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Old 09-30-2012, 11:17 PM   #21
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after having both I want a single with thrusters
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Old 09-30-2012, 11:29 PM   #22
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It's hard for me to imagine the crampness of an engine room with two primary engines, let alone with a genset in addition, in a 40-something-or-less vessel. Now I realize why most "trawlers" around me have such high superstructures.

For short and long-term cost efficiency, I chose a single engine (and no genset) with a bow thruster.

Marin sounds correct about twin-engined boats being inefficient running on a single engine, and I'd add based on verbal and written accounts, very difficult to maneuver.



(The Coot is leaning as I hadn't yet balanced the fuel among tanks after filling the starboard tanks.)
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Old 10-01-2012, 12:46 AM   #23
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I'd still get twins.

Not because a single is any worse or better.

I feel safer with twins, and that feeling even if unfounded is important to me.
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Old 10-01-2012, 03:14 AM   #24
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Marin sounds correct about twin-engined boats being inefficient running on a single engine, and I'd add based on verbal and written accounts, very difficult to maneuver.
We've had to maneuver and dock our boat on one engine twice. I would not call it at all difficult. You have to think through what you're going to do but once you have a plan it's pretty straightforward.

Underway with one shut down is annoying because the locked- off prop generates a lot of turbulence which buffets the rudder and steering. And you lose speed because of the drag of the unpowered prop and the need to compensate for the asymetrical thrust which creates more drag.
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Old 10-01-2012, 05:24 AM   #25
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If I was to do it again my preference would be 48/52 ft Raised Pilot House with a single engine, bow thruster and active fin stabilisers.
Plus flopper stoppers for when at anchor.
The other main items are main deck toilet/shower and a full walk around main bed.
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Old 10-01-2012, 07:46 AM   #26
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I would without a doubt do a single engine again...with a thruster either bow or stern. Room to work on everything, cheaper to maintain, and if it goes, yank it and put in another, no questions. Plus I like the skeg protection for the prop.
I would like to try a different style though as I dearly miss a real cockpit.
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Old 10-01-2012, 09:07 AM   #27
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How/Why do we continue to discuss the single/twin topic? Come on y'all. Give it a rest!
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Old 10-01-2012, 09:47 AM   #28
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How/Why do we continue to discuss the single/twin topic? Come on y'all. Give it a rest!
Because the OP asked.

If I were to upgrade now, I would look for a single screw with a wing engine and a bow thruster. My second choice would be twins which is what I currently have. The redundancy is worth all the negatives to me. If you are a good mechanic and have plenty of spares, or you're not planning on boating in areas with no tow service, then redundancy may not be all that important and will be just for convenience.
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Old 10-01-2012, 09:49 AM   #29
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It's hard for me to imagine the crampness of an engine room with two primary engines, let alone with a genset in addition, in a 40-something-or-less vessel. Now I realize why most "trawlers" around me have such high superstructures.
In this picture I was showing David James how cramped it is in the engine room.





That is the generator in the rear.
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Old 10-01-2012, 10:10 AM   #30
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(The Coot is leaning as I hadn't yet balanced the fuel among tanks after filling the starboard tanks.)
And here I thought you kept the bar stock to starboard.
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Old 10-01-2012, 10:38 AM   #31
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If you like the boat weather is a single or twin is secondary. The Eagle is a single with bow thruster. Weather the boat was twin or single it would not matter to me, but if a single it would have to have a bow thruster.
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Old 10-01-2012, 10:51 AM   #32
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We've had to maneuver and dock our boat on one engine twice. I would not call it at all difficult. You have to think through what you're going to do but once you have a plan it's pretty straightforward.

Underway with one shut down is annoying because the locked- off prop generates a lot of turbulence which buffets the rudder and steering. And you lose speed because of the drag of the unpowered prop and the need to compensate for the asymetrical thrust which creates more drag.

Yes, if you have a transmission that requires the shaft to be locked, it doesn't make operational sense. As for drag, it obviously goes up....but the real question is whether net efficiency also goes up. The numbers show that it does for our boat, although I believe it has to do with operating turbocharged engines at near idle rpm, and the fact that the props on a boat with enough installed power to actually reach semi-planing speeds are fairly large. So a single prop takes a pretty healthy bite at slow speeds (yes, also comparatively more drag for the one that's freewheeling). Again, for our boat the overall thrust/drag/engine efficiency ends up as an obvious net positive.

