Chris,
For the record, when I said tons I meant the actual mass of the boat in units of 2000 pounds and when I said kW I was talking Volts x Amps being draw from the battery to the propulsion system. It's a the simple ratio of Total Mass/Unit Power.
Capt. Carter
www.shipofimagination.com
On Saturday, March 22, 2014 4:37 PM, Chris Hudson <clh5_98@yahoo.com> wrote:
Ok, dumb question, someone please define a ton.
Chris
Sent from myPhone
I can tell you from experience that does not scale down to a 20 ton, 45 ft boat. Although I'm running at about .3 kiloWatts/ton and it works for me because I plan my passages carefully and built the boat for cruising the intracoastal waterway , I would not recommend a low ratio like this unless you only plan on operating exclusively on very calm waters or you have a backup, then it's fine.Although control of the boat can be maintained safely at this ratio of weight to power in some fairly adverse conditions, performance drops precipitously and with a combination of high waves, winds, and current, the sea could easily have it's way with you.My backup is a 38 hp diesel in parallel with my electric drive and I've even experienced a few extreme situations where this was marginal and that translate directly to about 1.5 KW/ton.Capt. Carter
James, you asked about electric power ratios on larger boats (ships?).My wife and I enjoyed our cruise on Royal Caribbean Voyager of the Seas.'I am sure its propellers, motor fairings and hull shape are optimized for efficiency at 22 knots.Using info available on line, the 1020 ft ship has a hull speed of 42 knots, is designed to cruise at 22 knots and we have been aboard when it was making 26 knots in 26 ft seas. It is rated at 142000 gross tons. Depending on how you accommodate the 3 bow thrusters of 3,000 kW each, and its 3 pods of 14,000 kW each, its power to displacement ratio is between 0.3 and 0.36 kW/ton.It looks like 1 kW/ton is a reasonable number when care is taken in hull and prop ... probably need more than either of those are less than optimum.--------------------------------------------------------------------------------See My sources and simple calculations for Voyager of the Seas below:--------------------------------------------------------------------------------ABB electric propulsion system comprises three ABB 14,000 kW Azipod units (two azimuthing and one fixed). Maneuverability is enhanced by three 3,000 kW thrusters supplied by Sweden's KaMeWa.142,000 gross tons.Built 1999, Years old 15, Passengers 3110- 3732, Crew 1180, so Souls on Board < 5000Cabins 1555, Space Ratio 37 sq ft/pax, Tons 137276,Speed 22.0 knots, Length 1020 ft, Beam 161 ftRefurbished: 2009Hull Speed = 1.34 Sqrt[1020 ft] = 42 knots.Power Ratio = (3*14000 kW)/(142000 tons) = 0.3 kW/tonor (3*14000 kW+9000 kW)/(142000 tons) = 0.36 kW/ton
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