I got more than I bargained for when I asked the question 5KW or 10KW. A lot of excellent dialogue here.
Each boat has it's own unique situation that will dictate what is right for them.I suppose it's ok to use the rule of thumb but that's all it is... I'd bet there are folks that selected their EP based upon the assumption of 1KW per ton & if could do it again might make a different selection.
On Saturday, March 22, 2014 5:32 PM, Electroprop <james@electroprop.com> wrote:
It is interesting to see the KW per ton for a ship.
Ship design has been fully optimized and the low KW power level is a result of this.
The propeller shaft rotates much slower, often as little as 100 rpm. The propeller shaft is horizontal. The propellers have been optimized with the right pitch, diameter, blade area ratio and cupping. The waterline length is almost the same as the ships length.
We constantly try to get closer to ship design but the constants of a small vessel mean that propulsion is oft time last on the list and is stuffed in wherever there is room left over. Much of the time this results in the engine behind the companion way stairs and a steep shaft angle. My Catalina 30 has the engine under the kitchen area which makes the propeller shaft very horizontal. Eric's Cheoy Lee and many other full keel designs like the The Darriel have a horizontal propeller shaft. This seems to make a huge difference in the efficiency of the propellers and more than counters the effects of putting a propeller in a tight aperture.
We should calculate a duration formula it quite simply with a bit of trigonometry and make a chart of efficiency compared to propeller shaft angle.
With respect to establishing a rule of 1 KW per ton, which incidentally is quoted in the West Marine Catalogue, we should first give credit to Eric Dysart, who to my knowledge was the first person to make the connection between displacement and power level and came up with the 1 KW per ton formula.
The conditions of the 1 KW per ton that we have been following are:
The vessel is a displacement sailboat hull.
The boat is no more than 7 tons and no more than 35 feet long
The waterline length (LWL) is at least 80% of the length over all (LOA)
The power is measured as continuous power consumed by the drive.
Intermittent Power is usually 40% more than continuous power.
The drive unit is at least 80% efficient, so the ratio is .8 KW per ton at the propeller shaft
The Ton that we are using is 2,000 lbs, also known as the short ton
The propeller shaft has no more than a 10 degree down angle, propeller shafts with more than a 10 degree down angle should be derated by multiplying by the cosine of the down angle.
The weight of the boat is fully laden, including the weight of the batteries, anchor chain etc.
What we have found is 1 KW per ton is getting us up to 90% of hull speed or up to 6 knots whichever is greater on vessels up to 35 feet long, weighing no more than 7 tons.
We will need to do an accurate charting of different boats to get a better data set for larger boats.
My intuition tells me that this should be 1.1 KW per ton at 38 feet, 1.2 KW per ton at 40 feet, 1.5 KW per ton at 45 feet, 2 KW per ton at 50 feet.
Anything less than these recommendations is "adventuresome" and not appropriate for all skippers.
If a vessel is continually exposed to high currents, wind or wave conditions, then the drive must be increased in size. Some vessels may go as high as 4 KW per ton or beyond if operating in very adverse conditions.
I welcome your thoughts and comments. It would be really cool if as a group we could come to a consensus. We are, after all, on the leading edge of this technology.
James
James Lambden
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103
805 455 8444
james@electroprop.com
www.electroprop.com
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103
805 455 8444
james@electroprop.com
www.electroprop.com
On Mar 22, 2014, at 9:04 AM, Carter Quillen wrote:
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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