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Author Topic: A "Thames Shove".  (Read 4661 times)

dreadnought72

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A "Thames Shove".
« on: November 14, 2008, 03:22:56 PM »

...Like a barge, only smaller.

On this thread, AndrewH mentions a possible eight-foot Thames barge to be sailed by offspring-control:}

I can't help thinking that this has potential - the joys of sailing, being on the water, and modelling, all-in-one. The removal of the risk of cracking your skull from having no boom is an added bonus. And - were others mad enough to take up the idea - the opportunity to race and have a laugh. The boat could fit in most sheds, and be easily transported. (Compared to most dinghies).

So what scale would a barge hull have to be to carry an adult around 80kg in mass?

A quick check of things bargey points to the lengths of the originals being around 85'. The length to beam ratio approximately 4.25, and the beam to draught about 3.12. Yet the listed displacements from the Thames Barge site hovers around 65 tonnes, which means either a block coefficient as low as 20%, which seems ridiculously "slippy" for a pointed box, or that this figure is the load. Could anyone reading this with a sailing barge model do me a favour: weigh it, and tell me what the scale is?

Many thanks!

Andy #1963#
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andrewh

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Re: A "Thames Shove".
« Reply #1 on: November 17, 2008, 01:09:09 PM »

Andy,

I have seldom seen such an interesting post so galvanised by indifference :}

Thames barges are flat-bottomed, square-sided and bluff ended, so their displacement is huge.
I'm sure there is a good  marine architect term like prismatic coefficient or displacement block factor or something;  I would say they have a shape factor about 85% of a brick of the same dimensions.

My offspring-controlled TSB was/is to be 8 foot long and 2 foot wide - we did a quick check and this would clear his structural bits (as they were then).  The main hatch is to be completely removable - to form a cockpit, and the keel/drop keel if required then lies between the lower legs.

If the plan was rectangular the base area would be 16 Square feet, (1.46M2), but it is probably 85% of that or 1.24M2   and for every 100 mm of displacement I make that 0.124 M3 displaced or 124Kg, (so it would support me if the hull is at least 200mm deep)

Angus can happily paddle a kayak standing up, so in some ways dynamic balance may be the way to go, rather than static pendulum stability.   

I tentatively planned to fit motors, batteries, etc in the rear compartment, together with servos in case the skiper and ballast were not available.

If a 4 foot model TSB is 1/24th scale, then an 8 footer should be 1/12 approx making the full size about 100 foot, which seems about right

As you so rightly say, :} no boom to clonk the head

On a historical note or two:  There is a half scale barge in existence and working (Cygnet - last seen (by me) at Snape in  spring 2007

Beautiful, but I can't get my mind round the tiller {:-{


And I have seen in a friend's book a scale TSB about 30 foot long - The pictures showed it in a landscape with narrow creeks which made me think of the Thames shore on the Kent side or possibly marshes in essex/suffolk.  My guess would have been the 1930s/40s.
 I wonder if anyone knows anything about her? 
If mayhemmers don't: nobody don't :((

andrew !1949!
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Tester

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Re: A "Thames Shove".
« Reply #2 on: November 17, 2008, 01:48:12 PM »

Hi Andrew

Re your 30 foot scale TSB in the book. This could be a Barge yacht, there is a small passage in Maurice Griffiths book "Sailing on a small income" published in 1953. He mentions E.B. Tredwen who first started to design and build minature barge yachts in the 1890's. They were 30ft by 9ft beam and drew around 2 or 3 ft and were based in Essex, so that may tie in with your picture.

I'll have a look through some of my old books and see if there is an illustration.

By the by plan MM902 from Nexus is a nice small scale barge yacht.

HTH
Richard
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dreadnought72

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Re: A "Thames Shove".
« Reply #3 on: November 17, 2008, 03:09:00 PM »

I have seldom seen such an interesting post so galvanised by indifference :}

Is it because I buried it in Design Office: Yachts and Sail? Who knows! But thank you, Richard, for the mention of barge-yachts ... they were lurking in my head somewhere.

My continuing online study (I'm sure work doesn't mind) is suggesting the block coefficient of the Cambria is around 53%. Quite sleek for a box, but then I suppose it's a sailing box - and a whole lot more believable than 20%. This means ...

(Pause while Excel fires up)

...that if my maths isn't stuffed, an 80 kg person and 120 kg hull+bulb comes in at:

105" by 25" by 8", or very near to 1/10th scale - and more or less AndrewH's estimate of eight feet by two feet by 200mm.

