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Martinb2347

Reduction ratio

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Evening all.   We are in the process of looking at either a repair or new propeller.   We need to provide the reduction ratio of our gearbox.  We have a lh150 om a lister sr3.   How do i find this out as i must confess i am at a loss.

 

Thanks

Martin

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Are you are able to decompress the engine, (the brass levers on the rocker covers), and wind it over by hand using a handle?  If so, then if you engage gear you can count how many full 360 degree revolutions you need to turn the engine over for to get one full revolution of the propshaft.  A bit of sticky tape on each of the moving parts, makes counting the revolutions easier.

 

Obviously if it takes 3 full rotations of the engine to get one full rotation of the prop shaft, then you have a 3:1 reduction, 2 revolutions, a 2:1 reduction, etc.....

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35 minutes ago, alan_fincher said:

Are you are able to decompress the engine, (the brass levers on the rocker covers), and wind it over by hand using a handle?  If so, then if you engage gear you can count how many full 360 degree revolutions you need to turn the engine over for to get one full revolution of the propshaft.  A bit of sticky tape on each of the moving parts, makes counting the revolutions easier.

 

Obviously if it takes 3 full rotations of the engine to get one full rotation of the prop shaft, then you have a 3:1 reduction, 2 revolutions, a 2:1 reduction, etc.....

... bearing in mind that the front pulley is on the camshaft, not the crankshaft, so multiply rotations by 2 to get engine rotations.

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22 hours ago, alan_fincher said:

Are you are able to decompress the engine, (the brass levers on the rocker covers), and wind it over by hand using a handle?  If so, then if you engage gear you can count how many full 360 degree revolutions you need to turn the engine over for to get one full revolution of the propshaft.  A bit of sticky tape on each of the moving parts, makes counting the revolutions easier.

 

Obviously if it takes 3 full rotations of the engine to get one full rotation of the prop shaft, then you have a 3:1 reduction, 2 revolutions, a 2:1 reduction, etc.....

If i am honest i am not entirely sure how to turn the engine manualy.  We have the levers but how do i then turn the engine?

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If you don't have the hand starting gear and starting handle you can reach in behind the flywheel shroud and turn the flywheel manually.

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If there is not a drop reduction box on the back of the gearbox it will be direct drive, 1.1.  If you have the large diameter alternator drive pulley on the camshaft, this should turn the engine by hand with the decompressors levers on.    Outhwise there should be the outer end of the camshaft sticking out in front of the alternator drive pulley. this can be used to turn the engine over. There should be a key way with a square key in it. Wrap some thin ally or loads of turns of oven foil or insulating tape around it and use Stilsons or big pipe grips on it, it will turn quite easily with the decompressor levers on. Don't forget the camshaft turns at exactly half the speed of the crankshaft so a half revolution of the camshaft is one complete revolutions of the crankshaft from which your gearbox is driven. Mark the camshaft pulley and a part of the casing with chalk marks to give ths exact half revolutions when they align up. You need to mark the propshaft half coupling and a bit of the casing with chalk too to determine the reduction ratio. You'll probably need someone watching the propshaft marks while you turn the camshaft and watch those marks.

As Liam above remarks that you can turn the crankshaft flywheel with a lever through the air cooling slots on the gearbox flywheel shroud, but these are usually covered with safety wire mesh, but they are removable.

Edited by bizzard

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IIRC the LH150 box is hydraulically actuated, will it turn the prop shaft at hand turning speed?

Edited by OptedOut
Wrong info

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2 minutes ago, OptedOut said:

IIRC the LH150 gearbox is hydraulically operated, will it turn the prop shaft when rotated slowly by hand?

Yes, they lock into forward gear automatically when the engine stops because the hyraulic oil pressure has ceased.

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If the engine runs and the rev counter works (assuming you have one?) then why not use a tachometer on the prop shaft? This is a cheap non-contact one, you just apply a small piece of reflective tape to the shaft and point the laser at it https://www.amazon.co.uk/Tachometer-Roeam-Non-Contact-2-5RPM-99-Reflective/dp/B07RXYRNFF/ref=lp_6286405031_1_17?s=industrial&ie=UTF8&qid=1581671950&sr=1-17

 

  • Greenie 1

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4 hours ago, GRLMK38 said:

If the engine runs and the rev counter works (assuming you have one?) then why not use a tachometer on the prop shaft? This is a cheap non-contact one, you just apply a small piece of reflective tape to the shaft and point the laser at it https://www.amazon.co.uk/Tachometer-Roeam-Non-Contact-2-5RPM-99-Reflective/dp/B07RXYRNFF/ref=lp_6286405031_1_17?s=industrial&ie=UTF8&qid=1581671950&sr=1-17

 

 

Bought one of those a few years back, well worth the money. Not sure how accurate the boat rev counter will be but the most  accurate results would be from checking the prop shaft and the camshaft pulley.

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3 minutes ago, OptedOut said:

 

Bought one of those a few years back, well worth the money. Not sure how accurate the boat rev counter will be but the most  accurate results would be from checking the prop shaft and the camshaft pulley.

 

Agreed - but even the boat's own rev counter* will be accurate enough to distinguish between 3:1 and 2:1 ratios!

 

 

* Despite its name. this is a purely secular instrument.

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Thanks for all the comments.  We r off to the boat next week so will give the guidance a go.  We dont have a rev counter, we have a very basic control panel.

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