Installation of new electric boat charging bollards

[IanD]

2 hours ago, Rishworth_Bridge said:

I'm not going to get into a detailed discussion with you, if only because I'm not an engineer so have only a qualitative understanding of the subject, but we have had this discussion within the Sustainable Boating Group (where we have a couple of engineers) and the conclusion seemed to be that it isn't the torque required to turn the prop which matters - you can easily do it by hand at zero revs - but that required to accelerate it instantaneously from 0 to 400 rpm when drive is engaged. I can't remember the acceleration formula but the torque requirement is much greater. An electric motor winds the speed up relatively gently so, as well as probably having more torque available, the demand is not so great.

On the subject of propeller efficiency I offer two comments. Working backwards from our overall performance by removing the 'stopped' and 'diesel efficiency' savings suggests that we benefit by about 30% from our larger prop. That said, while the 'stopped' saving can be calculated quite accurately, the 'diesel efficiency' one is only an educated guess based on generic diesel engine consumption curves so I wouldn't argue too hard. However, as propeller slippage (at 3 mph in open water) seems to vary from less than 40% for Firecrest (about 45% for us) to about 60% for boats with typical diesel installations it is hard to believe that the benefits of larger propellers aren't more than 10-20% unless there are compensating factors elsewhere in the overall efficiency equation. I can see that greater grip might require more power and that might be self-compensating, though I'm having a similar discussion with a retired aero engineer who provided me with an overall efficiency equation which contained the figure 0.5 for propeller grip (1-slippage, I assume). I'm still waiting for an answer to my question about what happens if this is changed to 0.4 or 0.6.

 

You need someone in the group who understands inertia -- the prop has a much smaller moment of inertia (at least 5x) than any diesel flywheel, even if you connected the prop instantaneously (which most gearboxes don't) the revs would only drop by at most 20% -- and much less than this with traditional diesels with massive flywheels. None of this matters for electric motors which just generate roughly constant torque from zero rpm up to the rated speed (maximum power) with no gearbox, they just accelerate the prop until motor and prop torque are equal.

 

If you do look at propeller efficiency curves (including slip) you'll find that the variation (in narrowboat terms) between a tiny high-speed eggbeater (the worst case) and a huge slow Bolinder-style prop (the best case) the efficiency difference is ~20%, with a "normal" narrowboat prop somewhere in the middle -- so the eggbeater might be ~10% worse than a typical prop and the huge slow prop might be ~10% better.

 

Slip in itself isn't bad, it's also linked to the P/D ratio and the BAR ratio of the prop -- just because there's more (or less) slip doesn't mean the prop is less (or more) efficient.

 

If you overprop an electric motor, all that happens is that at maximum voltage the speed (and power) drops, the motor power is less than rated, and the boat maximum speed drops. I had this discussion with Dave Jesse when he was building Perseverance, he fitted a huge 4-blade prop because it gave the highest possible efficiency. I warned him that this was far too big for his motor and estimated he wouldn't be able to get the (1800rpm rated) motor past 650rpm so less than 40% of rated power, which is exactly what he found on his shakedown cruise (the Ortomarine hybrid boat test).

 

Since an electric motor is constant torque, power is proportional to rpm. To give an easy-to-see example, take a motor rated at 15bhp at 1500rpm and let's see what happens with bigger props (figures from Vicprop calculator) to power/rpm/prop size/maximum speed (in open water, not a canal):

 

15hp/1500rpm : 13.5" x 7.8" prop, 5.9 knots -- optimum size prop for 100% motor power and maximum boat speed

12hp/1200rpm : 14.8" x 9.1" prop, 5.4 knots -- 80% power

9hp/900rpm : 16.6" x 11.3" prop, 4.9 knots -- 60% power

6hp/600rpm : 19.5" x 15.2" prop, 4.3 knots -- 40% power, prop much too big for motor -- similar to initial Perseverance prop

 

So some overpropping doesn't reduce maximum speed as much as you'd think (because power=speed^3) but does mean you're not getting the power out of the motor that you could -- you might as well have paid much less for a smaller motor with a correctly matched prop.

