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Old 06-17-2013, 07:28 PM   #1
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How effecient are Ferries? Quesiton for the NA's

Pondered this while cruising the San Juans this weekend. The Elwah can cruise by at I'm guessing 20 knts and barely cause a wake. Right behind is a tour boat running about the same speed with a big bone it's mouth, causing me to shut off the autopilot, turn into the wake and generally disrupting a very pleasant cruise back to Anacortes. As I enjoyed a good Cabernet back at the dock, i was wondering if this a function more of hull design, theoretical hull speed or what. Does the double ended design of the ferry help reduce wake?
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Old 06-17-2013, 09:12 PM   #2
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long waterline length
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Old 06-17-2013, 09:57 PM   #3
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The Elwha is 383 ft 2 in long, the the square root of her length at the waterline is ~19.5. So at 20 knots she is only going 1.02 times hull speed. Most boats will have a small wake at hull speed, but at speeds above hull speed the wake increases dramatically.

Elwha has 4 diesel electric engines producing 10,200 HP with a net weight of 1,322 tons and a gross weight of 2,813 tons. She carries up to 2,500 passengers and 144 vehicles.
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Old 06-17-2013, 10:29 PM   #4
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Re the Elwa I believe the hull speed would be 26 knots. To drive her at that speed would require perhaps twice the power (or more).

19.5 x 1.34 = 26 ... The formula being 1.34 X the square root of the WL.

The Elwa comes in at about 8hp per ton. The Willard in my avatar is 8 tons and has 40hp. That's 5hp per ton so the Elwa has more power per ton and I'm at a loss to explain why as I thought it would have about half that amount .. or less.
The answer is probably that only 2 or 3 of the 4 engines usually run in normal running. And the weight given is "net" so gross weight w all the vehicles on board would be way higher.

Re the OP yes the double end hull (they don't call these ferries canoes for nothing) is an extremely low drag hull. Minimal wave making resistance and minimal wetted surface. But of course the symmetrical hull is a function of the fact that the ferry runs in both directions. So for all practical purposes the only way to make this mono hull more efficient would be to make run slower or weigh less .. or both.
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Old 06-18-2013, 12:32 AM   #5
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It really is the length of the waterline driven at hull speed of a full displacement hull that produces a small wake, or a planing hull fully on step. The tour boat , Glacier Spirit is 80' and 2 -8V92's @ 500+ hp each running at 18.2-18.5 kts . She is running at semi displacement speed, not fully on plane. I have pushed her to over 22 kts empty and the wake gets much smaller. She will do the same speed loaded but she still isn't fully on step and the 4kts doubles the fuel consumption.
It always is amazing to see the difference in the merchant ships wake between loaded and empty as they navigate Puget Sound.
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Old 06-18-2013, 04:01 AM   #6
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Get ready to hang on! A 35-knot, catamaran ferry is passing.

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Old 06-18-2013, 04:07 AM   #7
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In Bay waters, eight-knot freighters hardly wrinkle the waters.

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Old 06-18-2013, 05:32 AM   #8
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We were on the flying cat ferry last week, I asked the crew how fast we were going...he said about 40 knots. Very smooth.

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Old 06-18-2013, 05:53 AM   #9
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The SQ rt of the actual LWL is known as SL .

1.34 times SL gives "hull speed" for fat heavy boats with a Length /Beam ratio of 3 or 4. .

With a L/B over 6 ( usually multihulls ) the rule no longer applies as mant can rin SL 3 or more with modest power.

They loose at low speeds as the high L/B ratio causes more wetted surface so cats make poorer low speed cruisers than fat tubs.

Hopefully light weight can make up for a bit , IF you don't have much gear aboard.

Always tradeoffs , no free lunch.
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Old 06-18-2013, 06:32 AM   #10
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Hull speed - Wikipedia, the free encyclopedia

As a ship moves in the water, it creates standing waves that oppose its movement. This effect increases dramatically in full-formed hulls at a Froude number of about 0.35, which corresponds to a speed-length ratio (see below for definition) of slightly less than 1.20 (this is due to a rapid increase of resistance due to the transverse wave train). When the Froude Number grows to ~0.40 (speed-length ratio about 1.35), the wave-making resistance increases further due the divergent wave train. This trend of increase in wave-making resistance continues up to a Froude Number of about 0.45 (speed-length ratio about 1.50) and does not reach its maximum until a Froude number of about 0.50 (speed-length ratio about 1.70).
This very sharp rise in resistance at around a speed-length ratio of 1.3 to 1.5 probably seemed insurmountable in early sailing ships and so became an apparent barrier. This leads to the concept of 'hull speed'.


The 1.34 in the formula is hardly cast in stone. Over the years I have seen it used for different hull shapes from 1.1 all the way up to 1.6/1.7.

And best of all...it's a theoretical number until one is actually determined for a finished hull...

So using 1.34 to "guess" at a hull speed is just that...a guess.
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Old 06-18-2013, 07:26 AM   #11
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Elwha has 4 diesel electric engines producing 10,200 HP with a net weight of 1,322 tons and a gross weight of 2,813 tons.
Geez guys ... if you are going to discuss the minutia of hull speed and power per can of beer sold per crossing times the square root of the number of Ipads onboard, at least get your basics straight.

