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How was Sara built?


Robert B.

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Hello everyone,

 

I spent a lot of time squeezed and contorted into various small spaces below decks yesterday whilst I tightened our stern gland and sponged out a quantity of greasy water from the bilge. I spent some time examining Sara's construction and it occurred to me that her frames  are not as thoroughly fixed as the doubling straps where adjacent hull sheets are abutted and riveted together.  You can see a frame in the image below with two such straps either side, this area being in the  curved butty style stern.

 

I always assumed that the frames would have been used to form the curve of the hull first with the hull plates applied over them (as in conventional ship building), but in places the frames do not conform too closely to the hull - at their lowermost ends I can usually get a finger between them and the baseplate. Looking closely and the image of the bare hull it is clear that none of the frames form a continuous a piece - they are more akin to an elongated knee as might be found in a conventional work boat. The hull plates are sandwiched between these extended areas and the base plate, showing that they were already in place before the frames were secured.

 

 

I take it then that Sara was first laid down as the base plate (in four sections) with the one piece stem  and stern, then the hull plating was rolled and riveted into place. The substantial frames then went into the hull, most (but not all) attached to both the hull sides and baseplate.  It seems a backwards way to do things and there must have been a reason for it, but I can't see it. is this the way a conventional work boat would have been built?  Are the frames purely intended to limit the spreading of the hull in the event that any loads were carried? Or are they to maintain hull shape when riding up onto (and plunging through) ice? They don't seem to have much to do with re-enforcement as there are none in the bow where the brunt of the ice breaking loads would have been borne.

 

 

I think I'll crack open another can on this fine evening whilst I ponder things...

 

 

Robert

 

 

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Don't know about Sara, but on the all steel Large Woolwich boats the bottom and side plating must have been all rivetted together first, and the rolled steel knees afterwards. The chine angle is rivetted to both the bottom plate and the side plates, including under where the knees are fitted. If the knees were put in first there would simply be no access for rivetting the chine angle. This is pretty straightforward for the straight parallel section in the middle, but must also have been done for the curved bits at bow and stern, where bending of the side plates is required.

 

Thinking further, I suppose it is possible that the knees were temporarily bolted in while the side plating was assembled, and the rivets between the knees completed, the knees could then be unbolted to allow the chine rivetting to be completed, with the knees following. And if the hull moved at all on unbolting the knees, then there could well be a bit of lack of fit when they were subsequently refitted.

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I suppose it is a little like wooden clinker building, where the planks overlap and are riveted together, and any framing is added afterwards. There is a limit to the size of clinker boats of around 100 feet, though most were/are under 70 feet. Above that and the sides become too difficult to control during construction. However, clinker boats do tend to have fabricated stem and stern posts, ie made from a post and apron. Carvel boats, as found around the Mediterranean, were built around frames, often with stem or stern posts from a single piece of wood. It was the combination of the two techniques which allowed countries around the European Atlantic coast to build boats large enough to trade internationally.

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The butt straps across the plate joints have closely speed rivets, to hold the joint tight and keep the hull watertight. But the frames/knees have much more widely spaced rivets. This means that if any rust forms between the plating and the frame, both knee and plating are free to bend to accommodate the expansion of the rust. After a few decades of rusting this may appear as a lack of fit.

 

It is notable that the GU Woolwich boats had closer spaced rivets on the knees than their Northwich cousins, and you can see this difference in that Northwich boats tend to have much wavier plating around the knees than the Woolwichs.

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Once a riveted joint has been closed and all the rivets are set there is/was a dockyard shipyard trade, the caulkers, whose job it was to come along and peen the edges of the riveted plates (usually with a blunt edged air chisel) so as to make them watertight  and prevent the rust getting between the two pieces and wedging them apart.

 

These days caulkers (albeit now amalgamated with a few other shipyard trades)  are employed in cutting off unwanted things, still using their air chisel  but with a sharper blade.

 

N

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Funny things canal boats. Its quite possible that in Sara's case the frames / knees hardly serve much purpose at all, they are not needed to connect the bottom to the sides, all those rivets did that. I guess they were very important in wooden boats as the side to bottom joint probably needs all the help it can get but if the yard had always done it that way they probably put them in Sara through habit. It must be massively over engineered for its purpose. Carrying craft are a bit different, coal and similar stuff must exert a heavy pressure on the sides and anything that keeps the boat straight and true seems like a good idea, Bashing stretched plates straight with a big hammer would be a thankless task.

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The 1850's iceboat Ross can be seen here. Whilst the overall shape of the hull is quite different (more of an ice 'ram') the method of construction, with alternating doubler plates and frames, is very familiar. 

 

I do wonder if Sara, Ross and others represent a transitional period in which the builders of the day were still following work patterns established in a previous era of wooden  craft construction. It is clear that she was constructed by a yard whose expertise was in boats rather than, say, boilers. I assume the frames have been delivered from the mill as long straight forms to be cut to length and bent on site? By my reckoning there are 35 metal plates that form her hull plus 20 frames and 20 doubling straps in her 30 foot length. The curves in the bow are quite complex and I assume the plates were offered up to check the fit at intervals whilst being rolled. Ultrasound shows them to be thinner in the middle and thickest at the edges, which makes sense as that is were they are drilled and held by the rivets. 

 

Talking of rivets, I notice that we have two varieties. A conical headed flat topped sort that I assume is of later origin for use with riveting guns/presses, and a hammered flat version that are probably original to her construction. These latter rivets only survive in areas along the bilge well below the waterline. Given that iceboats had a propensity for wasting rivets due to ice abrasion this probably represents her last major overhaul. 

 

2 hours ago, BEngo said:

Once a riveted joint has been closed and all the rivets are set there is/was a dockyard shipyard trade, the caulkers, whose job it was to come along and peen the edges of the riveted plates (usually with a blunt edged air chisel) so as to make them watertight  and prevent the rust getting between the two pieces and wedging them apart.

 

These days caulkers (albeit now amalgamated with a few other shipyard trades)  are employed in cutting off unwanted things, still using their air chisel  but with a sharper blade.

 

N

 

I had noticed that the margins of the plates are rather ragged. The blade used appears to have been an 1 1/2" inches wide.

 

Robert

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Yes I've noticed the Harts Hill Iron Co on NUTFIELD's knees and wondered if they were taken from the works on the Pensnett Canal to Northwich by boat.

 

Does anyone know where Yarwood's sourced the iron and steel side and bottom plates for their narrow boats?

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The knees on Sara are thick with paint, I might have a session with some stripper and sand paper to see if anything is revealed...

 

What is the history of Ice Dragon?  We would like some more pictures if you have them. for comparison purposes you understand!

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Here are some more pictures of Ice Dragon. I had previously posted some history in a topic 'Ice breaker tugs and Ailsa Craig engines', I hope you find it interesting. I'll post some more stuff as I think of it.

 

Enjoy the sun!

Adie.

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Edited by Adie
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