Cruising a twin screw Semi-displacement boat for minimum fuel consumption

How about a 2011 52' Nordic Tug, new, It's a single, first 52' single I've seen. *It's on G dock here at Cape Santa marina in Anacortes. *Go see Greg at the Nordic Tug office, I'm sure he would give you a tour.

*

Oh, asking price is 1.3 big ones

We have very specific requirements for liveability aboard the boat.* One of those requirements is that the boat have a good size salon and a good size outside area (e.g., sundeck, flybridge, cockpit).* The boats you listed have a tendency to chop up their interiors with large foredecks and a good size pilot house but leaving one with a small galley, a small salon, small flybridge and either a reasonable cockpit or one only useful* as a fishing platform.

Ouch!* If that was doable I wouldn't be worrying about fuel costs.

Here is what I dont understand....
A 40 footer runs just fine on a single 120HP engine. If You have twins and only run on one, the fuel consumption is almost the same? Does that mean if you run both, you are running each one individually on only half the consumption rate as a normal single would be running to give approximately the same overall consumption rate converted to MPG? Does this mean that the 2nd engine is only needed to provide enough power to turn it's own prop? Using the same logic, does this mean that if I had a single screw boat and were to drag something in the water behind me (Like a dinghy with maybe some weeds caught in the prop and just dragging) to create the same drag equivelant as a twin engine running only on one with a non-rotating prop and some rudder drag, that my fuel consumption rate would actually almost double?
I know there are always going to be some losses in power running on one but find it difficult to believe that it would be almost the same as running 2 considering the first engine is pushing up to 30,000 lbs of boat and the second engins only has to spin it own prop and overcome some drag on the rudder.
When in a sailboat, the rudder is typically around a 5 degree off of normal. When more than that, you have to trim your sails. There is a loss in power, no doubt, if you are more than 5 degrees off but not twice as much loss. You can feel it when you have a tiller on a heavy sailboat.

rostan wrote:
Ouch!* If that was doable I wouldn't be worrying about fuel costs.

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*Yeah, I agree!! *For 1.3 Million I wouldn't be looking at a Nordic Tug either. *Maybe a 61" Tolly for half the price or something like that. *They seem to be a niche boat and attract a certain croud. *My friends who have bought them were all long time sailboaters who finally came over from the dark side.* * That's a joke, don't get excited Marin. *A 39' has an asking price of 1/2 million!!

The part about them being sailboaters is true. *When asked why they chose a Nordic, they always point out the amenities, style, simularities to a sailboat (HUH), etc. *I don't get it, but "to each their own, said the man as he kissed the cow!"

LB


-- Edited by Edelweiss on Friday 12th of August 2011 08:28:05 AM

Tony,

The practice of running one engine on a twin overcomes the losses of asymmetrical thrust, prop drag ect by reducing the heat loss of one engine. Diesel engines are heat engines and any heat lost without doing any work is a total loss. The combustion chamber (including the piston crown) and cylinder area (the length of the stroke) is a very high heat loss as all this area is exposed to combustion temperatures. The heat goes into the cooling system and into the sea. There are many other ways heat is lost while an engine is running but when you stop one engine on a twin a great deal of heat loss is eliminated. Two engines producing 30hp each produce much more heat than one engine producing 60hp.

Tony B wrote:
Does this mean that the 2nd engine is only needed to provide enough power to turn it's own prop? ....I know there are always going to be some losses in power running on one but find it difficult to believe that it would be almost the same as running 2 considering the first engine is pushing up to 30,000 lbs of boat and the second engins only has to spin it own prop and overcome some drag on the rudder.

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*I don't have anywhere near the technical knowledge about this sort of thing as Eric, but the notion that in a twin one engine is doing all the work and the other engine is coasting is not correct.* Both engines are working equally hard (assuming the right prop pitches, gear ratios, etc.) to move the boat.* But since you have two engines moving the boat instead of just one, each engine does not have to work as hard as the one engine in the single-engine version of the same boat.* This is why the fuel consumption of a twin is not double that of the same boat with a single.

If you run a twin engine boat on one engine this one engine now has to do ALL the work of moving the boat PLUS overcome the drag caused by the freewheeling or locked unpowered prop plus the drag from the rudders that will now be at some angle to keep the boat going straight plus the drag caused by the hull not being evenly driven.* So this one engine now has to work harder--- perhaps much harder--- than the same engine in the same boat but configured as a single engine boat.* Whether the amount of fuel burned by the one operating engine in a twin equals the amount of fuel both engines would burn under normal conditions will depend on how much drag is being produced by not running both engines.

