Maximum weight for boat on the London canal network

[Tony Brooks]

32 minutes ago, k_lj said:

For whoever it was that asked.. I gave the dimensions as I understand that the surface area of an object determines the amount of water it displaces and therefore it's buoyancy.

 

No, no, no. As Jen said, it is the underwater volume that determine the draft of a given boat. A flat bottom, vertical sided barge with no keel, will have a far shallower draft than a deep V hull with a keel. A rounded bilge boat will be between the two. I don't think you told us about the hull form, although it seems likely it will be flat-bottomed, but we have no idea about the chines, which will have some effect on the draft.

 

Edited August 2, 2024 by Tony Brooks

[Tam & Di]

21 minutes ago, k_lj said:

For whoever it was that asked.. I gave the dimensions as I understand that the surface area of an object determines the amount of water it displaces and therefore it's buoyancy.

 You are using rather odd terminology. As noted, a floating object displaces its own weight of water. However the physical weight of a boat is only really relevant if you have to crane it out. On a canal the more relevant figures are its length, beam, and draft, as these are the figures that will determine where it can go.

 

You can see and measure the length and beam of your proposed boat, but if someone has suggested it weighs 80 tons the draft is far more than you say. If the draft is correct then it is the estimated weight that is wrong.

 

If the dimensions are correct you will be able to cruise London's waterways, but will be hitting the bottom quite often, especially on the canal sections. If the weight is correct you will be much deeper drafted and would not be able to move, period.

[agew]

On 02/08/2024 at 08:08, k_lj said:

 

 

Anyways.. Thanks again. Over and out! 🙂

Just to join the humour ....

 

Of course that is either 'over' if still in dialogue, or 'out' if you're off

 

Hope this is just as helpful  🙂

[system 4-50]

On 02/08/2024 at 08:17, Jen-in-Wellies said:

A rubber duck had co-authorship of the scientific paper

but this wasn't acknowledged as the duck was female.

[LadyG]

On 01/08/2024 at 16:55, k_lj said:

 

Thank you for the head's up Tony!

Hmm.. I've had conflicting advice on another thread from an experienced boater.

 Only a little.. though I've recently cruised a 60  X 12 ft dutch barge east to west all the way across London and back encountering no problem.. We all have to start somewhere right? 

 

But what is the plan, is it to potter round London or just find somewhere to moor and leave it thère?

[Tigerr]

Wouldn't the amount of ballast be a big weight, and draught factor. If this boat has a six inch heavy concrete ballast it would presumably weight a lot more than a similar hull with 3 inches of the same ballast, and also sit much lower into the water, putting the prop right into the rubble and old car engines.

I'd hate to come across a frankenboat like that on the K&A, mind you I've not been back there since 2017, and even then these beasts, water hogs, basically houses, had colonised the eastern end as well as the west.  Meet one and you are aground. 

Last year we went through Leicester in low conditions and passed two boats with smashed props, one of them a brand new wide beam, not just the prop gone but the shaft and he engine mounts out. Their summer cruising dream on the Soar gone in a mess of cranes and engineers bills. They'd come over from Australia, two days out. 

 

 

[IanD]

32 minutes ago, Tigerr said:

Wouldn't the amount of ballast be a big weight, and draught factor. If this boat has a six inch heavy concrete ballast it would presumably weight a lot more than a similar hull with 3 inches of the same ballast, and also sit much lower into the water, putting the prop right into the rubble and old car engines.

I'd hate to come across a frankenboat like that on the K&A, mind you I've not been back there since 2017, and even then these beasts, water hogs, basically houses, had colonised the eastern end as well as the west.  Meet one and you are aground. 

Last year we went through Leicester in low conditions and passed two boats with smashed props, one of them a brand new wide beam, not just the prop gone but the shaft and he engine mounts out. Their summer cruising dream on the Soar gone in a mess of cranes and engineers bills. They'd come over from Australia, two days out. 

 

 

How on earth do you do that? I've had a couple of prop jams which just stopped the engine dead instantly, but without any damage -- what did they do to cause such carnage?

[Tam & Di]

49 minutes ago, Tigerr said:

Last year we went through Leicester in low conditions and passed two boats with smashed props, one of them a brand new wide beam, not just the prop gone but the shaft and he engine mounts out. Their summer cruising dream on the Soar gone in a mess of cranes and engineers bills. They'd come over from Australia, two days out.

 I cannot even begin to imagine how that would happen in a well found ship.

[beerbeerbeerbeerbeer]

2 minutes ago, Tam & Di said:

 I cannot even begin to imagine how that would happen in a well found ship.

I thought that’s one of the jobs of a coupling, 

a weak point to avoid owt like that happening,

 

[Tigerr]

49 minutes ago, IanD said:

 

How on earth do you do that? I've had a couple of prop jams which just stopped the engine dead instantly, but without any damage -- what did they do to cause such carnage?

It was shallow, the pound below the main drag in Leicester. The bottom was a refuse tip. Foul beyond belief, I would not have been surprised if a corpse had shown up.

 Somewhere in there was a solid object, that got under the swim and into the prop. There was a rumour of a motorbike, but my money is on the locals chucking fridges etc in there. It stank too, really rotten. Not the best impression of Leicester tbh. 

Anyway, the damage was obvious - prop blades smashed, shaft knocked up, shearing the coupling, but also knackering the hull joint, and the engine knocked off its mounts. hull leak. 

I dont think they did anything other than drive forwards, possibly with enthusiasm, in what seemed to be clear water but was shallow. I admit when we went through I thought this is the worst pound I've seen in 20 yrs, including Birmingham. 

