Propulsion Power

[David Mack]

16 minutes ago, Antrepat said:

What's been said about real power delivered out of the propellor is interesting too.  It seems that hardcore whirling about beyond a certain amount of power is futile owing to the fluid dynamics of the channel and the fundamental inefficiency of a propeller especially in shallow water.  Tracy D'Arth mentioned a rule of thumb that you won't succeed in applying more than about 5hp no matter what you do.  I suspect the tendency towards bigger engines is owing to engine salemanship, like someone suggested, and questionable assumptions, rather than evidence that more and more power really helps do anything other than stir up the silt and cause cavitation and climate change.

 

Some years ago we took Fulbourne (full length ex working boat drawing 3 ft) up the Ashby, which has a reputation for being a bit shallow. As soon as we turned off the Coventry I felt the boat was running slower, and I increased the engine revs to compensate. Progress was tedious, noisy and frankly not very pleasant.  But we pushed on up to the far end, turned round and came back. As we passed one shallow spot I recognised from the way up, I realised that in the intervening few days I had got used to the shallowness, the engine was running at much lower revs, the stern end wasn't digging in and scraping the bottom, and as a result our speed over the ground was higher (or at least seemed it) compared with the way up. A salutary lesson in the benefits of using less power!

[Antrepat]

What David says about his experience on the Ashby is interesting.  Less is more.  It tends to add weight to the idea that applying excessive power achieves nothing or can actually make things harder under the most difficult conditions.

 

If I could sum up the impression I'm getting, it's this: don't sweat too much about having an underpowered boat as overpower is much more likely and, no matter how powerful your drive is, deploying more and more will result only in waste.  What your drive needs to do is push enough power through the inefficiencies to max out at about 5hp off the back of the propeller, since applying more power in will not result in more power out beyond that, under the circumstances.  From this perspective, an electric drive using that 5kW motor I was mentioning would need to be 75% efficient, the 8kW version 50% efficient, and a 20hp diesel 25% efficient.  How do these efficiencies compare with practical reality?

[Alan de Enfield]

5 minutes ago, Antrepat said:

If I could sum up the impression I'm getting,

 

Adding more power simply makes the back-end squat deeper so you drag thru the mud which slows you down, so you apply more power and squat deeper so .................

 

You don't need a lot of power to 'get going', the acceleration maybe slow but you'll get there (remember the worlds strongest man competition - hauling a truck from a standing start ..............)

When you do need POWER is when you need to try and stop quickly.

Edited February 26, 2021 by Alan de Enfield

[IanD]

33 minutes ago, Antrepat said:

What David says about his experience on the Ashby is interesting.  Less is more.  It tends to add weight to the idea that applying excessive power achieves nothing or can actually make things harder under the most difficult conditions.

 

If I could sum up the impression I'm getting, it's this: don't sweat too much about having an underpowered boat as overpower is much more likely and, no matter how powerful your drive is, deploying more and more will result only in waste.  What your drive needs to do is push enough power through the inefficiencies to max out at about 5hp off the back of the propeller, since applying more power in will not result in more power out beyond that, under the circumstances.  From this perspective, an electric drive using that 5kW motor I was mentioning would need to be 75% efficient, the 8kW version 50% efficient, and a 20hp diesel 25% efficient.  How do these efficiencies compare with practical reality?

There were a lot of numbers about this in the thread on electric boats.

 

Diesel generator at full output, 25% efficient.

Diesel engine at normal canal cruising speed (3kW), <20% efficient (>30% at full power), maybe only 10% when passing moored boats (1kW)

A good PMAC electric motor plus controller is >90% efficient

 

For normal cruising on a normal canal a good guesstimate is 3kW/4bhp at the prop (depending on exact speed), much more than this and you'll be making a breaking wash (except on wide deep canals). Power rises as cube of speed, so a lot more is needed on a river against the current (at least 15kW/20bhp?) if you ever intend to do this, maybe even a bit more if you want to tackle the Ribble link (where you also need to keep this up for several hours).

 

Quick stopping is another issue. When we hired the steamer Firefly many years ago (about 3shp flat out) it glided along very nicely, accelerated like a snail on Valium, and stopped like -- well, by hitting things if you got it wrong. That boat really shouldn't have speeded through that blind bridge without looking or hooting -- or hearing our whistle...

