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Boat flexing


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19 minutes ago, Jen-in-Wellies said:

Also internal steel beams, particularly cross beams welded to the base plate, or roof. A narrow boat might have good torsional rigidity, but a wide beam could need beefing up, or design changes to be similarly stiff in torsion. A mismatch in support across the width of the boat, front to rear will have a magnified effect on a fat boat.

 

Yes, if you just take a narrow boat and widen it, it will become weaker.

 

On my boat, as well as the cross members running athwartships at 18" centres welded to the inside the baseplate, it also has two lines of longitudinal bearers running fore-aft about 3ft in from both sides to strengthen the bottom of the boat.

 

But I have approximately 8 tonnes (2 layers) of broken concrete slab ballast under the floor, so despite the boat being pretty strong, if it's only supported by two trolleys at either end of the boat it's going to sag in the middle. 

Edited by blackrose
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This forum is so informative.

Next time I book in for a blacking I'm going to ask specifically about how the boat will be supported, and whether there will be a support under the engine.

If they say only two supports, I'll probably give them a miss. 

 

 

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1 minute ago, blackrose said:

I'm not sure if a support under the engine is any more critical than a support anywhere else? If the water tank was more than half full that could weigh more than the engine for example. 

Depends on the engine. Also depends on the size of the water tank. The marina where I moor ask you not to fill the water tank before slipping

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I don't think strength is an issue with canal boats - narrow or wide. They are massively strong and could be built very much lighter with a lot less steel in them. Add a bit of shape and a vee bottom and you get plenty of strength, and probably a faster, more nimble and better looking boat with perhaps less chance of sagging or trapping water across the width of the base plate and needing a lot less ballast to get the prop down. That ballast costs diesel money to drag about and is just dead weight. Not trying to be controversial or anything but I don't think some boatbuilders are terribly good designers.

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8 minutes ago, Bee said:

I don't think strength is an issue with canal boats - narrow or wide. They are massively strong and could be built very much lighter with a lot less steel in them. Add a bit of shape and a vee bottom and you get plenty of strength, and probably a faster, more nimble and better looking boat with perhaps less chance of sagging or trapping water across the width of the base plate and needing a lot less ballast to get the prop down. That ballast costs diesel money to drag about and is just dead weight. Not trying to be controversial or anything but I don't think some boatbuilders are terribly good designers.

 

I totally agree. When I was in uni and had access to fancy structural simulation software I had a play with different hull shapes etc and simulating impact, dead load etc. Just faffing about, nothing serious.  One day I might actually build something based off it...

 

As for bending, my plastic boat when supported totally incorrectly by the yard twisted by about 6"... it's totally fine now, but it broke the catch on my toilet door... 

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It's not the bits that flex and pop back to normal - I'd be more concerned with the bits that are flexing and maybe not popping back, like pipes and cables, or, something that shouldn't maybe getting trapped in the pop back. Is it a worry, or am I overly concerned? Also I see from the pics that the shower is not now watertight - fine if you know it has happened, but out of sight bits could be a problem down the line. Similarly is the flue pipe  not now airtight?

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1 hour ago, Bee said:

I don't think strength is an issue with canal boats - narrow or wide. They are massively strong and could be built very much lighter with a lot less steel in them. Add a bit of shape and a vee bottom and you get plenty of strength, and probably a faster, more nimble and better looking boat with perhaps less chance of sagging or trapping water across the width of the base plate and needing a lot less ballast to get the prop down. That ballast costs diesel money to drag about and is just dead weight. Not trying to be controversial or anything but I don't think some boatbuilders are terribly good designers.

 

Surely all other things being equal, a boat of the same dimensions made of lighter, thinner gauge steel would need more ballast not less to get it down in the water and provide stability? I really don't think ballast is just dead weight, it's serving an important purpose.

 

Also canal boat builders don't need to be good designers. The basic design is fairly universal. If you want a boat with good hydrodynamic properties don't buy a canal boat! 

