one net row to define a longitudinal chine. The hard chined example above was constructed this way. If transverse curvature is present then the number of net points one less than the order of the surface must be compacted (co-incident) to create a chine. The round bilged hull form above is 4th order so three net points lie on top of one another to create the chine at the transom, which fades away forward as the points move apart.
Modify your net to look more like the appropriate net shown above;
Insert net rows and columns using Controls/Add Column and Controls/Add Rows commands. Net columns can only be added from within the Plan or Profile windows, while net rows can only be added from within the Body Plan Window. After clicking Controls/Add Column from within the appropriate window simply click the net where you want the additional column inserted. To specify the order of the surface use Surfaces/Surface Properties and change the appropriate values. Set surface stiffness to 4 in longitudinal direction and transverse direction if round bilged – or 2 in the transverse direction if hard chined.
Editing the Control Point Net
Control points are moved individually by selecting one (clicking and holding the right mouse button down with the cursor overtop the appropriate control point) then dragging it to the desired location. When the button is released the control point will be dropped. Alternately, specific X, Y, and Z values for control points can be entered in the Control Points window. Multiple points can be manipulated at once be shift-clicking or dragging a window around multiple points. The allowable directions of motions can be limited to orthogonal directions by holding the shift key down during the dragging operation. Control points can be placed on top of one another by clicking on the point that is already in the desired location, shift clicking on a second net point, then using controls/compact to place the second point on the first. MaxSurf provides several utilities to assist in fairing and smoothing the control point net. controls/smooth controls and controls/straighten controls have multiple options but generally work by highlighting a range of control points then clicking the menu option to smooth or straighten the interior and edges of the selected control point range. The controls/snap to grid option is also helpful as is the ability to directly edit the control point XYZ locations in the Control Points window.
Advanced Editing
Bonding edges Intersecting Surfaces Bow knuckles
A design begins with one of the standard surfaces (or an existing design). You then modify the shape of the surface's four edges, followed by its interior. The longitudinal edges are best modeled before the transverse edges, and it is generally best to form the edges in the horizontal plane (the Plan view) prior to the vertical plane (the Profile view). Having defined the edges you may then go on to manipulate internal points in the net to create the required surface shape, this is best done in the Body Plan view.
Goal is to achieve a simple clean net, i.e. one that does not distort the parametric surface and has the minimum number of rows and columns to adequately define the surface.
Control box in the Body plan view shows the currently active net column and the currently displayed section. Data/Grid Spacing creates sections, waterlines, and buttocks.
Station lines, waterlines and buttocks are defined in the “Grid”
Set Surface flexibility (2 for hard chined, 3 for round bilged, 4or higher to fair better
Friday, June 4, 2010
Ship Stability
Ship stability is a complicated aspect of naval architecture which has existed in some form or another for hundreds of years. Historically, ship stability calculations for ships relied on rule-of-thumb calculations, often tied to a specific system of measurement. Some of these very old equations continue to be used in naval architecture books today, however the advent of the ship model basin allows much more complex analysis.
Master shipbuilders of the past used a system of adaptive and variant design. Ships were often copied from one generation to the next with only minor changes being made, and by doing this, serious problems were not often encountered. Ships today still use the process of adaptation and variation that has been used for hundreds of years, however computational fluid dynamics, ship model testing and a better overall understanding of fluid and ship motions has allowed much more in-depth analysis.
Transverse and longitudinal waterproof bulkheads were introduced in ironclad designs between 1860 and the 1880s, anti-collision bulkheads having been made compulsory in British steam merchant ships prior to 1860[1]. Prior to this, a hull breach in any part of a vessel could flood the entire length of the ship. Transverse bulkheads, while expensive, increase the likelihood of ship survival in the event of damage to the hull, by limiting flooding to breached compartments separated by bulkheads from undamaged ones. Longitudinal bulkheads have a similar purpose, but damaged stability effects must be taken into account to eliminate excessive heeling. Today, most ships have means to equalize the water in sections port and starboard (cross flooding), which helps to limit the stresses experienced by the structure, and also alter the heel and/or trim of the ship.
Add-on stability systemsThese systems are designed to reduce the effects of waves or wind gusts. They do not increase the stability of the vessel in a calm sea. The IMO International Convention on Load Lines does not mention active stability systems as a method of ensuring stability. The hull must be stable without active systems.
