Boat Stretching enquiry

[MtB]

On 8/6/2017 at 18:20, Peter-Bullfinch said:

Are you absolutely sure about lengthening? I can imagine a stretch, electrics, plumbing, repaint and engine swop could cost you towards £20,000.

Why not consider selling your boat and buying another the length you want? Perhaps consider a 57' length. There are compelling reasons for this size especially if you are considering going up north.

 

So getting back on topic...

I recognise the OP's boat name and I've had a discussion a while ago with her (presumably) other half about this project. Even if they find another good boat the right length, the chances of it being laid out inside how they need are vanishingly small, so a ton of internal conversion work will still be needed.

And the six months or six years they will spend looking at unsuitable boats means they will have no visible end date in sight and I think they really want it NOW. Quite a soul-destroying way to live, hunting for a non-existent boat to buy. I think stretching and modifying their existing boat is the right decision. 

[Wee Vee]

10 minutes ago, Mike the Boilerman said:

 

So getting back on topic...

I recognise the OP's boat name and I've had a discussion a while ago with her (presumably) other half about this project. Even if they find another good boat the right length, the chances of it being laid out inside how they need are vanishingly small, so a ton of internal conversion work will still be needed.

And the six months or six years they will spend looking at unsuitable boats means they will have no visible end date in sight and I think they really want it NOW. Quite a soul-destroying way to live, hunting for a non-existent boat to buy. I think stretching and modifying their existing boat is the right decision. 

Thanks Mike! You're absolutely right - on all counts.  

We've done a lot (as you know) of searching and researching to get to this point. We feel it's the right path to take, now we need to make sure we understand and mitigate the potential pitfalls before going for it.

Cheers again for your help and advice to date. 

[dmr]

I know nuffin about your boat so probly talkin rubbish.

Some stretched boats look very out of proportion as they end up with  a very small front deck with a very long cabin.

A cheap and easy way to lengthen a boat is to extend the front deck in front of the front bulkhead as this avoids cutting and joining most of the wiring etc. If your boat is at all traddy could you do this and then put a simulated sheety bit on the front???????????

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

[MtB]

1 minute ago, dmr said:

I know nuffin about your boat so probly talkin rubbish.

Some stretched boats look very out of proportion as they end up with  a very small front deck with a very long cabin.

A cheap and easy way to lengthen a boat is to extend the front deck in front of the front bulkhead as this avoids cutting and joining most of the wiring etc. If your boat is at all traddy could you do this and then put a simulated sheety bit on the front???????????

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

 

The Watchman is a beautifully proportioned trad boat which would look brilliant with a long tug deck added, in  my personal opinion. The faux sheeting idea is good too but wouldn't look quite so stylish...

[dmr]

5 minutes ago, Mike the Boilerman said:

 

The Watchman is a beautifully proportioned trad boat which would look brilliant with a long tug deck added, in  my personal opinion. The faux sheeting idea is good too but wouldn't look quite so stylish...

but then I hear a rumour that you have a particular fondness for long tug decks.

..........Dave

[MtB]

And Kelvin K2 engines. 

If only I could find a boat with both...

[Neil2]

5 hours ago, blackrose said:

Hull speed & waterline length: http://www.boats.com/reviews/crunching-numbers-hull-speed-boat-length/#.WYiV4NKGOt8

I know the theory behind displacement hull speed, but a longer hull is capable of greater speed only if the boat has adequate power.

The question I asked was how can adding length to a boat per se make it faster/more efficient, ie with the same engine.  The vast majority of narrowboats don't have powerful enough engines to get anywhere near their theoretical hull speeds.

 

 

 

 

[Neil2]

44 minutes ago, dmr said:

I know nuffin about your boat so probly talkin rubbish.

Some stretched boats look very out of proportion as they end up with  a very small front deck with a very long cabin.

A cheap and easy way to lengthen a boat is to extend the front deck in front of the front bulkhead as this avoids cutting and joining most of the wiring etc. If your boat is at all traddy could you do this and then put a simulated sheety bit on the front???????????