 

6hp/1500rpm : 11.3" x 6.1" prop, 4.3 knots -- same speed as above with much smaller cheaper motor and prop -- but more noise...

Edited May 6, 2022 by IanD

[IanD]

32 minutes ago, peterboat said:

We used to have a propeller designer on here, he advocated big propellers every time and explained why. Now given that it was his living I sort of believe him, it's a shame and our loss that he has left. 

All the stuff I post is taken from what he said, and numerous reliable resources about propeller design. The actual blade design is a bit of a black art (or a computer art nowadays) but the basic physics are not complicated -- a big slow prop is a bit more efficient and is certainly quieter and less prone to cavitation, but it's not a magic bullet... 😉

 

A couple of easy rules-of-thumb (assuming the prop is sized correctly for the engine) is to look at the blade tip speed at maximum power/rpm, which for a slow craft like a narrowboat should be about 25m/s, which means that (rpm*diam)=19000 with diameter in inches -- and P/D (pitch/diameter) should be somewhere around 0.6. If the tip speed is too high then a bigger gearbox ratio and propeller would be better and vice versa.

Edited May 6, 2022 by IanD

[Rishworth_Bridge]

I need more time to thinks about some of your points Ian but I would say that I wouldn't trust Vicprop as far as I could throw it. If you feed the correct figures for Ampère into it it suggest an 18" x 11" prop. That would almost certainly not even give the 6 mph maximum speed requested.  On the recommendation of Crowthers we originally fitted a 19" x 12" prop and were clearly under-propped. After our engineer gave them more information about our motor's characteristics they upgraded their recommendation to our current 20" x 14". This is much better but still revs so freely that I suspect we could go slightly larger still and still reach maximum revs. To get a sensible suggestion from Vicprop you need to tell it that you have double the power you actually have. Inputting 40 hp instead of our actual 20 produces a 21" x 14" recommendation, slightly bigger than what we have.

 

Putting numbers into your tip-speed equations, we score 20,000 and 0.7 which I think agrees that we are likely still to be slightly under-propped. Only the £1,000 cost of doing so deters me  from trying a larger prop.

[IanD]

5 hours ago, Rishworth_Bridge said:

I need more time to thinks about some of your points Ian but I would say that I wouldn't trust Vicprop as far as I could throw it. If you feed the correct figures for Ampère into it it suggest an 18" x 11" prop. That would almost certainly not even give the 6 mph maximum speed requested.  On the recommendation of Crowthers we originally fitted a 19" x 12" prop and were clearly under-propped. After our engineer gave them more information about our motor's characteristics they upgraded their recommendation to our current 20" x 14". This is much better but still revs so freely that I suspect we could go slightly larger still and still reach maximum revs. To get a sensible suggestion from Vicprop you need to tell it that you have double the power you actually have. Inputting 40 hp instead of our actual 20 produces a 21" x 14" recommendation, slightly bigger than what we have.

 

Putting numbers into your tip-speed equations, we score 20,000 and 0.7 which I think agrees that we are likely still to be slightly under-propped. Only the £1,000 cost of doing so deters me  from trying a larger prop.

The Vicprop figures agree pretty closely with recommended props for various diesel engines, very closely with Dave Jesse's experience, with other published figures I've found for absorbed propellor power, and with the discussions I've had with Finesse, so I think they're reasonably accurate.

 

So I'd certainly be interested to see what numbers you have for Ampere which suggest that they're wrong.

 

The tip-speed equation and P/D ratio are not precise requirements, but if the numbers for a boat are far away from these it suggests that design changes might improve things -- your numbers are close enough to suggest that this isn't the case for Ampere 🙂

[Rishworth_Bridge]

Data entered for Ampère. all units as required.

