Discussion:
Top Speed of 100 Mega Ton Iceberg Towed By 2 Tugs +1 Ship? 0.5 Knots
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Bret Cahill
2018-07-05 18:36:06 UTC
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Gear down the drive and fit them tugs with yuge 30 m diameter props or the propulsion efficiency will result in unnecessary burning of megatons of carbon!

https://www.cbsnews.com/news/south-africa-cape-town-water-crisis-plan-drag-iceberg-from-antarctica/

Is this a crank idea? Run the nubbers:

The iceberg is 200 X the size of the largest ship. Ignore efficiencies of scale (possibly to his favor), ignore the horrifically low propulsion efficiency from undersized props and ignore the lousy hydrodynamics of an iceberg (not to his favor).

Assume a 20,000,000 kw engine could get it moving at 20 knots.

Assume the 2 tugs + 1 ship with conventional props only put out 100,000 kw.

By the propeller rule the speed drops by (100,000/20,000,000)^0.33 = 0.17 or 83%

Or 3.4 knots ignoring the horrifically low propulsion efficiency.

Supposing you don't want to ignore the horrifically low propulsion efficiency?

The props were designed to go 3 - 5 times faster than this at maximum propulsion efficiency. Iterate once or twice and the horrifically low propulsion efficiency drops the speed down to maybe 1/2 knot.

He can use ocean currents _if_ they aren't too fast, _if_ he can get in and out of the currents with a top speed of half a knot.

Kind of like how they tell you at the beach to swim perpendicular to a rip tide. Half a knot might not be fast enough before you can no longer see land.


Bret Cahill
Bret Cahill
2018-07-06 00:03:14 UTC
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Post by Bret Cahill
Gear down the drive and fit them tugs with yuge 30 m diameter props or the propulsion efficiency will result in unnecessary burning of megatons of carbon!
https://www.cbsnews.com/news/south-africa-cape-town-water-crisis-plan-drag-iceberg-from-antarctica/
The iceberg is 200 X the size of the largest ship. Ignore efficiencies of scale (possibly to his favor), ignore the horrifically low propulsion efficiency from undersized props and ignore the lousy hydrodynamics of an iceberg (not to his favor).
Assume a 20,000,000 kw engine could get it moving at 20 knots.
Assume the 2 tugs + 1 ship with conventional props only put out 100,000 kw.
By the propeller rule the speed drops by (100,000/20,000,000)^0.33 = 0.17 or 83%
Or 3.4 knots ignoring the horrifically low propulsion efficiency.
Supposing you don't want to ignore the horrifically low propulsion efficiency?
The props were designed to go 3 - 5 times faster than this at maximum propulsion efficiency. Iterate once or twice and the horrifically low propulsion efficiency drops the speed down to maybe 1/2 knot.
He probably calculated it by thrust but this will work if you want to get started.
Post by Bret Cahill
He can use ocean currents _if_ they aren't too fast, _if_ he can get in and out of the currents with a top speed of half a knot.
Kind of like how they tell you at the beach to swim perpendicular to a rip tide. Half a knot might not be fast enough before you can no longer see land.
Bret Cahill
You could increase the propulsion efficiency by 2 orders of magnitude and the speed by 7X with 1000 submersible EV motors. The motors would be ganged on a long structure perpendicular to the direction of travel. The motors would be geared down to 10 rpm turning 3 m diameter props. The structure would have just enough buoyancy to float 4 m below the surface. The structure receives the yuge torque from each prop and provides attachments for several tow lines and electrical power.

Each motor would draw 100 kw from a 100 MW ship mounted power plant.

The structure could be modular. For bigger bergs make it longer or have several columns.
Bret Cahill
2018-07-06 18:11:17 UTC
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Post by Bret Cahill
Post by Bret Cahill
Gear down the drive and fit them tugs with yuge 30 m diameter props or the propulsion efficiency will result in unnecessary burning of megatons of carbon!
https://www.cbsnews.com/news/south-africa-cape-town-water-crisis-plan-drag-iceberg-from-antarctica/
The iceberg is 200 X the size of the largest ship. Ignore efficiencies of scale (possibly to his favor), ignore the horrifically low propulsion efficiency from undersized props and ignore the lousy hydrodynamics of an iceberg (not to his favor).
Assume a 20,000,000 kw engine could get it moving at 20 knots.
Assume the 2 tugs + 1 ship with conventional props only put out 100,000 kw.
By the propeller rule the speed drops by (100,000/20,000,000)^0.33 = 0.17 or 83%
Or 3.4 knots ignoring the horrifically low propulsion efficiency.
Supposing you don't want to ignore the horrifically low propulsion efficiency?
The props were designed to go 3 - 5 times faster than this at maximum propulsion efficiency. Iterate once or twice and the horrifically low propulsion efficiency drops the speed down to maybe 1/2 knot.
He probably calculated it by thrust but this will work if you want to get started.
Post by Bret Cahill
He can use ocean currents _if_ they aren't too fast, _if_ he can get in and out of the currents with a top speed of half a knot.
Kind of like how they tell you at the beach to swim perpendicular to a rip tide. Half a knot might not be fast enough before you can no longer see land.
Bret Cahill
You could increase the propulsion efficiency by 2 orders of magnitude and the speed by 7X with 1000 submersible EV motors. The motors would be ganged on a long structure perpendicular to the direction of travel. The motors would be geared down to 10 rpm turning 3 m diameter props. The structure would have just enough buoyancy to float 4 m below the surface. The structure receives the yuge torque from each prop and provides attachments for several tow lines and electrical power.
Each motor would draw 100 kw from a 100 MW ship mounted power plant.
The structure could be modular. For bigger bergs make it longer or have several columns.
The structure should be a flexible net covering a 1 km X 1km area, floating just below the waves -- 150 MW at noon if it's solar.

The motors should be in counter rotating pairs so there is no torgue on the next hinged and/or torsion bar segment.
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