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The Great Eastern Saga!

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Mark Beard30/04/2011 21:42:06
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There are several ways to work this problem.

The obvious way is to have a drum on a spindle and take the cable off tangentially, spinning the drum as the cable is unwound. This makes winding and unwinding the drum easy, but has a couple of disadvantages: 1) the inertia of the drum will cause cable overrun within the hold as the ship slows down; and 2) extra beam is required for the cable take-off.

The less obvious way is based on the principle of wire-guided missiles (can your imagination stretch from those to the Great Eastern?). A tapered bobbin with the narrow end astern. The take-off point is aligned with the spool axis and is further aft still. The attached sketch shows that the take-off point must be aft of the point projected by the outer layer of cable on the spool, to prevent it from fouling on the inner layers.

The advantage to this scheme is low un-spooling resistance, negligible inertia and simple axial take-off gear. This comes at the cost of less simple re-winding and the need to incorporate some twist into the cable as it is re-wound to avoid it twisting up when it is un-spooled .

Is this anything like what you have in mind? While you think of a diplomatic answer, I'll work out how much cable can be spooled this way.

Mark



Mark Beard30/04/2011 22:40:54
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163 forum posts
84 photos
Right:
 
The parameters I'm working to are 2mm cable diameter, minimum cable bend radius of 20mm and max radius of 35mm (to accommodate the 75mm diameter plus 2x cable radii plus 1.5mm clearance).


DRUM

The volume of the drum of wire is drum length x coil area

V = L x pi x (r2² - r1²

where V = the volume in cubic-mm
pi = 3,14159
r2 = the outer coil diameter in mm
r1 = the inner coil (former) diameter in mm

Cable length, assuming that the cable coils lie next to each other and occupy square secion places, is the volume divided by the square section across the cable diameter d, which is

CL = L x pi x (r2² - r1² / d²

So, for a simple drum on a 40mm former and 70mm outer diameter (leaving the spandrel section below the deck, adjacent to the hull, for the take-off gear), gives

L = 150
r1 = 20
r2 = 30
d = 2

CL = 150 x 3.142 x (900 - 400) / 4
= 58,912mm
= 58.9 metres


SPOOL

For a tapered spool, the angle of taper is given by the minimum radius 20mm and the 750mm (30-inch) take-off distance. 20mm in 750mm is 4mm in 150mm, which gives the minimum taper. So design with a 5mm taper, giving the bobbin a tapering in radius from 20mm to 25mm (diameter 40mm to 50mm).

Now because the take-off gear is axial, we can use almost the whole diameter available for the outer cable radius, so this will taper from 30mm to 35mm.

I don't propose calculating the precise volume without knowing that this is the scheme to go for, but will estimate is by taking the average radii of the tapered spool and using the drum formula above.

So, for this approximation:

L = 150
r1 = 22.5
r2 = 32.5
d = 2

CL = 150 x 3.142 x (1056 - 506) / 4
= 64,804mm
= 64.8 metres

So, either way I reckon you'll get around 60 metres in that volume, slightly more with the tapered spool. Is that enough?

Mark Beard30/04/2011 22:42:46
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163 forum posts
84 photos
Er, those smilies are supposed to be close brackets. Thanks forum software!
Bob Abell01/05/2011 05:11:38
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Wow! Mark!

You have been busy!

64 metres is disappointing!

Don't fancy the tapered spool idea, as it wouldn't look right in the hull....and it would twist the wire as it pulled off?

Prefer a parallel coil with a friction device resting on top......To prevent the drum overrunning

If that's all the cable we've got, then we'll have to live with it.

There is chance that single core cable could be made to work, with the earthing rods etc and an amplifier or something

I think a TV man is calling round, sometime next week and at least we make some progress, one way or the other.

Thank you very much for your assistance and tech stuff

Regards.....Bob
Bob Abell01/05/2011 09:46:48
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5099 forum posts
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Latest pictures of the winch
Slightly oversize........But needed to be very strong in case we have a snarl-up out to sea!
In which case the boat will stall with no damage done......Hopefully
 



Guess what I used for the pulley flanges?..........Pennies!

Edited By Bob Abell on 01/05/2011 09:47:12

Mark Beard01/05/2011 10:56:02
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163 forum posts
84 photos
Gosh Bob, you don't hang about! That looks amazing.
Mark Beard01/05/2011 11:57:16
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163 forum posts
84 photos
Did you spot my error in the drum cable calculation? The outer diameter was 70mm so the radius should be 35 not 30mm. Too much of a late night I think!
 
L = 150
r1 = 20
r2 = 35
d = 2

CL = 150 x 3.142 x (1225 - 400) / 4
= 97,205mm
= 97 metres
 
Think it makes a big difference? It should do, as the volume is proportional to radius squared, so a small increase in radius makes a big difference in volume (hence length).
 
If you could increase the outer diameter to 92mm, then I reckon you'd get 200m of 2mm cable in. But that of course may be too much for the model, unless your size limit is for passing a drum through a hatch, and winding the cable in situe would allow for more cable?
Bob Abell01/05/2011 20:10:25
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5099 forum posts
1523 photos

Had a trial run with some handy two core stuff and it ran through ok
 
The winch in it`s final position. The cable travels down towards the drum via a length of plastic tubing...To get through the mass of assorted wires inside the hull and into a clear area and hopefully, will flex sideways as the cable comes of the 6" wide drum

Bob Abell02/05/2011 20:03:27
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5099 forum posts
1523 photos
Looking at the archives again, Great Eastern`s max cable laying speed was 7 knots
 
This works out at 8 mph
 
The model is 1/100th scale and the model speed works out at 86" per min
 
In other words.......one hull length per min........1.44" per second.........Awfully slow!
 
It will be interesting to see what speed we get up to, the normal sailing speed is about 2 mph
 
Speaking to an electronic expert today, he reckons we can use fine earphone coaxial cable, and obtain a signal using computer on off signals?
This would make the cable drum size very practical
 
All good stuff!......Bob
ashley needham03/05/2011 10:45:05
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3205 forum posts
166 photos
I like all the calculations...but
 
i think a practice run is needed first, using a.n.other boat and any old setup, using different types of wire....because...
 
The wire will not be heavy enough to sink straight away like the real stuff, although no doubt it WILL sink, and tension on the wire may cause an appreciable length to be strung out over the stern looking less than realistic. The paying out attachment at the stern looks very "period" but the realist in me would like to see a funnel at the front end to guide the wire through better than a simple `ole.
 
It may be that, although running the cable across the pond in one length is the aim, just thinking out loud...that "an amount" of cable could be layed, and then a sleeved weight placed over it, and laying continues..with "weight" stops, to ensure the ship has something to pull against without the cable rising in an unseemly manner, and keeping the rest down and in a straight line.
 
The stops in laying need not be televised of course, and the fact that the ship will have laid a continuous length will not be compromised.
 
Ashley just thinking around the practicalities.

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