AVS for narrowboats

maxcampbell

maxcampbell

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I did start a similar thread on MoBo forum, but would appreciate some focus on the (to me) main point.

In a 50' steel narrowboat, flat bottomed, as seen on UK canals:

Assume 6'10" beam, 2' draft, 4'6" airdraft, 2 or 3 tons of ballast in a total displacement of 20 tons, 10mm bottom plate, 5mm sides & top (all steel) then what would you reckon the AVS would be?

What theoretical wave height would cause capsize?
 
Do not have the answer to this but I also will be interested to see the AVS for a narrow boat. My friend's father who built a number of narrow boats from his boatyard at Wigan assures me that he took his 60ft narrow boat across to the IOM (Douglas) a few times exiting the waterways at Preston, without a steering compass just using Blackpool Tower on a back/bearing on a handheld army compass,depending coming or going. None of your namby pamby raffting tricks for him. Impressive or what!
 
If you're really that interested - do the calcs for yourself. Just get a Basic Principles of Naval Architecture book out of the library for the formulae. If your local branch doesn't have anything, they can transfer in form any other branch that does.
 
Somebody tried seaging against advice and came to grief. But the guy who wrote the book Black Dog to Carcasson??? crossed the channel on a a good day.
What is your point? They are canal boats, AVS should not be of importance.
A
 
If you can tell us the roll period (the time it takes for the NB to roll to one side and back, it's possible to work out the metacentric height (GM), and then, as a very rough guide, using a square as the shape of the boat, work out the point at which the GM, CofBuoyancy, and CofGravity turn critical. It will be rather rough an dready.


BTW it's the wave slope you need to worry about, not the height.
 
sam_uk said:
You could put in some numbers here http://www.sailingusa.info/cal__avs.htm

Not sure how meaningful it would be
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Thanks for the link. I did try, but so much is dependant on relative weight of boat & keel that not sure it's relevant to a flat bottomed box. Do I count the weight of the bottom plate as keel weight, etc.
 
sarabande said:
If you can tell us the roll period (the time it takes for the NB to roll to one side and back, it's possible to work out the metacentric height (GM), and then, as a very rough guide, using a square as the shape of the boat, work out the point at which the GM, CofBuoyancy, and CofGravity turn critical. It will be rather rough an dready.


BTW it's the wave slope you need to worry about, not the height.
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Thanks. No idea on roll period - I haven't got a NB to go & rock. If I do get to one, how far do I rock it before timing? Take the point on wave slope, but apart from when they approach shore & start to break, isn't this proportional to height?
 
DownWest said:
Somebody tried seaging against advice and came to grief. But the guy who wrote the book Black Dog to Carcasson??? crossed the channel on a a good day.
What is your point? They are canal boats, AVS should not be of importance.
A
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Hi, last year as we left Bembridge after taking shelter from a blow we saw a narrowboat-about 55 foot- leaving the harbour bound for chichester. We think it had had a respray, probably at a better price than the mainland. Conditions were moderating but it would have been a lively ride .There was no report of any incidents so we assumed they arrived OK.
 
maxcampbell said:
I did start a similar thread on MoBo forum, but would appreciate some focus on the (to me) main point.

In a 50' steel narrowboat, flat bottomed, as seen on UK canals:

Assume 6'10" beam, 2' draft, 4'6" airdraft, 2 or 3 tons of ballast in a total displacement of 20 tons, 10mm bottom plate, 5mm sides & top (all steel) then what would you reckon the AVS would be?

What theoretical wave height would cause capsize?
Click to expand...

Before you try and find out these highly theoretical figures please remember that in a narrow boat the ballast is paving slabs or similar sitting on the flat bottom. Are you shure they will stay in place as otherwise you will have a Titanic moment and finish upside down with the ballast sitting in the cabin roof!

Narrow boats are Catagory D which says it all... One of the biggest problem with narrow boats in Catagory C waters is not the rolling but the inability to keep the foredeck and aft deck free of water due to the low freeboard.

Most insurers of narrow boats have specific references to where the boats can go and if you ask for cover on coastal waters they will put limitations on the range you are allowed to sail and the weather conditions as well as suggest you sail in company with someone.

I would suggest you go to Google Groups where you will find Narrow boat groups lurking and full of info about this.


https://groups.google.com/group/uk.rec.waterways/msg/18bbc3b5c7d6cf1f?hl=en
 
maxcampbell said:
I did start a similar thread on MoBo forum, but would appreciate some focus on the (to me) main point.

In a 50' steel narrowboat, flat bottomed, as seen on UK canals:

Assume 6'10" beam, 2' draft, 4'6" airdraft, 2 or 3 tons of ballast in a total displacement of 20 tons, 10mm bottom plate, 5mm sides & top (all steel) then what would you reckon the AVS would be?

