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Posted: Thu Jan 22, 2009 10:57pm

Post Subject: off-set engine

how far can you offset an engine. using 2 propshafts, with u-js and a centre-bearing, my first narrowboat had 2 propshafts with a centre-bearing, both propshafts in-line , on the angle, my 2nd narrowboat built with the engine offset again, with 2propshafts and a centre-bearing, but the propshafts were not inline, the centre-bearing, pushed 12inches to the left, so the propshafts , from above looked like this < , i had the boat put in the water @ hull marina , we got half way to beverley on the river hull, when the centre- bearing fell apart, when we took the props apart and lined them up , one of the shafts was 2in too long, the boatbuilder had bolted the engine down first then had the props made up, so instead of having aprop shortened, just pushed the centre- bearing to one side,i am very dissapointed, as its a sailaway, with no bandage on the exhaust,no id number on the hull, cheers barrie


Posted: Fri Jan 23, 2009 9:29am

Post Subject: off-set engine

Dear Barrie, my text books indicate the Cross or Hooks type joint (often referred to as UJs)can drive through at least 25 degrees. However this type of joint causes its output side to slow down and speed up twice in every revolution and the greater the angle the greater the speed difference. This may very well cause vibrations. To overcome this the joints should be used in pairs, aligned 90 degrees apart, and each driving through the same angle. This cases the speed variations in the second joint to cancel out the speed variations caused by the first. If you have an offset engine there will be a dogleg in the shafting and good practise would be to fit a thrust bearing between the actual prop-shaft coupling and the stern gland. Otherwise you run the danger of allowing prop thrust to cause the jack shaft (the one with the two joints in it) to dogleg even more an possible rip the engine mounts or bend the prop-shaft. I know many boats are not like this but it is still good practise. You also need to be aware that not all plumber-blocks (your centre bearing)are designed to take both radial and axial loads. (Axial loads are those produced by prop thrust). So if your engine is flexibly mounted so it has some fore and aft movement under prop thrust and if the centre bearing was only designed to accept radial loads the bearing is subject to failure. I would expect the type of bearing that is suitable for use on the prop-shaft to use a air of taper roller or angular contact ball bearings. It sounds to me as if your bearing is in the wrong place - especially if the engine is flexibly mounted - because engine shake must cause the centre of the jack shaft to "wave about" and a bearing on the jack-shaft will try to restrain this with the inevitable result of failure and possible excess vibration. I do not think you have a valid complaint about the exhaust bandage because a sail away is meant for you to complete but I am very concerned about no Hull Identification Number. This is a requirement for CE compliance and as such the lack of such a number and associated paperwork may make this an illegal sale. It will certainly prevent you from legally selling the boat for five years. Consult the enforcement authority (good luck - your local Trading Standards) or a surveyor/lawyer who is well versed in the CE legislation. Tony Brooks

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