Boat stretch

[IanD]

12 minutes ago, beerbeerbeerbeerbeer said:

If you hadn’t dipped out of doing the Trent you’d know. 
 

First thing I’d have done with a boat like yours is take it for a proper descent trial. Otherwise what’s the point? Any old shit like mine can trawl a ditch. 
A State of the Art boat needs testing properly. 
Drive it to the limit. 
 

 

Did that briefly on a deep wide section of the Bridgewater when nobody was looking -- whether this counts as "deep water" in the true sense of the word, I don't know.

 

At full power I think it was putting out about 15kW/20hp into the prop, pushed up a big bow wave and big wash -- *far* bigger than I'd ever use on the canals, and bigger than I ever saw going upstream on the Trent after rain -- and fairly flew along with GPS readings fluctuating somewhere around 7mph.

 

And no it wasn't silent at that speed/power, there was quite a lot of prop/wash noise as you'd expect -- but almost nothing from the motor... 😉

Edited November 16, 2023 by IanD

[David Mack]

42 minutes ago, IanD said:

It can't be deep for anchor purposes but shallow for boat speed purposes, can it?

Yes. Maximum depth matters for anchoring, minimum depth for avoiding grounding or severe shallow water effects.

[IanD]

13 minutes ago, David Mack said:

Yes. Maximum depth matters for anchoring, minimum depth for avoiding grounding or severe shallow water effects.

Understood. All I know is that when I went along the Trent (or the Soar) the boat went *way* faster at the same throttle setting than on a narrow canal, and didn't either squat at the stern as much or have any of the "boating uphill" feeling where the water level drops along the boat. Both were obviously *much* deeper than most of the canals.

 

24 minutes ago, Alan de Enfield said:

 

Do you not think that maybe your lack of 'real life' knowledge about some of the waterways is affecting your ability to post useful answers ?

Theory is great, but putting the theory into practice can be an eye-opener -  maybe a good analogy could be the statement made by a Prussian Field Marshall:

"No plan survives first contact with the enemy".

I don't claim to know everything about all the UK waterways and canals, though I have travelled over most of them (though not the Ribble Link -- at least, not yet) -- probably a couple of thousand miles total over the years, which I suspect is more than many... 😉

 

When it comes to deepwater boating -- meaning estuaries/seas, not "deep water" as above -- I'm happy to admit to having no experience. Or desire to do it, come to that... 🙂

Edited November 16, 2023 by IanD

[magnetman]

The biggest issue on canals is sucking the bottom. A longer boat is going to suck the bottom more than a shorter boat. 

 

Whether this means it needs more power is certainly an interesting question.

 

Of course canals were never designed for self propelled boats. I can imagine the boatmen of yore discussing how one horse was better than another and it was actually all about the diameter and material of the towing line. 

 

Sucking the bottom is a symptom of the fundamentally unsatisfactory nature of screw propellers. 

 

 

 

[Martin Kedian]

And breathe deeply and slowly Ahh 

[IanD]

3 minutes ago, magnetman said:

The biggest issue on canals is sucking the bottom. A longer boat is going to suck the bottom more than a shorter boat. 

 

Whether this means it needs more power is certainly an interesting question.

 

Of course canals were never designed for self propelled boats. I can imagine the boatmen of yore discussing how one horse was better than another and it was actually all about the diameter and material of the towing line. 

 

Sucking the bottom is a symptom of the fundamentally unsatisfactory nature of screw propellers. 

 

 

As already said, a longer boat displaces more water than a shorter boat, so in shallow canals more has to be moved backwards, so the boat goes slower and squats more and needs more power.

 

Just now, Martin Kedian said:

And breathe deeply and slowly Ahh 

 

Nice stretch job, BTW... 🙂

Edited November 16, 2023 by IanD

[beerbeerbeerbeerbeer]

4 minutes ago, IanD said:

 

Did that briefly on a deep wide section of the Bridgewater -- whether this counts as "deep water" in the true sense of the word, I don't know.

 

At full power I think it was putting out about 15kW/20hp into the prop, pushed up a big bow wave and big wash -- *far* bigger than I'd ever use on the canals, and bigger than I ever saw going upstream on the Trent after rain -- and fairly flew along with GPS readings fluctuating somewhere around 7mph.