 

Length: 55

Beam: 6.5

Draft: 2.5

Displacement: 51500

Engines: 1

HP: 20

RPM: 1000

Gearing: 1

Bearings: 1

Speed: 5 (won't allow 5.2)

 

This then comes back with 18.2" x 11.3" for a 3-bladed prop, suggesting at the same time that this will achieve 5.65 knots (6.5 mph). Upping the power to 40 hp produces the suggestion of 20.9" x 13.6" and a maximum speed of 7.13 knots (8.2 mph). We didn't investigate the maximum speed with our original, 19" x 12" prop as we were never in suitable water but can say that with our current 20" x 14" we can just about achieve our target 6 mph.

[PD1964]

What is the suitable water and water depth?

 One of Finesse’s first electric boats “Azzura” passed me a few years ago as I was moored up, very quiet on the outside, but I knew it was coming about 400m away by the sound of the high pitch of the prop through the water whilst I was inside my boat. Very Mosquito like and distinctive compared to a Diesel engine.

 

Edited May 7, 2022 by PD1964

[IanD]

5 hours ago, Rishworth_Bridge said:

Data entered for Ampère. all units as required.

 

Length: 55

Beam: 6.5

Draft: 2.5

Displacement: 51500

Engines: 1

HP: 20

RPM: 1000

Gearing: 1

Bearings: 1

Speed: 5 (won't allow 5.2)

 

This then comes back with 18.2" x 11.3" for a 3-bladed prop, suggesting at the same time that this will achieve 5.65 knots (6.5 mph). Upping the power to 40 hp produces the suggestion of 20.9" x 13.6" and a maximum speed of 7.13 knots (8.2 mph). We didn't investigate the maximum speed with our original, 19" x 12" prop as we were never in suitable water but can say that with our current 20" x 14" we can just about achieve our target 6 mph.

 

Don't forget that those speeds are in deep water, the measurements from the Ortomarine trials showed speed between 50% and 70% of the Vicprop numbers at the same power levels, depending on canal depth and width.

 

Are you using specification figures for the motor rpm/power or measured ones? Do you know what power and rpm you achieve in real life at full power with the 20" x 14" prop?

 

Just to check the Vicprop numbers, I compared them with the manufacturer's recommended prop sizes across the whole Beta Marine range, and they do come out a little bit smaller (an average of 4% lower pitch and diameter) -- but then the Vicprop notes say that they're sized for 90% of maximum rpm, and the recommended Beta Marine sizes are all multiples of an inch, so I wouldn't expect exact agreement.

 

Increasing the engine power entered into Vicprop by about 15% reduces the average prop size error to almost zero, so it looks like this is a good idea when using it to predict narrowboat prop size -- always assuming Beta Marine know how to size their propellers... 😉

 

If I do this for Ampere (23hp), Vicprop comes back with 18.7" x 11.7".

 

This all suggests that to get your prop (20" x 14") up to 1000rpm would (as you say) need much more than 20hp -- are you sure you get up to 1000rpm on 15kW?

 

If you do, there's something strange going on somewhere... 😞

[IanD]

4 hours ago, PD1964 said:

What is the suitable water and water depth?

 One of Finesse’s first electric boats “Azzura” passed me a few years ago as I was moored up, very quiet on the outside, but I knew it was coming about 400m away by the sound of the high pitch of the prop through the water whilst I was inside my boat. Very Mosquito like and distinctive compared to a Diesel engine.

 

That could have been before they got the custom-made direct-drive water-cooled motors that they use today, prop size on these is very similar to a typical diesel so prop noise should be the same at the same rpm/speed/power. The motor itself makes almost no noise, it's on flexible mounts with no belt whine or cooling fan noise -- unlike some electric boats...

 

Maybe prop singing was what you heard? This can affect any boat, and can usually be fixed with some tweaks to the prop.

Edited May 7, 2022 by IanD

[Rishworth_Bridge]

Almost all the figures I use are from specifications as I don't have the equipment to undertake independent measurements.

