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Old 08-08-2011, 08:58 PM   #1
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

In order to reduce overall fuel consumption on a twin boat I'm thinking about running a twin screw SD boat using one engine at low RPM for 4-6 hours and then at 75% for 1/2 hour before switching to the other engine?* I would assume that the loss in efficiency due to the rudder angle drag would be minor and I would be able run the one engine closer to it's 75% sweet spot.* I would also assume that an autopilot would be able to hold when running on that single engine.* Has anyone cruised this way and if so have you got any fuel consumption numbers (twin vs. single)?
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Old 08-08-2011, 10:50 PM   #2
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

The drag from the unpowered prop will be considerable. Not as much if it freewheels but some transmissions won't tolerate that. Also some boats need cooling water fed to the shaft log from the engine's raw water system, which it won't be if the engine isn't running. This is the case with our boat. So the shaft needs to be locked off which means the locked-off prop will be producing even more drag. (It's a popular myth that a locked prop produces less drag than a freewheeling prop but MIT showed in a series of tank tests quite awhile ago that this is just a myth.)

The founder of the Grand Banks owners forum ran some pretty comprehensive tests a number of years ago with his own boat, a twin-engine Alaskan 45, and in the end found that running on one with the unpowered prop freewheeling and locked did not save enough fuel to care about, and at some power settings didn't save any fuel at all or even used more. His conclusion was that unless one was doing some really long-distance cruising, in which case the best solution is to remove the unpowered prop altogether, running on one with a boat like a GB is not productive, particularly in light of the fact that fuel is probably the smallest factor in the cost of owning a boat, even at today's prices.

Running either a single or twin with less power definitely does save fuel. The only limiting factor is engine temperature. You don't want to back off the power to the point where the engine is running too cool. How detrimental this is will be dependent on the engine type-- some apparently tolerate it better than others. But even a 100 rpm reduction in power will reduce the fuel consumption by a not-insignificant amount if you're willing to go slower (we aren't :-) )
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Old 08-09-2011, 11:59 AM   #3
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Why don't you just buy a single engine boat in the first place?

*

http://www.yachtworld.com/boats/1998.../United-States

Here is a nice example.

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Old 08-09-2011, 09:01 PM   #4
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

I've done it for over 30 years with no problems. *We originally started doing it because we were running with another trawler, that had a single Ford Leman 120 and he cruised around 6 knots. *At that speed my engines would only reach 140 degrees and we were doing S turns (dodging U-boat torpedos?) in order to not run away from him. *Perkins didn't suggest running the 6.354's that cool and with such a lite load for long periods. *So I called Borg Warner and they told me that running one Velvet Drive transmission and free wheeling the other was not a problem (larger cruising sail boats, without feathering or folding props, do it all the time while under sail. *That said, check with the manufacture and be mindful of what Marin said about the shaft logs below. *

When the weather is good and we're not in a hurray, we run a little slower, between 7 and 8 knot, on one engine and we can see a difference on the fuel burn as well. We alternate engines every two hours (mainly just to keep the hours close for maintenance purposes) *The boat runs a lot quieter and yes it reduces maintenance, wear and tare, and we also enjoy the slower pace, you don't have to be on point looking intently for logs and debris behind every wave like when running at higher speeds. I've also noticed that the engine exhaust has cleaned up alot, they don't seem to smoke as much and the lube oil consumption has almost gone to nothing. *I think the added engine loading, actually has helped the engines run cleaner.

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-- Edited by Edelweiss on Tuesday 9th of August 2011 09:14:58 PM
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Old 08-09-2011, 09:38 PM   #5
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

As a FWIW data point, we run both our FL120s all the time and run them at 1650 rpm. That gives us a wee bit under 8 knots (we had the props pitched an inch down a couple of years ago, we used to run 8 knots). So far, after 13 years, the engines don't smoke or leave any soot on the transom and each one of them uses less than 1 guart of oil every 100-150 hours which is our oil change interval.

The operators manual for our BW Velvet Drives says the transmission can be freewheeled with no problems at "sailing or trolling " speeds, which is not too helpful of a guideline. But I take it to be "slow." Slower than we go, I think. Also, our shaft logs are cooled/lubed by water feeds from each engine's raw water system so if we have to shut an engine down we have to tie off the shaft. With the prop locked, the boat is no fun to drive at all. The turbulence off the locked prop buffets the rudder behind it something fierce and all this is fed back up the cable-chain steering to the helm (I guess it wouldn't be with hydraulic steering).

