Peter Duddigan Posted December 1, 2020 Report Share Posted December 1, 2020 I have been reading a lot of info from boaters on the topic of alternator speeds and drives and have come to the conclusion that speed is of the essence. My engine will be rumbling round at 600rpm cruising speed. I have two 70amp lucas altos with 60mm pulleys. I have a gearbox driveshaft from the crank with Two 100mm pulleys attached. I will belt drive two 50mm pulleys with these which will drive a two 150mm pulleys mounted on same shaft that will in turn belt drive the 60mm alto pulleys. This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle Link to comment Share on other sites More sharing options...
Alan de Enfield Posted December 1, 2020 Report Share Posted December 1, 2020 (edited) 13 minutes ago, Peter Duddigan said: This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle Alternator design and specifications tends to show that maximum output is between 4000 & 6000 RPM Try and get your ratio higher if you can edit to add a typical alternator output graph Edited December 1, 2020 by Alan de Enfield Link to comment Share on other sites More sharing options...
Jen-in-Wellies Posted December 1, 2020 Report Share Posted December 1, 2020 (edited) Alternators also have a maximum rpm too, above which they risk breaking up inside. The pulley ratio needs to be chosen so that at maximum engine revs you won't over speed it. More a problem for racing cars than inland waterways boats, but something to check if you are looking at unusually high ratios. Jen Edited December 1, 2020 by Jen-in-Wellies Link to comment Share on other sites More sharing options...
magnetman Posted December 1, 2020 Report Share Posted December 1, 2020 (edited) Gearbox driveshaft from the crank sounds interesting. Usually you would expect belt drive to obtain the reduction . ETA scrub that I see its an industrial engine with a gap between engine and gearbox. Sorry ! Edited December 1, 2020 by magnetman Link to comment Share on other sites More sharing options...
Alan de Enfield Posted December 1, 2020 Report Share Posted December 1, 2020 5 minutes ago, Jen-in-Wellies said: Alternators also have a maximum rpm too, above which they risk breaking up inside. The pulley ratio needs to be chosen so that at maximum engine revs you won't over speed it. More a problem for racing cars than inland waterways boats, but something to check if you are looking at unusually high ratios. Jen Wise words, but I think it would have to be a very high ratio - my Alternators (as used by Barrus etc) are rated at ; Cut in speed: 1200, Max speed: 15000 Link to comment Share on other sites More sharing options...
Mike Adams Posted December 1, 2020 Report Share Posted December 1, 2020 If you have room it is much better to go for a larger diameter alternator rather than lots of pulleys. The larger the diameter the lower speed they operate. something like this Link to comment Share on other sites More sharing options...
Peter Duddigan Posted December 1, 2020 Author Report Share Posted December 1, 2020 Good idea. Link to comment Share on other sites More sharing options...
Peter Duddigan Posted December 1, 2020 Author Report Share Posted December 1, 2020 I can up the ante to obtain 3000rpm at the altos at cruising revs. The Ruston Max's out at 1500rpm so max alto speed would-be around 7500. Link to comment Share on other sites More sharing options...
Tacet Posted December 1, 2020 Report Share Posted December 1, 2020 2 hours ago, Peter Duddigan said: I have been reading a lot of info from boaters on the topic of alternator speeds and drives and have come to the conclusion that speed is of the essence. My engine will be rumbling round at 600rpm cruising speed. I have two 70amp lucas altos with 60mm pulleys. I have a gearbox driveshaft from the crank with Two 100mm pulleys attached. I will belt drive two 50mm pulleys with these which will drive a two 150mm pulleys mounted on same shaft that will in turn belt drive the 60mm alto pulleys. This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle Doesn't that all come to gearing up at 15:1? Assuming the gearbox driveshaft is 1:1. If so, you will be spinning alternator at 9,000 rpm when the engine is running at 600 rpm Link to comment Share on other sites More sharing options...
nb Innisfree Posted December 1, 2020 Report Share Posted December 1, 2020 Innisfree has (or had before I sold it) a large frame Leece Neville (100amps @24v) at 2:1 reduction on a 3000rpm engine it could give full output at less than 2k engine rpm when cold, needed 2k+ when hot. Link to comment Share on other sites More sharing options...
magnetman Posted December 1, 2020 Report Share Posted December 1, 2020 (edited) 1 hour ago, Mike Adams said: If you have room it is much better to go for a larger diameter alternator rather than lots of pulleys. The larger the diameter the lower speed they operate. something like this That looks a bit like a Leece Neville ~160a (@12v nominal) large frame. I've got one of those somewhere on one of the boats. Not connected to anything it's just "somewhere on the boat". Proper bit of gear. Edited December 1, 2020 by magnetman Link to comment Share on other sites More sharing options...