 

I have always assumed that the problem with Vicprop and similar sites is that they are set up for diesels, which have significantly lower torque at maximum revs than at (approx) 2/3rds maximum. As props are basically sized for maximum rpm, it specifies them on the basis of this reduced torque. A Beta 43's torque at maximum revs, when I assume that it is putting out its full 43 hp, is just under 110 Nm - less than 75% of ours - though there is an interesting mathematical mismatch here. Power = revs*torque = 2,800 *110/1000 = 25.5kW = 34 hp. What happened to the other 9? Not sure where I got this idea from but isn't there a torque^2 factor somewhere in propeller theory? If so, we would sit at 185% of the Beta 43, close enough to the double I had observed.

 

As for reaching 1,000 rpm, we didn't monitor it closely as we were principally watching a rather dodgy speedo app to check that we could reach 6 mph. However, we certainly got damned close, too close to account for the discrepancies we are discussing.

[PD1964]

38 minutes ago, IanD said:

That could have been before they got the custom-made direct-drive water-cooled motors that they use today, prop size on these is very similar to a typical diesel so prop noise should be the same at the same rpm/speed/power. The motor itself makes almost no noise, it's on flexible mounts with no belt whine or cooling fan noise -- unlike some electric boats...

 

Maybe prop singing was what you heard? This can affect any boat, and can usually be fixed with some tweaks to the prop.

  It was one of their first, if not their first all elect boat and as you say before they started using their own designed engine and system. 
Their Crick boat this year is certainly different from what I’ve seen and will be technically more advanced then anything exhibited down there I suspect. So should be of interest to the elect tech boat wannabes and will get their vote for best boat I imagine. 

Edited May 7, 2022 by PD1964

[IanD]

1 hour ago, Rishworth_Bridge said:

Almost all the figures I use are from specifications as I don't have the equipment to undertake independent measurements.

 

I have always assumed that the problem with Vicprop and similar sites is that they are set up for diesels, which have significantly lower torque at maximum revs than at (approx) 2/3rds maximum. As props are basically sized for maximum rpm, it specifies them on the basis of this reduced torque. A Beta 43's torque at maximum revs, when I assume that it is putting out its full 43 hp, is just under 110 Nm - less than 75% of ours - though there is an interesting mathematical mismatch here. Power = revs*torque = 2,800 *110/1000 = 25.5kW = 34 hp. What happened to the other 9? Not sure where I got this idea from but isn't there a torque^2 factor somewhere in propeller theory? If so, we would sit at 185% of the Beta 43, close enough to the double I had observed.

 

As for reaching 1,000 rpm, we didn't monitor it closely as we were principally watching a rather dodgy speedo app to check that we could reach 6 mph. However, we certainly got damned close, too close to account for the discrepancies we are discussing.

Props are usually set up so that at maximum power (at rated rpm) the prop absorbs the full power of the engine, regardless of whether it's a diesel or electric, because otherwise you can't access all the power you've "paid" for. At all lower rpm the prop cannot absorb the full power of the engine, because as revs drop prop torque falls rapidly, and engine torque either rises (diesel) or stays flat (electric) -- see the attached plots from Hybrid Marine for a Beta 43 (peak power at 2800rpm), and the Beta datasheet. Propellor torque rises as rpm^2 so absorbed power rises as rpm^3 (blue line). Your calculations are wrong because you're using mixed units, 110nm at 2800rpm is 32kW, see Beta 43 datasheet...

 

BTW if I put in your rated 20hp+15% (23hp) at 1000rpm to Vicprop, it comes back with 18.7" x 11.7" and 5.9kts (6.8mph). If I put in 80% power (16hp+15%) at 800rpm (constant torque electric motor, power proportional to rpm) it comes back with 20.5" x 13.8" and 5.5kts (6.3mph) -- this is very close to your actual prop (20" x 14"), so I suspect this is what you'll actually be getting, about 20% lower than maximum motor power (12kW/16hp) but only losing 0.5mph off top speed, so not a big problem.

