Electric?

[dmr]

6 minutes ago, Alan de Enfield said:

Being a boater of simple-brain it's all beyond me, but this pic shows us doing 6 mph into a 5mph current, about 1500rpm

 

The two central 'disturbances' are the props, the two outer 'disturbances' are the hull 'edges'

 

 

 

That sums it up nicely, the outer disturbance is the same in both cases (towed and prop driven), the inner ones are the excess energy from prop drive. My engineers brain wants to know how much of the inner one is down to propulsion and how much due to prop inefficiency.

 

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

[Tacet]

11 hours ago, IanD said:

OK, here are the equations:

 

Force*distance=energy

Energy/time=power

Force*distance/time=power

Distance/time=speed

Force*speed=power

 

Speed here is how fast the thing doing the pushing (or pulling) is moving relative to whatever it's pushing (or pulling) against.

 

Ignoring the exact units, take a force of 1 unit to pull (or push) the boat, which is travelling at 1mph (land speed) upstream against a 3mph current.

 

The tractor is pushing against the ground so for it speed is 1mph, so power is 1 unit.

 

The propellor is pushing against the water so for it speed is 4mph, so power is 4 units.

 

It might seem magic, but it's physics really ?

Hmm.  So it uses no more power to pull a boat (from the bank) against a flow than it does in still water, assuming the same speed over the ground?

 

And if the boat is travelling with the 3mph flow, it still requires the same (positive) power to make it go only 1mph as soon as you start pulling from the bank?  Must be a special sort of rope

 

 

Edited October 14, 2020 by Tacet

[peterboat]

59 minutes ago, Midnight said:

Nail on head!
I met a fella at Ripon last year boasting about his 'all electric' boat covered in solar panels. When asked how many hours travel he gets he answered about five - not even enough to get down to York from Ripon. So I asked what he did when the batteries run down and he said "start the generator to charge them".
All electric boat hmmmmm!

I have a 36kwh battery bank over 10 hours without sun is achievable, the problem is some people play at electric me I thought it through and so have finesse but don't worry when diesel is 10 quid a litre I am sure you will still cruise all the time 

7 minutes ago, Tacet said:

Hmm.  So it uses no more power to pull a boat (from the bank) against a flow than it does in still water, assuming the same speed over the ground?

 

And if the boat is travelling with the 3mph flow, it still requires the same (positive) power to make it go only 1mph as soon as you start pulling from the bank?  Must be a special sort of rope

 

 

Very special I have tried holding my boat at gainsborough impossible had to tie it up again 

[nicknorman]

18 minutes ago, dmr said:

That sums it up nicely, the outer disturbance is the same in both cases (towed and prop driven), the inner ones are the excess energy from prop drive. My engineers brain wants to know how much of the inner one is down to propulsion and how much due to prop inefficiency.

 

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

Depends on the efficiency of the prop! For example pushing back slowly against a large area of water (large diameter, slower prop) is much better than pushing back faster against a small area of water (small diameter faster prop).

But obviously a fair bit of energy is wasted simply rotating the prop wake. Perhaps there is scope for improvement eg coaxial contra-rotating props, or fixed guide vanes (as per a Jet engine’s turbine stages) to extract some of the rotational energy from the wake and turn it into thrust. 
Probably incompatible with the Great Mattress of Tame Valley though.

[magpie patrick]

28 minutes ago, Tacet said:

Hmm.  So it uses no more power to pull a boat (from the bank) against a flow than it does in still water, assuming the same speed over the ground?

 

 

No the load is greater - the towing unit may not be up to the load

 

However, unlike a tug with a propeller, there is no risk of the towing unit being outpaced by the current. 