There are two problems with a short skinny hull like this. Since the crew is half the weight of the boat, and they're semi-prone, there's:

  • Not much chance for dynamic roll stability. (Needs a fin and bulb?)
  • A risk of getting the mass in the wrong place, in terms of fore-and-aft pitch. (Keep some mass back for fore-aft trim ballast?)

Regards,

Andy #1963#
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andrewh

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Re: A "Thames Shove".
« Reply #4 on: November 17, 2008, 04:06:55 PM »

Andy,

I may have been confusing (or confused)

Did I mean block coefficient? is that the ratio of "this hull volume to the rectangle of the same overall dimensions?"
I was calculating the displacement of an 8' x 2' TSB hull - ie how much water it displaces .  This is the same as the load it will carry and of course a CuM of water would be a tonne (fresh water)

So my estimate of  0.12cuM for every 100 mm of hull submerged means that (if the hull weighed 20Kg it would sink to the waterline of 100mm with 100Kg of person on board.
The hull is goung to be a very light ply box - even if it was made of 1/4 marine ply it would not be heavy

Its would also be rather like a white water kayak in length, beam and displacement
The position of the driver is rather like a kayak driver, and his/her ability to balance much the same. 
I have assumed that a centereboard of daggerboard will be necessary to give side area, and it might well have a HEAVY end to aid stability

Richard,
Thanks for that link, that could well be what was photographed, it was certainly a narrow tidal creek no doubt filled with soft, smelly and vicious mud (that's like viscous mud, but more personally out to get you , or at least a wellington)
I'm aware of a trio of "little" barges, of which Cygnet is one, but these are, I think, about 30 to 40 foot long
I can't even remember the book, and my friend is now flying indoor planes in heaven, (and sailing scale wherries), It might have been called  "Modelling Thames Barges" and most of the book was devoted to a static model of Katharine. 
Brain slowly engaging - I have a copy of a page at home - and some books carry the name on each page.  I am also in contact with Bernard's son, who will have inherited it :}

andrew
100kg and shrinking
Could I make the hull from Correx - we throw away 8x4 sheets every day?







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dreadnought72

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Re: A "Thames Shove".
« Reply #5 on: November 17, 2008, 04:24:02 PM »

Hi Andrew - nothing confusing in what you posted. That is the block coefficient - the volume displaced by the hull/(l*w*d of the hull at that point).

Here's the sort of scale I think I'm thinking about. (I think.)



Weighs more empty than an empty Mirror dinghy = good for stability. Has half the sail area = ditto.

Andy #1963#
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andrewh

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Re: A "Thames Shove".
« Reply #6 on: November 18, 2008, 10:20:53 AM »

I have not made a good job of settng out my suggested version of your superb drawing, and I have no idea if anything will appear

Can't quite understand the weight of hull, etc - morrors are staggeringly heavy - which is why most of them are still alive#

andrew
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dreadnought72

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Re: A "Thames Shove".
« Reply #7 on: November 18, 2008, 12:10:13 PM »

Hi Andrew - your image is great.

...My lawyers will be in touch shortly!  <*<

Seriously - yes to a deeper fin & rudder, and a bigger mizzen sail. And HOW could I have forgotten a flying jib-jamacallit? Good calls, all.

Hull mass: A back of an envelope calculation gets the hull around 20kg if using 3/8ths plywood. (Mirror/Enterprise style). That's without factoring in bulkheads/frames and solid wood for the keel and bow. I'd maybe go for 1/4 inch ply if I knew there was a substantial g.f. covering over it - but that then saves no weight. I don't know whether it's a touch of aircraftitis coming through, but building this structure for 25kg would be pushing it, I think.

I estimated "70kg" to cover building everything solidly (good wood, plentiful support, metal fittings & shrouds, etc.) while I'd be aiming for "perhaps 45-50kg". It's German-NASA methodology: when the parameters change you've got fudge room to accommodate it. Here, the room of 20-25kg helps. I can use it for ballast (more on that in a mo') or lose it for more freeboard.

Here's where I think the primary risk lies: the aft lines of the hull are very fine - and yet this is where the victim is placed. HAS to be placed. If his/her centre of mass is approximately at the base of the comfy-chair, where there's decreasing amounts of hull to support this mass, then more internal ballast could be placed forward to help counter it if needed.