Edited February 26, 2021 by IanD

[peterboat]

6 hours ago, dmr said:

 

Most people like to fit engines of about 30Hp, though increasingly think that 40Hp is preferable.

Some boats have old vintage engines of about 18Hp and these do struggle a bit on a river.

I am talking of 57 foot type boats here rather than smaller shallower ones.

The electric motor sellers rather tell us that the motors that they can produce are adequate, and sometimes even point out that smaller motors are fine because they produce full torque at zero speed (great in a locomotive, irrelevant in a boat). I would not want a boat with a 5hp electric motor.

 

My own view is that for serious boating about 30Hp is needed, especially to cope with rivers, emergency stops and getting "unstuck". Its actually quite difficult to do this with an electric motor, in part because we like to limit voltage to 48. At some point some more adventurous people might start using higher voltages.

 

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

 

I do Dave, I am at 72 volts, I tried at 60 volts but it didnt have the right acceleration or stopping power.

[peterboat]

18 hours ago, Antrepat said:

It's my naive impression that a lot of rule-of-thumbing goes on when it comes to selecting engines or motors of an appropriate power for a boat of a particular length, beam, hull design, material, and weight.  This impression has been formed having tried to work out how much power is really being deployed, and finding that no one anywhere seems to have published any specific data about this except for a guy with deep pockets, a battery that could power a village, and a small nuclear power station for a generator.  What's the drag for a narrow boat, a barge, a cruiser?  How does it vary with hull surface condition, length or draught?  If anyone knows, they don't seem to be saying, unless you count Lynch Motors' article or this chap who went round in circles on here trying to work something out a few years ago.  I could also be that boat builders and designers keep this kind of lore close to their chests so that the punters don't get ideas above their station.

 

This is probably where someone will pop up and post a link to a comprehensive public database and reveal the inadequacy of my web searching skill, but anyway, I did find what seemed to be a reasonable way of estimating and I thought it might be interesting to share it on here.  It's quite possible that this is a widely-known method and all I'll get is eye-rolling sighs and muttering of "another clueless newbie", but here we go.  I'm not a physicist, I'm not a mathematician, and I'm not a marine engineer, but this is what I've found out.

 

For about 150 years the horse was the predominant way to move a narrow boat, and although an unfettered horse will walk at 4mph, obviously pulling a 50-ton boat it's going to go a bit slower.  Lots of people seem to have wondered how fast passenger flyboats went (about 10mph, apparently - allegedly they got them to plane, which is pretty cool) but there wasn't much about how fast a regular plodder went.  2-3mph, reckons the blog of Whilton Marina.  A typical horse can do work at a rate of about 750W (actually 745.7W, or 33,000lb-ft/min - 50% more than the average pit pony, according to Mr Watt's observations), and this one horsepower seems to have fair validity, so it's a good start to assume that about 750W will move a laden wooden 72ft working narrow boat at perhaps 2mph, and an unladen one at perhaps 3mph.  I know the mass in itself doesn't limit the speed, but a laden boat would sit lower in the water so would have more drag.  It would of course also be harder to get going.

 

The power you need to move something at a constant speed with a resistive force (drag) is the exact counter-force required multiplied by the speed: P = Fv.  As the speed changes, the drag changes with the square of the speed, and is also a product of the cross-sectional (wetted) area of the hull (2.1*0.6=1.26 m^2), the density of the water (997 kg/m^3), and a factor called the drag coefficient.

 

The drag coefficient represents how the object's shape (boat hull) and the fluid behave.  We know that 745.7W will move a narrow boat at 2-3 mph, we know the density of water and the cross-sectional area of the typical hull, so we know the force that power was overcoming.  I believe that all means we can calculate the drag coefficient (https://en.wikipedia.org/wiki/Drag_(physics)) because .  (That's force = 1/2 x density x speed squared x drag coefficient x area.)

 

Suppose the speed is actually 2.5mph (1.12 m/s) - a bit of allowance for the fact that today's boats have steel hulls (a bit smoother?) and are nowhere near as heavy and hence low in the water as a laden working boat.  The power is 745.7W so, by P = Fv, the force applied at that speed is 667 N.  Now that we know that, we can calculate the drag coefficient CD as 0.8505.