Main article: Bilge keel A bilge keelA bilge keel is a long fin of metal, often in a "V" shape, welded along the length of the ship at the turn of the bilge. Bilge keels are employed in pairs (one for each side of the ship). A ship may have more than one bilge keel per side, but this is rare. Bilge keels increase the hydrodynamic resistance when a vessel rolls, thus limiting the amount of roll a vessel has to endure.
OutriggersMain article: outriggerOutriggers may be employed on certain vessels to reduce rolling. Rolling is reduced either by the force required to submerge buoyant floats or by hydrodynamic foils. In some cases these outriggers may be of sufficient size to classify the vessel as a trimaran, however on other vessels they may simply be referred to as stabilizers.
Antiroll tanksMain article: Antiroll TanksAntiroll tanks are tanks within the vessel fitted with baffles intended to slow the rate of water transfer from the port side of the tank to the starboard side. The tank is designed such that a larger amount of water is trapped on the higher side of the vessel. This is intended to have an effect completely opposite to that of the free surface effect.
ParavanesParavanes may be employed by slow moving vessels (such as fishing vessels) to increase stability.
Active stability systemsMany vessels are fitted with active stability systems. Active stability systems are defined by the need to input energy to the system in the form of a pump, hydraulic piston, or electric actuator. These systems include stabilizer fins attached to the side of the vessel, or tanks in which fluid is pumped around to counteract the motion of the vessel.
Stabilizer finsMain article: stabilizer (ship)Active fin stabilizers are normally used to reduce the roll that a vessel experiences while under way. The fins extend beyond the hull of the vessel below the waterline, and alter their angle of attack depending upon heel angle of the vessel. They operate in a very similar way to airplane wings. Cruise ships frequently use this type of stabilizer system because the high cost of incorporating it into the vessel can be justified. Pleasure yachts down to 15M LOA will increasingly choose active fin stabilization as the cost/benefit ratios are perceived to improve. This system may have any of the following disadvantages:
Because the fins may be retractable, they may take up valuable space in the engine compartment. When fins are not retractable, they constitute fixed appendages to the hull, possibly extending the beam or draft envelope; at a minimum, requiring attention for additional hull clearances. Altering the angle of attack requires the vessel to use fuel in supplying the power required to do so. However the power expended for fin motion may be offset by power recovered through more stable tracking on course. Power saved by following a more accurate course may be difficult to quantify. The fin and actuator mechanism is expensive to manufacture and fit into the vessel, especially when compared to a bilge keel. While the typical "active fin" stabilizer will effectively counteract roll for ships under way, some active fin systems have been shown capable of reducing roll motion when vessels are not under way. Referred to as Stabilization while not under way or Stabilization at Rest, these systems work by moving fins of special design, with the requisite acceleration and impulse timing to create effective roll cancellation energy.
Main article: Stabilization while not under way
Calculated stability conditionsMain article: Stability conditions (watercraft)When a hull is designed, stability calculations are performed for the intact and damaged states of the vessel. Ships are usually designed to slightly exceed the stability requirements (below), as they are usually tested for this by a classification society.
Intact stabilityIntact stability calculations are relatively straightforward and involve taking all the centers of mass of objects on the vessel and the center of buoyancy of the hull. Cargo arrangements and loadings, crane operations, and the design sea states are usually taken into account.
Damaged StabilityDamaged stability calculations are much more complicated than intact stability. Finite element analysis is often employed because the areas and volumes can quickly become tedious and long to compute using other methods.
The loss of stability from flooding may be due in part to the free surface effect. Water accumulating in the hull usually drains to the bilges, lowering the centre of gravity and actually increasing the metacentric height (GMt). This assumes the ship remains completely stationary and upright. However, once the ship is inclined to any degree (a wave strikes it for example), the fluid in the bilge moves to the low side. This results in a list.
Stability is also lost due to flooding when, for example, an empty tank is holed and filled with seawater. The lost buoyancy of the tank results in that section of the ship lowers into the water slightly. This creates a list unless the tank is on the centerline of the vessel.