Just the other side was a chap in a small outboard plastic boat - lost his prop too - same hazard I suspect. 

[IanD]

1 minute ago, Tigerr said:

It was shallow, the pound below the main drag in Leicester. The bottom was a refuse tip. Foul beyond belief, I would not have been surprised if a corpse had shown up.

 Somewhere in there was a solid object, that got under the swim and into the prop. There was a rumour of a motorbike, but my money is on the locals chucking fridges etc in there. It stank too, really rotten. Not the best impression of Leicester tbh. 

Anyway, the damage was obvious - prop blades smashed, shaft knocked up, shearing the coupling, but also knackering the hull joint, and the engine knocked off its mounts. hull leak. 

I dont think they did anything other than drive forwards, possibly with enthusiasm, in what seemed to be clear water but was shallow. I admit when we went through I thought this is the worst pound I've seen in 20 yrs, including Birmingham. 

Just the other side was a chap in a small outboard plastic boat - lost his prop too - same hazard I suspect. 

All I can say is they must really have been giving the engine some beans -- maybe with a widebeam in shallow water they had the throttle much wider open (higher rpm) than normal to try and get the boat moving at a decent speed? There's a lot of inertia/energy in a diesel engine at high revs, and if there was no clutch or sacrificial coupling to give way and the prop suddenly comes to a stop... 😞 

[beerbeerbeerbeerbeer]

25 minutes ago, Tigerr said:

It was shallow, the pound below the main drag in Leicester. The bottom was a refuse tip. Foul beyond belief, I would not have been surprised if a corpse had shown up.

 Somewhere in there was a solid object, that got under the swim and into the prop. There was a rumour of a motorbike, but my money is on the locals chucking fridges etc in there. It stank too, really rotten. Not the best impression of Leicester tbh. 

That’s a shame,

only been through Leicester once (with Dmr in 2023). 
Really liked it. 
I’d go again.

Hope others are put off. 

 

Eta: Whoops,

I mean ARE’NT put off!!

Edited January 16, 2025 by 5239

[Alan de Enfield]

Ever since starting narrowboating I've been surprised that, considering the risks, a common item used on outboard and even outdrive legs is not used on 'the canals'.

 

A simple shearpin or rubber hub incorporated into the prop ensures that anything giving resistance to the prop limits and minimises any damage as it 'disconnects' the prop at a known load.

 

If the shearpin comes into play, you simply insert an new one and carry on.

 

Prop damage limited, damage to brackets, bearings and gearbox very unlikely.

 

 

 

Edited January 16, 2025 by Alan de Enfield

[beerbeerbeerbeerbeer]

37 minutes ago, Alan de Enfield said:

Ever since starting narrowboating I've been surprised that, considering the risks, a common item used on outboard and even outdrive legs is not used on 'the canals'.

 

A simple shearpin or rubber hub incorporated into the prop ensures that anything giving resistance to the prop limits and minimises any damage as it 'disconnects' the prop at a known load.

 

If the shearpin comes into play, you simply insert an new one and carry on.

 

Prop damage limited, damage to brackets, bearings and gearbox very unlikely.

 

 

 

Dunno, all above my pay grade as they say,

but are shear pins for small engines?

without the gear box?

 

I’d be changing a shear pin every ten minutes if I had one,

with big engine, heavy prop and gear box ?

 

Ive a fenner tyre which stretches and allows some give until the engine stalls or the tyre slips and/or shears the bolts. 
Saves the gear box and stops the engine turn over off its mounts. 

 

 

[Alan de Enfield]

34 minutes ago, 5239 said:

Dunno, all above my pay grade as they say,

but are shear pins for small engines?

without the gear box?

 

Outboards and Outdrives have gearboxes - Fwd - N - Rev just the same as your NB.

 

Certainly, larger engines have a splined prop with a 'rubber'hub that slips at a known load.

I've had 200hp "straight 6" Volvo engines with that system.

 

 

This one is for 25-75hp which would cover most canal boats.

 

 

 

 

 

 

[Sailbadthesinner]

I suppise the logic is that a shearpin on an outboard propeller is easily replaced "on the hoof" but not so on a narrowboat. Better a damaged prop than no drive at all.

[Mike Adams]

In my experience most canal boat drive systems are very poorly engineered especially considering what would happen if you get a lock up on the propeller. Ideally the part that fails if the system cannot absorb the shock without damage should be cheap and easily replaceable. Something like a plastic R and D coupling in the prop shaft( you can remove the metal links). Most gearboxes used are far too small to do a proper job on a canal boat and the flywheel drive plates are too weak and don’t last. But of course the biggest problem is the operator who overrevs the engine when they get into shallow water and just pulls everything off the bottom instead easing off.

[magnetman]

I'm not sure how easy it would be to make scaled up shear pins. I think the remains might tend to get stuck in the hole and complicate replacement.

 

It is also worth remembering that the shear pins on outboards are actually drive pins. Without the pin the propeller is not turned by the motor. Obviously. 

 

Relying on a drive pin on a heavy Boat with large amount of torque could be problematic because if that pin breaks you have no drive at all. 

 

 

 

 

[MtB]

46 minutes ago, Mike Adams said:

But of course the biggest problem is the operator who overrevs the engine when they get into shallow water and just pulls everything off the bottom instead easing off.

 

And worse, revving the nuts off it sucks all the water from under the boat so it sinks 6" deeper into the mud. 

 

Far better to creep out of shallow water in tickover, or better still engage astern and reverse out of it, using the prop to pump extra water under the boat and deepen it by an inch or two....