 

There you go, the estimated and very approximate drag coefficient of a 72ft narrow boat; but wait, maybe I don't want to go at 2.5mph.  How much power do I need to go at other speeds?  Well, now we know the coefficient, we can just plug all the speeds into the force equation:

According to that, 10hp out the propeller would get you 5mph.  I don't know what the efficiency is of a typical diesel drive but I've heard it's not great, with loss through heat, mechanical friction and fluid viscosity in the engine.  The 40ft narrow boat I went on holiday on in October (Chas Hardern's terrific Thorin) slugged up to Ellesmere against the current and took a lot of welly to make 3mph - and Thorin is not underpowered by any means.  An electric drive with 50% efficiency would need to deliver 20hp continuous (yes, I know the canal limit is 4mph) to hold that speed.  Things I've read have talked of maybe 3hp to maintain a fair canal speed, and that does approximately fit in with the data here, but the chap with the big generator took proper measurements and reckoned 2.5kW for 3mph, 6kW for 3.5mph and 13kW for 4mph.  He was measuring his Amps, so this is before drive mechanical losses and electrical heat dissipation - just as well, since otherwise it implies a drag coefficient of 6.5, and with that, you'd need about seven horses to get your boat to 2.5mph, and to pull out the control rods of the nuclear power station to get to 4mph.  I'm going to assume that the steeper, deeper curve from the input power measurements is owing to system losses growing faster as power consumption increases.

 

My boat is diddy, only 30', so how does the drag vary with boat length?  I thought about this and I realised there are two components: the skin drag, which is the friction of the water passing the hull and is going to be proportional to length, and what I suppose you'd call the bow force, the effort required to push the water aside at the bow.  Most narrow boats seem to have basically the same bow shape (please don't jump on me, experts, this is just a convenient assumption) so I think it's a fair assumption that some component of the drag is actually constant between narrow boats regardless of length, at a given speed.  I have absolutely no idea what size this component is so I guessed at 20% of the total drag at my reference speed of 2.5mph.

 

I set up my spreadsheet to scale a proportion of the calculated drag force with length, but to include an unscaled component which I set as 20% of the 2.5mph force, and I recalculated for 30/72.

If some lunatic came up with the totally bonkers idea of trying to make a 30' narrow boat electric, he could now see that pushing 4kW of power would be needed for a continuous 4mph at 50% efficiency, and that eight hours of that would need 26kWh of power, and that even if he did manage to cram 1kW of solar on that roof between the vents and the gas flue and the stove chimney and the hatch, he'd probably only get 2kWh out of it on a grey day, and he'd be running a generator for 7 hours of the day, so he'd be best slowing the blazes down to 3mph where he'd only need 11kWh (14.6kWh allowing for domestic loads and bursts of full power).

 

I had a lot of fun doing this, and it might be complete rubbish, but it would probably be fun also to have a discussion about it, if anyone wants to comment.  Please, please let's not rehearse the old arguments about diesel vs electric.  I know all that and this is just an idea I'm exploring for amusement during lockdown!

So I have 2 electric boats both widebeams and on big deep waterways the broads cruiser runs at 24 volts and has a boaty shape cruising at 2 to 3 mph is a doddle on a few amps stopping takes everything its got. light weight shallow draft is the way to go for economical cruising big roof lots of solar perfik

My widebeam is a different kettle of fish 3.3 kw achieves 3 ish mph going faster isnt worth the effort, I have 4.6 kw of solar and a 6 kw genny and both those running would not not achieve 5mph for the day and thats with a 36 kwh battery bank

[Machpoint005]

52 minutes ago, Alan de Enfield said:

When you do need POWER is when you need to try and stop quickly.

 

Yes - this, especially in a narrow waterway when the boat coming towards you eats up most of your stopping distance.

 

Another point to bear in mind is that canal boat building and maintenance are very conservative businesses. If you get a problem on an unusual engine, you are likely to have to fix it yourself with parts nobody anywhere near you has in stock.

 

[Antrepat]

41 minutes ago, peterboat said:

I do Dave, I am at 72 volts, I tried at 60 volts but it didnt have the right acceleration or stopping power.

What length is your boat, Peter?  Is it a narrow boat?

 

Sorry, you already said.

Edited February 26, 2021 by Antrepat

[dmr]

26 minutes ago, peterboat said:

I do Dave, I am at 72 volts, I tried at 60 volts but it didnt have the right acceleration or stopping power.