And eight knots is painfully slow as it is. To reduce the speed to six or so would be intolerable and certainly not worth (to us) the savings of a few dollars worth of fuel. But then if we could afford it we'd have a boat that could do 30 knots. Actually, we have a boat that does 30 (mph). But it's only 17 feet long and not the sort of thing you'd want to take a three week cruise in with friends.* But it goes the right speed for us :-)
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Old 08-09-2011, 10:31 PM   #6
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

John P hath seen the light.
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Old 08-09-2011, 11:03 PM   #7
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Quote:
Edelweiss wrote:
...*We originally started doing it because we were running with another trawler, that had a single Ford Leman 120 and he cruised around 6 knots.
*Hmm.* My 80 h.p. John Deere is "happier" at around 1800 RPMs for 6.5 or so knots.
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Old 08-09-2011, 11:10 PM   #8
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Quote:
Marin wrote:
And eight knots is painfully slow as it is. To reduce the speed to six or so would be intolerable and certainly not worth (to us) the savings of a few dollars worth of fuel. But then if we could afford it we'd have a boat that could do 30 knots. Actually
*I don't know about that.*** When looking at water passing by the hull at six to seven knots, it*seems pretty fast to me.* But then I am a former sailor.* Besides, such speeds gives me time to think how to avoid traffic, as in slowing down to let the ferry pass and a hard turn to starboard to*avoid the sailing dinghy (as done in yesterday's bay cruise).
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Old 08-09-2011, 11:27 PM   #9
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Quote:
markpierce wrote:*When looking at water passing by the hull at six to seven knots, it*seems pretty fast to me.
It looks like we're bloody standing still to me.* Slow sucks, is my mantra :-)* When we run our Arima at 30 mph I don't find it any more difficult to watch out for stuff as I do when we're oozing along at 8 knots in the GB.* After all, even 30 mph is way slow compared to driving speeds.* I drive most of the time on my commute at 70 mph or so, and I don't find it hard to spot and avoid*debris in the road or*make maneuvering decisions or*whatever.* Seeing stuff in the water at 30 mph is like watching it in slow motion compared to that.*

The big problem is at 8 knots when it's so easy to get distracted (anything to relieve the ant-like pace, right?) that one can overlook even the obvious out in front of you.* This is why we use a five minute timer at the helm to remind us to check the engine instruments.* Way too easy to get distracted watching a whale or something.

Mind you I like our boat and we both enjoy our time in it and on the water immensely.* We are well aware of the efficiency of the GB and the price of going fast (more than we want to spend these days).* But that does not alter the fact that we both absolutely detest single-digit speeds.


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-- Edited by Marin on Tuesday 9th of August 2011 11:30:47 PM
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Old 08-09-2011, 11:51 PM   #10
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Quote:
markpierce wrote:
Quote:
Edelweiss wrote:
...*We originally started doing it because we were running with another trawler, that had a single Ford Leman 120 and he cruised around 6 knots.
*Hmm.* My 80 h.p. John Deere is "happier" at around 1800 RPMs for 6.5 or so knots.

*The trawler we were running with was a custom 44' with 14 1/2' beam live aboard. *It moved through the water like a hippo. *

I like to run 2000 RPM on one engine at 7.6 K over-the-ground on a flat tide, will run up to 2500 RPM max on one and 2800 RPM max with both running. *Don't do that often, the boats too noisy and the sound of diesel being sucked through fuel lines upsets my stomach.*
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Old 08-10-2011, 12:41 AM   #11
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Quote:
Marin wrote:markpierce wrote:*When looking at water passing by the hull at six to seven knots, it*seems pretty fast to me.
It looks like we're bloody standing still to me.* Slow sucks, is my mantra :-)*...

*What's the hurry?* If I want to go fast and far, I take an automobile, train,*airplane, or ship.* Boats*are for putzing and taking in the scenery. *(Ferries around here go upwards of 40 MPH.)* I*can usually*approach waterbirds at high cruising speed (7+knots) within 20 feet or less before they fly away.* *



(OK, OK.* We're going about 30 MPH in Arizona here.* But that is sedate for land-based travel.* And after all, it is a tourist railroad.)


-- Edited by markpierce on Wednesday 10th of August 2011 01:19:12 AM
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Old 08-10-2011, 10:35 AM   #12
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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*What's the hurry?