How can anyone discuss this stuff without even the most basic understanding of the numbers and terms they are using?

The figure of 2813 has absolutely nothing to do with the weight of the vessel. It is a regulatory number calculated with regard to internal volume.

Subtracting net from gross to come up with a ship's cargo weight is even more preposterous.

Displacement of Elwha is very close to 4000 tons, and the waterline length (the length that matters with regard to hull speed and such) is just under 360 feet. GIGO.

It takes less time to look it up than it does to post what should be very embarrassing nonsense.
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Old 06-18-2013, 08:47 AM   #12
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Geez guys ... if you are going to discuss the minutia of hull speed and power per can of beer sold per crossing times the square root of the number of Ipads onboard, at least get your basics straight.

How can anyone discuss this stuff without even the most basic understanding of the numbers and terms they are using?

The figure of 2813 has absolutely nothing to do with the weight of the vessel. It is a regulatory number calculated with regard to internal volume.

Subtracting net from gross to come up with a ship's cargo weight is even more preposterous.

Displacement of Elwha is very close to 4000 tons.

It takes less time to look it up than it does to post what should be very embarrassing nonsense.
I've read that being on this forum is really a social event and that real information is just a distant, secondary thing....
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Old 06-18-2013, 09:37 AM   #13
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I've read that being on this forum is really a social event and that real information is just a distant, secondary thing....
Fair enough but geez, if someone wants to write a science fiction post they should at least warn readers that what follows has no relevance to how or why a boat operates in some manner and the numbers supplied may or may not be applicable to anything at all.

This thread reads like the data was supplied by the same people who briefed Bush on Iraq's WMD program.

If it is just a social site then label it as such. But if it is going to hold itself out as a source of information for boat owners is it too much to expect contributors to at least understand what the numbers they post mean and if they are relevant and in the same context as the discussion?

Some people actually come here for information.
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Old 06-18-2013, 10:25 AM   #14
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It is a social event here. If you want facts and pure information just look it up.

Rick ... your analogy in your last post is priceless ...

"This thread reads like the data was supplied by the same people who briefed Bush on Iraq's WMD program."

However it's an obvious fact now.


psneeld wrote ... "The 1.34 in the formula is hardly cast in stone. Over the years I have seen it used for different hull shapes from 1.1 all the way up to 1.6/1.7."
Indeed and many hull shapes don't respond well to the formula like kayaks. But it gives us a jumping off point. And many people need numbers to explain or understand much of the physical world. The best example re this thread is that a dutch barge is practically round at either end and the Elwa ferry is very pointy at the ends but both may have the same WLL. Of course more numbers help like the PC but fully defining the physics of water and hull cannot be fully expressed accurately w numbers.
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Old 06-18-2013, 10:30 AM   #15
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WMDS were found. Not in large quantities. They were shipped to Canada for disposal.

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Old 06-18-2013, 01:44 PM   #16
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Fair enough but geez, if someone wants to write a science fiction post they should at least warn readers that what follows has no relevance to how or why a boat operates in some manner and the numbers supplied may or may not be applicable to anything at all.

This thread reads like the data was supplied by the same people who briefed Bush on Iraq's WMD program.

If it is just a social site then label it as such. But if it is going to hold itself out as a source of information for boat owners is it too much to expect contributors to at least understand what the numbers they post mean and if they are relevant and in the same context as the discussion?

Some people actually come here for information.
I do...but you know that....
Some no matter how many other references are provided like to keep passing along the "well it sounds good!"....
You alone have set me straight more than once...and not afraid to admit it or try to explain it away as we are just here to BS
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Old 06-18-2013, 07:35 PM   #17
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Those jet drives have to be fuel wasters. What is the efficiency of them? Props have to be much more efficient.
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Old 06-18-2013, 07:45 PM   #18
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would the "Elwha" have a diesel electric plant? or just straight diesels.?
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Old 06-18-2013, 08:17 PM   #19
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Vessel Information

Class: SuperType: Auto/Passenger FerryLength: 382' 2"Engines: 4Beam: 73' 2"Horsepower: 8,000Draft: 18' 9"Speed in Knots: 20Max Passengers: 1076Propulsion: DIESEL-ELECTRIC (DC)Max Vehicles: 144Gross Tonnage: 2813Tall Deck Space: 30City Built: San DiegoAuto Deck Clearance: 15' 1"Year Built / Re-built: 1967 / 1991Meaning of Elwha: Native American/Chinook: "elk."
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Old 06-18-2013, 08:19 PM   #20
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Those jet drives have to be fuel wasters. What is the efficiency of them? Props have to be much more efficient.
Jet Drives Picking Up Steam in Many Applications | BoatTEST.com

High Efficiency

Propulsive coefficients as good or higher than the best propeller systems achievable at medium to high planing speeds
Hole shots faster than prop-driven boats
Better fuel efficiency than most other propulsion options
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