David Mack Posted December 1, 2020 Report Share Posted December 1, 2020 2 hours ago, Peter Duddigan said: I have been reading a lot of info from boaters on the topic of alternator speeds and drives and have come to the conclusion that speed is of the essence. My engine will be rumbling round at 600rpm cruising speed. I have two 70amp lucas altos with 60mm pulleys. I have a gearbox driveshaft from the crank with Two 100mm pulleys attached. I will belt drive two 50mm pulleys with these which will drive a two 150mm pulleys mounted on same shaft that will in turn belt drive the 60mm alto pulleys. This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle 13 minutes ago, Tacet said: Doesn't that all come to gearing up at 15:1? Assuming the gearbox driveshaft is 1:1. If so, you will be spinning alternator at 9,000 rpm when the engine is running at 600 rpm The way I read it, he is proposing to gear up by a factor of 2 (100/50) from the crankshaft to the intermediate shaft, and by a factor of 2.5 (150/60) from the intermediate shaft to the alternator, giving an overall ratio of 5:1, so 3000 rpm alternator speed at an engine speed of 600 rpm (assuming the pulley sizes quoted are the effective diameter). But if there is room, it would be easier to fit a larger pulley (300mm for 5:1) to the crankshaft and drive the alternator directly. Or run a belt directly off the flywheel, which will spin the alternator at something like 5000 rpm. Link to comment Share on other sites More sharing options...
Tacet Posted December 1, 2020 Report Share Posted December 1, 2020 2 hours ago, Peter Duddigan said: I have been reading a lot of info from boaters on the topic of alternator speeds and drives and have come to the conclusion that speed is of the essence. My engine will be rumbling round at 600rpm cruising speed. I have two 70amp lucas altos with 60mm pulleys. I have a gearbox driveshaft from the crank with Two 100mm pulleys attached. I will belt drive two 50mm pulleys with these which will drive a two 150mm pulleys mounted on same shaft that will in turn belt drive the 60mm alto pulleys. This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle 26 minutes ago, Tacet said: Doesn't that all come to gearing up at 15:1? Assuming the gearbox driveshaft is 1:1. If so, you will be spinning alternator at 9,000 rpm when the engine is running at 600 rpm 2 minutes ago, David Mack said: The way I read it, he is proposing to gear up by a factor of 2 (100/50) from the crankshaft to the intermediate shaft, and by a factor of 2.5 (150/60) from the intermediate shaft to the alternator, giving an overall ratio of 5:1, so 3000 rpm alternator speed at an engine speed of 600 rpm (assuming the pulley sizes quoted are the effective diameter). But if there is room, it would be easier to fit a larger pulley (300mm for 5:1) to the crankshaft and drive the alternator directly. Or run a belt directly off the flywheel, which will spin the alternator at something like 5000 rpm. Maybe but how does it go from the 50 to the 150 pulley? The way I read it was the same as you, but that the intermediate shaft is driven by the 50mm pulley and the drive is taken off the same shaft by the 150 pulley. Thus an additional 3:1 which makes 15:1 Link to comment Share on other sites More sharing options...
David Mack Posted December 1, 2020 Report Share Posted December 1, 2020 5 minutes ago, Tacet said: Maybe but how does it go from the 50 to the 150 pulley? The way I read it was the same as you, but that the intermediate shaft is driven by the 50mm pulley and the drive is taken off the same shaft by the 150 pulley. Thus an additional 3:1 which makes 15:1 The 50mm and 150mm pulleys are on the same shaft, so rotate at the same speed. Link to comment Share on other sites More sharing options...
GUMPY Posted December 1, 2020 Report Share Posted December 1, 2020 Is there an exposed flywheel? On my JP I ran the belt round the flywheel against the ring gear to drive the alternator, worked well. Link to comment Share on other sites More sharing options...
