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Old 03-09-2009, 08:51   #1
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One Engine Running Two Shafts

Hi i am building a Tad Roberts Passagemaker Lite 486 in epoxy. I haven't placed the engine yet and was wondering if anyone had experience with running two shafts by one engine (with V-belts or so), in order to alternate engines during long crossing, or would it be non-sense? Ataraxia- Passagemaker Lite 486 - ataraxia.talkspot.com
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Old 03-09-2009, 09:02   #2
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I don't see any advantage. By running two propellers on two different shafts off of one engine, you would be increasing the overall friction of the drive train. Additionally one larger propeller that generates X amount of thrust is going to require less energy to turn than two smaller propellers that each generate half the thrust. There is an efficiency increase as propellers get larger and turn more slowly. I also think you would end up with a Rube Goldberg drive train that eats up a lot of space. There is a lot to be said about simplicity for keeping things reliable.

Even if it was advantageous, V-belts or even serpentine belts, even a few of them, do not have anywhere near the capacity to turn a propeller for a 48 foot boat. You would need a sizable chain, gears or hydraulic motors to do this.

As far as a twin screw maneuverability advantage, you won't get that either because that advantage comes with being able to have one propeller going ahead and the other going astern at two different controllable speeds with twin rudders to bounce the propeller wash off of.

A Rube Goldberg illustration...
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Old 03-09-2009, 09:07   #3
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Ataraxia- just for clarity I assume you mean a 2 engine/2 shaft installation with the capability to drive both the engines from one shaft? Some of Bob Beebe's designs employed an arrangement where 2 engines could drive a single central shaft using PTO's, belts and a single gearbox - see his original edition book. Another possibility is keeping engines and shafts separate and (ideally) using controllable pitch props to optimise emgine loading - see Sabb and Westmekan's web sites
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Old 03-09-2009, 09:08   #4
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There is an article in the August 2009 issue of "Pacific Yachting" titled "Single Engine, Twin Screws?". It discusses the repower of a 44 ft trawler.

I don't know if this helps with your twin-engine scenario.
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Old 03-09-2009, 09:11   #5
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I agree that one big prop is better than two small ones but I do not thrust crossing an ocean with just one engine. For the same reason I would like to be able to shift one engine to a different shaft...just in case but I still want to avoid to become as the professor in your cartoon :-)
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Old 03-09-2009, 09:12   #6
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I have BB's book but probably the wrong edition can you get me more info on it?
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Old 03-09-2009, 09:14   #7
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Quote:
Originally Posted by Paul Elliott View Post
There is an article in the August 2009 issue of "Pacific Yachting" titled "Single Engine, Twin Screws?". It discusses the repower of a 44 ft trawler.

I don't know if this helps with your twin-engine scenario.
any chance you can get me a copy of the article?
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Old 03-09-2009, 09:18   #8
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and yes I did mean 2 engines and 2 shafts
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Old 03-09-2009, 09:33   #9
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Quote:
Originally Posted by Ataraxia View Post
and yes I did mean 2 engines and 2 shafts
That changes everything.
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Old 03-09-2009, 10:55   #10
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If this is a 'new' installation then electric drive using submersible motors must be worth a quick costing out. It has the advantage of running one engine and both props for chugging along but at most efficient engine revs, and no shafts, gearboxes, rev matching etc.
Domestic power generation will be unlimited too and a dual installation almost guarantees water maker, etc.
It may take a bit more investigation to assess self adjusting props for electrics but lower prop drag when sailing is a big thing on longer trips.
Otheerwise simple one way couplings will rotate two shafts linked together. A forward reverse gear in each shaft could allow boat spinning but would be clumsy to operate.
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Old 03-09-2009, 11:13   #11
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I really dont see why you would want to worry about this. Even on a big wide cat with 2 engines/2 shafts, they are quite happy driving on a single engine, single shaft. when in cruising mode. or motor sailing.

Obviously when manoeuvring in harbour you need to fire up the second engine, and also if trying to push hard against wind/sea.

If you really must, then the best option would be to consider electric drive from two dedicated generators, with switching to be able to drive both or either from a single generator, and allowing the second generator to sync with the first to achieve the additional power when needed (similar to the honda stand alone portable generators).
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Old 03-09-2009, 11:25   #12
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I'm having a hard time rationalizing why you'd want to do this. The ability to cross connect shafts would only help you in the unlikely event of one engine breaking down at the same time as something breaking on the opposite shaft. Your original post referred to alternating engines, I'm guessing to even out the duty cycles of each? You're building a trawler, so it's pretty easy to freewheel one shaft while the other engine is running. Make sure you get transmissions that are rated for freewheeling. I guess the real question is how much energy is lost by keeping a 4-5 degree rudder on to compensate for the unbalanced thrust. I suspect, however, that if you rig a system to drive both shafts from one engine you will end up losing efficiency not only because of increased cost, weight and driveline losses, but also because you will face a choice of how to prop your boat. Prop it for one engine driving both shafts, you get a much smaller prop than if you dedicate one engine to one prop. You can't be set up for one operating mode then switch to the other without tremendous efficiency losses.

Brett
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Old 03-09-2009, 11:52   #13
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The steering offset while running on one of two engines is negligible if you mount feathering propellors. I've looked at some of your plans at the web address you posted, and I hope you meet your goals, but there are some purely external objections to a long narrow boat: Slips, insurance, and some gov't regulations are based on LOA and would penalize you unfairly. I don't suggest that these would tilt the balance sheet against you.

What are you considering for power? If you will be running a third engine for a generator, will it share spare parts with your motive engines?
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Old 03-09-2009, 13:52   #14
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When we were boat shopping we almost bought a Beebe 52 ft passage maker in Portland Oregon and had it hauled and did a sea trial. It had two Perkins engines stacked on top of each other connected by a rubber drive belt of the kind used in earth moving conveyor belts. You could engage either engine or both.The belt was incredibly noisy and the whole set up was awkward and weird. Made for a very strange engine room. The bottom engine was so low in the bilge it would have been a nightmare to work on.
We ended up not buying it because the surveys came back iffy and it had been used as an live aboard for too long without making a passage.
We settled on a single engine, a DD 871, with a reputation for running forever, and a bow thruster for docking.
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Old 03-09-2009, 22:09   #15
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Oh yes, I would investigate diesel-electric drive while building a trawler. This is proven technology with just one catch: your efficiency goes down as compared to diesel drive.

Two electric motors with shafts and propellers and two big gensets.

cheers,
Nick.
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