 

The bigger prop does mean slightly lower rpm (and noise) when cruising at typically 3kW/4hp, 585rpm with the matched prop compared to 505rpm with your bigger prop.

 

Edited May 7, 2022 by IanD

[peterboat]

35 minutes ago, PD1964 said:

  It was one of their first, if not their first all elect boat and as you say before they started using their own designed engine and system. 
Their Crick boat this year is certainly different from what I’ve seen and will be technically more advanced then anything exhibited down there I suspect. So should be of interest to the elect tech boat wannabes and will get their vote for best boat I imagine. 

Is it the square sterned one? The one that left a couple of months ago?

[PD1964]

37 minutes ago, peterboat said:

Is it the square sterned one? The one that left a couple of months ago?

No, I doubt that would work as a show boat, as the couple are living in it, so doubt they would want strangers rummaging through their draws.
 I hear they’re taking the big white one everyone’s working on at the minute, a new approach to inland boating, they always do well with the bigger boat👍

Edited May 7, 2022 by PD1964

[IanD]

2 minutes ago, PD1964 said:

No, I doubt that would work as a show boat, as the couple are living in it, so doubt they would want strangers rummaging through their draws. I hear they’re taking the big white one everyone’s working on at the minute, a new approach to inland boating.  

You mean this one?

[peterboat]

10 minutes ago, IanD said:

You mean this one?

I have seen that progress from sheet metal to what's on the picture, it's a big beast which lived in the bottom shed fo a long time, that's if it's the same boat 🤣

[PD1964]

1 hour ago, IanD said:

You mean this one?

No, this one😂 changes every day, today was handrails(folding)👍

1 hour ago, peterboat said:

I have seen that progress from sheet metal to what's on the picture, it's a big beast which lived in the bottom shed fo a long time, that's if it's the same boat 🤣

Same boat, I hear this is going to Crick, I haven’t heard if a Narrow is going, but could all change by the time of the show.

Edited May 7, 2022 by PD1964

[Victor Vectis]

Good Gawd!

That boat is just 'orrible.

 

(If it had a mother she would be the only person that could love it)

[David Mack]

If that is a boat intended for boating, how are you supposed to get on and off the bows?

[Slow and Steady]

2 minutes ago, David Mack said:

If that is a boat intended for boating, how are you supposed to get on and off the bows?

You're not!

A triumph of style over practicality and I'm not even sure about that. It is certainly different though.

[ditchcrawler]

On 07/05/2022 at 17:33, PD1964 said:

No, I doubt that would work as a show boat, as the couple are living in it, so doubt they would want strangers rummaging through their draws.
 I hear they’re taking the big white one everyone’s working on at the minute, a new approach to inland boating, they always do well with the bigger boat👍

A lot of show boats are privately owned few month old boats, often with their owners onboard at the show with people looking in their draws

[PD1964]

1 hour ago, ditchcrawler said:

A lot of show boats are privately owned few month old boats, often with their owners onboard at the show with people looking in their draws

Maybe, but I wouldn’t want 100’s of strangers walking through my home touching things. 

[peterboat]

Just now, PD1964 said:

Maybe, but I wouldn’t want 100’s of strangers walking through my home touching things. 

Strange but true, I have been on numerous boats at the show, that are owned and the owners sometimes have been present.

[Slow and Steady]

Just now, peterboat said:

Strange but true, I have been on numerous boats at the show, that are owned and the owners sometimes have been present.

Maybe they got a discount for agreeing to do this.

[peterboat]

1 hour ago, Slow and Steady said:

You're not!

A triumph of style over practicality and I'm not even sure about that. It is certainly different though.

It's extremely large and I sure will be well appointed, it's going to have a lot of solar and it's electric drive 

[Momac]

1 hour ago, David Mack said:

If that is a boat intended for boating, how are you supposed to get on and off the bows?

Plenty of boats have no access on and off the bows