It would be an interesting science project for members with small engine twins (that can be free wheeled), and who have a flow scan or a prop chart, to do several comparison speed/rpm runs (twin versus single). Then calculate single versus twin fuel burn for the same speed(s). I'm guessing the improvement would be smaller than for large engine, semi-planing twins. But real numbers would sure beat speculation. In that regard, next summer I'll lock down a shaft for a short test to compare free wheeling to locked prop....not that I'll ever lock one in normal operation.

Regarding turbulence and crab/rudder angle, it's a virtual non-issue for our boat in open water (free wheeling prop). And I'd always start the second engine for docking. I'm going to put this in operation for long runs next summer. Still need to mod the raw water cooling feed to cross connect the stuffing boxes to get flow to the side with engine off. I'll post long term results as I expect differences versus the prop chart.
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Old 10-01-2012, 01:19 PM   #33
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The comparison I am most familiar with is one that was done with an Alaskan 45 with a pair of Amarine (John Deere) engines of about 120 hp and props that could be freewheeled. The owner (Bob Lowe) conducted an extensive test and charted the results. The basic conclusion was that running on one with this type of boat and engines yields little to no fuel savings and under some power conditions the efficiency actually goes down.
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Old 10-01-2012, 01:24 PM   #34
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Do you have a link to that data?
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Old 10-01-2012, 01:37 PM   #35
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Do you have a link to that data?
Bob, who is one of the founders of the Grand Banks owners forum, posted it there. I've sent it to other people on this forum in the past but it's no longer on my computer or this iPad. Bob's original post is in the GB forum archives.
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Old 10-02-2012, 12:54 AM   #36
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[QUOTE
Marin sounds correct about twin-engined boats being inefficient running on a single engine, and I'd add based on verbal and written accounts, very difficult to maneuver. [/QUOTE]

Au contraire, Marin and Mark.

Our twin screw Tolly cruising on a single engine at 6 to 6.5 knots (a bit below calced hull speed of 7.58 knots) improves NMPG by approx .75 NMPG over the best NMPG available when two engines run simultaneously. Our BW trany have no prob with freewheel prop at slow speed, and, just for leaning to the side of caution I rotate engine use each hour. Guarantees the 71C trany discs stay lubed and engine hours remain close to the same. Also, if water/wind conditions get too rough where single engine usage becomes hard to easily handle the boat – just start the second screw and great control is instantly in your hands again!

To OP on this thread – I recommend twin screw planing hull with hard chine... from experience and for several reasons. – IMHO!

Good luck with your boat search... May Neptune and his Mermaids’ luck stay with you!
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Old 10-02-2012, 01:23 AM   #37
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[QUOTE=Art;106115]
Quote:
Marin sounds correct about twin-engined boats being inefficient running on a single engine, and I'd add based on verbal and written accounts, very difficult to maneuver.

Au contraire, Marin and Mark.
The fuel savings (or not) from running a twin on one will vary with the type of hull, the speed, the weight of the boat, the engines, and a whole bunch of other variables. The tests I've seen charted used boats like ours-- relatively heavy (30,000 pound-plus), keeled, semi-planing boats with relatively low powered engines (FL120s and the like). These comparison tests showed that at best the fuel savings were minimal and under some conditions the efficiency was actually worse than running the boat on both engines.

How these comparison tests relate to a light (17,000 pounds) planing cruiser with gas engines I have no idea but the two types of boats are so different I would be hesitant to apply the results from one type to the other type.
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Old 10-02-2012, 02:22 AM   #38
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And here I thought you kept the bar stock to starboard.
Well, the beer is kept in the refrigerator, and it is on the starboard side. Wine (red) is kept under the saloon table along the centerline, however. Sorry, no stronger drink aboard (presently).
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Old 10-02-2012, 02:23 AM   #39
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[QUOTE=Marin;106119]
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The fuel savings (or not) from running a twin on one will vary with the type of hull, the speed, the weight of the boat, the engines, and a whole bunch of other variables. The tests I've seen charted used boats like ours-- relatively heavy (30,000 pound-plus), keeled, semi-planing boats with relatively low powered engines (FL120s and the like). These comparison tests showed that at best the fuel savings were minimal and under some conditions the efficiency was actually worse than running the boat on both engines.

How these comparison tests relate to a light (17,000 pounds) planing cruiser with gas engines I have no idea but the two types of boats are so different I would be hesitant to apply the results from one type to the other type.
Agreed!
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Old 10-02-2012, 02:33 AM   #40
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It's hard to imagine dragging a propeller and having the rudder creating drag to offset a working off-set-from-boat-centerline propeller could be efficient.
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