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Post by peterboat on Mar 13, 2019 9:53:00 GMT
So you engineers out there consider this, having fitted my tacho to the electric motor its spinning at 933 rpm going forward, and 980rpm in reverse, this is with the boat moored, so I would expect moving the prop speed to increase slightly. Its direct drive and it really needs to spin at 1650 rpm otherwise it creates to much heat. The propeller is 19 x 10 and on the old diesel it would have rotated at 1500 rpm, so I am going to repitch the prop and am thinking 19 x 6.5 if it can be done, this is removing about one third of its pitch so I am hoping for a third gain in motor rpm. I do not want to go down the gear box route as they rob power, any thoughts people?
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Post by bargemast on Mar 13, 2019 10:54:58 GMT
So you engineers out there consider this, having fitted my tacho to the electric motor its spinning at 933 rpm going forward, and 980rpm in reverse, this is with the boat moored, so I would expect moving the prop speed to increase slightly. Its direct drive and it really needs to spin at 1650 rpm otherwise it creates to much heat. The propeller is 19 x 10 and on the old diesel it would have rotated at 1500 rpm, so I am going to repitch the prop and am thinking 19 x 6.5 if it can be done, this is removing about one third of its pitch so I am hoping for a third gain in motor rpm. I do not want to go down the gear box route as they rob power, any thoughts people?
If you give all your info about the power of your electric motor and the revs needed to a prop specialist, they should be able to tell you what the pitch has to be, and normally be able to reset the pitch on your prop accordingly. It's a dark art, some are quite good and manage to get it right, or at least close enough to what you want. Good luck with it. Peter. |
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Post by peterboat on Mar 13, 2019 11:34:26 GMT
So you engineers out there consider this, having fitted my tacho to the electric motor its spinning at 933 rpm going forward, and 980rpm in reverse, this is with the boat moored, so I would expect moving the prop speed to increase slightly. Its direct drive and it really needs to spin at 1650 rpm otherwise it creates to much heat. The propeller is 19 x 10 and on the old diesel it would have rotated at 1500 rpm, so I am going to repitch the prop and am thinking 19 x 6.5 if it can be done, this is removing about one third of its pitch so I am hoping for a third gain in motor rpm. I do not want to go down the gear box route as they rob power, any thoughts people?
If you give all your info about the power of your electric motor and the revs needed to a prop specialist, they should be able to tell you what the pitch has to be, and normally be able to reset the pitch on your prop accordingly. It's a dark art, some are quite good and manage to get it right, or at least close enough to what you want. Good luck with it. Peter. The problem is that electric motor makers dont quote torque figures, my motor if geared two to one at 6 kw would already be able to spin the propeller at the same speed as the 50 HP diesel before it!! |
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Post by bargemast on Mar 13, 2019 11:57:54 GMT
If you give all your info about the power of your electric motor and the revs needed to a prop specialist, they should be able to tell you what the pitch has to be, and normally be able to reset the pitch on your prop accordingly. It's a dark art, some are quite good and manage to get it right, or at least close enough to what you want. Good luck with it. Peter. The problem is that electric motor makers dont quote torque figures, my motor if geared two to one at 6 kw would already be able to spin the propeller at the same speed as the 50 HP diesel before it!! Too bad they don't supply the torque figures with these motors, something I can't understand (I'm probably far from being the brightest spark around) is how a 6 kw motor with a two to one reduction will be able to spin the propellor at the same speed as a 50 HP diesel which is giving about 37kw. With the two to one reduction I have no big problem believing that that would then give 12 kw, which is still only one third of 37 kw, but of course, everything is possible. Maybe one day, even I will be able understand this matter. Peter. |
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Post by peterboat on Mar 13, 2019 12:16:46 GMT
Join the club, it draws 400 amps at 69 volts when it giving its all, and at cruising speed its 50 amps which equates to 3.3kw according to the control board, its all double dutch to me
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Post by TonyDunkley on Mar 13, 2019 12:36:52 GMT
If you give all your info about the power of your electric motor and the revs needed to a prop specialist, they should be able to tell you what the pitch has to be, and normally be able to reset the pitch on your prop accordingly. It's a dark art, some are quite good and manage to get it right, or at least close enough to what you want. Good luck with it. Peter. . . . . . . . . . . . . , my motor if geared two to one at 6 kw would already be able to spin the propeller at the same speed as the 50 HP diesel before it!! No it wouldn't, Peter, because the power a propeller needs, or absorbs, is proportional to rpm cubed, instead of being directly proportional to the rpm. This relationship is defined by what's known as the Propeller Law curve and is often plotted on graphs alongside engine power curves. The power absorbed by a correctly matched propeller at two thirds maximum rpm is a lot less than the engine that's turning it is capable of producing at two thirds it's maximum rated rpm. |
