Boat stretch

[blackrose]

10 hours ago, truckcab79 said:

... it must cross the owners mind to just sell and buy another that better matches their needs.  But I can understand why they’re attached enough to want to spend on their boat to make it perfect for them.  Credit to them for investing in it and to your for your work.  👍

 

Most NB stretch success or failure depends on the steelwork and this is a very good example, but some owners forget about the additional demands on the engine. In most cases the existing engine will still push the boat along a canal but it might affect the boat's performance on rivers.

[Martin Kedian]

2 hours ago, blackrose said:

 

Most NB stretch success or failure depends on the steelwork and this is a very good example, but some owners forget about the additional demands on the engine. In most cases the existing engine will still push the boat along a canal but it might affect the boat's performance on rivers.

Engine size is important and should be taken into consideration 

[Tracy D'arth]

Will you be taking out any ballast?

[Liam]

11 minutes ago, Tracy D'arth said:

Will you be taking out any ballast?

As its now a longer boat, it would probably mean adding ballast into the new bit.

[Martin Kedian]

6 minutes ago, Liam said:

As its now a longer boat, it would probably mean adding ballast into the new bit.

No removal of ballast more added as displacement area is bigger 

 

[Tracy D'arth]

I understand the increase in displacement but just wondered as Stowe boats run deep usually.

[Martin Kedian]

Yes they do especially as they are a tug style we put 3 two inch pavers in each floor space  

[BilgePump]

8 hours ago, blackrose said:

 

Most NB stretch success or failure depends on the steelwork and this is a very good example, but some owners forget about the additional demands on the engine. In most cases the existing engine will still push the boat along a canal but it might affect the boat's performance on rivers.

Really good point. We had a 30hp 45' NB stretched to 60' many years ago. Was still plenty adequate power for everywhere it went but probably would have been better with a few more ponies to try something with strong flooding or tidal like the Ribble link.

 

That's looks to be very nice work  @Martin Kedian has done there

[Martin Kedian]

20 minutes ago, BilgePump said:

Really good point. We had a 30hp 45' NB stretched to 60' many years ago. Was still plenty adequate power for everywhere it went but probably would have been better with a few more ponies to try something with strong flooding or tidal like the Ribble link.

 

That's looks to be very nice work  @Martin Kedian has done there

Thank you a solution can be to change the prop to a heigh efficiency one 

[IanD]

40 minutes ago, BilgePump said:

Really good point. We had a 30hp 45' NB stretched to 60' many years ago. Was still plenty adequate power for everywhere it went but probably would have been better with a few more ponies to try something with strong flooding or tidal like the Ribble link.

 

That's looks to be very nice work  @Martin Kedian has done there

 

If you look at the power needed to propel a narrowboat in deep water -- with or without a current -- it hardly varies with length, and in fact it drops slightly with longer boats because the hull speed limit goes up with length. This is because unlike most boats narrowboats don't get wider or deeper in the water as they get longer. Some deep-water numbers from the Vicprop displacement hull calculator (all with 43hp i.e. a Beta 43):

 

36'x7'x2' 24000lb 7.62kts (hull speed 8.04kts)

48'x7'x2' 32000lb 7.99kts (hull speed 9.28kts)

60'x7'x2' 40000lb 8.03kts (hull speed 10.38kts)

72'x7'x2' 48000lb 8.55kts (hull speed 11.37kts)

72'x7'x3' 72000lb 7.47kts (hull speed 11.37kts)

 

Yes I know these are approximations and the weight isn't exactly proportional to length, but the trend is clear. Of course if longer boats have deeper draught and weigh more this will slow them down, see the last line, so they'll need more power.

 

The reverse is however true in narrow shallow canals where most of the power goes into pushing the weight of water backwards past the boat, so longer heavier boats take more power than shorter lighter ones. On the other hand, when do you ever need anything close to full power in this case?

 

The same applies if you want to start or stop in a hurry/emergency, more length/weight needs more power.

 

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.

 

Before anyone questions the Vicprop numbers, they're based on solid marine engineering principles and are generally accepted as being pretty a good guess in deep water, which is what the Ribble link and tidal Trent are... 😉

Edited November 16, 2023 by IanD

[magnetman]

On canals there is more need to do manoovering than on open water.

 

If one is sucking the bottom it uses a lot of energy.

 

 

[IanD]

3 minutes ago, magnetman said:

On canals there is more need to do manoovering than on open water.

 

If one is sucking the bottom it uses a lot of energy.

 

 

Agreed. But it's extremely rare to use anything near full power in such cases... 😉

[matty40s]

Milan when first launched.

 

[magnetman]

Yes I remember that boat.

 

It used to get around a bit.

 

 

I think its a real Stowe Hill boat not a contract hull builder job.