This is why to really get a benefit from running on one engine, the unpowered prop should be removed (or at least feathered if the boat has that kind of capability).

But I believe the objective of putting two engines in a boat has more to do with things like redundancy, maneuverability, and---* if the hull configuration makes this possible--- speed.* Maximum fuel efficiency is not the objective with a twin-engine boat.

Thank you Marin.

I think if one had lots of variables stacked up against running single like a wide spacing between screws (props) small or otherwise undesirable rudders ect there may be a point where running single on a twin will burn more fuel instead of less. Another element that I think has'nt been mentioned is the question of the fact that without counter rotating props one engine will run more efficient than the other. Think of a single screw. If you had a right hand prop the boat would want to turn to port and the rudder needs to be deployed to some degree to counteract that tendency. So if you were trying to run a twin with the port engine only and it's prop was right hand the "prop walk" would be turning the boat to port and the asymmetrical thrust would be turning the boat to stbd so it would require less rudder deflection to hold a course than if that port prop was left hand. If you had counter rotating props both would be advantageous or disadvantageous. Can't remember which way most twins turn but most all are one way. But if you had all the variables stacked up in your favor it could be almost as efficient as a single engined boat. So if you had a short wide boat w counter rotating props very far apart and the port prop was left hand and the rudders were small and the keel was small.........toooo many variables. It may work great and it may not work at all but I think (opinion) that on most boats it will work to a significant degree.

nomadwilly wrote:Can't remember which way most twins turn but most all are one way.

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I'm not sure if this is correct, Eric, at least not with more modern producton twins.* Twin engine GBs, for example, have counter-rotating props as have most of the other twins I've seen in the Seaview Yard in Bellingham (when I've bothered to check).* The prop configuation is always the same.* The port prop is a left-hand prop (turns counterclockwise when viewed from the rear) and the starboard prop is a right-hand prop (turns clockwise when viewed from the rear).* There is a reason for this specific rotation setup that has to do with what the prop wash does when the transmission is put in reverse and the direction of the combined prop walk when one transmission is put in forward and the other in reverse.* In short, the boat is more maneuverable with this rotation setup.

Some twins have counter-rotating engines.* The gas-powered 28' Uniflite I fished on in Hawaii had a pair of counter-rotating Chryslers.* I believe most twin-engine diesel boats have engines that turn the same way.* FL120s, for example, all turn the same way (counterclockwise when viewed from the rear).* Reversing the direction of the starboard prop is accomplished by an extra gear in the starboard BW Velvet Drive which is why the final drive ratios of the port and starboard transmissions are different.

Twin engine boats from the 60s or so back may tend to have props that all turn the same way.* The three props on the PT boats of WWII all turned the same way, which when combined with the boats' tiny spade rudders made them all but unmaneuverable at idle speeds.

PS--- The prop shop where we had our props re-pitched and balanced a few years ago gave me a tip on how to tell if a prop is right or left handed.* If you face the prop from the rear, if the blade on the left is pitched so you can reach out with your left hand and put your palm on the blade with your hand slanted away from you, it's a left hand prop.* If the blade on the right is pitched so you can reach out with your right hand and put your palm on the blade with your hand slanted away from you, it's a right handed prop.


-- Edited by Marin on Friday 12th of August 2011 02:23:49 PM

Marin wrote:nomadwilly wrote:Can't remember which way most twins turn but most all are one way.

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I'm not sure if this is correct, Eric, at least not with more modern producton twins.**The prop configuation is always the same.* The port prop is a left-hand prop (turns counterclockwise when viewed from the rear) and the starboard prop is a right-hand prop (turns clockwise when viewed from the rear).* There is a reason for this specific rotation setup that has to do with what the prop wash does when the transmission is put in reverse and the direction of the combined prop walk when one transmission is put in forward and the other in reverse.* In short, the boat is more maneuverable with this rotation setup.

*Any twin screw commercial vessel will have opposite turning propellers.* As Marin says, the port prop is left-hand and the strb prop is right-hand.* This is referred to as outboard turning.* Interestingly, this tends to be more of a West Coast occurence as most Gulf of Mexico built twin screws will be inboard turning (just the opposite).

While there is a design argument for outboard turning props to help the twisting motion, I handled an inboard turning tug years ago, and the twisting effect did not appear to be significantly lessened.* Any measureable difference may depend on a number of external factors.