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Post by peterboat on Mar 13, 2019 12:51:02 GMT
. . . . . . . . . . . . , my motor if geared two to one at 6 kw would already be able to spin the propeller at the same speed as the 50 HP diesel before it!! No it wouldn't, Peter, because the power a propeller needs, or can absorb, is proportional to rpm cubed, instead of being directly proportional to the rpm. This relationship is known as the Propeller Law curve and is often plotted alongside engine power curves. The power absorbed by a correctly matched propeller at two thirds maximum rpm is a lot less than the engine that's turning it is capable of producing at two thirds it's maximum rated rpm. Tony it can already turn the prop at 933 rpm and that is in direct drive so two to one gearing would double its torque so it would easy manage 1500 rpm at the propeller, Electric motors without load can and will spin out of control until they explode, they arnt governed like diesels to protect them. HP isnt the needed to turn a prop its torque that will do it. The problem for me is I dont want a gearbox as its something else to go wrong, its far easier to remove pitch from the propeller, increasing motor speed with reduced load.
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Post by bargemast on Mar 13, 2019 13:21:11 GMT
. . . . . . . . . . . . , my motor if geared two to one at 6 kw would already be able to spin the propeller at the same speed as the 50 HP diesel before it!! No it wouldn't, Peter, because the power a propeller needs, or absorbs, is proportional to rpm cubed, instead of being directly proportional to the rpm. This relationship is defined by what's known as the Propeller Law curve and is often plotted on graphs alongside engine power curves. The power absorbed by a correctly matched propeller at two thirds maximum rpm is a lot less than the engine that's turning it is capable of producing at two thirds it's maximum rated rpm. As I said Tony, I know nothing about electric motors and their power torques, but I know that they're in no way comparable with an internal combustion engine. On my barges I've all kinds of props that were supposed to be the best, they all moved the boats, but only one was really very good, with lots of trust in forward and in reverse aswell, which is quite exceptional, they were German props of the make OSTERMANN from Köln, of which I still have 2 bronze spare props with a 98 cm diam, that were for a GM 8V-71 of 240 Hp @ 1800n with a 3.83 : 1 Twin Disc MG-509 reduction gearbox, and of which one is brand new, and the other only did one return trip between the Netherlands and France. One day in the near future, I have to make a list with all the gear I still have, and try to sell everything, as I won't buy another (working) barge anymore. Peter. |
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Post by peterboat on Mar 13, 2019 13:35:07 GMT
No it wouldn't, Peter, because the power a propeller needs, or absorbs, is proportional to rpm cubed, instead of being directly proportional to the rpm. This relationship is defined by what's known as the Propeller Law curve and is often plotted on graphs alongside engine power curves. The power absorbed by a correctly matched propeller at two thirds maximum rpm is a lot less than the engine that's turning it is capable of producing at two thirds it's maximum rated rpm. As I said Tony, I know nothing about electric motors and their power torques, but I know that they're in no way comparable with an internal combustion engine. On my barges I've all kinds of props that were supposed to be the best, they all moved the boats, but only one was really very good, with lots of trust in forward and in reverse aswell, which is quite exceptional, they were German props of the make OSTERMANN from Köln, of which I still have 2 bronze spare props with a 98 cm diam, that were for a GM 8V-71 of 240 Hp @ 1800n with a 3.83 : 1 Twin Disc MG-509 reduction gearbox, and of which one is brand new, and the other only did one return trip between the Netherlands and France. One day in the near future, I have to make a list with all the gear I still have, and try to sell everything, as I won't buy another (working) barge anymore. Peter. Thats a big prop if only you had the right size for me, anyway its a job to do when I get back from holiday as removing it will be no mean feat whilst its still in the water |
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Post by TonyDunkley on Mar 13, 2019 13:56:24 GMT
No it wouldn't, Peter, because the power a propeller needs, or can absorb, is proportional to rpm cubed, instead of being directly proportional to the rpm. This relationship is known as the Propeller Law curve and is often plotted alongside engine power curves. The power absorbed by a correctly matched propeller at two thirds maximum rpm is a lot less than the engine that's turning it is capable of producing at two thirds it's maximum rated rpm. Tony it can already turn the prop at 933 rpm and that is in direct drive so two to one gearing would double its torque so it would easy manage 1500 rpm at the propeller, Electric motors without load can and will spin out of control until they explode, they arnt governed like diesels to protect them. HP isnt the needed to turn a prop its torque that will do it. The problem for me is I dont want a gearbox as its something else to go wrong, its far easier to remove pitch from the propeller, increasing motor speed with reduced load.
