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pdscaz
pdscaz

Posted: Tue Aug 11, 2015 12:43pm

Post Subject: marine patrs

will narrow boat parts ever advance from the 1950's the rattle from my engine was a worn centre plate on examination it had almost worn away for an item that does not move (unlike a car clutch plate) is really unacceptable the same goes for engine mounts a regular replacement something that would be a scandal in a car today

Tony-B
Tony-B

Posted: Tue Aug 11, 2015 8:46pm

Post Subject: marine patrs

Ah well, best not ask the Vauxhall owners etc. who have had to fork out for a new dual mass flywheel then. I can understand why you think the drive plate does not move but the crankshaft accelerates and decelerates twice in every revolution. This is known as torsional oscillation and has to be absorbed before it gets into the gearbox. Otherwise it causes tooth chatter and wear. The rubber/plastic/steel springs in the flywheel are constantly being compressed and then extending just as they are in a car clutch plate. The older steel type wears at about the same rate as the same parts in a car clutch plate given the same operating conditions but the conditions are not the same. First of all cars are not driven for long periods on idle and if they were the torsional oscillation damping parts would wear. Cars also tend not be subject to very fast emergency gear changes from fast ahead to full astern and if they were the clutch plates will fail in a similar way to drive plates, let alone the car gearbox and drive shafts. I have seen identical failure modes on drive plates and car clutch plates that came from “boy racer” type drivers who did a lot of fast down changes. In both cases the rivets holding the centre parts together had been sheered. You could solve this problem by specifying a fluid flywheel type drive where the oscillations are taken up by oil but the cost would be prohibitive. I would point out that not all engines use a drive plate as such. Those employing the Lister LH150 (and probably the L100) do not neither id the BMC A and B type marine gearboxes. My Bukh uses a rubber member between a pair of steel spiders to transmit the drive but even so the rubber hardens and really should be changed an very long intervals. However I dread to think how much a Bukh DV36 costs today. Most people buy on price and as such really should not complain when items that are built down to a price fail after many years of service. If my engine had a drive plate I would inspect it at around 6 to 8 yearly intervals to ensure it is changed before it fails.

Tony-B
Tony-B

Posted: Wed Aug 12, 2015 8:49am

Post Subject: marine patrs

Remembered that you also commented about engine mounts. Again you are not comparing like with like. At one time the BMC range of diesels used front mount rubbers direct from cars and high up relative to the engine bed. These rarely failed. The same could not be said for the rear mounts. Car engine mounts only have to carry the weight of the engine and torque reaction from the crankshaft. This tend to make it twist sideways. Cars absorb this by either having at least one pair of mounts comparatively high up and set at an angle or they use a high level tie bar (remember how those on Minis regularly wore out). Any fuel or oil leaks on cars do not pool below the engine or run across engine feet onto the rubber mounts, they just drop onto the road. If the car mounts were contaminated they would fail as readily as those in a boat. On the vast majority of boats the mounts also have to absorb the propeller thrust and its the mounts that actually push the boat along. This is in addition to the forces absorbed by car engine mounts. If you want flexible engine mounts then you can extend their life by removing the prop thrust and any forces generated by slight out of alignment shafts. This is achieved by fitting a thrust bearing and two element flexible coupling like an Aquadrive. This also extend the life of the stern gland and packing. I am also convinced a lot of mounts sold are actually machinery mounts and may not be designed to absorb the forces applied to them in boats. To maximise the life of engine mounts one needs to remove the prop thrust, ensure no oil or fuel ever gets onto the rubbers, and ensure the ones specified are adequate fro the jobs they have to do. Really we are back to using equipment that is a general purpose component rather than is designed specifically for the job. It comes back to cost again. My Bukh with internal balance weights to minimise engine vibration and fitted with an Aquadrive only seems to suffer mount failure when they are contaminated by oil or fuel but they are expensive for their size. All mount problems can be easily solved by having a rigidly mounted engine and a CORRECTLY aligned shaft.

pdscaz
pdscaz

Posted: Fri Aug 14, 2015 11:18am

Post Subject: marine parts

Hi again. I'm afraid I'm not convinced the reference to mini's is the nineteen fifties dangling engine mounts from an engine is not the future the wear on centre plates is unacceptable considering low hours of use it takes to fail if trucks cars or farm tractors / machinery suffered the same our roads and fields would be littered with vehicles going nowhere because of mechanical failure.Engine Mounts, if you are old enough you might have owned a morris 1000 parked it on a slope put it into reverse gear lifted the clutch pedal only to be shaken not stirred like a ball bearing in a tin can when the rubber separated from the metal on the engine mounts a tradition still found today in narrowboats (abet not so memorable) for the record we are taking about 40hp perkins 4cyl engine a PRM150 gearbox with a python drive 2004 in year and 2300hrs on the clock

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