 

And no it wasn't silent at that speed/power, there was quite a lot of prop/wash noise as you'd expect -- but almost nothing from the motor... 😉

Is briefly enough?

Run it at full pelt for a few hours at least. 

Everything on your boat is brand new, blast the hell out of it and see what it does. 
 

By the way you’re suddenly

vague: 

Somewhere around 7mph,

you think 20hp,

deep water?

 

come on get that boat out and blast the shit out of it, 

well as much  as you can with an electric engine, 

not like you’ll blow a gasket or pop a cog 

 

 

[magnetman]

The recessed panels were very nicely executed. 

2 minutes ago, IanD said:

 

As already said, a longer boat displaces more water than a shorter boat, so in shallow canals more has to be moved backwards, so the boat goes slower and squats more and needs more power.

Nice stretch job, BTW... 🙂

 

 

What about in deep canals ?

 

I suppose the Caledonian and the MSC are probably deep. 

 

I suspect that most other canals are much less than 7ft deep (3 x depth rule with a draught of 28 inches. 

 

[IanD]

Just now, beerbeerbeerbeerbeer said:

Is briefly enough?

Run it at full pelt for a few hours at least. 

Everything on your boat is brand new, blast the hell out of it and see what it does. 
 

By the way you’re suddenly

vague: 

Somewhere around 7mph,

you think 20hp,

deep water?

 

come on get that boat out and blast the shit out of it, 

well as much  as you can with an electric engine, 

not like you’ll blow a gasket or pop a cog 

 

 

I'm an engineer, and if numbers are not 100% accurate I use words like "about" because that's the correct thing to do... 😉

 

"About 15kW/20hp" -- OK, if you want more detail the DC power draw was varying from second-to-second between about 16kW and 17kW, I know the motor efficiency at this power is about 95%, I don't know about the motor controller but I think it's similar, put all these together and the answer is "about 15kW/20hp" -- with some uncertainty either way. Still probably a damn sight more accurate than any similar power figure for a diesel though, assuming you had one... 😉

 

Same with "somewhere around 7mph" -- as has often been pointed out GPS receivers aren't very accurate at low speeds and the readings fluctuate, in this case bounding around between maybe 6mph and 8mph but I didn't take any recording so "about 7mph" is the best I can do.

 

"Deep water" is just based on how the boat went and looking at the wake and how much it squatted -- no I didn't jump overboard with a measuring stick.

 

I'm sure of I'd given any accurate figures like 14.7kW or 7.1mph the first thing you'd have done is question their accuracy and how I measured them... 😉

 

When I get it onto a nice deep wide river where I can thrash it flat out for some time with a clear conscience then that's what I'll do, but I'm not going to do that on a canal like the Bridgewater -- I suppose if I had you'd have berated me for having a breaking wash? (which I certainly did).

 

Any other bones you'd like to pick, or is that it? 😉

[Francis Herne]

As a third party to the debate so far without any personal investment (and a 30ft boat owner painfully aware of hull speed), there's quite a familiar pattern here from other threads:

 

5 hours ago, IanD said:

If you look at the power needed to propel a narrowboat in deep water -- with or without a current -- it hardly varies with length

[...]

So yes, on most UK canals a longer boat does need more power -- but not for river use or against currents, which was what was mentioned.

 

You've made a theoretical statement which is - within certain assumptions - correct, significant and quite interesting. I didn't realise that hull speed vs. drag balanced each other that neatly on narrowboats.

 

In this case you even stated (some of) the assumptions to begin with!

 

People disagreed with the validity of the assumptions for reaching your final conclusion:

 

  - That a high required speed will always be in deep water, because the Ribble Link (which was one of the cases mentioned even before your post) is notoriously fast-flowing while being about three feet deep mid-channel at some points. Some river sections can be too, albeit not quite that shallow.

 

 - That (implied by the second line I quoted) straight-line maximum speed meaningfully reflects convenient or safe handling on rivers, because effects like flow pushing the bow outward on upstream bends usually make navigation unsafe before you're literally going backwards.

 

5 hours ago, IanD said:

I did say that those numbers were in deep water (e.g. a river) -- if people want to argue with basic hydrodynamics in this case (which is what Vicprop uses) they're onto a loser.

