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Posted: Wed Jun 09, 2010 9:43pm

Post Subject: prop shaft alignment

Hi I have 57ft narrowboat with a ford escort engine.How can I check if the prop shaft is accurately aligned to the stern housing and how to reset it if it is not aligned properly cheers bob smith


Posted: Thu Jun 10, 2010 8:26am

Post Subject: prop shaft alignment

Please note that you align the shaft to the engine NOT the "stern housing" - whatever that may be. As you give no information about the type of stern gland and the type of flexible coupling in use (if any) I will give you a general guide, however there are some "flexible" couplings that have a special bolt head that allows you to check alignment without disconnecting the coupling. If you have a long Centaflex coupling, and Aquadrive or Python drive, or if your boat uses a "car type" propeller shaft with two universal joints something would have to be very wrong for you to even think about alignment problems. I find it frustrating when people seem to do their own diagnosis and then expect me to tell them how to cure a problem that may or may not have been correctly diagnosed and then do not supply sufficient information. A potential problem with flexible couplings is that they may have distorted over time so their two faces are no longer parallel whilst others may flop about from new so you really need a metal "dummy coupling" that is identical to the two faces of the flexible but absolutely solid with both faces parallel. Some people make them out of wood. If you have one remove the flexible and bolt the dummy to the gearbox coupling. If not you will just have to do your best so free the shaft half coupling from the flexible. Push the shaft back so the half coupling comes free of the flexible. You may have to slacken the stern gland a little to facilitate this. As the shaft slides backwards a land on one half should pull free of a recess on the other. It's hard to describe but easy to see. The shaft may then jump up or down or, if the gland is badly worn or flexible it could drop. If there is a any up & down or side to side play in the shaft pack it with wood or something to hold it right in the centre of the free play but so you can still slide it up in and out. Pull the shaft forewords so the land(s) and recess(es)on the coupling and flexible come together. If they slide into each other without any effort form you to push the shaft up/down/sideways the shaft is in radial alignment so now jump over the next few sentences to ZZZ. If the land hits the face of the coupling you need to move the engine so the two halves do slide together. On some boats there are metal shims between the engine feet and the beds and on others the engine is held partway up a stud fixed in the flexible mount by two nuts. Whatever way it is fixed slacken whatever holds it down to an equal amount. Check that no gap has opened up between the engine foot and whatever it is secured to. A very thin feeler gauge helps here. I there is a gap wedge that end of the engine a very small amount and then use a feeler gauge whilst adjusting/packing the mounting point with the gap so the feeler gauge shows the same size gap on both sides of the engine. Let the engine back down onto its mounts. Now, taking great care to do exactly the same to both sides of the engine start adjusting the nuts or inserting/removing shims to lift or drop the back of the engine until the land and recess on the couplings slide together. ZZZ Now get a 0.05mm feeler gauge and whilst holding the shaft half coupling into the dummy or flexible run the feeler around the joint. It is almost certain you will find a larger gap on one part of the coupling than another so you need to move the engine up or down at one end (again taking great care to ensure you do the same to both sides of the engine) or sideways on its mounting until you do have the same gap all round. Note: many gearbox manufacturers specify the maximum out of alignment as being no more than 0.05mm. This stage tends to take a time because as you lift the front of the engine the back will drop and so on. When you think you have the alignment correct tighten the engine mounts and recheck. If you can not the turn the shaft by hand (with the half couplings pushed together) either the alignment is still not correct or the gland is tight. Finally fit the bolts, replacing the dummy coupling with the flexible if you used one, and run the engine to see if it tends to rock fore and aft on its mounts. If it does and the gland gets hot either you have a bent shaft or the alignment is not correct. Recheck the bolts every day for at least a week of cruising because they may well work loose. If they do you probably need new self locking nuts for them. Tony Brooks

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