Fluid Flow

[hughc]

There has been an interesting discussion, albeit on an inappropriate thread, about the shape of rudders and hulls.  The phrase, 'a brick in a drain', referring to the way a narrow boat swims  was used to suggest that improvements to the shape effects only miniscule improvements. In free water improvements to swim shapes etc. will have an effect but in a restricted flow channel incremental improvements will have a greater 'relative' effect. It is true that the average modern narrow boat has a poor hydrodynamic performance but then many seem to be built down to a price by and for a market that is more interested in maximising internal space than in how a boat swims. Have any tank tests been carried out to determine the best hull shape for a restricted channel ?  Many small improvements will have a result 'greater than their parts.'

[BEngo]

The only ones I am aware of were undertaken by Alvechuchurch and all related to the introduction of their "bulbous bow". I believe BW were also involved but can't remember who the academics were.  Thee was an article about the results in WW.

 

I did once try to interest the fluid mechanics folk at RNEC  Manadon in  doing some experimental work in their flume  but understandably they did not think it was very 'Naval' so was not the sort of thing they could fund.

 

N

 

 

[Bee]

Good question. It is usually thought that modern narrow boats follow their cargo carrying predecessors. I think its more likely that many follow the cheap and cheerful designs of the early years of leisure boat designs, boxy, built to a price, right angles everywhere, easy to weld 'sacrificial edge' along the bottom and more. I sometimes think that Springers might have advantages with a bit of a chine, slightly 'v' bottom and with a bit of work at the ends might be quite good.  Canals usually have a 'saucer shaped' profile and hulls really ought to have a similar cross section and fine tapered ends with a bit of sheer to look good. Thing is of course that it would cost more to build, more to fit out and there's a good chance that without those flat sides it would be a sod to reverse 

[Tony Brooks]

1 hour ago, Bee said:

Good question. It is usually thought that modern narrow boats follow their cargo carrying predecessors. I think its more likely that many follow the cheap and cheerful designs of the early years of leisure boat designs, boxy, built to a price, right angles everywhere, easy to weld 'sacrificial edge' along the bottom and more. I sometimes think that Springers might have advantages with a bit of a chine, slightly 'v' bottom and with a bit of work at the ends might be quite good.  Canals usually have a 'saucer shaped' profile and hulls really ought to have a similar cross section and fine tapered ends with a bit of sheer to look good. Thing is of course that it would cost more to build, more to fit out and there's a good chance that without those flat sides it would be a sod to reverse 

 

You mean that the majority are not a sod to reverse? The ones that seem better at reversing are the ones with long swims.

[blackrose]

All the speeds canal boats move hull shape, fluid flow & hydrodynamics seem like a bit of an academic pastime to me. 

[ditchcrawler]

1 hour ago, blackrose said:

All the speeds canal boats move hull shape, fluid flow & hydrodynamics seem like a bit of an academic pastime to me. 

I suggest you go and try bow hauling Saturn and compare it with pulling a modern narrowboat

[blackrose]

25 minutes ago, ditchcrawler said:

I suggest you go and try bow hauling Saturn and compare it with pulling a modern narrowboat

 

What's bow hauling got to do with anything? Lower resistance might save you a bit of fuel over the long term, but as long as an engine can push a modern narrowboat at canal speeds then the difference to fluid dynamics, etc is marginal at best. 

Edited June 8, 2023 by blackrose

[ditchcrawler]

Just now, blackrose said:

 

What's bow hauling got to do with anything? It might save you a bit of fuel over the long term, but as long as an engine can push a modern narrowboat at canal speeds then the difference to fluid dynamics, etc is marginal at best. 

It demonstrates the effort/force required to move boats with different hull shapes 

[blackrose]

15 minutes ago, ditchcrawler said:

It demonstrates the effort/force required to move boats with different hull shapes 

 

Yes, so as long as you have more than the effort/force required you'll be fine. It's all relative and in the grand scheme of boats no big displacement canal boat hull is ever going to score highly in terms of hydrodynamics and handling. My point is simply that at the speeds we travel the differences are marginal and we probably tend to overthink these things.

Edited June 8, 2023 by blackrose

[Captain Pegg]

18 minutes ago, blackrose said:

 

What's bow hauling got to do with anything? Lower resistance might save you a bit of fuel over the long term, but as long as an engine can push a modern narrowboat at canal speeds then the difference to fluid dynamics, etc is marginal at best. 

It affects fuel consumption. Which isn’t marginal in the long term. I’ll reference my comment on the thread about Ferrous. The trim of the boat influenced speed by 0.25mph. That’s a fairly big proportion.

 

However I think that with modern craft the big flat expanse of the bottom is the predominant influence on how they behave.

Edited June 8, 2023 by Captain Pegg

[magnetman]

It could become a hot topic once electric propulsion is mandated on canal boats.

 

No problem for big old diesel lumps but electric is a whole nother kettle of fish.

 

 

[IanD]

18 minutes ago, magnetman said:

It could become a hot topic once electric propulsion is mandated on canal boats.

 

No problem for big old diesel lumps but electric is a whole nother kettle of fish.

 

 

Long swims are a matter of diminishing returns; going from horribly blunt/short ones to maybe 10' or so (IIRC that's what TT used on my boat) makes a big difference to wake generation and how easily the hull slips through the water, going beyond this (e.g. to 15') less so.

