Understanding Engine Powers
While its often said that ship handling is an art, But it is very helpful to have a perspective of forces involved i.e. Propulsion, tugs, thrusters, tugs, tides, wind, moorings etc. Below are some easy to remember and apply pointers in day to day use.
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SHP=BHP(approx.)
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100 SHP=1 T BP
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1 HP = 746 W = 0.746 KW
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Transverse Thrust will be generally approx. 10% of engine power at max. It will be stronger in loaded vessel as compared to vessel in ballast, especially for vessel with fuller stern body construction.
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Rudder force= 45 % of engine power.(Approx)
Few points to understand from above table:-
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Rudder gives a very effective turning force (even more than tug) and is very effective if vl is stopped or has headway as pivot point will be far forward and rudder is acting at right astern. So It has more force, large lever hence can produce very strong turning moment.
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Transverse thrust is a rather weak force and cannot be trusted/used alone to execute a turning movement. However we must have fair idea of strength of transverse thrust and how much counter force will be required from tug to check it.
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Transverse thrust may create a strong turning effect when vessel is moving with head way, thus pivot point will be far forward, a rather weak transverse thrust will create a strong turning moment as explained below.
Turning Moment= Force X Distance from Pivot point
(Large distance from pivot point situated far forward due to headway)
Positioning of Tugs and their effect on Maneuvering:-
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Generally if tugs are fast on side (Fwd shoulder and Aft break of accommodation), they will be little less effective in turning as their distance from pivot point is reduced. In order to achieve maximum turning moments tugs should be fast at center leads fwd and aft, provided they can safely work in that configuration.
In next chapter we will discuss the types of propulsion systems and their effect on shiphanding.