 

That must be the way to go. I think most cars are quite a bit higher. I believe various extra safety regs kick in at various voltages but I can't remember the details.

 

A nice slow revving electric motor with a direct coupling to a biggish prop would be the ideal, did you manage this? I have had a quick look at a few motors and all develop their rated power at quite high speeds forcing the use of a reduction system.

 

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

[peterboat]

11 minutes ago, Antrepat said:

What length is your boat, Peter?  Is it a narrow boat?

57 x 12 very heavy widebeam 

12 minutes ago, dmr said:

 

That must be the way to go. I think most cars are quite a bit higher. I believe various extra safety regs kick in at various voltages but I can't remember the details.

 

A nice slow revving electric motor with a direct coupling to a biggish prop would be the ideal, did you manage this? I have had a quick look at a few motors and all develop their rated power at quite high speeds forcing the use of a reduction system.

 

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

Direct drive forklift motor it's a dc series motor lots of torque but not high revs has worked ok for over 2 years now 

[cuthound]

14 minutes ago, dmr said:

 

That must be the way to go. I think most cars are quite a bit higher. I believe various extra safety regs kick in at various voltages but I can't remember the details.

 

A nice slow revving electric motor with a direct coupling to a biggish prop would be the ideal, did you manage this? I have had a quick look at a few motors and all develop their rated power at quite high speeds forcing the use of a reduction system.

 

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

 

The IET regulations state that 120 volts DC or less is considered low risk, but most of the higher voltage batteries I have seen (typically 400 v DC for a UPS) broke the battery into 50 volt sections when it was isolated.

[peterboat]

2 minutes ago, cuthound said:

 

The IET regulations state that 120 volts DC or less is considered low risk, but most of the higher voltage batteries I have seen (typically 400 v DC for a UPS) broke the battery into 50 volt sections when it was isolated.

My I3 battery bank is 400 volts built up of 48 volts blocks, if I was doing it again I would be tempted by a secondhand pack as they aren't expensive 

[TheBiscuits]

1 hour ago, IanD said:

steamer [...]

 

That boat really shouldn't have speeded through that blind bridge without looking or hooting -- or hearing our whistle...

 

 

Funny things steam whistles.  A few years ago I very nearly hit @DHutch on Emily Anne at Litherland swing bridge - he had clearly whistled but my brain didn't register it as a boat noise at all until I came round the blind corner and he was right there in the bridge narrows.  I wasn't speeding though, so stopped before bouncing off him!

[IanD]

12 minutes ago, cuthound said:

 

The IET regulations state that 120 volts DC or less is considered low risk, but most of the higher voltage batteries I have seen (typically 400 v DC for a UPS) broke the battery into 50 volt sections when it was isolated.

I'm pretty sure that the threshold above which extra safety precautions are needed is 60V DC, which is why all the automotive mild hybrids and hybrid boats use 48V nominal systems.

 

Once you go above this precautions similar to mains voltages are needed, so most BEV are 400V to reduce current and some of the newer ones are 800V.

[dmr]

20 minutes ago, peterboat said:

57 x 12 very heavy widebeam 

Direct drive forklift motor it's a dc series motor lots of torque but not high revs has worked ok for over 2 years now 

 

Its been a very long time since I did any electric motor stuff, but I thought serious motors do high start up/low speed torque and high speeds/lighter loads  which is not ideal for a boat? Could you easily  modify it?

 

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

[peterboat]

5 minutes ago, dmr said:

 

Its been a very long time since I did any electric motor stuff, but I thought serious motors do high start up/low speed torque and high speeds/lighter loads  which is not ideal for a boat? Could you easily  modify it?

 

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

That's why 3 mph is ok it's a light load 3.3 kw full power is 23 kw plus and its good stopping power 

[cuthound]

20 minutes ago, IanD said:

I'm pretty sure that the threshold above which extra safety precautions are needed is 60V DC, which is why all the automotive mild hybrids and hybrid boats use 48V nominal systems.

 

Once you go above this precautions similar to mains voltages are needed, so most BEV are 400V to reduce current and some of the newer ones are 800V.