Edited by blackrose
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1 hour ago, Mike Tee said:

It's not the bits that flex and pop back to normal - I'd be more concerned with the bits that are flexing and maybe not popping back, like pipes and cables, or, something that shouldn't maybe getting trapped in the pop back. Is it a worry, or am I overly concerned? Also I see from the pics that the shower is not now watertight - fine if you know it has happened, but out of sight bits could be a problem down the line. Similarly is the flue pipe  not now airtight?

 

Pipes running through bulkheads could be susceptible if the hole they're running though is too tight.

 

The pictures show the tiled wall of the bathroom not the shower. Flue pipes should never be fixed in the roof or deck fitting. The flue should be able to expand/contract in both axes. The pipe is still gas tight but now it's not watertight so I'll have to reseal it in the roof fitting with hight temp silicone. 

Edited by blackrose
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1 hour ago, Bee said:

I don't think strength is an issue with canal boats - narrow or wide. They are massively strong and could be built very much lighter with a lot less steel in them. Add a bit of shape and a vee bottom and you get plenty of strength, and probably a faster, more nimble and better looking boat with perhaps less chance of sagging or trapping water across the width of the base plate and needing a lot less ballast to get the prop down. That ballast costs diesel money to drag about and is just dead weight. Not trying to be controversial or anything but I don't think some boatbuilders are terribly good designers.

 

Why the trend from 6mm base plates to 10mm (or more) in the past (say) 20 years then? 

 

The shape is a compromise between practicality and on-water performance, with a passing nod to looks (buyers tend to want their boat to look boat-shaped, maybe, with a pretty bow or something like that).

 

Ballast IS dead weight on a canal boat but then its fairly cheap to ballast a too-light hull design - in both materials and diesel consumption - than design & build it with a thicker base, or re-engineer eg the prop (might need 2x small props instead?), rudder, swim etc.

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35 minutes ago, Paul C said:

Why the trend from 6mm base plates to 10mm (or more) in the past (say) 20 years then? 

 

Mines 30 years old and has a 15mm base plate ;)

Doesn't flex when on the blocks, picture from 2020 repaint

 

image0000001.jpg

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I think the trend to really thick baseplates is because people never used to paint them - hence they rusted. Steel was cheaper a few years back and it was a good selling point, after all 10mm is pretty much 'fit and forget' plus in many old docks it was impossible to get under the boat to scrape and paint it, hence the myth that the bottom never needed painting, in fact many docks were built for wooden bottomed boats that did not need painting. A lot of boat design is done for cost reasons and then customers assume that it is done that way for good reasons and the circle goes on - and on. One of these days I might put my money where my mouth is and build a better boat!

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1 hour ago, Loddon said:

Mines 30 years old and has a 15mm base plate ;)

Doesn't flex when on the blocks, picture from 2020 repaint

 

image0000001.jpg

I love the arched windows on either side of the bow doors. Don't think I've seen those before.

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24 minutes ago, ditchcrawler said:

Put a laser dot down it and then say that. Even the North Sea trawlers flex when put in the water

Doesn't flex that I can notice.

Parglena used to in so much as certain doors wouldn't shut when out of the water.

36 minutes ago, MrsM said:

I love the arched windows on either side of the bow doors. Don't think I've seen those before.

The windows in the front doors are arched as well. Simone put a lot of thought into the boat when it was built

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43 minutes ago, ditchcrawler said:

Put a laser dot down it and then say that. Even the North Sea trawlers flex when put in the water

All steel vessels flex, not just trawlers but much larger vessels such as tankers, cargo ships and passenger vessels , especially  when  afloat. Canal craft also need adequate support when on the blocks, and also when being craned in or out of the water when ideally they should employ the use of spreaders.

 

Howard

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59 minutes ago, howardang said:

All steel vessels flex, not just trawlers but much larger vessels such as tankers, cargo ships and passenger vessels , especially  when  afloat. Canal craft also need adequate support when on the blocks, and also when being craned in or out of the water when ideally they should employ the use of spreaders.