Marin wrote:

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*Twin engine GBs, for example, have counter-rotating props as have most of the other twins I've seen..............

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That's been my experience....In fact, I don't thnk I've ever see a boat with twins that were not counter rotating.

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OK good. I've never had anything to do w twins so that's good to learn. The guys wanting to run twins single should rejoice as that configuration has prop walk that helps keep the boat on course running on one engine. But why on earth is it different on the east coast??? There is a discussion going on on BoatDesign.net about counter rotating props and the reason for "outbd turning props" on inboards is said to minimize the tendency for the props to suck air. Inbd turning props are said to have a tendency to lift the bow more than outbd turning. Toe in and toe out of the prop shafts seem to have an effect on directional stability toe in being the favored direction. And they talk about different toe in/out being required for various differences in deadrise due to the spanwise water flow in the deeper hulls. At any rate I see no areas of concern for trawlers with the exception of what I talked about in my previous post and that's basically a + or - for running single w a twin.

Eric--- The opposite turning props on the east coat/Gulf doesn't make sense to me, either. When you put the starboard transmission in reverse and the port transmission in forward this will yaw or pivot the boat around in a clockwise direction, stern moving to port. With a left-hand prop on the port side and a right-hand prop on the starboard side, both props will now be turning counter-clockwise, which means the propwalk from both props will move the stern to port. So you have the combined pivoting motion of the opposing thrust AND the propwalk together swinging the stern to to port, which means the boat will pivot or yaw clockwise that much faster.

With counter-rotating props turning the opposite direction--- right-hand prop on the port side, left-hand prop on the starboard side--- if you put the starboard transmission in reverse and the port transmission in forward the opposing thrust will still be trying to pivot the boat clockwise and move the stern to port, right? But both props will now be turning clockwise so the propwalk of both props will be trying to move the stern to starboard, thus negating some of the pivoting force to port from the opposing thrust. Therefore, the boat will yaw or pivot clockwise that much slower and thus be slower to maneuver.

It's a tough thing to visualize maybe--- I was sitting here moving my fingers in circles to makes sure I had it all correct

And.... my guess is that Grand Banks, for example, does not build boats with left hand props-port, right hand props-starboard for the west coast and right hand props-port, left hand props-starboard for the east and Gulf coasts. I bet they build them all the same way. And I suspect most all the other production boat companies that build twin-engine boats do the same.


-- Edited by Marin on Friday 12th of August 2011 10:42:07 PM

I believe you are correct, Most modern twins have counter rotating props to take advantage of the superior handling, especially when docking (" the prop walk effect") and*slow speed maneuvering using reverse / forward thrust to change the boats heading. However, There are a number of notable exceptions. *I owned a 73 (I think) 28' Fiberform with twin V6 / OMC outdrives and they did not counter rotate. *The issue was the reverse on the older OMC was not beefy enough to run in reverse at high speed/torque. A counter rotating engine was apparently not a consideration. *It was fine for normal cruising and usually not an issue. *You learned quickly that placing the port engine in reverse when sliding into a starboard side slip, would move the stern away from the dock, not to it and get in a crosswind or strong current while docking, it behaved like any other single engine, lite, high speed hull. *Sail. . . sail away, comes to mind!!

I have seen a few larger boats with non-counter rotating props, but I suspect they had an engine replacement and the original counter- clockwise engine was replaced with a clockwise engine. *One that comes to mind was a 32' or 34' Luhrs with twin horizontal Perkins 6.354's. *Must have been cheaper to replace CCR engine with a standard rotation CR engine and maybe the owner thought he would replace the drives at a later date.

Who said that east coast boats are bass akwards? Me thinks it was Don. I surely don't think it could be but do you suppose it's possible that Don's observations were** ............... do I dare say it ...... incorrect???? Don, are you there w something to throw?

nomadwilly wrote:
.....but do you suppose it's possible that Don's observations were** ............... do I dare say it ...... incorrect????

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* * * * I'm willing to lay down a good size bet that Don's observations are not incorrect! I'm also willing to bet that he* has more "on the water experience" than most on this Forum.

nomadwilly wrote:
Who said that east coast boats are bass akwards? Me thinks it was Don.

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*No, it wasn't Don.* Jay said that most twin-engine Gulf Coast boats will have counter-rotating props that turn the opposite direction (inboard).* Then you asked why it was different on the east coast.