In stability calculations, when a tank is holed, its contents are assumed to be lost and replaced by seawater. If these contents are lighter than seawater, (light oil for example) then buoyancy is lost and the section lowers slightly in the water accordingly.
For merchant vessels, and increasingly for passenger vessels, the damage stability calculations are of a probabilistic nature. This is a concept in which the chance that a compartment is damaged is combined with the consequences for the ship, resulting in a damage stability index number that has to comply with certain regulations.
Required stability
In order to be acceptable to classification societies such as the Bureau Veritas, American Bureau of Shipping, Lloyd's Register of Ships and Det Norske Veritas, the blueprints of the ship must be provided for independent review by the classification society. Calculations must also be provided which follow a structure outlined in the regulations for the country in which the ship intends to be flagged.
For U.S. flagged vessels, blueprints and stability calculations are checked against the U.S. Code of Federal Regulations (CFR) and SOLAS conventions. Ships are required to be stable in the conditions to which they are designed for, in both undamaged and damaged states. The extent of damage required to design for is included in the regulations. The assumed hole is calculated as fractions of the length and breadth of the vessel, and is to be placed in the area of the ship where it would cause the most damage to vessel stability.
In addition, U.S. Coast Guard rules apply to vessels operating in U.S. ports and in U.S. waters. Generally these Coast Guard rules concern a minimum metacentric height or a minimum righting moment. Because different countries may have different requirements for the minimum metacentric height, most ships are now fitted with stability computers that calculate this distance on the fly based on the cargo or crew loading. CargoMax or MACS3 are popular computer programs used for this task.
Master shipbuilders of the past used a system of adaptive and variant design. Ships were often copied from one generation to the next with only minor changes being made, and by doing this, serious problems were not often encountered. Ships today still use the process of adaptation and variation that has been used for hundreds of years, however computational fluid dynamics, ship model testing and a better overall understanding of fluid and ship motions has allowed much more in-depth analysis.
Transverse and longitudinal waterproof bulkheads were introduced in ironclad designs between 1860 and the 1880s, anti-collision bulkheads having been made compulsory in British steam merchant ships prior to 1860[1]. Prior to this, a hull breach in any part of a vessel could flood the entire length of the ship. Transverse bulkheads, while expensive, increase the likelihood of ship survival in the event of damage to the hull, by limiting flooding to breached compartments separated by bulkheads from undamaged ones. Longitudinal bulkheads have a similar purpose, but damaged stability effects must be taken into account to eliminate excessive heeling. Today, most ships have means to equalize the water in sections port and starboard (cross flooding), which helps to limit the stresses experienced by the structure, and also alter the heel and/or trim of the ship.
Add-on stability systemsThese systems are designed to reduce the effects of waves or wind gusts. They do not increase the stability of the vessel in a calm sea. The IMO International Convention on Load Lines does not mention active stability systems as a method of ensuring stability. The hull must be stable without active systems.
Main article: Bilge keel A bilge keelA bilge keel is a long fin of metal, often in a "V" shape, welded along the length of the ship at the turn of the bilge. Bilge keels are employed in pairs (one for each side of the ship). A ship may have more than one bilge keel per side, but this is rare. Bilge keels increase the hydrodynamic resistance when a vessel rolls, thus limiting the amount of roll a vessel has to endure.
OutriggersMain article: outriggerOutriggers may be employed on certain vessels to reduce rolling. Rolling is reduced either by the force required to submerge buoyant floats or by hydrodynamic foils. In some cases these outriggers may be of sufficient size to classify the vessel as a trimaran, however on other vessels they may simply be referred to as stabilizers.
Antiroll tanksMain article: Antiroll TanksAntiroll tanks are tanks within the vessel fitted with baffles intended to slow the rate of water transfer from the port side of the tank to the starboard side. The tank is designed such that a larger amount of water is trapped on the higher side of the vessel. This is intended to have an effect completely opposite to that of the free surface effect.
ParavanesParavanes may be employed by slow moving vessels (such as fishing vessels) to increase stability.
Active stability systemsMany vessels are fitted with active stability systems. Active stability systems are defined by the need to input energy to the system in the form of a pump, hydraulic piston, or electric actuator. These systems include stabilizer fins attached to the side of the vessel, or tanks in which fluid is pumped around to counteract the motion of the vessel.
