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Old 10-04-2015, 07:32 PM   #21
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All the CH46 and CH47 helos have twin engines, and twin rotors, so both engines go into a differential, and then the power is split into two rotors, one aft and one front rotating in opposite directions. That's how they did it since the 1940's and many are still flying...
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Old 10-04-2015, 07:59 PM   #22
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Two engines geared to a single screw is a pretty common thing in commercial ships. The one I'm on now has that setup. Over half of the ships in our fleet have that setup, actually. It's not unheard of for one of the engines to be out of service, while the vessel continues to run on the other until repairs can be made, albeit very... very slowly.

I know a lot of steam turbine driven ships had that setup as well. They would frequently have a high pressure turbine and a low pressure turbine geared to a single screw via two gear ratios. Some of the gear boxes I've seen have been really enormous.
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Old 10-04-2015, 08:14 PM   #23
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Originally Posted by RT Firefly View Post
Greetings,
Mr. OC. I think, in the case of the Swedish rescue vessels it was for extra HP for towing if necessary with the benefit of redundancy. The VP prop could make the best use of either the one or two engines. What I would have loved was a single driving two props but I'm WAY too old to embark on THAT exercise.
Mr. RT, I was think in regard to the Romsdahl 65 with 2 different manufacturers engines. As he is only planning to leave one in place, it seems two weren't needed to achieve desired speed. Hence my comment on redundancy.

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Old 10-04-2015, 09:08 PM   #24
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Greetings,
Mr. OC. Ah, I see now. In the case of the Romsdahl, I couldn't guess. The most likely would be for redundancy I suppose since HP/torque at various RPM would most likely be different with no way to synchronize them without very sophisticated electronics IF such a thing was even doable.
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Old 10-04-2015, 09:57 PM   #25
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Kort nozzles are ducted propellers typically found on displacement boats and especially favored by tugs, they increase thrust:

https://en.wikipedia.org/wiki/Ducted_propeller

Flanking rudders are additional rudders often found in front of the propeller "flanking" (one each either side of) the prop shaft. They increase maneuverability. Again often found on tugs and especially push boats on rivers.




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Please elaborate. What are Kort nozzles and flanking rudders? How are they used, etc. Thanks!
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Old 10-04-2015, 10:09 PM   #26
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Twin engines

With modern electronically controlled engines , it's probably unnecessary today to run two small engines as new large engines run clean and efficient at low speed. I looked at a Bebe designed 50' trawler that had been built with two industrial diesels. They used a chain drive coupling that proved to be unreliable.
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Old 10-04-2015, 10:51 PM   #27
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twin engines, single shaft

I've been aboard this one at the dock.

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Old 10-05-2015, 12:44 AM   #28
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Wonder if that setup was for combined HP or simply redundancy.

Ted
No idea. He cruises at 6.5 knots which seems to be slow for a 65 foot vessel, which suggests to me that the normal operation would be two operating engines. But I really don't know.
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Old 10-05-2015, 12:52 AM   #29
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I've been aboard this one at the dock.

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This one looks very similar to the one I mentioned. The write up suggests that the engines can be combined, so I guess there is a way to run both and not get into synchronisation challenges. (Although they are the same model of engine.)
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Old 10-05-2015, 12:53 AM   #30
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Quote:
Originally Posted by Keysdisease View Post
Kort nozzles are ducted propellers typically found on displacement boats and especially favored by tugs, they increase thrust:

https://en.wikipedia.org/wiki/Ducted_propeller

Flanking rudders are additional rudders often found in front of the propeller "flanking" (one each either side of) the prop shaft. They increase maneuverability. Again often found on tugs and especially push boats on rivers.


Nozzles also greatly reduce prop walk. Flanking rudders are very useful when going astern, when a typical rudder would be of little use.
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Old 10-05-2015, 11:59 AM   #31
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This one looks very similar to the one I mentioned. The write up suggests that the engines can be combined, so I guess there is a way to run both and not get into synchronisation challenges. (Although they are the same model of engine.)
Yes. They're both Grieg class Romsdahl vessels, 65 ft.
I would tend to think that minor synchronization will be adjusted by the
engine governors as they supply fuel according to load. Set the governor on
one engine to rpm, then advance the second to approximately same rpm.
As power is increased on one the other should reduce. The belt system maintaining matching rpm.
The engines have in-out pto type clutches that are operated at the clutch.
Propeller is full reversible controllable pitch.
The owner runs at about 1250 rpm.

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Old 10-05-2015, 02:04 PM   #32
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Interesting; I hadn't realized it's semi-common... and the history about available 6-71s and needing more HP is useful, too!


Seems like a good concept. I keep thinking a main engine and a fair sized genset, couple-able (?) together this way, would make a useful combination, slightly better than adding a whole 'nother wing engine/feathering prop to the mix. Although... I'd guess adding another layer of complexity in the gear box would have to enter into the trade-off analysis...


I remember some months ago there was some discussion about one engine running two independent props... seems like this could be a useful inverse version of that for recreational boats...