Lots of places to see, lots of things to do, and you only go round once.* The slower you go the less you experience in that one go-round.
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Old 08-10-2011, 06:13 PM   #13
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

I fly a floatplane that goes 110 miles per hour. I used to fly a plane that did about 180 mph. I fly IN planes that go 500-plus miles per hour. I drive cars at up to 85-90 mph when I can get away with it. I find this notion that 30 mph is too fast to see anything or enjoy the surroundings to be pretty funny....
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Old 08-10-2011, 09:21 PM   #14
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Quote:
*
*Argueable, no.... if you go to fast you miss the experience some might say...

this is an opinion, as are all the rest of the above comments.* I used to believe in the faster is better mode, but once I started RVing on land and water I like the slow pace mode more often then not... there is a time and place for fast and slow...
I agree. . . I used to live for fast cars, boats and airplanes.* But I have learned to slow down. I have more patience now and enjoy life to it's fullest.
Nothing more enjoyable than slow cruising through the islands at dawn or just before sunset watching the marine life, porpoises rolling on the surface chasing bait, salmon jumping all around us.* Anchored up in Eagle Harbor watching Bald Eagles gliding overhead.*
Life is good. . . . . . but sometimes you just need to slow down to enjoy it.
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Old 08-10-2011, 09:53 PM   #15
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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Edelweiss wrote:*
Nothing more enjoyable than slow cruising through the islands at dawn or just before sunset watching the marine life, porpoises rolling on the surface chasing bait, salmon jumping all around us.* Anchored up in Eagle Harbor watching Bald Eagles gliding overhead.*

*We see and experience all that and more at a staggering, blinding, heart-stopping*30 mph when we're out fishing up north.* In fact we see more of it because we're covering a lot more ground than we do at a glacial 8 knots.

If we were talking about driving around at 60 knots or going through the islands in an unlimited hydroplane I'd agree with the point.* But we have a 30 mph boat and we've been running it in these waters since 1987.* And this notion that at 20 or 30 mph you can't see and experience and enjoy what you can at 8 knots is just nuts, at least to me.* In my opinon, 30 knots is not considered fast because it is, it's considered fast because most other boats are so bloody slow.*

If somebody wants and likes to go slow, that's fine.* I have no quarrel with that at all.* We met a fellow at Patos Island several years ago who uses his engine as little as possible.* He times all his trips through the islands*to simply drift with the currents.* Works for him, he has a great time, and I'd be a fool to tell him he's wrong.

But this idea that fast is Bad and you won't see anything and can't enjoy your surroundings.... bollocks, as they say in the UK.* I'm here to tell you that 30 mph ISN'T FAST.* Go out and drive home at that speed sometime.* It's so frickin' slow you'll not only notice every tree and bush along the way,* you'll be counting their leaves :-)
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Old 08-10-2011, 11:44 PM   #16
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Guys, waxing poetic about speeds is nice but the topic is "Cruising a twin screw Semi-displacement boat for minimum fuel consumption." Can one save fuel and improve engine life by alternately running one engine at a higher RPM while free wheeling the prop of the off engine (assuming no tranny problem)? That's the question.
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Old 08-11-2011, 12:02 AM   #17
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

I thought it was already answered awhile back--- not enough to make much difference.
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Old 08-11-2011, 12:28 AM   #18
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Not really.* I'd like to see some hard numbers.* I'm finding it hard to believe that the drag from the non-operating but rotating prop negates the fuel savings from shutting down one engine.
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Old 08-11-2011, 04:18 AM   #19
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

I'd like to see some hard numbers.

Create your own, find out the diameter of your prop , run at a modest cruise speed , and tow a hunk of plywood as a drogue that is about 50% larger than the prop diameter.

The speed reduction will be similaR TO A DRAGGING BUT ROTATING SLOWLY PROP.
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Old 08-11-2011, 04:53 AM   #20
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Quote:
Marin wrote:Lots of places to see, lots of things to do, and you only go round once.* The slower you go the less you experience in that one go-round......But this idea that fast is Bad and you won't see anything and can't enjoy your surroundings.... bollocks, as they say in the UK.* I'm here to tell you that 30 mph ISN'T FAST.* Go out and drive home at that speed sometime.* It's so frickin' slow you'll not only notice every tree and bush along the way,* you'll be counting their leaves :-)
*Sorry Marin, but I can't quite reconcile the above with the fact we all know you just love narrow-boating in this very bollocking UK you speak of...and from whence I have just come as you know, doing just that, (and had a ball, and highly recommend it to anyone), and yet the maximum speed allowed or even possible is ~ 4 mph...?** Que...?
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