 

For most boats internal space is a bigger priority anyway nowadays, and swims much longer than 10' start to affect what you can fit into the cabin at the ends. Given the resteicted width/space inside a narrowboat, this doesn't seem like a good tradoff to me -- but people who want a super-elegant hull and aren't bothered about the space so much may well disagree... 😉

 

The Alvechurch bulbous bow was never a good idea on canals, these work best to reduce drag in deep water at one particular design speed by reducing the bow wake -- fine for oceangoing ships which spend almost all their time at a fixed cruising speed, but not for a canal boat where bow wake drag is also much smaller than skin drag or the drag caused by pushing the water back past the hull in a narrow shallow canal.

[magnetman]

I wonder if some work may be carried out regarding introducing extra chines into the hull sides. I realise this can reduce internal full headroom width but depending on the layout this may not be a significant problem. 

 

A well designed forward swim leading in to an angled chine from side to base could be quite handy for efficiency and also for mooring. 

 

 

[ditchcrawler]

30 minutes ago, magnetman said:

I wonder if some work may be carried out regarding introducing extra chines into the hull sides. I realise this can reduce internal full headroom width but depending on the layout this may not be a significant problem. 

 

A well designed forward swim leading in to an angled chine from side to base could be quite handy for efficiency and also for mooring. 

 

 

But its easier to make a box with a triangle bit on each end

31 minutes ago, magnetman said:

I wonder if some work may be carried out regarding introducing extra chines into the hull sides. I realise this can reduce internal full headroom width but depending on the layout this may not be a significant problem. 

 

A well designed forward swim leading in to an angled chine from side to base could be quite handy for efficiency and also for mooring. 

 

 

But its easier to make a box with a triangle bit on each end For those that do Facebook you may find Dan's little project interesting (4) Dan Cauldwell - Slow and steady...... losing the plank wobbles one... | Facebook

[Captain Pegg]

I tried the tiller at 90 degrees thing today as mentioned by @haggis, @Francis Herne and others elsewhere on the forum.

 

I had absolutely zero rotation. Water spewing out both sides of the stern and a creeping forward motion dead straight ahead even at and above normal cruising revs.

 

This is obviously a boat specific thing and perhaps explains why 45 degrees using forward and reverse gears is taught as that will work on all boats.

[Francis Herne]

13 minutes ago, Captain Pegg said:

I tried the tiller at 90 degrees thing today as mentioned by @haggis, @Francis Herne and others elsewhere on the forum.

 

I had absolutely zero rotation. Water spewing out both sides of the stern and a creeping forward motion dead straight ahead even at and above normal cruising revs.

 

This is obviously a boat specific thing and perhaps explains why 45 degrees using forward and reverse gears is taught as that will work on all boats.

That's certainly a different result to mine!

 

I'm wondering if it's down to how 'balanced' the rudder is - on Lark the entire blade, less an inch or so, is aft of the stock.

 

I've seen some narrowboat rudders where about 1/3rd of the blade is forward of the stock to reduce steering forces - putting one like that over 90° would just be a blanking plate behind the entire prop and presumably lead to what you describe. Would be interesting to know how Vulpes' is set up.

 

(Come to think of it, the incident you mentioned with the tiller being thrown sideways while going forward implies some amount of balancing more than zero? An object striking the rudder in front of the stock would do that with torque proportional to the distance, behind should push it into line)

Edited June 8, 2023 by Francis Herne

[beerbeerbeerbeerbeer]

I did a quick 360 today in Cambrian wharf 

45(ish) on tiller to get it turning, 

keep it going,

some reverse to halt any forward momentous,

then let it keep turning under its own momentous 

then a bit more 45 (ish) and straighten up and away,

 

could “turn on a sixpence”

 

I could rob a Post Office. 

 

 

48’ boat, 

give me 49’ and I’ll spin it with room to spare. 

[Captain Pegg]

13 minutes ago, Francis Herne said:

That's certainly a different result to mine!

 

I'm wondering if it's down to how 'balanced' the rudder is - on Lark the entire blade, less an inch or so, is aft of the stock.

 

I've seen some narrowboat rudders where about 1/3rd of the blade is forward of the stock to reduce steering forces - putting one like that over 90° would just be a blanking plate behind the entire prop and presumably lead to what you describe. Would be interesting to know how Vulpes' is set up.

 

(Come to think of it, the incident you mentioned with the tiller being thrown sideways while going forward implies some amount of balancing more than zero? An object striking the rudder in front of the stock would do that with torque proportional to the distance, behind should push it into line)

There’s plenty of rudder ahead of the stock.

 

When I had the tiller wrenched from my hand I was in the process of kicking weed off the prop and had probably just engaged reverse albeit still moving forward. So I could have drawn something into the prop from behind or even below.

[stagedamager]

19 hours ago, ditchcrawler said:

But its easier to make a box with a triangle bit on each end

But its easier to make a box with a triangle bit on each end For those that do Facebook you may find Dan's little project interesting (4) Dan Cauldwell - Slow and steady...... losing the plank wobbles one... | Facebook

Ooh thanks for the link!

Here's some photos showing the latest progress, this is all about how a boat swims through the water. Anderton kept to the original design to the end of their operations, and the boats were renowned for how cleanly they swam. 

 

Edit to add linky....

 

 

Edited June 9, 2023 by stagedamager