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1 hour ago, Mike Adams said:

I was assuming you had a narrowboat. The main problem with such a large propeller with little pitch will be with weed and plastic. Anything around the prop will load the prop much more than a smaller one because it is pushing the rubbish at a higher speed and have more more effect on the prop performance because of the lower pitch. In theory a larger propeller would be more efficient but would need fairly thin blades - not a good idea on a canalboat. If a large prop with a very low pitch worked well you would see more of them about which is not my experience however good luck with it. The method of producing the power at the shaft is not important nor is torque at low speed since the propeller offers little resistance at very low speed.

That's a very interesting observation that I have not really thought about before. We are 21 by 20 and its very rare to get a prop fouling that actually stops the engine, mostly the engine keeps on turning but the boat just goes slower and slower.

 

...............Dave

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1 hour ago, Mike Adams said:

I was assuming you had a narrowboat. The main problem with such a large propeller with little pitch will be with weed and plastic. Anything around the prop will load the prop much more than a smaller one because it is pushing the rubbish at a higher speed and have more more effect on the prop performance because of the lower pitch. In theory a larger propeller would be more efficient but would need fairly thin blades - not a good idea on a canalboat. If a large prop with a very low pitch worked well you would see more of them about which is not my experience however good luck with it. The method of producing the power at the shaft is not important nor is torque at low speed since the propeller offers little resistance at very low speed.

 

1 minute ago, ditchcrawler said:

If you have spoken to these experts why ask us, non of us have experience of electric propulsion

Because there are other electric boaters out there that might have had the same experience as I am having with a similar size electric boat, the experts have in one case made the fastest electric boat made and some of the others have much bigger boats than mine

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3 hours ago, peterboat said:
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?
 
PS The motor is only 6 KW but torque is huge of these DC series electric motors, so traditional prop sizing charts dont work

 

I feel that gearing down is the way to go, toothed belts? They are more efficient than gearboxes. 

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.As hinted at above you need a new prop, not the existing one re-pitched.  

 

I agree that diameter is more useful than pitch, but only within some practical limits- the prop has to be a sensible shape both in its hydrodynamic proportions and in  mechanical form, so that it is reasonably efficient or you will simply waste your limited power.

 

  At 19 x 6.5 it might be right to absorb the motor power, but the boat  may be  slow.  6.5 in pitch at 1500 rpm will give a no slip speed of ~9mph.  Slip will be somewhere between 50 and 70% depending on water depth,  width, rudder angle, swim design, yada yada.  So flat out will give you about 3-4mph.  Possibly OK on the cut, not much use for river work and your batteries will be producing a lot of current

 

19 x 6.5 is a pitch /diameter ratio of about 1/3, which is low for a canal boat.  Heavy displacement boats typically run P/D  of at least 0.5 up to 1 or more.

 The prop's propensity to pick up everything will be much enhanced!

 

I doubt you will actually be able to reduce the pitch successfully by as  much as you would like.  A 1/3 reduction is a big change to the blades angles.  As a reference, Crowthers could/would  not reduce the pitch of my 21 x 13 prop by more than 1 inch.

 

The golden rules for a prop selection are:

 

Estimate the power you need for the speed you want, or can get out of your hull allowing for its form (displacement/planing/ prop riding/ hydroplane etc.  all have different power/speed requirements)

Choose the pitch so that, allowing for slip, the prop will go through the water at the boat speed you want and the prop speed the power unit can deliver.

Select the diameter so that the available power is all absorbed at the full-power prop speed.

 

Check that the resulting pitch- diameter makes sense.  If not, try again,  with a different power unit set-up- reduction gearing/different characteristic/whatever.

 

So in your case:

 

What hull speed will 6 kW give you in open water? 

 What is the pitch needed to give about 1.5 times that speed at 1500 motor  rpm ?  1500 rpm allows for some losses in the stern gear and because

What is the diameter at that pitch to absorb 6 kW?

 

Is the pitch/diameter ratio that results more than ~0.7 and less than ~1?

 

The prop calculators don't care about the type of motor- just its power output and shaft speed.   Any or all of them will give you a fair starting point, as will a decent prop maker.

 

 

The other factor I expect you will have to consider is the current draw at canal  cruising speed, as opposed to that needed for optimising the motor/prop combi.  That is what will determine battery endurance and, possibly,  life.

N

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For 8hp at 1650rpm the Vicprop calculator says you only need a tiny prop, 11.3" x 5.6" for a 3-blade prop. The reason is without gearing down you need the prop to turn pretty fast but have very little power available to do this.