Stabilizer finsMain article: stabilizer (ship)Active fin stabilizers are normally used to reduce the roll that a vessel experiences while under way. The fins extend beyond the hull of the vessel below the waterline, and alter their angle of attack depending upon heel angle of the vessel. They operate in a very similar way to airplane wings. Cruise ships frequently use this type of stabilizer system because the high cost of incorporating it into the vessel can be justified. Pleasure yachts down to 15M LOA will increasingly choose active fin stabilization as the cost/benefit ratios are perceived to improve. This system may have any of the following disadvantages:
Because the fins may be retractable, they may take up valuable space in the engine compartment. When fins are not retractable, they constitute fixed appendages to the hull, possibly extending the beam or draft envelope; at a minimum, requiring attention for additional hull clearances. Altering the angle of attack requires the vessel to use fuel in supplying the power required to do so. However the power expended for fin motion may be offset by power recovered through more stable tracking on course. Power saved by following a more accurate course may be difficult to quantify. The fin and actuator mechanism is expensive to manufacture and fit into the vessel, especially when compared to a bilge keel. While the typical "active fin" stabilizer will effectively counteract roll for ships under way, some active fin systems have been shown capable of reducing roll motion when vessels are not under way. Referred to as Stabilization while not under way or Stabilization at Rest, these systems work by moving fins of special design, with the requisite acceleration and impulse timing to create effective roll cancellation energy.
Main article: Stabilization while not under way
Calculated stability conditionsMain article: Stability conditions (watercraft)When a hull is designed, stability calculations are performed for the intact and damaged states of the vessel. Ships are usually designed to slightly exceed the stability requirements (below), as they are usually tested for this by a classification society.
Intact stabilityIntact stability calculations are relatively straightforward and involve taking all the centers of mass of objects on the vessel and the center of buoyancy of the hull. Cargo arrangements and loadings, crane operations, and the design sea states are usually taken into account.
Damaged StabilityDamaged stability calculations are much more complicated than intact stability. Finite element analysis is often employed because the areas and volumes can quickly become tedious and long to compute using other methods.
The loss of stability from flooding may be due in part to the free surface effect. Water accumulating in the hull usually drains to the bilges, lowering the centre of gravity and actually increasing the metacentric height (GMt). This assumes the ship remains completely stationary and upright. However, once the ship is inclined to any degree (a wave strikes it for example), the fluid in the bilge moves to the low side. This results in a list.
Stability is also lost due to flooding when, for example, an empty tank is holed and filled with seawater. The lost buoyancy of the tank results in that section of the ship lowers into the water slightly. This creates a list unless the tank is on the centerline of the vessel.
In stability calculations, when a tank is holed, its contents are assumed to be lost and replaced by seawater. If these contents are lighter than seawater, (light oil for example) then buoyancy is lost and the section lowers slightly in the water accordingly.
For merchant vessels, and increasingly for passenger vessels, the damage stability calculations are of a probabilistic nature. This is a concept in which the chance that a compartment is damaged is combined with the consequences for the ship, resulting in a damage stability index number that has to comply with certain regulations.
Required stability
In order to be acceptable to classification societies such as the Bureau Veritas, American Bureau of Shipping, Lloyd's Register of Ships and Det Norske Veritas, the blueprints of the ship must be provided for independent review by the classification society. Calculations must also be provided which follow a structure outlined in the regulations for the country in which the ship intends to be flagged.
For U.S. flagged vessels, blueprints and stability calculations are checked against the U.S. Code of Federal Regulations (CFR) and SOLAS conventions. Ships are required to be stable in the conditions to which they are designed for, in both undamaged and damaged states. The extent of damage required to design for is included in the regulations. The assumed hole is calculated as fractions of the length and breadth of the vessel, and is to be placed in the area of the ship where it would cause the most damage to vessel stability.
In addition, U.S. Coast Guard rules apply to vessels operating in U.S. ports and in U.S. waters. Generally these Coast Guard rules concern a minimum metacentric height or a minimum righting moment. Because different countries may have different requirements for the minimum metacentric height, most ships are now fitted with stability computers that calculate this distance on the fly based on the cargo or crew loading. CargoMax or MACS3 are popular computer programs used for this task.