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

 

41 minutes ago, Mike the Boilerman said:

 

The Watchman is a beautifully proportioned trad boat which would look brilliant with a long tug deck added, in  my personal opinion. The faux sheeting idea is good too but wouldn't look quite so stylish...

This is the way I was thinking of doing it.  Almost all narrowboats would look better with longer front wells, and doing it this way avoids the expense of repainting the cabin.  

I'm just waiting for someone to convince me that I don't need a new engine...  

[MtB]

1 minute ago, Neil2 said:

 

This is the way I was thinking of doing it.  Almost all narrowboats would look better with longer front wells, and doing it this way avoids the expense of repainting the cabin.  

I'm just waiting for someone to convince me that I don't need a new engine...  

 

I'd say 'need' is a very subjective term. My 68ft boat used to have a 35hp BD3 then I replaced it with a 22hp Kelvin K1. The boat still goes along perfectly well, it's just not quite as nimble.

I'd say you'll notice a marginal reduction in performance (in acceleration and in stopping) due to the extra weight, but steady state cruising along will be unaffected unless you have to use full power already to achieve cruising speed. You'll just have the speed control set slightly higher for any given speed through the water.

Are you convinced now? Where do I send the bill?? 

[WotEver]

38 minutes ago, Neil2 said:

The question I asked was how can adding length to a boat per se make it faster/more efficient, ie with the same engine.

Precisely. I understood your question (and I share your doubts). Blackrose's link I believe suggests that the opposite is true:

 

It certainly doesn't confirm it. 

[MtB]

1 hour ago, WotEver said:

Precisely. I understood your question (and I share your doubts). Blackrose's link I believe suggests that the opposite is true:

 

It certainly doesn't confirm it. 

 

I have in the past, seen texts that claim to illustrate that for a given width, the longer the waterline the faster the boat will go in displacement mode.  

This I suspect is the origin of the suggestions that a longer boat for a given width requires less power. The logic doesn't actually follow if you think about it properly though.

[WotEver]

8 hours ago, Mike the Boilerman said:

 

I have in the past, seen texts that claim to illustrate that for a given width, the longer the waterline the faster the boat will go in displacement mode.  

This I suspect is the origin of the suggestions that a longer boat for a given width requires less power. The logic doesn't actually follow if you think about it properly though.

Quite. 

The fact that a longer hull has a higher theoretical top speed is purely down to wake interaction and has no bearing on how much power is required to achieve the top speed. 

Wake interaction is particularly irrelevant on canals where a breaking wash is verboten. 

[Neil2]

8 hours ago, Mike the Boilerman said:

 

I have in the past, seen texts that claim to illustrate that for a given width, the longer the waterline the faster the boat will go in displacement mode.  

This I suspect is the origin of the suggestions that a longer boat for a given width requires less power. The logic doesn't actually follow if you think about it properly though.

Yes, just to re-emphasise the point, take a displacement vessel such as a narrowboat that cannot already acheive it's theoretical maximum hull speed (ie most narrowboats), how can making the waterline longer increase the efficiency of such a vessel?  How can a heavier boat require less power?

If there is something I'm not quite grasping here I wish someone would point it out.      

[MtB]

Intuitively, I would expect most of the effort expended in pushing a boat along at a steady speed well below theoretical maximum speed to be used up in:

1) Parting the water at the bow, and

2) Accelerating it backwards either side of the hull as the boat passes.

As the work required to do each of these remains the same i.e. neither varies with boat length, the next thing to consider is the 'friction' of the water against the length of the parallel part of the hull. This resistance I would estimate to be trivial compared to 1 & 2 above, but even so it is being increased by about 30% of 'trivial' by stretching the boat 11ft. 

I'd say when steady state cruising the boat will feel little different from how it is now in terms of speed, but given an 11ft stretch will weigh a couple of tons or so, the boat will feel noticably less nimble, i.e. stopping will take longer/more space, as will getting going from a standstill. Perhaps in the order of 10% more distance to stop and 10% more distance to accelerate up to cruising speed. If the OP is willing to accept a small reduction in performance of this sort of magnitude, there is no need to change the engine. 

