Marine Engines Application and Installation Guide

1 Marine Engines Applicationand Installation GuideLEKM7142 Boat PerformanceLEKM7143 Driveline/Mounting/Alignment/Auxiliary LEKM7144 Control Systems/Instrumentation andMonitoring Systems/Starting Systems/Start-up Systems/ServiceabilityLEKM7145 Cooling SystemsLEKM7146 Ventilation & Exhaust SystemLEKM7147 Lubrication Systems & Fuel SystemsLEKM7148 Dredge EnginesMaterials and specifications are subject to change without notice. 2000 Caterpillar Inc. Printed in PerformanceTolerances on Hull, Propeller and EnginePropeller SizingDucted Propellers (Kort Nozzles)Hull TypesRules of ThumbThe performance of the boat is the result of acomplex interaction of all three aspects of theinstallation; the engine , the hull, and on Hull,Propeller and EngineProper component sizing is very important tothe life and performance of the entirepropulsion system.

2 There are tolerances inseveral aspects of the propulsion system. Inworst-case conditions, the result can be shortlife and/or unsatisfactory performance. Forexample: the effect of these tolerances isshown below in Figure : The engine power may be expected to varydue to manufacturing tolerance by as much as3% on either side of its rated or 100% propeller power absorption may be asmuch as 5% higher, or lower, than originallyexpected. This could result frommanufacturing tolerance in pitch, surfacefinish, and blade hull resistance may vary as much as 20%from calculated values or previous experiencedue to inevitable differences in weight SizingThe propeller is as important as the hull orthe engine to the performance of the propeller directly influences: top speed,fuel efficiency, and engine Information While many operators will choose to operateat reduced throttle settings while cruising, theengine must be able to reach its rated speed(rpm) when the boat is ready for sea; fullyloaded with fuel, water, and stores.

3 For theultimate in engine life and economy, expectedengine operating speeds during sea trialsshould be approximately 1-3% over full loadrated engine speed (rpm).Eliminating engine Overloadingon Overwheeled Vessels When the engine speed (rpm) measuredduring the sea trial of a vessel fails to attainthe required sea trial speed, the reasongenerally is one of the following:Excessive hull fouling Solvable bycleaning the hull and re-running the sea engine power Resolved bymeasuring and recording engine performanceparameters such as inlet, exhaust and transmission or propeller A detailed discussion of the resolution of thiscondition follows.

4 Simply, this discussion willbe restricted to fixed pitch (rpm)Expected EngineSpeed DuringSea Trials(rpm)28002800240024002300210019251 80018002830-28902830-28902425-24702425-2 4702320-23602120-21601945-19801820-18501 820-1850 Table of engine rpm at Sea Trials160 Hull Propeller Engine1501401301201101009080706050403020 1000 20406080100120 Percent Design Hull SpeedTolerances on PowerPercent engine Power(+20%)HullDemand(-10%)PropellerMatc hLineEngine Power (53%)Figure (53%)9 engine fuel setting adjustment Manyvessel operators and shipyards want toincrease the engine fuel (rack) setting whentheir engine does not reach rated speedduring sea trials.

5 At first glance, this seems tobe the easiest and least costly , in such a situation, this solution isincorrect, even if the engine speed (rpm) doesincrease to the expected rated the fuel (rack) setting will result inreduced engine life, increased wear, or inworst case, early engine failure. The vesseloperators engine repair and maintenancecosts will likely far exceed the cost ofreplacing or modifying the existingtransmission or idle adjustment Another oftenconsidered alternative is increasing high idleengine speed to the specified free runningspeed. This will not provide the desiredresults since the fuel stop is already at themaximum fuel position, and an increase inhigh idle will not result in any appreciablespeed sized propeller and/orreduction ratio The correct, but morecostly, remedy is to install a properly matchedpropeller and/or transmission ratio to allowthe engine to operate within it s driveline component changes There is another alternative which we willconsider in cases where driveline componentchanges cannot or will not be method consists of a reduction of boththe engine fuel setting and the high idlespeed.

6 Of course, the engine power and ratedspeed are reduced in the process; however,we are taking advantage of the fact that thepropellers power demand drops off muchfaster than the engine power capability whenengine and propeller speed is reduced(referto Figure ).The net result is that the engine will performwithin its Application limits and theengine/propeller match have been following formula generally applies for astandard fixed pitch propeller:or by rewriting the equationWhere: hp1 = engine power produced at the fullthrottle speed recorded during the sea power level is determined by referring tothe appropriate Marine engine performancecurve corresponding to the original enginerating sold by the dealer and reading thepower on the curve at the recorded Calculated propeller power demand atthe new reduced engine speed (rpm)proposed for this engine speed (rpm) observed andrecorded during the original sea trial priorto fuel setting and high idle modifications.

7 (This speed should always be measured witha precision tachometer.)N2= New, reduced engine speed (rpm) whichmust be determined in order to provide anacceptable engine , transmission, and = hp1 x3N2N1hp1=3hp2N1N23002001300 Propeller hpDemandhp Capability1400 1500 engine Speed (rpm)1600 1700 1800 1900400500 Marine engine Performance Curve3412 TA (388 kw (520 hp) at 1800 rpm)Figure example: Consider a 3408B DITA engine ,sold at a continuous rating of 365 hp at 1800 rpm; During the sea trial, the maximumattainable engine speed was only 1620 engine was operating in an unacceptableoverload (or lug) condition.

8 The MarineEngine Performance Curve (for a continuousrating of 272 kw (365 hp) at 1800 rpm)indicates that the engine was producing (andthe propeller was demanding) 344 hp at thelimited speed of 1620 rpm. This powerrequirement exceeds the approved continuousrating of 330 hp at 1620 rpm. The solution isto further reduce the rpm until the approvedengine rating, as shown on the 3408B marineengine rating curve, exceeds the this example we will calculate the powerrequired if the rated engine rpm was reducedto the engine speed by 70 rpm hasresulted in a decrease in propeller demand of43 hp. The approved engine continuous ratingat 1550 rpm is 314 hp and the propellerdemand has been reduced to 301 the initial trials, the recorded vessel speedwas knots for this 21 m long the engine from 344 hp @ 1620 rpmto 314 hp @ 1550 rpm would decrease thevessel speed to knots, a relativelyinsignificant difference, especially consideringthe gain in engine Pitch Correction An overpitched propeller must have its pitchreduced to allow the engine to reach ratedrpm.

9 The pitch must be reduced by anamount proportional to the engine rpm following formula defines thisrelationship:Where:Prequired= pitch the propeller must have toallow the engine to run at rated rpmPpresent= pitch of the propeller which ispreventing the engine from reaching its ratedrpmEngine rpmwhile overloaded = engine rpmunder normal working conditions whenequipped with the propeller whose pitch is toogreatRated engine rpm = appropriate engine rpmfound on applicable engine specification sheetPropeller Errors and PropellerMeasurement Fast boats need more precise propellers thanslow speed pitch errors which would beinsignificant on a 10 knot river towboat.

10 Willcost a high speed patrolboat or yacht 2 or 3knots of its top on fast boats must be preciselymanufactured if design performance is to beattained and they must remain within nearlynew specifications to prevent severeperformance deterioration. This is particularlytrue of propellers leading and trailing errors in profile, almost too small to bedetected by feel, can constitute sites forinitiation of cavitation. In severe cases, thiscan result in blade failure or loss after as littleas 24 hours of high speed industry professionals can relateinstances where new propellers have beenfound to be several inches out of the specifiedpitch.