 

Howard

 

Easier said than done in a lot of yards.

 

Also when boats are craned the spreaders just prevent the strops from coming into contact and putting pressure on the cabin sides. You've still got a big boat supported at only two points on the hull. 

Edited by blackrose
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11 hours ago, blackrose said:

 

Easier said than done in a lot of yards.

 

Also when boats are craned the spreaders just prevent the strops from coming into contact and putting pressure on the cabin sides. You've still got a big boat supported at only two points on the hull. 

Hence the need to choose the yard carefully if possible. Re your first point, not if the spreaders are in the form of a rectangular frame (both fore and aft and thwartships) as some yards/crane companies provide, either as a permanent fixture or one which bolts together as required. 

 

Howasrd

 

 

 

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All of the forces ,stresses and deflections due to lifting and supportong ashore can be calculated or at least estimated .

But to do so properly isn't a 5 minute job.

But in simple terms, if the supports are say at the ends plus the mid point of the boat length the deflection will be a very small proportion of the deflection if the boat is supported at each end only.

Span to the power 4 comes into the deflection calculation if we assume the boat is a beam of constant section and the load is evenly distributed.

 

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45 minutes ago, howardang said:

Hence the need to choose the yard carefully if possible. Re your first point, not if the spreaders are in the form of a rectangular frame (both fore and aft and thwartships) as some yards/crane companies provide, either as a permanent fixture or one which bolts together as required. 

 

Howasrd

 

Hence it's easier said than done. Many of us don't have a broad choice of yards we can get to within a reasonable distance of our moorings, and for those like myself who live aboard that choice may become even more limited. For my boat I only know one yard on the Nene which happens to be a day's cruise away, though there may be others further downstream that I don't know about. I work so I really can't spend a week getting to the perfect boat yard plus the time off work to do the work. It's got to be within a days cruise. 

 

My boat has been out the water in all sorts of places including tidal river beds and craned in/out so the extent of hull support has been varied. I know the hull can take it but the fit-out can shift around inside. However I'd never reject a yard on the basis of the hull support offered. Next time I come out at Blackthorne I'll just ask them to put the 2 rail bogeys a bit closer together. 

 

I don't think I've seen the spreaders you're talking about. Do you have a picture? I've been craned using spreaders several times but they've only ever used 2 strops. 

Edited by blackrose
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4 minutes ago, blackrose said:

 

Hence it's easier said than done. Many of us don't have a broad choice of yards we can get to within a reasonable distance of our moorings, and for those like myself who live aboard that choice may become even more limited. For my boat I only know one yard on the Nene which happens to be a day's cruise away, though there may be others further downstream that I don't know about. I work so I really can't spend a week getting to the perfect boat yard plus the time off work to do the work. It's got to be within a days cruise. 

 

My boat has been out the water in all sorts of places including tidal river beds and craned in/out so the extent of hull support has been varied. I know the hull can take it but the fit-out can shift around inside. However I'd never reject a yard on the basis of the hull support offered. Next time I come out at Blackthorne I'll just ask them to put the 2 rail bogeys a bit closer together. 

 

I don't think I've seen the spreaders you're talking about. Do you have a picture? I've been craned using spreaders several times but they've only ever used 2 strops. 

I use Castleford boatyard because the dry dock is so good and the dry dock really is dry! Also you can get under the boat with ease. They do glass blasting there which provides an excellent finish for Zinga or two pack 

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The narrowboat next to me last week was Zingered and epoxied with Hemple Hempadur. The owner used a belt sander to key the existing epoxy before applying 4 more coats of the same stuff. On the last day before we were due to go back into the water he noticed that some areas of the paint were bubbling. When he punctured one of the blisters it went right back to the Zinga so it had lifted off the old epoxy. 

 

He blamed the heat on the hull but it wasn't blistered all over, just in a few places and my boat was in the same heat. I'm wondering if it was a bit of contamination + heat? 

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