The tug on the bank has to pull a bigger load at the same speed

 

The tug on the water has to pull the same load at a higher speed

 

In both cases, the lower the flow the easier the tow

[Mike Todd]

55 minutes ago, Alan de Enfield said:

Being a boater of simple-brain it's all beyond me, but this pic shows us doing 6 mph into a 5mph current, about 1500rpm

 

The two central 'disturbances' are the props, the two outer 'disturbances' are the hull 'edges'

 

 

 

Fluid resistance can sometimes be counter intuitive:

 

Golf balls have dimples which create vortices so one might expect that it would have greater resistance than a smooth ball. However, a smooth ball will only travel a fraction (IIRC about one third) of the distance a conventional golf ball will given the same 'hit'.

[bizzard]

I had a girlfriend with dimples, she could run everso fast against the wind.  And if she ran for a bus, never failed to catch it.

Edited October 14, 2020 by bizzard

[IanD]

26 minutes ago, nicknorman said:

Depends on the efficiency of the prop! For example pushing back slowly against a large area of water (large diameter, slower prop) is much better than pushing back faster against a small area of water (small diameter faster prop).

But obviously a fair bit of energy is wasted simply rotating the prop wake. Perhaps there is scope for improvement eg coaxial contra-rotating props, or fixed guide vanes (as per a Jet engine’s turbine stages) to extract some of the rotational energy from the wake and turn it into thrust. 
Probably incompatible with the Great Mattress of Tame Valley though.

You're right that a bigger slower prop is always better, but this is always limited by practical restrictions like draught and displacement. Wouldn't stop me using a 20" x 16" 4-blade prop with a 2.8:1 gearbox on a Beta 43 instead of an 18" x 12" one with a 2:1 gearbox though (numbers from Beta and propcalc) if I had the choice, an extra 2" of draught in exchange for better starting/stopping power and lower tiller vibration seems a good deal to me -- also the reference works on marine propellers recommends keeping below 1000rpm to minimise noise and cavitation which this does.

 

If there were easy fixes to improve prop efficiency then you'd think big cargo ships would already be doing to to save valuable fuel -- they already use the biggest diameter prop that will fit with a large number of carefully shaped blades (not standard turbine) but that's about it.

 

Having said that there's a new radical propeller design which claims (and which seems to be backed up by tests) significant efficiency increase and fuel savings by removing tip vortices:

 

https://boattest.com/Sharrow-Engineering-Propeller

 

It'll be interesting to see how successful this is, if those savings are delivered on big ships (remains to be seen) you'd expect it to be widely adopted. Or maybe it only really helps with heavily-loaded high-speed propellers like on fast boats, in which case it'll appear there instead of on ships.

 

[doratheexplorer]

2 hours ago, Midnight said:

Nail on head!
I met a fella at Ripon last year boasting about his 'all electric' boat covered in solar panels. When asked how many hours travel he gets he answered about five - not even enough to get down to York from Ripon. So I asked what he did when the batteries run down and he said "start the generator to charge them".
All electric boat hmmmmm!

I wonder if there's a single all-electric, continuous cruising liveaboard with no fossil-fuel burning back-up at all?  I doubt it.  So if ICEs are banned on the cut at some point, there either needs to be some kind of breakthrough or infrastructure investment, or this way of life is over.

[Alan de Enfield]

32 minutes ago, doratheexplorer said:

So if ICEs are banned on the cut at some point,

Its looking like it will be propulsion only.

[doratheexplorer]

4 minutes ago, Alan de Enfield said:

Its looking like it will be propulsion only.

So I'll just fit an electric drive and get a diesel genny?  Are there any environmental benefits to that?

[mrsmelly]

2 minutes ago, doratheexplorer said:

So I'll just fit an electric drive and get a diesel genny?  Are there any environmental benefits to that?

Nope. Just as daft as hybrid cars and range extender cars. It sounds good to those who dont actualy think it through.

[magpie patrick]

There are towns in America, small ones I admit, that depend on diesel generators. They are too far off grid to be, erm, on grid... 

