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Old 11-05-2013, 03:38 PM   #21
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With all the single v. twin discussion, I think the OP misses the central point. It takes the same energy to push a boat at any given speed. In theory, it should take exactly the same amount of diesel fuel burnt to produce that set amount of energy, whether the energy is supplied by a single powerplant or twins. In practice of course, as several posters have noted, there is a difference in fuel consumption between a single and twins although an almost negligible difference.

One thing to note-from my experience, many twin displacement/semi-displacement hulls with twins are overpowered. This seems to be the case a bit more with older boats. I know Steve D'Antonio is big on this theory, perhaps to an extreme. Many newer displacement boats will carry almost identical HP whether in a single or twins.
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Old 11-05-2013, 03:55 PM   #22
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I do not know who made the get home I had. I looked like a truck power steering unit mounted on the front of my 7.5 KW two cylinder Onan generator with two fan belts; with hoses to a hydraulic motor on a metal platform bolted to the top/back of my borg warner velvet drive transmission. The hydraulic motor had a small sprocket. I had to install a size 60 motorcycle chain around the motor sprocket and another larger sprocket sandwiched between the tansmission and shaft couplings when I needed to use the get home. The pump on the generator had an electric clutch like an automobile air conditioner compressor. I flipped the switch with the transmission in neutral and went home at about 4 knots.

One time about 40 miles out in the Gulf of Mexico I got water in my main engine thru the exhaust while anchored. I burned up my starter and had to use the get home. I went through two sets of fan belts because the adjustment feature was limited. Changed that shortly afterwards.
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Old 11-05-2013, 04:00 PM   #23
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[QUOTE=THD;189570]One thing to note-from my experience, many twin displacement/semi-displacement hulls with twins are overpowered.

I believe this. My twin engine boat would perform very nicely with about half the HP in 2- 4 cylinder engines vrs the 2- 6 cylinder engines I have.
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Old 11-05-2013, 04:07 PM   #24
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Very cool. What do they cost, assuming you already have hydraulics on the boat?
I haven't got that far yet so the answer is "I don't know." I've learned, however, that I'd have to add a 12KW genny or larger to run the hydraulics and my little boat doesn't have near enough room for that. (I think SomeSailor's remark about "more than a second engine" is probably correct.)
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Old 11-05-2013, 04:09 PM   #25
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This example was interesting on what I assume would be similar style hulls.

54 ft Defever
Quote:
These 4 cylinder diesels (Cat D 330T x 2) are running between 1400-1500 rpm at a cruise speed of 7-8 knots. I have never been able to figure out the fuel burn exactly, but I estimate at less than 5 gallons per hour total.
So 19 l/h
54'Defever Offshore Pilot House Trawler

Vs

60 ft trawler
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This vessel cruises effortlessly at 8knots with a fuel burn of about 10 liters per hour with a single Gardiner 8cyl 200 hp
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Old 11-05-2013, 04:25 PM   #26
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I haven't got that far yet so the answer is "I don't know." I've learned, however, that I'd have to add a 12KW genny or larger to run the hydraulics and my little boat doesn't have near enough room for that. (I think SomeSailor's remark about "more than a second engine" is probably correct.)
WESMAR offers the APU in two sizes. The 200 series has a maximum rating of 25 hp and can take up to a 2.38-inch-diameter prop shaft. Pricing starts at $10,695. The 300 series maxes out at 100 hp, takes a prop shaft of up to 3.38-inch diameter, and lists for $12,400. Prices do not include installation. Expensive? Compared to the cost of a second engine and the required running gearónot to mention the additional mechanical complexity and hydrodynamic dragóitís a good deal. And, as they say in those MasterCard commercials, being a hero in an emergency is priceless.
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Old 11-05-2013, 04:45 PM   #27
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Vessel Assist and an adequate anchoring system could provide the get home protection for singles in many locations in this country. Of course, the more remote areas may not have this option.

But weren't we talking about single-and twin-engine efficiencies?
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Old 11-05-2013, 05:01 PM   #28
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The 200 series has a maximum rating of 25 hp and can take up to a 2.38-inch-diameter prop shaft.

