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Boat stretch

Martin Kedian

By Martin Kedian
in Boat Building & Maintenance

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Jon57 Rising Star

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Posted
Posted
12 minutes ago, beerbeerbeerbeerbeer said:

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

Yes it is 

 

17 minutes 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.

Untill you have a nice soft landing on a mattress. 😕

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ditchcrawler Veteran II

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Posted
Posted
2 hours ago, magnetman said:

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.

 

 

I know of one case and then the skin tanks weren't adequate to cool the larger engine. Probably today a lot of Narrowboats have more Hp than they need, I have  

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IanD Veteran II

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Posted
Posted
13 hours ago, TheBiscuits said:

 

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.

 

 

 

 

 

The river sections are what I was referring to when I said "Ribble Link" -- my error 🙂

14 hours ago, magnetman said:

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.

 

 

Indeed it is, any program is only as good as the data that goes in and the algorithms used.

 

The Vicprop calculator says that it should only be used to give a first-pass estimate of things like boat speed and prop size -- so when it says "6.54kts" or "17.3"x11.5" prop" these shouldn't be taken as gospel absolute numbers. However the underlying trends seem to make sense if you play around with the numbers -- if it says a 72' boat is 10% faster in a straight line in deep water than a 48' one the difference is likely to be correct. 

 

The other canal effects discussed like shallow depth are indeed not taken account of by this, but this doesn't mean they can't be -- a narrowboat in a typical canal is actually a pretty good scale model for something like the EverGiven in the Suez/Panama canals, and a *lot* of analysis has been done for cases like this for obvious reasons...

 

To be accurate, a computer program didn't say you didn't need a bigger engine when a boat was lengthened, it just said that you didn't need it to maintain speed against a current -- there are other reasons you might want a bigger engine, as have been pointed out at great length... 😉

13 hours ago, Martin Kedian said:

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

Yes the hull.shape makes a difference to speed and handling, obviously, but this is separate from the length issue unless you're  also going to put a new bow or stern on. Paint probably doesn't unless it's *really* rough or you're covered in barnacles and weed because skin drag is small at canal speeds...

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MtB Veteran II

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Posted
Posted
1 hour ago, IanD said:

there are other reasons you might want a bigger engine, as have been pointed out at great length... 😉

 

Like stopping for example. Add three or four tonnes to the weight of a boat and the extra monentum will make the brakes seem less effective than before the stretch, creating the impression of a need for more power.

 

 

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IanD Veteran II

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Posted
Posted (edited)
1 hour ago, Machpoint005 said:

 

There seem to be several posters who don't understand what an engineering approximation is.

 

 

Also what seems to be failure to read what was actually said, just to try and make an argument -- which already had what I thought was a clear description of exactly when it applied in my very first post:

 

"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."

 

(maybe I should have explicitly said "when travelling in a straight line", but I thought this was blindingly obvious given the description...)

 

Somehow this turned into an argument that I was claiming a bigger engine wasn't needed, when  this was exactly the opposite of what I actually said -- and then into another argument about how computer programs aren't accurate and shouldn't be relied on, again ignoring what was written. It often seems that some people just love to argue against anything that wasn't being done a century ago... 😞

Edited by IanD
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IanD Veteran II

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Posted
Posted
Just now, Jon57 said:

Not without putting a bigger engine in. 😉

Since 4WD cars are so popular nowadays -- better grip etc -- why not put *another* engine in, maybe at the bows? Could even arrange it so it could also be used to move the bows from side to side if required... 😉

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Jon57 Rising Star

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Posted
Posted
Just now, IanD said:

Since 4WD cars are so popular nowadays -- better grip etc -- why not put *another* engine in, maybe at the bows? Could even arrange it so it could also be used to move the bows from side to side if required... 😉

No just put the bowthruster tube longways. Simple. 

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IanD Veteran II

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Posted
Posted (edited)
5 minutes ago, Jon57 said:

No just put the bowthruster tube longways. Simple. 

But then you couldn't use it as a bow thruster... 😞

 

Oh I forgot, lots of people on here think they're the spawn of the devil so that would be A Good Thing... 😉

 

Next suggestion : pod motors...

Edited by IanD
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Jon57 Rising Star

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Posted
Posted
48 minutes ago, IanD said:

But then you couldn't use it as a bow thruster... 😞

 

Oh I forgot, lots of people on here think they're the spawn of the devil so that would be A Good Thing... 😉

 

Next suggestion : pod motors...

Think we could have a pole on this. 😏

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