What theoretical wave height would cause capsize?
Click to expand...

It will be interesting to see what the mathermaticians come up with but would be surprised if it exceeded about 50 degrees at which point swimbo will be stiffing pillows into the low level ventilation to stop flooding as well as sticking duct tape over all the cracks in the deck boards forward and aft. This is why adventurous narrowboaters do it in pairs!
 
Bilgediver said:
I would suggest you go to Google Groups where you will find Narrow boat groups lurking and full of info about this.


https://groups.google.com/group/uk.rec.waterways/msg/18bbc3b5c7d6cf1f?hl=en
Click to expand...

Yeah, done that - thread still going on Canalworld - but I seem to be upsetting them by even mentioning the salty stuff. Am aware of all (well, a lot) of the other issues - but can't get a hold of how stable a flat but narrow boat is - OK, it's only theory, and I'm not intending to put to sea in one tomorrow, but they're well ballasted, and flat bottomed, but narrow - what's that come to?
 
maxcampbell said:
they're well ballasted
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Are they? Doesn't seem like it would be necessary in a canal.

The problem I see (purely empirically I'm afraid) is that they're so shallow there's little leverage for what ballast they have. Add that to what Bilgediver said about the ballast sitting loose in the bottom and the description I read of a narrowboat being rescued at sea "rolling over and over like a log" doesn't seem that far-fetched.

See also Nathan's story posted a while back about a narrowboat getting grounded and then swamped on a tidal river near his marina, and being very difficult to refloat.

Pete
 
Why would a motor driven boat need a keel "for grip in the water"? If it's 55' long & 6'6" wide, even 12" of draft will offer plenty of "grip" for steerage, leeway is not an issue, I don't think anyone is proposing sails.

Barges & lots of other shallow draft traditional sailing craft use "loose ballast". It doesn't just lie on the floor does it? It is fastened down by flooring panels & is unlikely to break free even if the boat rolled 360.

Nathan's story was about a boat that grounded & when the tide went out it sat stern down. As the tide returned there was inadequate bouancy to float the stern with the bows up in the air on a bank so the tide came over the top, flooding the boat.

Most narrowboats attempting sea passages will seal the bows & stern as far as possible and make them self-draining areas. Narrowboats have made 50-60 mile passages around & across the Irish Sea.
 
I would like others see swamping as the big problem and then the free surface effect from the water taken on will make a real mess of your stability calcs .
 
maxi77 said:
I would like others see swamping as the big problem and then the free surface effect from the water taken on will make a real mess of your stability calcs .
Click to expand...

That's why they seal the fore & aft doors & lockers & then fit canvas spray covers & cut scuppers to drain any water on the fore or aft decks. Side doors & windows are also sealed or covered. If the water getting in is restricted then bilge pumps will easily cope with it.

I don't think any one who gave it a moment's consideration would take a totally unmodified narrowboat to sea.
 
Searush said:
Barges & lots of other shallow draft traditional sailing craft use "loose ballast". It doesn't just lie on the floor does it? It is fastened down by flooring panels & is unlikely to break free even if the boat rolled 360.
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That's all true for seagoing craft, but does the same apply to narrowboats? They don't expect to ever heel at all, let alone roll over. Even with a crane you couldn't roll a narrowboat much more than 30º in the canal near my parents' house - the chine would hit the bottom.

Steel, tar paint, patio slabs, plywood, carpet seems an entirely plausible set of layers for the bottom of a narrowboat to me, with no real fastenings at all.

Pete
 
prv said:
Steel, tar paint, patio slabs, plywood, carpet seems an entirely plausible set of layers for the bottom of a narrowboat to me, with no real fastenings at all.

Pete
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That sounds right....and the flooring is not designed to hold the ballast in place!.

I have friends with a very sea worthy narrow boat and even they have very tight restriction put on them for passages on the Clyde Estuary and the River Forth between Grangemouth and Granton.. In fact the restrictions were so tight on one occasion they launched at Granton to get to Leith Docks and then had to be in company with other craft. No loose flors and no loose ballast! but Cat D!!!!
 
prv said:
That's all true for seagoing craft, but does the same apply to narrowboats? They don't expect to ever heel at all, let alone roll over. Even with a crane you couldn't roll a narrowboat much more than 30º in the canal near my parents' house - the chine would hit the bottom.

Steel, tar paint, patio slabs, plywood, carpet seems an entirely plausible set of layers for the bottom of a narrowboat to me, with no real fastenings at all.

Pete
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You may be right, but all the ones I have been on, or seen, have had floorboards or ply fastened down over the ballast & wiring/plumbing & insulation. Don't forget they are based on a commercial design intended to carry tons of cargo, so a ton or so of bricks, pig iron or concrete is required in the bows to counter-balance the engine, batteries, fuel, water, gas bottles & crew all normally carried at the aft end.
 
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