Hull
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and / or deckhouse, where present. The line where the hull meets the water surface is called the waterline.
The structure of the hull varies depending on the vessel type. In a typical modern steel ship, the structure consists of watertight and non-tight decks, major transverse and longitudinal members called watertight (and also sometimes non-tight) bulkheads, intermediate members such as girders, stringers and webs, and minor members called ordinary. transverse frames, frames, or longitudinals, depending on the structural arrangement. The uppermost continuous deck may be called the "upper deck," "weather deck," "spar deck," "main deck" or simply "deck.". The particular name given depends on the context - the type of ship or boat, the arrangement, or even the area where it sails. Not all hulls are decked (for instance a dinghy).
In a typical wooden sailboat, the hull is constructed of wooden planking, supported by transverse frames (often referred to as ribs) and bulkheads, which are further tied together by longitudinal stringers or ceiling. Often but not always there is a centerline longitudinal member called a keel. In fiberglass or composite hulls, the structure may resemble wooden or steel vessels to some extent, or be of a monocoque arrangement. In many cases, composite hulls are built by sandwiching thin fiber-reinforced skins over a lightweight but reasonably rigid core of foam, balsa wood, impregnated paper honeycomb or other material.
The shape of the hull is entirely dependent upon the needs of the design. Shapes range from a nearly perfect box in the case of scow barges, to a needle-sharp surface of revolution in the case of a racing multihull sailboat. The shape is chosen to strike a balance between cost, hydrostatic considerations (load carrying and stability) and hydrodynamics (speed, powering, and dynamic motion behavior). The frame or body of a ship, exclusive of masts, engines, or superstructure.
After this they can be categorized as:.
Displacement the hull is supported exclusively or predominantly by buoyancy. They travel through the water at a limited rate which is defined by the waterline length. They are often heavier than planing types, though not always. Semi-displacement, or semi-planing the hull form is capable of developing a moderate amount of dynamic lift, however, most of the vessel's weight is still supported through buoyancy Planing Royal Navy World War II MTB planing at speed on calm water showing its Hard. chine hull - note how most of the forepart of the boat is out of the waterthe planing hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed. The dynamic lift reduces the wetted surface and therefore also the drag. They are sometimes flat-bottomed, sometimes V-bottomed and sometimes round-bilged. The most common form is to have at least one chine, which makes for more efficient planing and can throw spray down. Planing hulls are more efficient at higher speeds, although they still require more energy to achieve these speeds. (See: Planing (sailing), Hull speed). [Edit] Most used hull formsAt present, the most widely used form is the round bilge hull. [1].
The inverted bell shape of the hull, with smaller payload the waterline cross-section is less, hence the resistance is less and the speed is higher. With higher payload the outward bend provides smoother performance in waves. As such, the inverted bell shape is a popular form used with planing hulls.
Further information: Smooth curve hullSmooth curve hulls are hulls which use, just like the curved hulls, a sword or an attached keel.
Semi round bilge hulls are somewhat less round. The advantage of the semi-round is that it is a nice middle between the S-bottom and chined hull. Typical examples of a semi-round bilge hull can be found in the Centaur and Laser cruising dinghies.
S-bottom hull (A), compared to a hard (B) and soft (C) chine hullS-bottom hulls are hulls shaped like an s. In the s-bottom, the hull runs smooth to the keel. As there are no sharp corners in the fuselage. Boats with this hull have a fixed keel, or a kielmidzwaard. This is a short keel which still sticks a sword. Examples of cruising dinghies that use this s-shape are the yngling and Randmeer.
Further information: Chine (boating) A chined hull consists of straight plates, which are set at an angle to each other. The chined hull is the most simple hull shape because it works with only straight planks. These boards are often bent lengthwise. Most home-made constructed boats are chined hull boats. Benefits of this type of boating activity is the low production cost and the (usually) fairly flat bottom, making the boat faster at planing. Chined hulls can also make use of a sword or attached keel.
Chined hulls can be divised up into 3 shapes:.
V-bottom chined hulls flat-bott chined hulls and multi-chined hulls.