 

I've been out of the game for a few years now, but my copy of BS7671:2018 states

"Voltage, nominal. Voltage by which an installation (or part of an installation) is designated. The following ranges
of nominal voltage (rms values for AC) are defined:"

– Extra-low. Not exceeding 50 V AC or 120 V ripple-free DC, whether between conductors or to Earth.

– Low. Exceeding extra-low voltage but not exceeding 1000 V AC or 1500 V DC between conductors, or
600 V AC or 900 V DC between conductors and Earth.

 

– High. Normally exceeding low voltage.

 

 

Perhaps someone still working with the regulations can confirm?

 

[dmr]

1 minute ago, cuthound said:

 

I've been out of the game for a few years now, but my copy of BS7671:2018 states

"Voltage, nominal. Voltage by which an installation (or part of an installation) is designated. The following ranges
of nominal voltage (rms values for AC) are defined:"

– Extra-low. Not exceeding 50 V AC or 120 V ripple-free DC, whether between conductors or to Earth.

– Low. Exceeding extra-low voltage but not exceeding 1000 V AC or 1500 V DC between conductors, or
600 V AC or 900 V DC between conductors and Earth.

 

– High. Normally exceeding low voltage.

 

 

Perhaps someone still working with the regulations can confirm?

 

 

Calling 1500v DC a low voltage suggests you worked in a very different world to many of us ?

 

Electronic engineers regard 3.3 as low, 5 as medium, and 12 as getting dangerous.

 

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

[Antrepat]

Quote

Diesel generator at full output, 25% efficient.

Diesel engine at normal canal cruising speed (3kW), <20% efficient (>30% at full power), maybe only 10% when passing moored boats (1kW)

A good PMAC electric motor plus controller is >90% efficient

 

Are these all full-system efficiencies (post-propeller), or pre-propeller?  20% pre-propeller sounds atrocious, but 90% post-propeller sounds astonishing.  30hp at 2/3 power would be 20hp at 20% efficiency which would of course be 4hp, 3kW.

 

If I wanted to cruise for 8 hours at 3kW, does that mean 8x3kWh (never mind starting, stopping, manoeuvering) coming out the battery, or delivered out of the prop with additional losses to be accounted for to estimate battery size and power input capacity?  It makes an enormous difference, potentially.  Lynch quote 93% optimal for the Marlin motors, but seem to use 50% efficiency in their system estimations, which would mean 8x6kWh.  From all I've read, that seems far more than is practically consumed by real boats going at normal speeds.

 

(Obviously my original estimates were based on actual propulsive work done, which is why understanding the efficiency is crucial to making it useful to estimate powertrain specifications.)

 

Quote

Power rises as cube of speed, so a lot more is needed on a river against the current (at least 15kW/20bhp?) if you ever intend to do this, maybe even a bit more if you want to tackle the Ribble link (where you also need to keep this up for several hours).

 

Ah, of course, P=Fv and F=d A v^2 CD, so P=d A v^3 CD.  Duh.  I didn't work that through.

[peterboat]

11 minutes ago, Antrepat said:

 

Are these all full-system efficiencies (post-propeller), or pre-propeller?  20% pre-propeller sounds atrocious, but 90% post-propeller sounds astonishing.  30hp at 2/3 power would be 20hp at 20% efficiency which would of course be 4hp, 3kW.

 

If I wanted to cruise for 8 hours at 3kW, does that mean 8x3kWh (never mind starting, stopping, manoeuvering) coming out the battery, or delivered out of the prop with additional losses to be accounted for to estimate battery size and power input capacity?  It makes an enormous difference, potentially.  Lynch quote 93% optimal for the Marlin motors, but seem to use 50% efficiency in their system estimations, which would mean 8x6kWh.  From all I've read, that seems far more than is practically consumed by real boats going at normal speeds.

 

(Obviously my original estimates were based on actual propulsive work done, which is why understanding the efficiency is crucial to making it useful to estimate powertrain specifications.)

 

 

Ah, of course, P=Fv and F=d A v^2 CD, so P=d A v^3 CD.  Duh.  I didn't work that through.

In my case 3.3 kw is what the motor is consuming to maintain 3mph in deep and wide water 

[dmr]

The very low efficiencies for a diesel engine are based on the fraction of the available potential energy of the fuel that is converted into mechanical output. Lots gets wasted as exhaust and coolant heat.

 

You could do something similar for electric by including power station losses, grid losses, charging losses etc etc.