Most of this is somewhat meaningless to me.
My original intent was to get some ideas of fuel economy and here is what I need. I need real numbers so here is a situation.........Say for instance, I have 2 identical 40' trawlers except one has a single 120HP engine and the other has twin 120 HP engines. Now, say that I burn 2 gal/hr on the single screw at 6 knots. That gives me roughly 3 NMPG in dead calm and no curent. If I wanted to go the same 6 NM's (distance) with the twins how much fuel will I consume and at what approx. speed? Would I be actually traveling faster with less fuel burn per engine or is it not possible to get the same overall 3NMPG?
If you want to use a different figure to be more precise, that is fine. I just want to know how much more it would cost if anything to run twins vs. a single screw. I would like to know this because in my hunt for a trawler should I just discount the idea of twins or are twins in the running?
I can convert these numbers into percentages and dollars. Things like 10% different dont excite me, but a 25% difference definitely would. That could mean the the difference between a $15K or a $20K fuel budget per year if we do long distance cruising and the price of fuel keeps rising.

Tony, please forgive me but I'm somewhat confused. The last post I read of yours* indicated that you were going to spend $12K on your sail boat and keep it.

So, which way are you going? Sailing or trawlering?

Tony

$15 -20K per year in fuel? How do you* propose to do this without owning an obviously fuel hungry twin burning about 10 to 20 gph ? A fuel efficient twin burning 4-5 gph at 8 knots isn't what you need.

SeaHorse II wrote:
Tony, please forgive me but I'm somewhat confused. The last post I read of yours* indicated that you were going to spend $12K on your sail boat and keep it.

So, which way are you going? Sailing or trawlering?

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*If I can't sell my sailboat within a month or so, I will probably just go ahead and upgrade my sailboat. If I do sell my sailboat, I will buy a trawler. Either way, I will be ready to do some river running by late spring. As for the $12K, I will probably need that to upgrade any trawler I buy also.

******* Sunchaser wrote:

$15 -20K per year in fuel? How do you propose to do this without owning an obviously fuel hungry twin burning about 10 to 20 gph ? A fuel efficient twin burning 4-5 gph at 8 knots isn't what you need

Actually, I don't know what I need, that is why I am asking here. I seem to get a lot of rudder and drag theory but no one including your post has given me any quantitive information. You seem to know what I wont be burning, so how much will I burn traveling 6 NM in A) A single 120 HP and how much will I burn in B). with twin 120's. in identical 40' trawlers? I guess I can assume the actual speeds will be different.

Thanks in Advance

Tony B



-- Edited by Tony B on Saturday 13th of August 2011 08:05:54 PM


-- Edited by Tony B on Saturday 13th of August 2011 08:08:07 PM

DavidM wrote:
...*The basic twin engine prop is more efficient because it can operate in clean water, ie it is not behind the deadwood ...

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*Meaning, shaft, propeller and rudder aren't protected by a keel.

DavidM - why didn't you say so in the first place? LOL. I guess I didn't ask it properly.

****************** That clears up the mystery.

MarkPierce - "Meaning, shaft, propeller and rudder aren't protected by a keel."

********************* You just had to bring that up, didn't you? I guess some issues just will never be put to bed.

Tony B wrote:
MarkPierce - "Meaning, shaft, propeller and rudder aren't protected by a keel."

********************* You just had to bring that up, didn't you? I guess some issues just will never be put to bed.

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*"Put to bed."* So, how long has this thread been going (dual engines seeking the efficiency of single engines), as well as several like it in the last several months?* I'd say protecting the running gear is more important than having redundant/expensive engine trains.




-- Edited by markpierce on Saturday 13th of August 2011 10:43:38 PM

It's all situational, if you have sandy bottoms then the keel is certainly a bonus. *Fishing in Alaska, we would power right over sandbars, sliding on the keel and lash side by side with another boat and allow the tide to run out from under us. *We would go high and dry on the keels resting on the sandy bottom until the tide came back in. You wouldn't do that on a hard bottom, the damage would be significant and might take your rudder shoe off.

The one time I actually touched bottom in my present boat (going into Fishermans Bay on Lopez Isl 25 years ago) fortunately I didn't have that extra two feet of Keel hanging under me to hook up on the granite bottom and leave me high and dry on an outgoing tide. *it wouldn't have been pretty*

Instead I motored into the bay on the other engine, rented a prop puller from OMC marine, scuba dove with the spare, replaced my damaged prop. I was out of there the next day. Total cost about $100 to have the prop straighted and balanced at HDF propeller.*

Most boats are NOT optimized for fuel burn, as it costs more and owners don't much care to pay for dream perfection.