A protrusion below the waterline forward is called a bulbous bow and is fitted on some hulls to reduce the wave making resistance drag and thus increase fuel efficiency. Bulbs fitted at the stern are less common but accomplish a similar task. (See also: Naval architecture) A keel may be fitted on a hull to increase the transverse stability, directional stability or to create lift. Control devices such as a rudder, trim tabs or stabilizing fins may be fitted.
Bow is the frontmost part of the hull.
Stern is the rear-most part of the hull.
Port is the left side of the boat when facing the Bow.
Starboard is the right side of the boat when facing the Bow.
Waterline is an imaginary line circumscribing the hull that matches the surface of the water when the hull is not moving.
Midships is the midpoint of the LWL (see below). It is half-way from the forwardmost point on the waterline to the rear-most point on the waterline.
Baseline an imaginary reference line used to measure vertical distances from. It is usually located at the bottom of the hull.
Metrics Principal hull measurements "LWL & LOA" "Beam, draft & Depth" Hull forms are defined as follows:.
Block Measures that define the principal dimensions. They are: Length overall (LOA) is the extreme length from one end to the other. Length at the waterline (LWL) is the length from the forwardmost point of the waterline measured in profile to the stern-most point of the waterline. Length Between Perpendiculars (LBP or LPP) is the length of the summer load waterline from the stern post to the point where it crosses the stem. (See also p / p) Beam or breadth (B) is the width of the hull. (Ex: BWL is the maximum beam at the waterline) Depth or moulded depth (D) is the vertical distance measured from the top of the keel to the underside of the upper deck at side. [2] Draft (d) or (T. ) is the vertical distance from the bottom of the hull to the waterline. Freeboard (FB) is the difference between Depth and draft. Form Derivatives that are calculated from the shape and the Block Measures. They are: Volume (V or ∇) is the volume of water displaced by the hull. Displacement (Δ) is the weight of water equivalent to the immersed volume of the hull. Longitudinal Centre of Buoyancy (LCB) is the longitudinal distance from a point of reference (often Midships) to the centre of the displaced volume of water when the hull is not moving. Note that the Longitudinal Centre of Gravity or centre of the weight of the vessel must align with the LCB when the hull is in equilibrium. Vertical Centre of Buoyancy (VCB) is the vertical distance from a point of reference (often the Baseline) to the centre of the displaced volume of water when the hull is not moving. Longitudinal Centre of Floatation (LCF) is the longitudinal distance from a point of reference (often Midships) to the centre of the area of waterplane when the hull is not moving. This can be visualized as being the area defined by the water's surface and the hull. Coefficients [3] help compare hull forms as well:.
1) Block Coefficient (Cb) is the volume (V) divided by the LWL x BWL x T. If you draw a box around the submerged part of the ship, it is the ratio of the box volume occupied by the ship. It gives a sense of how much of the block defined by the Lpp, beam (B) & draft (T) is filled by the hull. Full forms such as oil tankers will have a high Cb where fine shapes such as sailboats will have a low Cb. 2) Midship Coefficient (Cm or Cx) is the cross-sectional area (Ax) of the slice at Midships (or at the largest section for Cx) divided by beam x draft. It displays the ratio of the largest underwater section of the hull to a rectangle of the same overall width and depth as the underwater section of the hull. This defines the fullness of the underbody. A low Cm indicates a cut-away mid-section and a high Cm indicates a boxy section shape. Sailboats have a cut-away mid-section with low Cx whereas cargo vessels have a boxy section with high Cx to help increase the Cb. 3) Prismatic Coefficient (Cp) is the volume (V) divided by Lpp x Ax. It displays the ratio of the underwater volume of the hull to a rectangular block of the same overall length as the underbody and with cross-sectional area equal to the largest underwater section of the hull. This is used to evaluate the distribution of the volume of the underbody. A low Cp indicates a full mid-section and fine ends, a high Cp indicates a boat with fuller ends. Planing hulls and other highspeed hulls tend towards a higher Cp. Efficient displacement hulls travelling at a low Froude number will tend to have a low Cp. 4) Waterplane Coefficient (Cw) is the waterplane area divided by Lpp x B. The waterplane coefficient expresses the fullness of the waterplane, or the ratio of the waterplane area to a rectangle of the same length and width. A low Cw figure indicates fine ends and a high Cw figure indicates fuller ends. High Cw improves stability as well as handling behavior in rough conditions.