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Old 10-05-2015, 04:09 PM   #33
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Single diesel, hydraulic drives to twin shafts
Fisher catamaran
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Old 10-05-2015, 06:28 PM   #34
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Here's a photo of the one engine/two prop kit that was marketed some years ago. Lots of whirling failure points. I believe the Ramsey silent chain drives mentioned in post #11 would be a more elegant engineering solution. As I said, Ramsey has already done the 2 engine/1 prop deal using two 600 HP engines on one shaft.

Name:  boat geared up.jpg
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Here's a link to Ramsey http://ramseychain.com/prod_sc_pt.asp
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Old 10-22-2015, 11:29 PM   #35
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Yes-Faulk Gear Company many years ago had and may still manufacturer a gear that has four singles on a single shaft.
I had personal dealings with a WW11 era tug having four 671 Gray Marine engines.
Funny story. Our company often hired a company owning this tug to assist in coming cargo ships. On one occasion on an assist out the sea pilot onboard requested "Quarter power"of the tug. Nothing happend so the pilot asked for "Half power " Still nothing happen to reflect the tug making progress. The pilot now showing some frustration ask for "Full power ".
During this issue, my employer was listening on the vhf. Obviously with full power requesed and no movement of the heavyweight ship,asked the tug master how many engines were on line? "One" was the reply. "G-d damn it capt. We are paying for four. Give us four!!'"
True story
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Old 10-22-2015, 11:41 PM   #36
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On the subject of two engines on a single shaft. We have a fellow here in Ketchikan who installed s single new low block jimmy engine using the inside engine bed rails where before there were two enginesointed.He then plumbed the single to two hydraulic pumps one reversed to the other. These were plumbed to the reduction gears. Working well as he achieved his desired operating outcome.
The beauty of the Installation being able to mount the hydraulics in such a manner that alignment issues are eliminated.
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Old 10-23-2015, 04:13 AM   #37
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I was engineer on a lcm 8 here in Alaska that had four 6-71 piggybacked running two shafts. A very slick set-up with Falk gears could start one engine with the other. Operate on one but I had to build a metal plate to keep exhaust from coming out the blower of the down main.
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Old 10-23-2015, 06:28 AM   #38
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This is a very interesting conversation.

does anyone know where you can get a lightweight gearbox for leisure use that will let you convert a twin into a single engine/ twin outputs...?

There's lots and lots of very cheap twin engined trawlers out there , cheaply priced simply because they are twins that nobody can afford to run.
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Old 10-23-2015, 10:17 AM   #39
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Rusty,
Most of those boats have a keel that can offer an easy conversion w/o a kinky gear. Just pull up the twin shafts and run one down the middle. Then you've got a spare engine too. I'm sure it wouldn't be quite that simple unless ballast was put in the keels of twins.

But people don't want to go 6 knots. I'm fairly happy w it but from the talk on the forum over the years .. like Marin for example. He wouldn't be caught dead w such a wimpy boat. But the only way I'd have his boat is if it were a single. But most think more like Marin.

The twin shaft/single engine settup in semi-planing's post #34 I think is only cost effective for converting gas boats to single diesel. There was an article in PMM some time ago and as I recall they said that was what it was designed/intended for. They were looking to retain the maneuverability features of the twin. Instead of the settup in post 34 two engines half the power of the single shown would be my choice for redundancy. Perhaps that's why they didn't sell.
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Old 10-23-2015, 11:08 AM   #40
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Rusty,
Most of those boats have a keel that can offer an easy conversion w/o a kinky gear. Just pull up the twin shafts and run one down the middle. Then you've got a spare engine too. I'm sure it wouldn't be quite that simple unless ballast was put in the keels of twins.

But people don't want to go 6 knots. I'm fairly happy w it but from the talk on the forum over the years .. like Marin for example. He wouldn't be caught dead w such a wimpy boat. But the only way I'd have his boat is if it were a single. But most think more like Marin.

The twin shaft/single engine settup in semi-planing's post #34 I think is only cost effective for converting gas boats to single diesel. There was an article in PMM some time ago and as I recall they said that was what it was designed/intended for. They were looking to retain the maneuverability features of the twin. Instead of the settup in post 34 two engines half the power of the single shown would be my choice for redundancy. Perhaps that's why they didn't sell.
Here's some real numbers for prop drag: very interesting figures....



In the graph above, you can see that at 5 knots, a fixed three-blade prop with its shaft locked creates almost half as much drag as the entire hull. The drag can be halved by allowing the prop to spin, but the gearbox may suffer. By contrast, the drag of a feathering prop is negligible, and the drag of a folding prop is too small to plot on a graph of this scale.

>The hull resistance curve for the Océanis 323 was calculated for YM by the Wolfson Unit, at the University of Southampton, using data from the Delft University Systematic Series. The propeller drag curves are based on data from SSPA Maritime Consulting, using Volvo S-drives. This data were verified by YM’s on-the-water drag test.


Read more at Folding and feathering propeller test
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