For 200 hours a year , it doesn't pay.

Looking at the D-G prop handbook it shows that efficiency could be 10% or more improved with a deep reduction gear and large diameter 2 blade prop.

Large diameter props cost more (regardless of blade count) so are seldom seen even on single engine displacement "cruisers"..

Some (few) cruising sailboats will run a proper 2 blade and many do get 10 nmpg at SL 1.

The "loop" is usually about ,6000 miles , the difference between 2 gph or 2.5 gph at the usual 6 stat mph is not that big a deal.

Motor 24,000 for a world cruise and tankage becomes a bigger hassle than cost.

Mark: Every time I see this cradle I think "what a shame you had to give it up!" I know, I know.....you had no place to store it but nevertheless....a shame.

-- Edited by markpierce on Saturday 13th of August 2011 10:43:38 PM

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*

I feel the protected running gear in invaluable For me it may make the difference
between just backing off a sand bar and continuing as vrs pulling the boat to repair running gear.../.... well it has never happened to me but I have heard.....

SeaHorse II wrote:Mark: Every time I see this cradle I think "what a shame you had to give it up!" I know, I know.....you had no place to store it but nevertheless....a shame.

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*

*Walt,*thanks for reminding me.*

Tony B wrote:
*

Say for instance, I have 2 identical 40' trawlers except one has a single 120HP engine and the other has twin 120 HP engines. Now, say that I burn 2 gal/hr on the single screw at 6 knots. That gives me roughly 3 NMPG in dead calm and no curent. If I wanted to go the same 6 NM's (distance) with the twins how much fuel will I consume and at what approx. speed? Would I be actually traveling faster with less fuel burn per engine or is it not possible to get the same overall 3NMPG?*

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I don't have specific numbers because to get those you need an accurate fuel flow measurement system and you need it on two identical boats, one single and one twin.* So the best I can do is offer a rough-ish comparison.

With our twin-engine GB36, 1650 rpm on both engines gets us about 8 knots and the boat burns about 5 gph (total) to do that.* To get 8 knots with the same boat with one engine, the engine needs to be run at about 1800-1900 rpm.* At that rpm the engine will burn about 3-4 gph.* A big variable here will be prop configuration, of course.

So you need to pick your poison.* Going the same distance at the same speed, the twin will burn more than the single, but certainly not double the amount.* Probably more like 2/3 the amount but that's a guesstimate.* If you want to burn the same amount of fuel in each boat over the same distance the engines in the twin will have to be run slower than the engine in the single.* How much slower is what requires some accurate measurements.* But I would hazard that if we're talking about a 5 to 8 knot speed range, the engines in the twin might have to be run too slow for their own good.

Frankly, I think debating a single vs twin on the basis of fuel burn (assuming the same kind of boat and engines) is perhaps debating the wrong thing.* In the typical, slow cruiser with low-powered engines like the FL120, the difference in fuel burn between the single and twin is not going to be so different as to negate the other advantages and disadvantages of either configuration.* When we decided to acquire an older GB we didn't care if it was a single or a twin.* The boat that best met our needs and budget happened to be a twin, and having run it for 13 years now neither one of us would want to go back to a single, which is what we chartered before buying our own boat.

But our preference for a twin over a single has nothing to do with fuel burn. The factors of our preference are redundancy, maneuverability, and the fact I like running multiple engines * If we preferred a single-engine boat, the main factors would not be fuel savings but things like half the service and maintenance cost, ease of engine servicing in an uncrowded engine room, simpler systems, less noise, a more protected prop, and so on.* The fuel savings would be a nice bonus, of course, but they would not be the reason we opted for a single engine boat.

Not sure how you come up with an annual fuel cost of $15k-$20k.* Most boat owners run their boats less than 100 hours a year.* But I believe 100 hours a year is considered the average use of a recreational boat for most people, full-time cruisers excepted.* Assuming a fuel consumption of 5 gph (twin-engine boat) and a fuel price of $5/gal, 100 hours a year is $2,500 in fuel costs.* If one cruises 500 hours a years, given the same fuel burn and gallon price, that's $12,500 in fuel for the year.

I have no clue what kind of annual hours are typically racked up on the rivers, lakes, and waterways that comprise the Loop nor do I know how much of the year you intend to be actually runninng the boat.* But if you're assuming you'll be spending $15k to $25k in fuel each year, I see a twin-engine boat in your future


-- Edited by Marin on Wednesday 24th of August 2011 09:10:06 AM