 

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

[Antrepat]

2 minutes ago, peterboat said:

In my case 3.3 kw is what the motor is consuming to maintain 3mph in deep and wide water 

Twice the beam, 57ft, but without the issues of a narrow and shallow channel.  Hmm.  This is a damned minefield...

[Alan de Enfield]

1 minute ago, dmr said:

You could do something similar for electric by including power station losses, grid losses, charging losses etc etc.

 

 

And then conclude "maybe diesel isn't so bad after all"

[Antrepat]

1 minute ago, dmr said:

The very low efficiencies for a diesel engine are based on the fraction of the available potential energy of the fuel that is converted into mechanical output. Lots gets wasted as exhaust and coolant heat.

 

You could do something similar for electric by including power station losses, grid losses, charging losses etc etc.

 

Yes indeed, grid power comes a long way and passes through numerous losses before reaching sockets, and I believe there's surprisingly low theoretical limits on the thermal efficiency of a power station anyway, aren't there?

 

So is that efficiency of 20-30% referring to power at the shaft of the engine, or does it include propeller loss too?  That's what I can't tell.  If the 20-30% is halved again by prop losses, does that make 50% a good actual-mechanical-work-done efficiency for electric (where the motor itself is optimally turning >90% of the power coming out the battery into mechanical force on the prop shaft)?

[IanD]

33 minutes ago, Antrepat said:

 

Are these all full-system efficiencies (post-propeller), or pre-propeller?  20% pre-propeller sounds atrocious, but 90% post-propeller sounds astonishing.  30hp at 2/3 power would be 20hp at 20% efficiency which would of course be 4hp, 3kW.

 

If I wanted to cruise for 8 hours at 3kW, does that mean 8x3kWh (never mind starting, stopping, manoeuvering) coming out the battery, or delivered out of the prop with additional losses to be accounted for to estimate battery size and power input capacity?  It makes an enormous difference, potentially.  Lynch quote 93% optimal for the Marlin motors, but seem to use 50% efficiency in their system estimations, which would mean 8x6kWh.  From all I've read, that seems far more than is practically consumed by real boats going at normal speeds.

 

(Obviously my original estimates were based on actual propulsive work done, which is why understanding the efficiency is crucial to making it useful to estimate powertrain specifications.)

 

 

Ah, of course, P=Fv and F=d A v^2 CD, so P=d A v^3 CD.  Duh.  I didn't work that through.

All the numbers I was talking about were power at the prop shaft. Propeller efficiency varies but is usually in the 50%-60% region, so 3kW/4hp at the shaft is just over 1.5kW/2hp power in the water.

 

Lynch's motors are pretty rubbish to be honest, modern brushless PMAC motors are far better with peak efficiency >96%, probably around 94% under actual conditions. Add 4% loss for the controller and you get 90% from battery to motor. On top of 55% typical prop efficiency, this gives you 50% from battery (3kW) to water (1.5kW).

 

For an electric boat you could assume 3kW (from the battery) when cruising on the open canal, 1kW when passing moored boats, and 0kW when sitting in a lock, so your overall power per day depends on how much time you spend doing what. If you take an 8 hour day to be 4 hours cruising/2 hours passing/2hours in locks this means you consume 14kWh. If you do the western part of the Cheshire Ring in a day like I did (25 miles IIRC, no locks) you'll use more, maybe 25kWh or so.

 

You also need to allow for any solar power if you have panels fitted, and any onboard power. Even with as much solar as you can fit on a narrowboat (yielding maybe 8kWh/day in summer, 1kWh/day in winter?), unless you're only going to cruise 2-3 days a week in summer (and not at all in winter) you'll need an onboard diesel generator.

 

Crunch all these numbers and you end up with maybe 40% lower fuel consumption than a diesel, assuming you use the generator a lot. If all the power comes from solar, your direct fuel cost is zero. None of this comes anywhere close to making up the extra cost of installation compared to a diesel even over a long lifetime, but that's not the reason people are choosing electric boats -- or series hybrids to be more accurate -- it's the silence and zero vibration when travelling. In future when/if the canal gets charging points they'll also be much greener, but this is probably many years away.

 

Don't do it for the money, do it because you want to. So long as you have deep pockets...

Edited February 26, 2021 by IanD