HOW TO SELECT THE RIGHT SIDE PO­WER UNIT THRUSTER

Superyacht - - Technical -

Bow Th­ru­sters are nor­mal­ly cho­sen ac­cor­ding to how mu­ch la­te­ral pu­sh they need to sup­ply. Su­ch pu­sh or th­ru­st is ba­sed on the fol­lo­wing pa­ra­me­ters: Boat’s La­te­ral Wind Area: in other words all that is ex­po­sed abo­ve the wa­ter­li­ne whi­ch is af­fec­ted by la­te­ral wind pres­su­re. In prac­ti­cal terms the ex­po­sed sur­fa­ce area is re­du­ced by about 20- 25% to take in­to ac­count the fact that the la­te­ral sur­fa­ces con­cer­ned ( su­per­struc­tu­re, top­si­des) are not com­ple­te­ly flat nor per­pen­di­cu­lar and the­re­fo­re in so­me way en­han­ce the wind to sli­de off exer­ci­sing less pres­su­re. - Po­si­tion of the Bow Thruster. De­ter­mi­nes the de­gree of le­ve­ra­ge wi­th whi­ch the Bow Thruster pu­shes to one side or the other. The mo­re the Bow Thruster is in­stal­led in the bow the mo­re the tor­que ob­tai­ned de­li­vers grea­ter ca­pa­ci­ty to ro­ta­te, fur­ther fa­ci­li­ta­ting ma­noeu­vra­bi­li­ty ( whi­le re­mem­be­ring the pre­cau­tions men­tio­ned in this ar­ti­cle) - Wind Streng­th – This is, as pro­ba­bly kno­wn to all, ex­pres­sed as per the Beau­fort sca­le ran­ging from for­ce 1 to 10. For exam­ple in a for­ce 5 wind speed va­ries bet­ween 8 to 10.7 metres per se­cond. From this in­di­ca­ti­ve ran­ge of wind speed ap­ply­ing the fol­lo­wing for­mu­la p = 1/ 2 rv whe­re v is ve­lo­ci­ty and r is stan­dard air den­si­ty equal to 1.23 kgm/ m3, we can ob­tain the mat­ching pres­su­re ran­ge whi­ch will be com­pri­sed bet­ween 41 e 74 N/ m2. Let’s now take a prac­ti­cal exam­ple as fol­lo­ws: Sup­po­sing we ha­ve a 10 me­tre long boat wi­th an ex­po­sed la­te­ral wind area equal to 12m2. We wi­sh to in­stall a Bow Thruster whi­ch can ea­si­ly ma­noeu­vre in a for­ce 5. Fir­st we need to cal­cu­la­te ne­ces­sa­ry tor­que: MT = FV x SL x A x 0.8

Whe­re FV = esti­ma­ted wind ve­lo­ci­ty cho­sen as equal to 60 N/ m2 SL = ex­po­sed la­te­ral wind area equal to 12 m2 A = di­stan­ce bet­ween the wind’s pres­su­re cen­tre and the Pi­vot Point ( whi­ch is si­tua­ted at prac­ti­cal­ly the ex­tre­me stern of the boat). This di­stan­ce is near­ly equal to half the boat’s si­ze ( 5m). 0.8 = is the ap­pli­ca­ble coef­fi­cient gi­ven for wind pres­su­re on non flat sur­fa­ces.

Put­ting the­se va­lues in­to the for­mu­la we ob­tain: MT = 60 x 12 x 5 x 0.8 = 2880 N/ m2 To cal­cu­la­te the th­ru­st nee­ded by the Bow Thruster we di­vi­de nee­ded tor­que ob­tai­ned MT by the leng­th of the le­ve­ra­ge of the Bow Thruster whi­ch is the B di­stan­ce bet­ween the Bow Thruster and the Pi­vot Point. The di­stan­ce is as­su­med as being of 9 metres.

The­re­fo­re: TH­RU­ST = MT / B = 2880 / 9 = 320 N ( 33 Kgf)

The­re­fo­re we‘ ll choo­se a Bow Thruster wi­th a th­ru­st of at lea­st 33 Kgf or mo­re.

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