The structure of the hull varies depending on the vessel type. In a typical modern steel ship, the structure consists of watertight and non-tight decks, major transverse and longitudinal members called watertight (and also sometimes non-tight) bulkheads, intermediate members such as girders, stringers and webs, and minor members called ordinary. transverse frames, frames, or longitudinals, depending on the structural arrangement. The uppermost continuous deck may be called the "upper deck," "weather deck," "spar deck," "main deck" or simply "deck.". The particular name given depends on the context - the type of ship or boat, the arrangement, or even the area where it sails. Not all hulls are decked (for instance a dinghy).
In a typical wooden sailboat, the hull is constructed of wooden planking, supported by transverse frames (often referred to as ribs) and bulkheads, which are further tied together by longitudinal stringers or ceiling. Often but not always there is a centerline longitudinal member called a keel. In fiberglass or composite hulls, the structure may resemble wooden or steel vessels to some extent, or be of a monocoque arrangement. In many cases, composite hulls are built by sandwiching thin fiber-reinforced skins over a lightweight but reasonably rigid core of foam, balsa wood, impregnated paper honeycomb or other material.
The shape of the hull is entirely dependent upon the needs of the design. Shapes range from a nearly perfect box in the case of scow barges, to a needle-sharp surface of revolution in the case of a racing multihull sailboat. The shape is chosen to strike a balance between cost, hydrostatic considerations (load carrying and stability) and hydrodynamics (speed, powering, and dynamic motion behavior). The frame or body of a ship, exclusive of masts, engines, or superstructure.
After this they can be categorized as:.
Displacement the hull is supported exclusively or predominantly by buoyancy. They travel through the water at a limited rate which is defined by the waterline length. They are often heavier than planing types, though not always. Semi-displacement, or semi-planing the hull form is capable of developing a moderate amount of dynamic lift, however, most of the vessel's weight is still supported through buoyancy Planing Royal Navy World War II MTB planing at speed on calm water showing its Hard. chine hull - note how most of the forepart of the boat is out of the waterthe planing hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed. The dynamic lift reduces the wetted surface and therefore also the drag. They are sometimes flat-bottomed, sometimes V-bottomed and sometimes round-bilged. The most common form is to have at least one chine, which makes for more efficient planing and can throw spray down. Planing hulls are more efficient at higher speeds, although they still require more energy to achieve these speeds. (See: Planing (sailing), Hull speed). [Edit] Most used hull formsAt present, the most widely used form is the round bilge hull. [1].
The inverted bell shape of the hull, with smaller payload the waterline cross-section is less, hence the resistance is less and the speed is higher. With higher payload the outward bend provides smoother performance in waves. As such, the inverted bell shape is a popular form used with planing hulls.
Further information: Smooth curve hullSmooth curve hulls are hulls which use, just like the curved hulls, a sword or an attached keel.
Semi round bilge hulls are somewhat less round. The advantage of the semi-round is that it is a nice middle between the S-bottom and chined hull. Typical examples of a semi-round bilge hull can be found in the Centaur and Laser cruising dinghies.
S-bottom hull (A), compared to a hard (B) and soft (C) chine hullS-bottom hulls are hulls shaped like an s. In the s-bottom, the hull runs smooth to the keel. As there are no sharp corners in the fuselage. Boats with this hull have a fixed keel, or a kielmidzwaard. This is a short keel which still sticks a sword. Examples of cruising dinghies that use this s-shape are the yngling and Randmeer.
Further information: Chine (boating) A chined hull consists of straight plates, which are set at an angle to each other. The chined hull is the most simple hull shape because it works with only straight planks. These boards are often bent lengthwise. Most home-made constructed boats are chined hull boats. Benefits of this type of boating activity is the low production cost and the (usually) fairly flat bottom, making the boat faster at planing. Chined hulls can also make use of a sword or attached keel.
Chined hulls can be divised up into 3 shapes:.
V-bottom chined hulls flat-bott chined hulls and multi-chined hulls.