 

There are car recharging points in the Australian outback that are powered by diesel generators - same applies, too far off grid

 

There are villages in Romania that... you get the picture, but they don't use electric vehicles

 

There are places where the only electricity supply is going to be local - IF cars and boats are to go all electric then these places need to be able to service them. Otherwise, in the short term FF cars and boats are retained to reach them (and belch their fumes in town 90% of the time) whilst in the long term they end up even further off grid because cars and boats can't reach them.

[dmr]

26 minutes ago, magpie patrick said:

There are towns in America, small ones I admit, that depend on diesel generators. They are too far off grid to be, erm, on grid... 

 

There are car recharging points in the Australian outback that are powered by diesel generators - same applies, too far off grid

 

There are villages in Romania that... you get the picture, but they don't use electric vehicles

 

There are places where the only electricity supply is going to be local - IF cars and boats are to go all electric then these places need to be able to service them. Otherwise, in the short term FF cars and boats are retained to reach them (and belch their fumes in town 90% of the time) whilst in the long term they end up even further off grid because cars and boats can't reach them.

Alderney in the Channel Islands was run on diesel, dunno if it still is, there was always talk of a cable to France but not sure if it happened. Leccy was expensive.

 

All the people and much of the goods, including fresh veg, came in by plane, they are gonna be in trouble when fossil fuel is banned.

 

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

 

 

[peterboat]

55 minutes ago, mrsmelly said:

Nope. Just as daft as hybrid cars and range extender cars. It sounds good to those who dont actualy think it through.

Yet my daughters Vauxhall ampera range extender hasnt used any petrol this year in its running around so are they daft? she has the ability to go to Leeds and back daily on electric but if need be she can go much further and still run around London for free

42 minutes ago, magpie patrick said:

There are towns in America, small ones I admit, that depend on diesel generators. They are too far off grid to be, erm, on grid... 

 

There are car recharging points in the Australian outback that are powered by diesel generators - same applies, too far off grid

 

There are villages in Romania that... you get the picture, but they don't use electric vehicles

 

There are places where the only electricity supply is going to be local - IF cars and boats are to go all electric then these places need to be able to service them. Otherwise, in the short term FF cars and boats are retained to reach them (and belch their fumes in town 90% of the time) whilst in the long term they end up even further off grid because cars and boats can't reach them.

That might be now but things are changing wind turbines/solar farms are on the increase in the states and Teslas battery storage helps out if the wind doesnt blow or the sun doesnt shine

[David Mack]

3 hours ago, dmr said:

But I think (and I do mean think rather than know) that the plume of water is not an intrinsic part of prop operation, its a result of prop slip and hence inefficiency?

As props gets bigger they are more efficient because the slip is less. Question....is the plume just bigger and slower or is it actually less? I think its less.  In the extreme a very big prop would have zero slip so all the energy would go into moving the boat forwards rather than pushing the water backwards?

 

I think you're wrong here.

Imiagine sitting in a small dinghy, on a windless lake with a load of heavy weights in the bottom of the boat. If you just sit still, nothing happens, and the boat doesn't move. But if you vigorously hurl the weights horizontally over the stern in rapid succession, conservation of momentum tells you that the boat will move forwards. Isn't the propeller doing just the same?

 

2 hours ago, IanD said:

Having said that there's a new radical propeller design which claims (and which seems to be backed up by tests) significant efficiency increase and fuel savings...

 

Didn't Axiom say that...?

Edited October 14, 2020 by David Mack

[IanD]

58 minutes ago, David Mack said:

 

I think you're wrong here.

Imiagine sitting in a small dinghy, on a windless lake with a load of heavy weights in the bottom of the boat. If you just sit still, nothing happens, and the boat doesn't move. But if you vigorously hurl the weights horizontally over the stern in rapid succession, conservation of momentum tells you that the boat will move forwards. Isn't the propeller doing just the same?

 

 

Didn't Axiom say that...?

Axiom said their prop was better in reverse -- which is was, by being worse going ahead. So it was expensive and consumed more fuel.

 

This one seems different, I suggest you go and read the article before pouring cold water on it (ho, ho...)

