EHPEngine Strut Reduction Screw Gear Bearing Seals THP DHP BHP SHP Brake Horsepower (BHP) - Power output at the shaft coming out of the engine before the reduction gears
EHPEngine Strut Reduction Screw Bearing Gear Seals THP BHP DHP SHPShaft Horsepower (SHP)- Power output at the shaft coming out of the reduction gears
EHPEngine Strut Reduction Screw Gear Bearing Seals THP BHP DHP SHP Delivered Horsepower (DHP) - Power delivered to the propeller - DHP=SHP – losses in shafting, shaft bearings and seals
EHPEngine Strut Reduction Screw Gear Bearing Seals THP BHP DHP SHP Thrust Horsepower (THP) - Power created by the screw/propeller - THP=DHP – Propeller losses - THP is the end result of all HP losses along the drive train
Basic Nomenclature:• Hub The hub of a propeller is the solid center disk that mates with the propeller shaft and to which the blades are attached. Ideally the hub should be as small in diameter as possible to obtain maximum thrust, however there is a tradeoff between size and strength. Too small a hub ultimately will not be strong enough.• Blades Twisted fins or foils that protrude from the propeller hub. The shape of the blades and the speed at which they are driven dictates the torque a given propeller can deliver.• Diameter The diameter (or radius) is a crucial geometric parameter in determining the• amount of power that a propeller can absorb and deliver, and thus dictating the amount of• thrust available for propulsion. With the exception of high speed (35 Knots+) vehicles• the diameter is proportional to propeller efficiency (ie. Higher diameter equates to higher• efficiency). In high speed vessels, however, larger diameter equates to high drag. For• typical vessels a small increase in diameter translates into a dramatic increase in thrust• and torque load on the engine shaft, thus the larger the diameter the slower the propeller• will turn, limited by structural loading and engine rating.
Basic Nomenclature:• Revolutions per Minute (RPMs) RPM is the number of full turns or rotations of a propeller in one minute. RPM is often designated by the variable N. High values of RPM are typically not efficient except on high speed vessels. For vessels operating under 35Knots speed, it is usual practice to reduce RPM, and increase diameter, to obtain higher torque from a reasonably sized power plant. Achieving low RPM from a typical engine usually requires a reduction gearbox.• Pitch The pitch of a propeller is defined similarly to that of a wood or machine screw. It indicates the distance the propeller would “drive forward” for each full rotation. If a propeller moves forward 10inches for every complete turn it has a 10inch nominal pitch. In reality since the propeller is attached to a shaft it will not actually move forward, but instead propel the ship forward. The distance the ship is propelled forward in one propeller rotation is actually less than the pitch. The difference between the nominal pitch and the actual distance traveled by the vessel in one rotation is called slip.
Screw Propeller• Variable Pitch (the standard prop): - The pitch varies at the radial distance from the hub. - Improves the propeller efficiency. - Blade may be designed to be adjusted to a different pitch setting when propeller is stopped.• Controllable Pitch : - The position of the blades relative to the hub can be changed while the propeller is rotating. - This will improve the control and ship handling. - Expensive and difficult to design and build
Right and Left Hand Props Left Hand Right Hand
Suction Face Leading EdgeTrailing Edge Pressure Face
Propeller Walk• Due to a difference in the pressure at the top and bottom of the prop (due to boundary layer), the lower part of the prop works harder.• This leads to a slight turning moment.• Right hand props cause turns to port when moving ahead.
Skewed Screw Propeller Highly Skewed Propeller Advantages - Reduce interaction between propeller and rudder wake. - Reduce vibration and noise Disadvantages - Expensive - Less efficient operating in reverseDDG51
Propeller TheoryPropeller Theory • Speed of Advance Vwater= 0 P Wake Region VS VW Q Vwater= VS • The ship drags the surrounding water so that the wake to follow the ship with a wake speed (Vw) is generated in the stern. • The flow speed at the propeller is, VA = VS − VW Speed of Advance
Propeller Cavitation• Cavitation : Definition - The formation and subsequent collapse of vapor bubbles on propeller blades where pressure has fallen below the vapor pressure of water. - Bernoulli’s Equation can be used to predict pressure. - Cavitation occurs on propellers (or rudders) that are heavily loaded, or are experiencing a high thrust loading coefficient.
Blade Tip Cavitation Navy Model Propeller 5236 Flow velocities at the tip are fastest so that pressure drop occurs at the tip first. Sheet CavitationLarge and stable regionof cavitation covering thesuction face of propeller.
Propeller CavitationConsequences of Cavitation 1) Low propeller efficiency (Thrust reduction) 2) Propeller erosion (mechanical erosion as bubbles collapse, up to 180 ton/in² pressure) 3) Vibration due to uneven loading 4) Cavitation noise due to impulsion by the bubble collapse
Propeller Cavitation• Preventing Cavitation - Remove fouling, nicks and scratch. - Increase or decrease the engine RPM smoothly to avoid an abrupt change in thrust. rapid change of rpm ⇒ high propeller thrust but small change in VA ⇒ larger CT ⇒ cavitation & low propeller efficiency - Keep appropriate pitch setting for controllable pitch propeller - For submarines, diving to deeper depths will delay or prevent cavitation as hydrostatic pressure increases.
Propeller Cavitation• Ventilation - If a propeller or rudder operates too close to the water surface, surface air or exhaust gases are drawn into the propeller blade due to the localized low pressure around propeller. The prop “digs a hole” in the water. - The load on the propeller is reduced by the mixing of air or exhaust gases into the water causing effects similar to those for cavitation. -Ventilation often occurs in ships in a very light condition(small draft), in rough seas, or during hard turns.
Other forms of propulsionA one horsepower cable-drawn ferry!
Ship rudder• rudder is the most important ship control surface• a fin-like projection under the counter and below the waterline – placed as far aft as practical• mounted onto a circular shaft referred to as the stock – penetrates the hull through bearings
Area and shape of rudders• no fixed rule for determination of the size• in practice, rudder area, expressed as a fraction of the product of the length and draft or centerline plane area, often selected by comparison with a ship with similar maneuverability requirements• (Rudder Area)Cargo Ships = 0.017 * LOA * T
Area and shape of rudders - types• rudder consists of two parts: the blade (flat part) against which the water pressure acts and the stock (shaft) which transmits motion of the steering gear to the blade• there are 3 types of rudders:balanced: a portion of the blade area is disposed symmetrically through the rudder height and fwd of stockunbalanced: blade is entirely aft of stocksemi-balanced: area fwd of stock does not extend to the full height of the blade aft of the stock – upper portion may be considered unbalanced and the lower portion, balanced
Area and shape of rudders Modern double-plate, semi-balanced rudder in a single screw ship