 

On narrow/shallow canals a longer/heavier boat does need more power, as I also explained -- so no contradiction with what people have found, then... 😉

 

You then dismissed the challenges to your assumptions by simply restating them, including the factually-wrong one about the Ribble Link that you included directly in later posts.

 

Any further comment is portrayed as "disagreeing with basic physics" rather than the actual disagreements with the assumptions (or whether the assumed case covers enough in practice to support your conclusions) that you still haven't meaningfully addressed.

 

Then everyone repeats themselves five times with no further progress, leading to great frustration all round, including us poor folk who merely have to read through it.

 

Please try a bit harder to understand which part of your statements people are trying to (constructively) criticise - often things you've implied rather than the theory directly stated - before kicking off the usual dismissal-followed-by-mutual-flamewar thing.

Edited November 16, 2023 by Francis Herne

[IanD]

14 minutes ago, magnetman said:

The recessed panels were very nicely executed. 

 

What about in deep canals ?

 

I suppose the Caledonian and the MSC are probably deep. 

 

I suspect that most other canals are much less than 7ft deep (3 x depth rule with a draught of 28 inches. 

 

 

The boundary where depth effects start to have an effect is about 3x draught, they get severe below 2x ("shallow water"), but are negligible above 5x ("deep water") -- so most canals are shallow water, bigger rivers are deep water, deep canals (how deep?) and shallower rivers (how shallow?) are somewhere in between.

 

There's no "brick wall" dividing line, as the water gets shallower the depth effects (drag and squat) get worse, gradually at first and then more rapidly as the water gets shallower.

[Alan de Enfield]

27 minutes ago, magnetman said:

 

 

I suspect that most other canals are much less than 7ft deep (3 x depth rule with a draught of 28 inches. 

 

 

If you look at C&RTs dredging manuals, where any dredging is done, pretty much all canals are dredged to  1 to 1.3 metre, so you are looking at a multiplyer of  ~1.5x draft

 

The Llangollen is dredged to 0.7mts and relies on boats ploughing the channel for them

Edited November 16, 2023 by Alan de Enfield

[Guest]

1 hour ago, IanD said:

I'm giving up here,

 

Cough........

[IanD]

45 minutes ago, Francis Herne said:

 

As a third party to the debate so far without any personal investment (and a 30ft boat owner painfully aware of hull speed), there's quite a familiar pattern here from other threads:

 

 

You've made a theoretical statement which is - within certain assumptions - correct, significant and quite interesting. I didn't realise that hull speed vs. drag balanced each other that neatly on narrowboats.

 

In this case you even stated (some of) the assumptions to begin with!

 

People disagreed with the validity of the assumptions:

 

  - That a high required speed will always be in deep water, because the Ribble Link (which was one of the cases mentioned even before your post) is notoriously fast-flowing while being less than three feet deep mid-channel at some points. Some river sections can be too, albeit not quite that shallow.

 

 - That (implied by the second line I quoted) straight-line maximum speed meaningfully reflects convenient or safe handling on rivers, because effects like flow pushing the bow outward on upstream bends usually make navigation unsafe before you're literally going backwards.

 

 

You then dismissed the challenges to your assumptions by simply restating them, including the obviously factually-wrong one about the Ribble Link that you included directly in later posts.

 

Any further comment is portrayed as "disagreeing with basic physics" rather than the actual disagreements with the assumptions (or whether the assumed case covers enough to be useful information) that you still haven't meaningfully addressed.

 

Then everyone repeats themselves five times with no further progress, leading to great frustration all round, including us poor folk who merely have to read through it.

 

Please try a bit harder to understand which part of your statements people are trying to (constructively) criticise - often things you've implied rather than the theory directly stated - before kicking off the usual dismissal-followed-by-mutual-flamewar thing.

 

I've been trying to spot which arguments were valid and which were just misunderstanding, but it's not always easy especially when people don't seem to have read what was actually written... 😉

 

I'm pretty sure that the only time most canal boats ever get anywhere near full power (apart from emergency stops) is going upstream on rivers, that's when you *need* the power to make way against an opposing current, equivalent to making maybe 6mph or so in still water. Most of the rivers where this happens are quite wide and deep -- not everywhere, but generally -- especially at times when you get a lot of flow, which tends to be after rains when the level is high -- and when you need the power anyway. In these "deep-water" cases you won't need more power because the boat is longer, which is what I said.