Two questions:
  1. How do you propose to provide the 25 HP and at what cost?
  2. What if the reason the boat isn't moving is a broken shaft, strut, prop or shaft through hull?
The answers to these two questions are why twins or separate get home engines are desired by some.
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Old 11-05-2013, 05:34 PM   #29
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[QUOTE
Moonstruck
120 hp Lehman, single screw
7 knots, 1.5 gph

SomeSailor
120 hp Lehman, twin screw
8 knots, 4 gph total, or 2 gph per engine

So, SomeSailor has twice as many horses in the engine room, drinking twice as much diesel, but cruises at about the same speed as Moonstruck. Why don't trawler skippers just shut down one of their engines, once they are clear of the marina?[/QUOTE]

Big fan of single screw here but you are comparing 2 different boats at 2 different speeds which are near their most critical speeds as far as fuel consumption is concerned. You need better information b/4 you can make a decision correctly. I will add that the partial redundancy provided by twins is, for me, outweighed by the exposed running gear, added complication, noise, fuel burn, maintenance, & cost on twins.
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Old 11-05-2013, 08:37 PM   #30
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In my opinion on a trawler the ideal set up is a single screw with a thruster (bow or stern or both). For ocean and remote cruising a hydraulic drive off the generator would be a good thing. With the reliability of a well maintained diesel engine and towing services for a back up, I think that twins are unnecessary in most cases.
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Old 11-05-2013, 09:13 PM   #31
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My BW Velvet Drive Manual limits speed to "trolling speed" with one engine unused. Locked off, I suppose speed would not matter.
If you have a "get home" set up, a separate shaft/prop makes sense,as it covers drive train as well as engine failure. A saildrive unit, like an outdrive or leg, could be easier than a second shaft, though outdrives need maintenance. Mounting the gearbox underwater has always seemed odd.
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Old 11-05-2013, 10:10 PM   #32
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If I had twins, I'd run them at the same time. If I wanted to operate with a single engine, I'd acquire a boat with a single engine, which I did.
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Old 11-06-2013, 12:47 AM   #33
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You folks might want to check out the Chris Craft forum for this thread:

Chris Craft Constellation 500s

Some pretty good numbers and comments regarding the handling of the reduction gears while single shaft running with twins.

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Old 11-06-2013, 02:03 AM   #34
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My present trawler is 40 feet goes 7 knots on two 165 HP volvos and uses about 6 gallons per hour.
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OB

You need to get a bigger boat! My vessel with twin 225TIs gets about 4.3 - 4.6 gph (including genset) at 7.7 knots and 1700 RPM. It has been locked on these numbers since new with calibrated tank fills and hour meters the method for calculation.
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Old 11-06-2013, 05:54 AM   #35
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Bay Pelican, a single engine Krogen 42, is set up with a separate Yanmar 27 hp get home engine with its own shaft and prop. The prop is a Max Prop for minimum drag when not being used. The engine is offset to starboard.

This is a typical arrangement for most of the the smaller trawlers (less than 60 feet /18 meters) in the Eastern Caribbean.

I use this engine on a regular basis when in harbors just to exercise it as a get home system which is not used will freeze up.

The Yanmar is also used for docking (very infrequent for us) and as a back up generator since being a sail boat engine I was able to put on a 150 amp alternator.

In 15 years and thousands of miles my main engine has only failed once. That was due to air in the fuel lines and the air shut down the auxiliary engine as well.

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Old 11-06-2013, 06:15 AM   #36
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Engines are not usually specified by a NA , only a power size .

The purchasing folks or add guys have far more say than the boats designer about the product.

Thats how folks get Volvo (very inexpensive for OEM) and way too large engines.
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Old 11-06-2013, 06:40 AM   #37
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Moonstruck
120 hp Lehman, single screw
7 knots, 1.5 gph

SomeSailor
120 hp Lehman, twin screw
8 knots, 4 gph total, or 2 gph per engine

So, SomeSailor has twice as many horses in the engine room, drinking twice as much diesel, but cruises at about the same speed as Moonstruck. Why don't trawler skippers just shut down one of their engines, once they are clear of the marina?

So others have said, but I'll pile on: 8 knots is not 7 knots.

And LWL matters; it's not apparent whether these two boats are the same length or not.

And "hull speed" as usually used in common discussion is not the most efficient speed for all boats; instead, that usually means a boat's maximum theoretical full displacement speed (the 1.34... formula) and the most fuel efficient speed is less that than. Beebe's book has some discussion of it...

In these two example boats, it might have been that the most fuel efficient speed is 5 or 6 knots... and it may well have been that they both burn about the same amount -- or only slightly different amounts -- at their respective fuel efficient speeds.

Ref shutting down one engine... as mentioned, the current Chris Craft discussion touches on several points. It's not out of the question, but has pros and cons, and could depend on specific products in a specific boat.

-Chris
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Old 11-06-2013, 06:55 AM   #38
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Gentlemen,

The real question regarding singles or duals should be - 15 or 18 year single Malts?
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Old 11-06-2013, 08:52 AM   #39
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Old 11-06-2013, 09:37 AM   #40
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It is.....pier 5
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