The type of power source that drives any propeller is irrelevant to how much power, or torque, will be needed to drive a given propeller at a given speed, or rpm. Torque is simply an element of power - the two terms cannot be considered as separate entities because power is the rate of doing work, and is calculated from torque and rpm. If you want to calculate the torque the electric motor is producing at the rpm it can achieve coupled directly to the existing propeller then use the equation : Torque (in ft.lbs) = Horsepower X 5252 Rpm If you want to use the Propeller Law curve with torque instead of horsepower, then the torque a propeller needs at any given rpm is proportional to the rpm squared. |
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Post by TonyDunkley on Mar 13, 2019 14:45:33 GMT
I might have missed the answer elsewhere, but just out of interest Peter, are you aiming to get the same performance out of your boat with the electric motor as you had with the diesel ?
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Post by Deleted on Mar 13, 2019 18:38:51 GMT
Electric motors have a completely different power curve to IC engines.
I'd say larger diameter and reduced pitch. Buy another prop keep the existing one in case of a return to diesel in future.
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Post by tonyqj on Mar 13, 2019 19:17:34 GMT
Electric motors have a completely different power curve to IC engines. I'd say larger diameter and reduced pitch. Buy another prop keep the existing one in case of a return to diesel in future. Consensus elsewhere suggests that a large diameter with small pitch will be more prone to picking up rubbish than a small diameter with greater pitch. |
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Post by TonyDunkley on Mar 13, 2019 19:20:10 GMT
Peter, what you're proposing to do with that big reduction in the pitch isn't going work out at all well.
If you keep the maximum rpm at the propeller at 1500, then you'll lose a big chunk of boat speed, and if you increase shaft rpm to compensate for the reduced lift, then with a 19'' x 6.5'' prop operating at such high rpm on a heavy displacement hull your propeller efficiency will drop by a huge amount. In fact you'll probably lose more propulsive effort from the very low propeller efficiency than you would ever do from mechanical and friction losses in a gearbox, or a timing/tooth belt drive.
What is the rated horsepower and maximum rpm for the motor you've got ?
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Post by Deleted on Mar 13, 2019 19:49:22 GMT
Electric motors have a completely different power curve to IC engines. I'd say larger diameter and reduced pitch. Buy another prop keep the existing one in case of a return to diesel in future. The power curve is irrelevant - that virtually Maximum torque is available from rest makes no odds to the propeller - Tony Dunkley has nailed that bit of the problem. peterboat you need some reduction, 1:1 has its limitations on smaller boats (like our Elysians) but are employed because often there physically isn't the space to swing a larger prop with a reduction gearbox. Acceptable performance can be had using a 1:1 ratio without taxing the engine too much. On your big boat the opposite is true, a finely pitched 19" prop on a 1:1 ratio will be awful, no shove, no stopping, it may well spin up ok (rule of thumb is +/- 1" on a given diameter prop is worth -/+ 200 rpm) but it will be bloody terrible to get your boat along the water. as Tony suggests You can do the reduction with belts and pulleys, no need for a gert big gearbox in between, you can reverse the motor direction electrically - something not so easy done on an IC engine! |
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