 

With a 2:1 gearbox/toothed belt this changes to 17.1" x 11.1" 3-blade which is not far off your existing 19" x 10".

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6 minutes ago, nb Innisfree said:

I feel that gearing down is the way to go, toothed belts? They are more efficient than gearboxes. 

I have done that on my other electric boat, but that had a totally different dc motor, Revs are the enemy of a loaded series motor also if the belt breaks the motor could blow itself to pieces!! If you hit the throttle on a diesel it winds itself up to maximum revs, mine just goes for it instantly! with a big plume of water behind it, it also hits 1000rpm before dropping to 933, I will consult Woodwards when back from holiday really must go now as I have lots still to do

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5 minutes ago, BEngo said:

I doubt you will actually be able to reduce the pitch successfully by as  much as you would like.  A 1/3 reduction is a big change to the blades angles.  As a reference, Crowthers could/would  not reduce the pitch of my 21 x 13 prop by more than 1 inch.

N

Crowthers successfully reduced ours from 14" down to 12", though they did say if it had not been one of their props it might not have been successful

 

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9 minutes ago, BEngo said:

.As hinted at above you need a new prop, not the existing one re-pitched.  

 

I agree that diameter is more useful than pitch, but only within some practical limits- the prop has to be a sensible shape both in its hydrodynamic proportions and in  mechanical form, so that it is reasonably efficient or you will simply waste your limited power.

 

  At 19 x 6.5 it might be right to absorb the motor power, but the boat  may be  slow.  6.5 in pitch at 1500 rpm will give a no slip speed of ~9mph.  Slip will be somewhere between 50 and 70% depending on water depth,  width, rudder angle, swim design, yada yada.  So flat out will give you about 3-4mph.  Possibly OK on the cut, not much use for river work and your batteries will be producing a lot of current

 

19 x 6.5 is a pitch /diameter ratio of about 1/3, which is low for a canal boat.  Heavy displacement boats typically run P/D  of at least 0.5 up to 1 or more.

 The prop's propensity to pick up everything will be much enhanced!

 

I doubt you will actually be able to reduce the pitch successfully by as  much as you would like.  A 1/3 reduction is a big change to the blades angles.  As a reference, Crowthers could/would  not reduce the pitch of my 21 x 13 prop by more than 1 inch.

 

The golden rules for a prop selection are:

 

Estimate the power you need for the speed you want, or can get out of your hull allowing for its form (displacement/planing/ prop riding/ hydroplane etc.  all have different power/speed requirements)

Choose the pitch so that, allowing for slip, the prop will go through the water at the boat speed you want and the prop speed the power unit can deliver.

Select the diameter so that the available power is all absorbed at the full-power prop speed.

 

Check that the resulting pitch- diameter makes sense.  If not, try again,  with a different power unit set-up- reduction gearing/different characteristic/whatever.

 

So in your case:

 

What hull speed will 6 kW give you in open water? 

 What is the pitch needed to give about 1.5 times that speed at 1500 motor  rpm ?  1500 rpm allows for some losses in the stern gear and because

What is the diameter at that pitch to absorb 6 kW?

 

Is the pitch/diameter ratio that results more than ~0.7 and less than ~1?

 

The prop calculators don't care about the type of motor- just its power output and shaft speed.   Any or all of them will give you a fair starting point, as will a decent prop maker.

 

 

The other factor I expect you will have to consider is the current draw at canal  cruising speed, as opposed to that needed for optimising the motor/prop combi.  That is what will determine battery endurance and, possibly,  life.

N

Great post and very helpful I already know that 50 amps gives me 3mph because I tested it before I had the tacho on it, It would go faster at full throttle but then the issue is excess heat being produced because the motor isnt spinning at its optimum revs, I have had it on prop charts and they do seem to give different results which is not good so as I say I will visit Woodwards and see what they come up with. My mate however does have a couple of props so I may play with those first who know one might be right

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34 minutes ago, dmr said:

How can you say "more horsepower than torque" when they are different units????

I don’t say it, the author of the site says it. He simply means that you’ll be below the crossover point of the two curves. If you read it in context it makes sense. 

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14 minutes ago, WotEver said:

I don’t say it, the author of the site says it. He simply means that you’ll be below the crossover point of the two curves. If you read it in context it makes sense. 

I was using "you" in the non specific sense. ?

I have just looked at the link and it is a direct numerical equality....hence his statement is only valid for imperial engines and won't apply to metric engines ??

 

................Dave

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38 minutes ago, dmr said:

I was using "you" in the non specific sense. ?

