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Old 08-12-2011, 07:37 PM   #41
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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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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Old 08-12-2011, 09:24 PM   #42
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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.
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Old 08-12-2011, 10:39 PM   #43
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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
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Old 08-13-2011, 12:57 AM   #44
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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.
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Old 08-13-2011, 09:50 AM   #45
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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?
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Old 08-13-2011, 10:13 AM   #46
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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.....but do you suppose it's possible that Don's observations were** ............... do I dare say it ...... incorrect????
* * * * 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.
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Old 08-13-2011, 01:18 PM   #47
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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Who said that east coast boats are bass akwards? Me thinks it was Don.
*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.
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Old 08-13-2011, 04:29 PM   #48
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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.
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Old 08-13-2011, 04:55 PM   #49
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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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Old 08-13-2011, 06:54 PM   #50
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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.
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Old 08-13-2011, 08:03 PM   #51
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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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?
*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
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Old 08-13-2011, 08:29 PM   #52
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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...*The basic twin engine prop is more efficient because it can operate in clean water, ie it is not behind the deadwood ...
*Meaning, shaft, propeller and rudder aren't protected by a keel.
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Old 08-13-2011, 08:45 PM   #53
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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.
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Old 08-13-2011, 10:39 PM   #54
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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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.
*"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
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Old 08-13-2011, 11:51 PM   #55
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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.*
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Old 08-14-2011, 06:15 AM   #56
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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.
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Old 08-14-2011, 08:59 AM   #57
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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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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Old 08-14-2011, 10:44 AM   #58
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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.....
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Old 08-16-2011, 03:55 PM   #59
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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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.
*

*Walt,*thanks for reminding me.*
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Old 08-24-2011, 09:07 AM   #60
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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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?*
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
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