A protrusion below the waterline forward is called a bulbous bow and is fitted on some hulls to reduce the wave making resistance drag and thus increase fuel efficiency. Bulbs fitted at the stern are less common but accomplish a similar task. (See also: Naval architecture) A keel may be fitted on a hull to increase the transverse stability, directional stability or to create lift. Control devices such as a rudder, trim tabs or stabilizing fins may be fitted.
Bow is the frontmost part of the hull.
Stern is the rear-most part of the hull.
Port is the left side of the boat when facing the Bow.
Starboard is the right side of the boat when facing the Bow.
Waterline is an imaginary line circumscribing the hull that matches the surface of the water when the hull is not moving.
Midships is the midpoint of the LWL (see below). It is half-way from the forwardmost point on the waterline to the rear-most point on the waterline.
Baseline an imaginary reference line used to measure vertical distances from. It is usually located at the bottom of the hull.
Metrics Principal hull measurements "LWL & LOA" "Beam, draft & Depth" Hull forms are defined as follows:.
Block Measures that define the principal dimensions. They are: Length overall (LOA) is the extreme length from one end to the other. Length at the waterline (LWL) is the length from the forwardmost point of the waterline measured in profile to the stern-most point of the waterline. Length Between Perpendiculars (LBP or LPP) is the length of the summer load waterline from the stern post to the point where it crosses the stem. (See also p / p) Beam or breadth (B) is the width of the hull. (Ex: BWL is the maximum beam at the waterline) Depth or moulded depth (D) is the vertical distance measured from the top of the keel to the underside of the upper deck at side. [2] Draft (d) or (T. ) is the vertical distance from the bottom of the hull to the waterline. Freeboard (FB) is the difference between Depth and draft. Form Derivatives that are calculated from the shape and the Block Measures. They are: Volume (V or ∇) is the volume of water displaced by the hull. Displacement (Δ) is the weight of water equivalent to the immersed volume of the hull. Longitudinal Centre of Buoyancy (LCB) is the longitudinal distance from a point of reference (often Midships) to the centre of the displaced volume of water when the hull is not moving. Note that the Longitudinal Centre of Gravity or centre of the weight of the vessel must align with the LCB when the hull is in equilibrium. Vertical Centre of Buoyancy (VCB) is the vertical distance from a point of reference (often the Baseline) to the centre of the displaced volume of water when the hull is not moving. Longitudinal Centre of Floatation (LCF) is the longitudinal distance from a point of reference (often Midships) to the centre of the area of waterplane when the hull is not moving. This can be visualized as being the area defined by the water's surface and the hull. Coefficients [3] help compare hull forms as well:.
1) Block Coefficient (Cb) is the volume (V) divided by the LWL x BWL x T. If you draw a box around the submerged part of the ship, it is the ratio of the box volume occupied by the ship. It gives a sense of how much of the block defined by the Lpp, beam (B) & draft (T) is filled by the hull. Full forms such as oil tankers will have a high Cb where fine shapes such as sailboats will have a low Cb. 2) Midship Coefficient (Cm or Cx) is the cross-sectional area (Ax) of the slice at Midships (or at the largest section for Cx) divided by beam x draft. It displays the ratio of the largest underwater section of the hull to a rectangle of the same overall width and depth as the underwater section of the hull. This defines the fullness of the underbody. A low Cm indicates a cut-away mid-section and a high Cm indicates a boxy section shape. Sailboats have a cut-away mid-section with low Cx whereas cargo vessels have a boxy section with high Cx to help increase the Cb. 3) Prismatic Coefficient (Cp) is the volume (V) divided by Lpp x Ax. It displays the ratio of the underwater volume of the hull to a rectangular block of the same overall length as the underbody and with cross-sectional area equal to the largest underwater section of the hull. This is used to evaluate the distribution of the volume of the underbody. A low Cp indicates a full mid-section and fine ends, a high Cp indicates a boat with fuller ends. Planing hulls and other highspeed hulls tend towards a higher Cp. Efficient displacement hulls travelling at a low Froude number will tend to have a low Cp. 4) Waterplane Coefficient (Cw) is the waterplane area divided by Lpp x B. The waterplane coefficient expresses the fullness of the waterplane, or the ratio of the waterplane area to a rectangle of the same length and width. A low Cw figure indicates fine ends and a high Cw figure indicates fuller ends. High Cw improves stability as well as handling behavior in rough conditions.
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