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Old 13-04-2012, 15:43   #1
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Thrust Configuration Options ?

Hi All. I'm seeking sage advise on the following question please. Feel free to redirect me if there is already a thread in play.

I'm looking at two similar boats for a liveaboard/cruise in retirement situation.
Both have the config that I like in terms of layout ... basically 42 foot Grand Banks Trawler style.

One is powered by a 120HP Ford Lehman, and the other by two of the same.
For a similar price, am I better off with the single engine, and fit a bow thruster, or two donks ? ( The twin has one 320 hours out of an overhaul, and one at 3200 hours)

The obvious topics of fuel consumption, running and overhaul costs as a given aside, what about the less obvious aspects. Manouverability, safety, noise, virbration ?

The cruising grounds I intend to use on the East Coast of the top half of the North Island of New Zealand are well serviced by Coastguard. VHF, and cell-phone coverage.

I am leaning towards the single + bow thruster, but will value your imput.
Smooth Seas
Regards Mach Buffett
( A sailor converting to power )

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Old 13-04-2012, 16:12   #2
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Re: Thrust Configuration Options ?

If you are not worried about fuel and maintance cost, two is better than one. You have redundancy, is is easy to dock, more power when the sh.. hits.

I get knocked down...But get up again...
You're never going to keep me down....

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Old 13-04-2012, 16:21   #3
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Re: Thrust Configuration Options ?

As a trawler owner-my advice is to go with the twins. If the single is a 120hp and the twin are both 120 HP in the same or very similar hulls, the twins will actually be more efficient. You will not find as much difference in fuel usage as you might think. A single 120 pushing a 42' GB is going to work pretty hard for extended periods. The twins, much less so. Manueverability is much better with the twins. In a 42 GB, you just about spin it in its own length. Redundancy is important (for me a deal breaker). The old 120 Lehmans are good solid engines, easy to work on and will run pretty much forever if properly cared for.
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Old 13-04-2012, 17:13   #4
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Re: Thrust Configuration Options ?

Mach, go for the single, 120hp is plenty to push the boat you are thinking of to hull speed. Maintenance costs less, protected propellor, lower fuel consumption. Most displacement launches in the Hauraki Gulf are singles and you will not see too many commercial fishing boats under 60' with twins. Forget about the idea of lots of power to blast your way home, when one of those dirty westerly kicks in, the seas limit how fast you can go. In the unlikely event of a disabling failure, as you noted there is Coastguard and plenty of other friendly boaties always willing to help.
Twins are of course more manoeverable but with a bow thruster you won't go wrong.
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Old 13-04-2012, 19:52   #5
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Re: Thrust Configuration Options ?

Twins will give more manouverability than a thruster AND redundancy.
Thruster not going to help get you home.
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Old 16-04-2012, 02:00   #6
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Re: Thrust Configuration Options ?

Thanks for the responses so far, the points are all valid and noted.

Could you please comment on noise and vibration. Are twins proportionately
louder than singles ?

Is vibration an issue, or does it effectively disappear once engines are cruise harmonized ?

Apart from breakdown/get -home situations, does running on one engine in the cruise have any negative effects on a free-wheeling prop ? Or do you put the non-operating engine in reverse and accept the additional drag ?

Are there any twin operators who run on one as a normal cruise config ?

The reason I ask, is the disparity in engine hours on the twin in question. At age 60, the recently refurbished one could conceivably see me out. ( Maybe the other one too if I can just get the rum intake to sit where I'd like it.)

As with all things nautical, inevitably it will be a compromise, ... I just can't afford to cock it up this time.

Having been a professional pilot for 43 years, I'm well versed in most of the arguments for singles versus twins. (or up to four ), But ... guess what ?? I'm still open minded.

All opinions gratefully received

Regards Mach Buffett

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