Tacet Posted December 1, 2020 Report Share Posted December 1, 2020 20 minutes ago, Tacet said: Maybe but how does it go from the 50 to the 150 pulley? The way I read it was the same as you, but that the intermediate shaft is driven by the 50mm pulley and the drive is taken off the same shaft by the 150 pulley. Thus an additional 3:1 which makes 15:1 13 minutes ago, David Mack said: The 50mm and 150mm pulleys are on the same shaft, so rotate at the same speed. Same RPM but surely different linear belt speed? Link to comment Share on other sites More sharing options...
David Mack Posted December 1, 2020 Report Share Posted December 1, 2020 36 minutes ago, Tacet said: Same RPM but surely different linear belt speed? Yes linear belt speeds will be different, because the pulleys on the intermediate shaft are different sizes. Intermediate shaft rotates at twice crankshaft speed. Alternator rotates at 2.5 times intermediate shaft speed = 5 times crankshaft speed. Link to comment Share on other sites More sharing options...
Peter Duddigan Posted December 2, 2020 Author Report Share Posted December 2, 2020 Yes, my thoughts too. I'm going to try it, proof in the pudding and all that. I could run from the flywheel end but the flywheel is 550mm diameter. Link to comment Share on other sites More sharing options...
Peter Duddigan Posted December 2, 2020 Author Report Share Posted December 2, 2020 That'll give an alternator speed of 5000rpm on tickover, a top speed of around 17000. No thanks. A larger pulley on alternator would reduce this but I'm not going that route. The belt length would be silly. I'll prob up the idler to 200mm, giving an alto speed of 4000 plus at cruising and a max of 7500 Link to comment Share on other sites More sharing options...
jonesthenuke Posted December 2, 2020 Report Share Posted December 2, 2020 Following on from earlier suggestion of using a larger frame alternator, we have Prestolite 8MR2070 units on our boat, so have some data to hand. There are trade offs in designing the drive speed (which can be lower on these units than car alternators). I would avoid running the alternator too fast, perhaps its better to optimise the output whilst avoiding high speeds where efficiency drops and noise increases. However do not run too slow as cooling is reduced and torque will increase. Link to comment Share on other sites More sharing options...
GUMPY Posted December 2, 2020 Report Share Posted December 2, 2020 5 hours ago, Peter Duddigan said: That'll give an alternator speed of 5000rpm on tickover, a top speed of around 17000. No thanks. A larger pulley on alternator would reduce this but I'm not going that route. The belt length would be silly. I'll prob up the idler to 200mm, giving an alto speed of 4000 plus at cruising and a max of 7500 Yes the belt is long but there were no failures in the 10 years I had the boat and charge rates were excellent flywheel is 590 on the JP. I think @frangarused to have the same system, if he still has that will have been running 25 years. You would need to choose your alternator well mine was from a Jaguar XJS ? Link to comment Share on other sites More sharing options...
Peter Duddigan Posted December 2, 2020 Author Report Share Posted December 2, 2020 I'm impressed ?? Link to comment Share on other sites More sharing options...
Peter Duddigan Posted December 2, 2020 Author Report Share Posted December 2, 2020 There's nothing like using your ideas and creating something that works. Using the flywheel is a good idea but I'll stick to my plan to up the drive speed via pulleys. Thanks everyone for your thoughts and knowledge. I'll let you know how I get on? Link to comment Share on other sites More sharing options...
frangar Posted December 2, 2020 Report Share Posted December 2, 2020 1 hour ago, Loddon said: Yes the belt is long but there were no failures in the 10 years I had the boat and charge rates were excellent flywheel is 590 on the JP. I think @frangarused to have the same system, if he still has that will have been running 25 years. You would need to choose your alternator well mine was from a Jaguar XJS ? Yep still going...Alternator now changed from a A133 from a Jag to a A127 from a Discovery!!...used the same principle on a couple of other boats with large external flywheels...gives you a great charge rate at tickover and spins the fan fast enough to ensure proper cooling. Change belts every 3/4 years just because they get a bit polished and its easier to do planned maintenance than have to worry about it when it breaks. 1 Link to comment Share on other sites More sharing options...
Peter Duddigan Posted December 2, 2020 Author Report Share Posted December 2, 2020 Great. I may run an extra alternator in that fashion later on. Many thanks? Link to comment Share on other sites More sharing options...
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