I suppose what I'm saying is that were I to build one, I'd not want to find things scuppered at this (possibly smallest?) scale by it being too low at the stern and uncorrectable. If built solidly, then there's no reason why, for example, a big dod of concrete couldn't be added at the sharp end to sort design errors out.

(It'd be great for ramming, too.)

Andy #1963#
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andrewh

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Re: A "Thames Shove".
« Reply #8 on: November 18, 2008, 12:58:47 PM »

Andy

Your lawyer called - he is discussing matters with my cousin Luigi. 
I have suggested that "de minimis non curat lex" covers the case

The barge I have in mind has fairly full lines at the stern - its Ricky Webb's Kitty which has the appropriate aspect ratio, and carries the volume welll aft

Pic from my untidy youth

This is the format I am imagining, only a little larger  - four times, to be precise
I will go for 3mm B&Q ply or near equivalent with 1/2 inch stringers/longerons
along these lines:


Sorry about my cad package  - its a manual version with elastomeric delete function

andrew

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dreadnought72

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Re: A "Thames Shove".
« Reply #9 on: November 18, 2008, 02:51:20 PM »

Faced with Luigi's extended family, and his knowledge of concrete, we've decided to drop the (centreboard?) case.

Meanwhile:

The drawing's a cracker - but I'm a little sceptical of 1/8th ply running two feet across as the base. It'll need plenty of cross-stringers? In truth, I was never totally happy standing on the 3/8th's inch skin of my Enterprise. It felt "minimal/enough".

This is interesting. It's about the same length and sail area as my sketch. Bulb is 20kg and carried about 30" deep. At four feet beam (twice the barge's) it's described as virtually uncapsizeable. They're designed for disabled people to enjoy affordable sailing...which makes me think the dimensions we've both arrived at are workably correct.

I seem to remember a small (indoor!) keelboat contest a few years ago. Based on American cup boats, or something similar?

Regards,

Andy #1963#
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Tester

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Re: A "Thames Shove".
« Reply #10 on: November 18, 2008, 03:01:05 PM »

Do you mean the International 2.4 Metre class, I belive they had a competition or demo in Paris some years ago.

http://www.inter24metre.org/files/about24.htm

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dreadnought72

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Re: A "Thames Shove".
« Reply #11 on: November 18, 2008, 03:26:49 PM »

Them's the fellas!

Well remembered. Yes, something was televised from France. They (from the media profile) seem to have disappeared though.

Andy #1963#
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Arrow5

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Re: A "Thames Shove".
« Reply #12 on: November 18, 2008, 03:33:52 PM »

Andy is this you in a previous venture ?
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Arrow5

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Re: A "Thames Shove".
« Reply #13 on: November 18, 2008, 03:36:26 PM »

...and at an early fitting eh, I mean design stage
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andrewh

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Re: A "Thames Shove".
« Reply #14 on: November 18, 2008, 03:39:59 PM »

Andy,
I remember these little fellers too, always thought that I would love them when floating, but would have trouble getting out if they filled up?
 You really must remove your stillettos when sailing.

3mm ply does for me with Lemon8


Its a bit flexy but it has water on the other side supporting it :}  Being honest: I added a couple of  inside ribs to stiffen the bottom

I wanted a good bit of stiffness in the bottom - since the boat is built on itm, and was thinking of a sheet of blue foam 25mm thick with either ply or fibreglass on it both sides (as well as the 1/2 square chine rails)

Hi Andrew - your image is great.
...My lawyers will be in touch shortly!  <*<

.... a substantial g.f. .....

Andy #1963#

Please don't call me shortly! :}
And tell me more, please, about this substantial girl friend

andrew
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dreadnought72

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Re: A "Thames Shove".
« Reply #15 on: November 18, 2008, 03:59:23 PM »

Andy is this you in a previous venture ?

Not me - but look, from his second picture - I reckon he's just walking along the bottom and not sailing at all. The cheat!

Andrew - ok, you've demonstrated 3mm ply is enough. Does that mean I can add another 40kg of ballast? And while I note your desire for a stiff bottom, I'd still prefer the comfy chair%%

Andy #1963#
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Tester

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Re: A "Thames Shove".
« Reply #16 on: November 25, 2008, 09:01:42 PM »

Hi

You must see this youtube video of the small 2.4's sailing in the Solent, they are about 2 mins into the video. Just came across them while looking at the round Britain trip.
http://uk.youtube.com/watch?v=sr8jDOR05yI&feature=PlayList&p=0724921CE4D5BF56&index=2

Richard

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