On the other hand if someone stretching their boat requires it to perform exactly as before, then the engine power will need to be increased and probably the blade changed too. 

Edited August 8, 2017 by Mike the Boilerman

[Neil2]

3 hours ago, Mike the Boilerman said:

Intuitively, I would expect most of the effort expended in pushing a boat along at a steady speed well below theoretical maximum speed to be used up in:

1) Parting the water at the bow, and

2) Accelerating it backwards either side of the hull as the boat passes.

As the work required to do each of these remains the same i.e. neither varies with boat length, the next thing to consider is the 'friction' of the water against the length of the parallel part of the hull. This resistance I would estimate to be trivial compared to 1 & 2 above, but even so it is being increased by about 30% of 'trivial' by stretching the boat 11ft. 

I'd say when steady state cruising the boat will feel little different from how it is now in terms of speed, but given an 11ft stretch will weigh a couple of tons or so, the boat will feel noticably less nimble, i.e. stopping will take longer/more space, as will getting going from a standstill. Perhaps in the order of 10% more distance to stop and 10% more distance to accelerate up to cruising speed. If the OP is willing to accept a small reduction in performance of this sort of magnitude, there is no need to change the engine. 

On the other hand if someone stretching their boat requires it to perform exactly as before, then the engine power will need to be increased and probably the blade changed too. 

I understand that extra displacement may be  offset by the beneficial effect of the extra waterline length, but I assumed there must come a point quite early on where the extra displacement starts to "take over", and with a steel narrowboat obviously more length means a lot more weight compared to say grp boats.     

I've been playing around with data on the Vicprop site - it's intended for prop size calculations based on vessel size, weight etc. but it displays power calculations as well.

Obviously this is all highly approximate but some of the results I got are very interesting.  

Adding length to the boat does increase maximum speed, or conversely it reduces the HP needed to attain a certain speed, provided the extra weight added is below a certain level.  What the Viprop site suggests is that the weight has to be pretty substantial before it makes any difference.  In my case, going from say 45 to 55 feet, it's only when you get to four or five tons does the extra weight start to affect the theoretical maximum speed.  I reckon a 10' stretch would add about a ton of steel to the boat, and obviously less if it's just the front well, so there's a huge margin of error.  In fact in terms of efficiency through the water there is quite an improvement to be had in load capacity.  I found this very surprising. 

What is more difficult to assess is the effect of extra weight on acceleration and, more important, slowing down.  But again, if it's only a ton or so I can't see it making a massive difference, I think as you say it would manifest itself in the boat feeling a little less responsive, that's all.  

 

[alan_fincher]

There is a clear difference between steady progress through the water once cruising speed has been reached, and actually starting the boat and getting it up to normal cruising speed or stopping it again from cruising speed down to zero miles per hour.

Is seems completely intuitive to me that if a boat is stretched, (and hence made considerably heavier), that whilst an existing engine may be adequate to keep the stretched boat moving along steadily, there will still be a performance penalty when running up to speed, or trying to stop from speed.

I would suggest that it is the latter that causes most people to upgrade their engine/gearbox/propeller - to deal with a heavier boat, not a longer boat.  Some feel it necessary, some don't - I have known similar sized boats get similar sized stretches, and although both had broadly equivalent engines, in one case the owner felt they then had to do an engine change, but in the other they did not.

My full length ex-working boat, with a heavy full length cabin conversion, and many tons of added ballast bowls along well, but any change of speed, (in particular stopping) is not achieved anything like as  fast as with an unconverted empty unballasted boat of the same type, even if the engine/gearbox/prop arrangements are very similar.

[WotEver]

32 minutes ago, alan_fincher said:

My full length ex-working boat, with a heavy full length cabin conversion, and many tons of added ballast bowls along well, but any change of speed, (in particular stopping) is not achieved anything like as  fast as with an unconverted empty unballasted boat of the same type, even if the engine/gearbox/prop arrangements are very similar.

Newton didn't get everything wrong

[dmr]

8 hours ago, Neil2 said:

Yes, just to re-emphasise the point, take a displacement vessel such as a narrowboat that cannot already acheive it's theoretical maximum hull speed (ie most narrowboats), how can making the waterline longer increase the efficiency of such a vessel?  How can a heavier boat require less power?