 

If the water is shallower/narrower (most canals) then other factors take over like displacement, and boat size/weight affects starting/stopping and manoeuvring -- and in these cases a longer boat will need more power -- but then in shallow/narrow canals full power is almost never needed except in an emergency (e.g. starting/stopping). This is the case everyone seems to keep focusing on as trying to disprove the above "deep-water" case, but it's not the same case. If parts of the Ribble Link and Trent are shallower, then they're obviously not deep-water -- my error for not spotting this.

 

There's no hard-and fast "brick-wall" boundary between the two cases, more a continuous increase in drag/squat as the water gets shallower. The case where most power is needed is most likely to be in relatively deep water, and usually a longer boat won't need more power. In the "shallow-water" case (most canals) where a longer boat needs more power than a shorter one, the absolute power level will usually be lower so a bigger engine is less likely to be needed, the smaller one can still provide plenty of power.

 

Plenty of "usuallys" and "oftens" inserted to make it clear that this isn't black-and-white, there are lots of shades of grey in between.

 

Is that better? 😉

 

P.S. Thanks for the cool and impartial analysis, much appreciated to help bring the heat down 🙂

 

Edited November 16, 2023 by IanD

[magnetman]

Heat?

 

This whole thing came up because a computer program says when you lengthen your boat there is no need to upgrade the engine then someone said they spoke to more than one other person who had their boat lengthened and felt afterwards that they could do with a bigger engine.

 

It is a basic case of computer program versus person with boat.

 

It could just be a psychological thing but I do think putting too much faith in computer programs for engine and prop design on canal boats is unwise.

 

 

[Francis Herne]

1 hour ago, IanD said:

I've been trying to spot which arguments were valid and which were just misunderstanding, but it's not always easy especially when people don't seem to have read what was actually written... 😉

[...]

Is that better? 😉

 

Well... no. I've read everything written here (which was quite tedious, thus the objection). Assuming bad faith or that everyone else is an idiot right off the bat is never a good start. But anyway:

 

Part of what was written, in the original post that kicked this off, was:

Quote

So yes, on most UK canals a longer boat does need more power -- but not for river use or against currents, which was what was mentioned.

 

You didn't write "-- but not in a straight line in deep water".

 

The examples given of fast-flowing, non-deep rivers and currents are valid objections to the original statement. You've never acknowledged these, just narrowed the scope to exclude them and then argued that they're out of scope.

 

Excluding them for the moment anyway:

 - On a deep, flowing river, a longer boat has more drag (~linear with length) when it's not in-line with the flow. This requires more power to overcome.

 - Current on bends pushes the bow or stern outward (>linear in length, because the moment arm is longer on top of the increase in side area). This requires more steering authority to overcome - either more speed through more power, or more rudder angle which creates drag and cosine losses requiring more power.

EDIT: perhaps it's just linear. I overlooked that the rudder gets more moment arm too.

 

I stated the second of these clearly in my last post, and the first was mentioned by someone else upthread, yet you completely ignore that and continue to say

Quote

In these "deep-water" cases you won't need more power because the boat is longer, which is what I said.

 

Back to the shallow-water case...

Quote

[...] but then in shallow/narrow canals full power is almost never needed except in an emergency (e.g. starting/stopping). This is the case everyone seems to keep focusing on as trying to disprove the above "deep-water" case, but it's not the same case. If parts of the Ribble Link and Trent are shallower, then they're obviously not deep-water.

The Ribble Link in normal conditions, and parts of the Trent in some conditions (and the Soar, and probably elsewhere), are both very fast-flowing and very shallow. Full power is needed. There are a number of boats that aren't fast enough to safely navigate the Ribble Link.*

 

People mentioning these aren't confusing the deep-water and shallow-water cases, they're providing counterexamples to your assertion that speed and power in the shallow-water case are unimportant.

