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Old 13-07-2015, 02:56   #1
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Singleor duble screw?

Hi Everybody,

I was wondering about a trawler full displacement hull...

Same hull, same everything, same weight, ecc

Lets say a 46ft

One lets say with a 250 hp single screw


The same with 2x 125 hp double screw.

Wich will be the more efficient? The more miles per galoon? The faster?

Lets think they have both the same gearbox ration, just the one with 2 engine an half big propeller.

I know is like a docksise neverending discussion but i would like to know your opinion, just about cruise speed 7/8 kts 😀
Thanks to who will answer
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Old 13-07-2015, 04:21   #2
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Re: Singleor duble screw?

The single screw will be more efficient, especially on the maintenance end.

This of course will be at the sacrifice of maneuverability and redundancy.
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Old 13-07-2015, 06:05   #3
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Re: Singleor duble screw?

You'd probably get more response on trawlerforum.com, but you'd be best off to do a search there, first... because it's been discussed probably 100 times or more, I think.


IIRC, the consensus -- such as it might be -- seems to be that first, you can't find many examples in the real world, so empirical data is thin on the ground, and second, in close but not perfect comparisons in the real world, the twins might be slightly less efficient but not enough to matter. Especially if you're concerned about related ideas about safety (redundancy) and maneuverability. (And then of course the discussion usually turns to singles and bow thrusters...)


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Old 13-07-2015, 07:01   #4
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Re: Singleor duble screw?

Thanks for the answer. I was just reading right now�� i knowit will be a non stop discussion...
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Old 13-07-2015, 08:04   #5
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Re: Singleor duble screw?

Unless the single screw also has a bow thruster, you will find the twin screws to be more handy for maneuverability.

There is not a whole lot of difference in efficiency between the twin or the single. If you want more efficiency then slow down a knot or two.

You will also be less likely to find yourself dead in the water with a twin screw.
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Old 13-07-2015, 08:43   #6
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Re: Singleor duble screw?

having owned both I will have to agree with the above post. Single screw and bow thruster
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Old 13-07-2015, 10:41   #7
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Re: Singleor duble screw?

The trick is most twin screw boats are not the same. They typically are planning boats in trawler dress so you are really comparing a single 200hp vs twin 300hp.


As others have said, speed is the biggest factor. The single going 10kts will likely burn more than the twins going 5kts for the same distance covered.
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Old 13-07-2015, 10:56   #8
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Re: Singleor duble screw?

If you're going on the ocean, twin screw gives you 100% backup. Usually you don't need it, but in the last 35 years, I have come in twice on a single engine.
I've never needed a bow thruster with the maneuvering ability of twin engines. Slowing down saves more fuel than just running a single engine. You can run on 1 engine when fishing, etc.
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Old 13-07-2015, 19:01   #9
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Re: Singleor duble screw?

Thanks all

I was matching between the same trawler with displacement hull and the same speed, lets say oceanic going speed like 7 8 knts...

What if the engine would be electric? Do you still choose one or two with half power? Obviously with a big backup diesel generator...
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Old 13-07-2015, 19:50   #10
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Re: Singleor duble screw?

Quote:
Originally Posted by stefano_ita View Post
Thanks all

I was matching between the same trawler with displacement hull and the same speed, lets say oceanic going speed like 7 8 knts...

What if the engine would be electric? Do you still choose one or two with half power? Obviously with a big backup diesel generator...
Diesel electric systems are more complex, have more systems and are there for at greater risk of failure. 2 electric motors driven by a single gen set will fail if the gen set fails.



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Old 14-07-2015, 01:25   #11
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Re: Singleor duble screw?

Yep i know, but have one genset is like have a one engine boat...well its better the genset...always the same rpm, less problem...i was planning electric engine with batteries for 5 hours cruise at 6/7 kts, solar panels that can charge up all the battery in a day or permit to cruise at 2/3kts just with sun power, and bigger genset for fast cruise and cloudy days or cruise on hight latitude
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Old 14-07-2015, 04:59   #12
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Re: Singleor duble screw?

Quote:
Originally Posted by stefano_ita View Post
Yep i know, but have one genset is like have a one engine boat...well its better the genset...always the same rpm, less problem...i was planning electric engine with batteries for 5 hours cruise at 6/7 kts, solar panels that can charge up all the battery in a day or permit to cruise at 2/3kts just with sun power, and bigger genset for fast cruise and cloudy days or cruise on hight latitude
Cool idea. I know a guy that's putting a system somewhat similar together on a 30' sailboat.

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Old 14-07-2015, 05:09   #13
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Re: Singleor duble screw?

Both singles and twins have their advantages & disadvantages, so it's down to personal preference and style/size of boat.

At that size there is plenty of space for twins, so accessibility should not be an issue.

Singles offer slightly better fuel efficiency, but much less difference than most people expect. You are still using the same amount of hp to push the boat. The difference is less than 10% in most cases.

Probably a more important advantage of singles is having the prop & rudder protected by the keel.

Twins offer better manoeuvrability, and redundant engine if one fails.

Another option is a centered main engine and wing engine as a backup with a folding prop, similar to many Nordhavn's.
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Old 14-07-2015, 20:29   #14
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Re: Singleor duble screw?

It would be 2x 45kw electric engine with 270n/m of turque from 0 to 1400rpm and 45kw at 1500rpm... 100kwh of battery for every engine, plus 20kw of genset...should work nice, im thinking a lot about reduction gear or direct but 99% reduction 2:1 with big 28 x 24 pitch propeller (x2)...slow and efficient
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