I have just looked at the link and it is a direct numerical equality....hence his statement is only valid for imperial engines and won't apply to metric engines ??

 

................Dave

Metric engines just don’t have the same power as good old-fashioned imperial engines, obviously...

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45 minutes ago, WotEver said:

Metric engines just don’t have the same power as good old-fashioned imperial engines, obviously...

Actually somebody with a vintage engine told me exactly that,  they don't use modern horsepower but proper British brake horsepower.

 

.............Dave

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2 hours ago, dmr said:

Actually somebody with a vintage engine told me exactly that,  they don't use modern horsepower but proper British brake horsepower.

 

.............Dave

Actually you are correct long stroke slow revving engines are perfik for boats, powering a large propeller, same as steam engines and big DC series electric motors. I will try to borrow another propeller before chopping mine down

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On 13/03/2019 at 15:09, dmr said:

How can you say "more horsepower than torque" when they are different units????

 

................Dave

Precisely. That is a nonsense statement.

Also someone is getting confused using USA units rather than the metric system.

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2 hours ago, yabasayo said:

That is a nonsense statemen

Someone else who hasn’t read the link to understand the meaning...

 

2 hours ago, yabasayo said:

someone is getting confused using USA units rather than the metric system.

Yes, because American units are much more powerful than metric ones aren’t they :D

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On 13/03/2019 at 15:27, IanD said:

For 8hp at 1650rpm the Vicprop calculator says you only need a tiny prop, 11.3" x 5.6" for a 3-blade prop. The reason is without gearing down you need the prop to turn pretty fast but have very little power available to do this.

 

With a 2:1 gearbox/toothed belt this changes to 17.1" x 11.1" 3-blade which is not far off your existing 19" x 10".

The first technically correct post on this subject.

ie you need a prop which can absorb the rated continuous output power of the motor through a combination of diameter and pitch which suits a slow, heavy displacement boat. The figures given by the OP suggest that his current prop is overloading the motor by approx 100 %  - which is why it gets hot.

Edited by yabasayo
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13 hours ago, yabasayo said:

The first technically correct post on this subject.

ie you need a prop which can absorb the rated continuous output power of the motor through a combination of diameter and pitch which suits a slow, heavy displacement boat. The figures given by the OP suggest that his current prop is overloading the motor by approx 100 %  - which is why it gets hot.

I would say about 35% oversized and with luck I will confirm it next week, I have done an electric boat before but with a different motor so gearing on that was the way forward, DD series motors are slow spinning high torque Beastie's and the right propeller with direct drive will give me the best results

I would like to see 1650 rpm flat out which will give me 700 rpm cruising fingers crossed

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40 minutes ago, Mike Adams said:

I hope you are correct. I think at 700rpm the prop will only absorb 0.75KW if it takes 6KW to turn it at 1650rpm. Maybe the one horse is enough! It always used to be.

Normally around 3kW/4bhp at the prop is needed for canal cruising as a rule of thumb. If 6kW turns the prop at 1650rpm, 3kW will turn it at about 1200rpm. 700rpm, no way...

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On 21/03/2019 at 13:27, IanD said:

Normally around 3kW/4bhp at the prop is needed for canal cruising as a rule of thumb. If 6kW turns the prop at 1650rpm, 3kW will turn it at about 1200rpm. 700rpm, no way...

3.3kw already gives me 3 mph at that point the tacho wasnt fitted so I didnt know what rpm it was spinning at next week a slightly smaller prop with less pitch will be fitted then I will see how that fares.

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As it happens I have been given a 16 x 14 prop so that is getting cleaned up and fitted, so I should know soon how that performs, its 3 inch smaller which if it was the same pitch should give me 600rpm or so but it isnt so I am in unknown waters with this?

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31 minutes ago, ditchcrawler said:

Out of interest how have you coupled the motor to the propshaft?

That was easy, I had a centre flex on there which I removed, I also had a flange on the motor which I removed, I machined up a new flange with a centering piece on it, and the holes to match the cetreflex coupler and put the whole thing together. spins true, the motor is a drive motor so it has thrust bearings in it so no issues there either.

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37 minutes ago, peterboat said:

That was easy, I had a centre flex on there which I removed, I also had a flange on the motor which I removed, I machined up a new flange with a centering piece on it, and the holes to match the cetreflex coupler and put the whole thing together. spins true, the motor is a drive motor so it has thrust bearings in it so no issues there either.

OK I was just thinking if you fouled the prop and tried to stall the motor. That wont happen will it.

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