 

1 hour ago, IanD said:

There's no hard-and fast "brick-wall" boundary between the two cases, more a continuous increase in drag/squat as the water gets shallower. The case where most power is needed is most likely to be in relatively deep water, and usually a longer boat won't need more power. In the "shallow-water" case (most canals) where a longer boat needs more power than a shorter one, the absolute power level will usually be lower so a bigger engine is less likely to be needed, the smaller one can still provide plenty of power.

All the "usually"s make this version of the statement almost correct. (I argue a longer boat does need more power on any "usual" river - there are few dead-straight rivers - but not so much in usual flows that it's likely to be a problem).

 

They also exclude enough real-world situations, unusual overall but still frequently encountered by some boats, that it doesn't support your conclusions explicitly stated in earlier posts and never retracted.

 

 *(CRT advise boats using it should be capable of at least 6mph in deep water -- not because it is deep but because that's the only figure that's practical to replicate and measure for comparison)

Edited November 16, 2023 by Francis Herne
edited a few bits for clarity, particularly rivers, and corrected myself

[Martin Kedian]

Why do I have the feeling my post has been hacked does the underwater shape of the boat have and effect on this ie a chine length of swim distance from counter to prop pitch and size of prop shape and length of bow  double or single curvature. Bitumen or two pack 

any answers on a postcard

[magnetman]

How deep was the water v the thickness of the paint coating?
 

It is possible that no paint could be the answer.

[Martin Kedian]

2 minutes ago, magnetman said:

How deep was the water v the thickness of the paint coating?
 

It is possible that no paint could be the answer.

Perhaps we could Teflon coat the hull to 100 microns 

[magnetman]

This could be an interesting idea.  One could put a smaller engine in the boat and it would go faster.

 

Could be a game changer in the era of excessively stressful diesel prices.

 

 

[TheBiscuits]

1 hour ago, IanD said:

Ribble Link

 

The Ribble Link is Savick Brook.  I think some of the confusion here is that you're claiming the shallow muddy ditch with locks is deep water, which it isn't when navigation is permitted. When the water is flooding over the tops of the locks, you'd not want to be on it in a boat!

 

The River Ribble itself however certainly is deep water, in fact CRT won't allow passage up to the Ribble Link on anything less than a 9m tide or passage down the link on less than an  8.5m tide.

 

 

 

 

[magnetman]

The CRT can't allow or disallow passage on tidal water.

 

 

[TheBiscuits]

4 minutes ago, magnetman said:

The CRT can't allow or disallow passage on tidal water.

 

No, but they only allow you through the rotating sea lock at certain times.  You can still go for a jolly on the River Ribble when there's enough water in the River Douglas, but you're staying out for 24 hours at least ...

 

Preston Docks or Douglas boatyard by prior arrangement are the only realistic options for narrow boats.

[dmr]

4 hours ago, beerbeerbeerbeerbeer said:

and it all gets tricky when the boat comes to a rest on top of a moped on the bottom of the cut,

 

so you’re keeping the revs low because there’s not enough water to draw,

but then you’re not going fast enough to have momentum (or is it inertia?🤷‍♀️?) to travel over the object,

so does one;

creep between low  pounds,

give a blast of revs to get a good ‘push’ across

or ring CRT and ask why the depth isn’t 5 times deeper than than the boats draft?

 

ETA: boat stretch looks ‘bostin’ 👍

 

One of lifes big decisions.

You hit someting in a bridge 'ole and the front raises up, do you do an emergengy stop, or just push on and hope? or use extra power to climb over it?

If you don't stop you might get even more stuck ???? 😀

We hit a big one just above Rochdale a few weeks ago.

 

In reality it all happens so fast there is rarely time to make a decision, and in 90% of cases if you just carry on the boat climbs right over the top.

 

But if you do get stuck its time like this when a lot of extra power really helps.

[beerbeerbeerbeerbeer]

1 minute ago, dmr said:

 

One of lifes big decisions.

You hit someting in a bridge 'ole and the front raises up, do you do an emergengy stop, or just push on and hope? or use extra power to climb over it?

If you don't stop you might get even more stuck ???? 😀

We hit a big one just above Rochdale a few weeks ago.

 

In reality it all happens so fast there is rarely time to make a decision, and in 90% of cases if you just carry on the boat climbs right over the top.

 

But if you do get stuck its time like this when a lot of extra power really helps.

It’s scary when the boat lists big time and you hope to ride it out