 

[blackrose]

39 minutes ago, IanD said:

 

Actually if you look at the power needed to propel a narrowboat in deep water -- with or without a current -- it hardly varies with length, and in fact it drops slightly with longer boats. This is because unlike most boats narrowboats don't get wider or deeper in the water as they get longer. Some deep-water numbers from the Vicprop calculator (all with 43hp i.e. a Beta 43):

 

60'x7'x2' 8.03kts

60'x7'x2' 8.03kts

60'x7'x2' 8.03kts60'x7'x2' 8.03kts

72'x7'x2' 8.55kts

 

That's the theory... But how do you explain so many accounts of people having their boats stretched and then finding their existing engine is lacking in power. I've heard it quite a few times. Perhaps that's the difference between theory and reality.

[IanD]

4 minutes ago, blackrose said:

 

That's the theory... But how do you explain so many accounts of people having their boats stretched and then finding their existing engine is lacking in power. I've heard it quite a few times. Perhaps that's the difference between theory and reality.

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... 😉

Edited November 16, 2023 by IanD

[magnetman]

Out of interest does Vic Prop have any settings related to under dredged and rather narrow saucer shaped waterways with landfill sites under bridges and fluctuating levels?
 

And locks.

 

 

 

It seems like an almost impossible task to calculate for a canal as they are all so different.

 

Open deep water is simple.

[IanD]

Just now, magnetman said:

Out of interest does Vic Prop have any settings related to under dredged and rather narrow saucer shaped waterways with landfill sites under bridges and fluctuating levels?
 

And locks.

 

No, like I said the Vicprop numbers are in deep water, and in narrow/shallow canals different rules apply. Try reading what I wrote... 😉

[magnetman]

Why did you bring up deep water when the topic is about a narrow boat which has been lengthened?

 

 

[IanD]

1 minute ago, magnetman said:

Why did you bring up deep water when the topic is about a narrow boat which has been lengthened?

 

 

Because the exact quote I was replying to was suggesting that a bigger engine might be needed for a lengthened narrowboat in rivers and against currents -- which is "deep water" in the hydrodynamic sense, where Vicprop applies 🙂

Edited November 16, 2023 by IanD

[magnetman]

Arguing must be terribly boring if you are always right.

 

I'm not sure the ribble link is very deep is it?
 

 

 

Does Vic Prop indicate how deep the water needs to be before it is classed as 'deep water'?

 

 

It would be interesting to know how much water below the bottom of the boat is needed to avoid interaction with the land beneath, or to the sides.

 

 

 

I think they might mean open seaways which are dissimilar to inland tidal waterways for a number of different reasons and in a number of different ways.

 

 

 

Presumably there is a formula for the boat draught v the water depth. Something like having draught x 1.5 of water depth below the boat.

 

There will have been studies and a credible source of info such as Vic will have all of this. I might go and look at Mr Prop's website.

 

 

Edited November 16, 2023 by magnetman

[IanD]

3 minutes ago, magnetman said:

Arguing must be terribly boring if you are always right.

 

I'm not sure the ribble link is very deep is it?
 

Does Vic Prop indicate how deep the water needs to be before it is classed as 'deep water'?

 

It would be interesting to know how much water below the bottom of the boat is needed to avoid interaction with the land beneath, or to the sides.

 

I think they might mean open seaways which are dissimilar to inland tidal waterways for a number of different reasons and in a number of different ways.

 

 

If you want detailed answers to questions like that, I suggest you go and do your own research like I did -- but of course that would mean reading what other people wrote, which I know you're not a fan of... 😉

[blackrose]

40 minutes 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... 😉

 

I'm not arguing with basic hydrodynamics. I'm saying there may be a difference between theory and practice. I think you rely too much on theory. When I lived on the Thames I met people who complained their engine was now struggling with their newly stretched boat.

[magnetman]

It could be a problem if the engine was already not ideally propped.

 

Hull speed is slightly higher as it is a function of length but the power needed to move the boat at normal speeds may well be higher for a longer boat. There is going to be more skin friction for a longer boat so it seems unlikely it would use less power even if it has a higher calculated hull speed.

 

I wonder how many people with a  `60ft narrow with Beta 43 actually get 8 knots on open water. 8 knots is really quite swift.

 

 

[IanD]

Just now, blackrose said:

 

I'm not arguing with basic hydrodynamics. I'm saying there may be a difference between theory and practice. I think you rely too much on theory. When I lived on the Thames I met people who complained their engine was now struggling with their newly stretched boat.

 

Like I said, it's not just theory, the calculations for how ships and boats behave in water -- at least the easy one to analyse which is deep water -- have been known for many years, checked against actual measurements on many occasions, and the entire worldwide ship and boatbuilding industry depends on them being correct.

 

If you think that all the theory and measurements and analyses from people who do this for a living should be ignored in view of anecdotal views of "people", than that's your prerogative. But I should point out that lots of people -- including plenty of boaters -- believe things which turn out to be wrong when actually tested against the evidence... 😉