[Jen-in-Wellies]

1 hour ago, David Mack said:

But if you vigorously hurl the weights horizontally over the stern in rapid succession, conservation of momentum tells you that the boat will move forwards.

The future of boat propulsion right here ^^^^^^^^. Forget diesel engines, electric motors, batteries. All we need is a supply of weights at each sani station to restock. In the winter, drain the canal and retrieve them. To many boats using the canal over the summer and towards the end there won't be enough depth left, so drain and retrieve early.

 

[Murflynn]

6 hours ago, dmr said:

I think we all agree that the force is the same regardless of where it comes from, its the speed and hence energy that is different.  The towing tank is not accurate, among other things I don't think water resistance simply scales with model size, but the ship science people at Southampton have a lot of experience and know all the correction factors.

 

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

there is no linear scaling between full size and models.   I suggest you read up about Froude numbers.  I can assure you that the towing tank results are extremely accurate.

[dmr]

1 hour ago, Murflynn said:

there is no linear scaling between full size and models.   I suggest you read up about Froude numbers.  I can assure you that the towing tank results are extremely accurate.

I remember doing all that Froude stuff at university, I didn't like it then so don't want to look again. Are we not sort of saying the same,? the towing tank does not simply scale but with correct knowledge and experience it works.  If I made a model of a ship and pulled in through the bath and measured the force I expect I would get a very wrong answer.

 

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

[Sea Dog]

3 hours ago, peterboat said:

Yet my daughters Vauxhall ampera range extender hasnt used any petrol this year in its running around so are they daft? she has the ability to go to Leeds and back daily on electric but if need be she can go much further and still run around London for free

Hasn't she been carrying an unused built-in petrol generator around all year to no advantage then?  Instead of paying for that petrol generator up front purely as a contingency, she could have bought a pure electric vehicle and had a substantial sum put by to hire something else if ever she needed it which, patently, she didn't.  Just a thought... 

 

[dmr]

3 hours ago, David Mack said:

 

I think you're wrong here.

Imiagine sitting in a small dinghy, on a windless lake with a load of heavy weights in the bottom of the boat. If you just sit still, nothing happens, and the boat doesn't move. But if you vigorously hurl the weights horizontally over the stern in rapid succession, conservation of momentum tells you that the boat will move forwards. Isn't the propeller doing just the same?

 

 

 

I think there are two ways of looking it at. Throwing a plume of water backwards to propel the boat forward (and conserve momentum) is one way.  Its a bit more complicated than this as water is funny non linear stuff, the pop-pop boat proves this, and also we can move our boat sideways (without the prop turning) by swishing the rudder.

 

The other visualisation is to think about prop pitch, if the water was sort of solid how far would the boat move forwards with one prop revolution? it would move forward a "pitch" with the prop sort of crawling through the water without sending any water backwards.

 

I suspect the reality is somewhere between the two, and the split is prop slip.

 

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

 

.

[peterboat]

2 minutes ago, Sea Dog said:

Hasn't she been carrying an unused built-in petrol generator around all year to no advantage then?  Instead of paying for that petrol generator up front purely as a contingency, she could have bought a pure electric vehicle and had a substantial sum put by to hire something else if ever she needed it which, patently, she didn't.  Just a thought... 

 

Absolutely not we got the Ampera at a bargain price,  she would have to pay double or maybe treble to get something as good or as big, these cars were made at the Cadillac factory in the USA, soit was lot of car for little money 

[Sea Dog]

16 minutes ago, peterboat said:

Absolutely not we got the Ampera at a bargain price,  she would have to pay double or maybe treble to get something as good or as big, these cars were made at the Cadillac factory in the USA, soit was lot of car for little money 

OK, fair enough - you'll have done the research I'm sure!

 

[peterboat]

35 minutes ago, Sea Dog said:

OK, fair enough - you'll have done the research I'm sure!

 

Oh yes i was going to buy it myself but Jaynes parking space at work wouldn't work for it.