Powerboat resistance and seakeepingMarkku Hentinen 2016

• Resistance components

Dominantin slowvessels

Significant in fast boats

Hull resistance• Viscous resistance due to viscosity of the water• Residual resistance, mainly due to maintaining the wave

pattern created by the advancing hull, and spray

Resistance component distribution• Resistance component distribution of semi-planing (semi-

displacement) and planing boats as a function of speedInduced pressure resistance

Hydrodynamic and hydrostatic liftdistribution in planing boats

Pressure distribution under a planing plate

Forces acting on a planing plate

As the trim increases, thelift increases, but also theinduced drag increases.On the other hand, thewetted surface decreasesas the trim angleincreases.Combining these effects,an optimal trim angle canbe found as a function ofm, V and .

Systematic resistance test series for round-bilge, semi-planing vessels

NPL base model

Planing boats

• Prismatic hulls• Savitsky’s semiempiric

method• Effect of chine and spray

rails to be estimated

Spray rails and wetted surface

Systematic resistance test series for hardchine planing boats

• Series-62 (Clements series), deadrise =12.5º

• Delft 25º series, based on Series 62 hull form but with =25ºdeadrise

• Analytical (semiempirical) calculation methods popular, becauseaccuracy is quite good

• Appendage resistance more difficult to calculate

Example calculation using Savitsky’s basic methodBase version Heavy engine Added length

LoaLwlBoaBchDsplSW (static)CG from transomß

Seakeeping• Commonly used calculation methods are based on linearity of the motions:

motion amplitudes are directly proportional to wave amplitude. In boats, thedimensions, frame shapes and large relative size of the waves lead to thesituation in which excitations and damping are dependent of the motionamplitude. Especially in fast boats the motions are normally stronglynonlinear. Slamming forces on planing hull bottom still increase thenonlinearity.

• If the motions can be determined, the evaluation of seakeepingcharacteristics is still difficult because of insufficient criteria for boats. Theaccelerations due to slamming, in terms of both amplitude and frequency,are larger than indicated in normal seasickness criteria. On the other hand,vibration analyses are valid for still higher frequencies. New criteria havebeen developed for fast passenger vessels, but for planing boats they arestill too limiting.

Vertical accelerations• Motions in waves are divided to six components: Surge x(t), sway y(t),

heave z(t), roll (t), pitch (t) and yaw (t).• In practice heave, roll and pitch are the most significant; in planing boats

just heave and pitch.• Vertical accelerations are substantial for crew comfort and operations. Crew

and passengers should be seated on areas where vertical motions are assmall as possible: more and more aft as the speed of the boat increases

• According to the seasickness criteria for ships, people are most sensitive tomotions having a period T = 2...5 s. Even 0,1 g acceleration incur thenseasickness.

• On fast boats the accelerations at CG are typically clearly over 1 g, but theirshort period decreases tendency to seasickness.

• From the operational ability point of view high accelerations have also otherharmful effects: keeping one’s balance and walking is more difficult, andreading small displays or using switches can be tricky.

• High accelerations and slamming also stress the structures of the boat, aswell as equipment and their fastenings.

Nonlinear motions• Broaching: sudden turning sideways in following seas:

– As the boat proceeds in following seas about at the speed of the waves, ayawing moment is created when the bow is at the wave trough (water particlesmoving against boat speed) and the stern at the crest (water particles moving atthe same direction than the boat). The effect of the rudder also decreases as itsspeed through the water decreases. The boat looses part of its directionalstability, as the longitudinal center of the lateral area moves forward (stern liftedout of the water, bow deeper). The phenomena is pronounced in boats havingbroad, full stern and sharp fore sections. The boat may become out of controland heel heavily. At worst broaching may lead to capsizing.

• Porpoising: in small, planing boats a small length-breadth ratio and too largetrim angle (because of LCG is too much aft) may get the boat to porpoise.The phenomena may even lead to bow directing upwards and then invertingof the boat.

• Chine walking: Unstable rolling “from chine to chine”. Can be a problem inlight, very fast, hard chine boats. The phenomena is often related to sprayrails that are extended to transom. The flow separates from them withoutmeeting the chines The hull is planing high with narrow waterline. If thecenter of pressure is moving fast to one side as the boat heels, a powerfulrighting moment is created, which induces a fast returning motion andheeling to the chine, etc.

Parameters affecting verticalaccelerations

• According to the formula developed by Savitsky, the acceleration atthe CG of a planing boat in irregular waves (Pierson-Moskowitz) is:

• The accuracy of the equation is mentioned to be 0,2 g. At the bowthe accelerations are naturally larger and can be calculated by

•

= trim angleC = loading coeff. = /Bx³Bx = chine beamH1/3 = significant wave heightß = deadriseFn = Froude number

• Trim angle. Accelerations are directly related to trim angle. The dynamictrim can be reduced by power trim (outboard or stern drive engines). Trimangle also affects to heave and pitch amplitudes, and the added resistancecaused by the waves.

• Deadrise affects mainly to accelerations, which are roughly inverselyproportional to deadrise. Motion amplitudes and added resistance by wavesare not much affected by deadrise. It shall be noted, that if other parametersremain the same as the deadrise is increased, the dynamic trim angleincreases – which has negative effects to seakeeping.

• Loading coefficient affects mainly to vertical accelerations, that areapproximately inversely proportional to loading coefficient. At high speedalso motion amplitudes and added resistance due to waves decreaseslightly, if the loading coefficient is increased. Chine beam affects to theloading coefficient C = /Bx³ in third power. Decreasing width 10% thusdecreases accelerations almost 30%, if the other parameters (also trim) arekept constant.

Reducing vertical accelerations

• Accelerations are also directly proportional to length-breadth –ratio. This is howevermisleading, because large L/B leads to small breadth, which has its effect in thirdpower in loading coefficient

• Additionally, the dimensions of chine and spray rails affect to the running trim,accelerations and green water (visibility, deck wetness). Frame shape affects to thetime history of slamming pressure, and thus the maximum peak acceleration.

• Means to reduce vertical accelerations are thus:– Narrow beam– Small dynamic trim angle (running trim)– Large deadrise

• This leads to as long and narrow hull as possible without sacrificing other essentialcharacteristics too much.

• Frame shape at bow area shall normally be convex, because it is not likely that alarge part of a convex frame meets water surface simultaneously. Additionally, thevertical force as a function of time is more even than with straight V-shaped frames.Inverted bell is a good shape in direct head seas, but in other directions slamming atconcave surface can be heavy. Double chine may be an efficient design in someboats.

Reducing vertical accelerations (2)

Resistanceat highspeed

Resistanceat humpspeed

Seakeeping:accelerations

Seakeeping:motionamplitudes

Manoeuvringcharacteristics

L/Bincreases

increases decreases Normallyimproving

Bottomloadingincreases

Optimumcan be found

Optimumcan be found

decrease Decreaseslightly

LCG movesaft

Optimumcan be found

Increase increase Increase Normallyimproving

Deadriseincreases

Increases increases decrease contradictory

Spray railsincrease

Maydecrease

decrease increase contradictory

Manoeuvring

wind

thrust