If there is something I'm not quite grasping here I wish someone would point it out.      

 

The waterline length thing calculates the maximum speed that a boat can achieve with a reasonable engine (you can go as fast as you want if your engine is infinitely big), it has nothing to do with how much power is needed to propel that boat at lower speeds, especially in a muddy ditch.

There is an easy way to visualise this, a boat makes a sort of bow wave that rises up just after the bow, then sinks to a trough a little further towards the back. This wave travels at constant speed through the water so as the boat goes faster the trough moves towards the back of the boat, and at some "maximum" speed the back falls into this trough and so the boat needs to climb out of a hole to go any faster. So its easy to see that this has nothing to do with the power needed to propel the boat at lower speeds. 

Look at the Beta website (or Barrus) and you will see that engine suppliers believe that longer boats need bigger engines. All that displaced water has to squish a much longer distance to get round the side of the boat. I don't think its really friction, its more that water likes to go at its own pace down a narrow channel (between boat and bank) and the longer that channel more pushing it needs.

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

[hughc]

Given power above the minimum in free water the maximum speed of a displacement boat will be between 1.1 and 1.4 times the square root of the waterline length. Where the boat sits on this curve depends on the hull design. Regards, HughC.

[David Mack]

18 hours ago, Neil2 said:

I reckon a 10' stretch would add about a ton of steel to the boat

But if your stretched boat is going to draw tbe same at bow and stern as before, then a 10 ft stretch will require a couple of tons of extra ballast as well. (3m long x 2m wide x 0.5m deep = 3 tonnes additional displacement)

[alan_fincher]

40 minutes ago, David Mack said:

But if your stretched boat is going to draw tbe same at bow and stern as before, then a 10 ft stretch will require a couple of tons of extra ballast as well. (3m long x 2m wide x 0.5m deep = 3 tonnes additional displacement)

My thoughts as well!

If you could add 10 feet  of boat for only a ton of extra weight, then it is highly unlikely the boat will then sit deep enough to not cause issues, (assuming it was right in the first place!)

If you are adding (say) 20% to the length, you probably need to be adding not far off that to the weight, surely?

[WotEver]

1 hour ago, hughc said:

Given power above the minimum in free water the maximum speed of a displacement boat will be between 1.1 and 1.4 times the square root of the waterline length. Where the boat sits on this curve depends on the hull design. Regards, HughC.

Why do we keep talking about maximum hull speed? The boat is limited in speed by regulations, not hull length. The question is about whether or not a longer hull requires more power to push it along at the same speed as a shorter boat. 

I believe the answer is obvious. 

[Wee Vee]

Thanks for all the input on hull speed/water displacement calculations chaps, but this has rather moved away from my original enquiry.  Our existing engine has been checked and will do the job, the extra ballast required will be calculated by the company who stretch the boat.

We were more interested in people's experience of having their boats stretched – the pitfalls and things to look out for.

[Neil2]

29 minutes ago, Wee Vee said:

Thanks for all the input on hull speed/water displacement calculations chaps, but this has rather moved away from my original enquiry.  Our existing engine has been checked and will do the job, the extra ballast required will be calculated by the company who stretch the boat.

We were more interested in people's experience of having their boats stretched – the pitfalls and things to look out for.

I'll offer my apologies Wee Vee for being at least partly responsible for dragging this thread off topic, it's rather in the nature of this forum...  

However, as the OP I think your wishes should be respected so I'll suggest those of us that might want to carry on arguing about technical matters should "take it outside" and start another thread.  

[Wee Vee]

19 minutes ago, Neil2 said:

I'll offer my apologies Wee Vee for being at least partly responsible for dragging this thread off topic, it's rather in the nature of this forum...  

However, as the OP I think your wishes should be respected so I'll suggest those of us that might want to carry on arguing about technical matters should "take it outside" and start another thread.  

Hehe, thanks Neil! I'd hate to spoil the party, so I'll happily start you a new thread if you like.  
I'm actually impressed by how much science you were all applying.