Twins on a single shaft?

Ted · Oct 5, 2015

Conrad said:
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.

Ted

ranger58sb · Oct 5, 2015

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...

-Chris

boatpoker · Oct 5, 2015

Single diesel, hydraulic drives to twin shafts
Fisher catamaran

go-planing · Oct 5, 2015

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.

Here's a link to Ramsey http://ramseychain.com/prod_sc_pt.asp

Al · Oct 22, 2015

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
Al-Ketchikan

Al · Oct 22, 2015

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.
Al-Ketchikan

rncheney · Oct 23, 2015

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.

Rustybarge · Oct 23, 2015

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.

Nomad Willy · Oct 23, 2015

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.

Rustybarge · Oct 23, 2015

manyboats said:
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

Rustybarge · Oct 23, 2015

It takes 21hp to push boat at 7kts with 130kg drag: 6.19hp/kg.

20kg of drag from freewheeling prop = 3.23hp loss.

If 18hp burns about 1 gal so thats about 0.18 gal loss.

Maths is not my strong point. Lol.

Anyone like to make up the numbers for twins ...

Nomad Willy · Oct 23, 2015

Rusty,
It sounds like you're directing the prop drag info at me. Don't know why if true as I have never contemplated or otherwise supported running one engine on a twin and draging the prop locked .. or freewheeling.
We were talking about shaft drag before but not the props.

Rustybarge · Oct 23, 2015

manyboats said:
Rusty,
It sounds like you're directing the prop drag info at me. Don't know why if true as I have never contemplated or otherwise supported running one engine on a twin and draging the prop locked .. or freewheeling.
We were talking about shaft drag before but not the props.

Sorry, wasn't directed at you. :flowers:

I've never seen actual numbers for the amount drag a prop produces, only guesses.

Its very useful to know about 3 1/2 HP......

Twins on a single shaft?

Ted

Guru

ranger58sb

Guru

boatpoker

Guru

go-planing

Senior Member

Al

Guru

Al

Guru

rncheney

Member

Rustybarge

Guru

Nomad Willy

Guru

Rustybarge

Guru

Rustybarge

Guru

Nomad Willy

Guru

Rustybarge

Guru

Similar threads

Latest posts