Are twin diesels worth more than a single?

mvweebles said:

31-second video of a tour boat on an Amsterdam canal making a very tight turn. Single engine, no thruster. Looks like rudder swings through at least 120-degrees.

I fancy myself a decent close-quarter driver. This is away above my pay grade.

Peter

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That boat bumps into the seawall. Not hard to turn if you bounce off walls.

Twins are worth more to me. I have had both.

Bayview,
They were to the guy that bought the boat new too.
Many thousands.
And he probably knew what he was doing.

https://youtu.be/9Q7oxIbD3MY

Always wondered if anyone ever used an outboard for a "get home" engine? A remarkably small outboard will move a 36 foot trawler at 4-5 mph in smooth water.

And, for guys running twins...if you shut down one engine..How do you cool the stuffing box and transmission with the prop freewheeling?

SoWhat said:

I just did a highly scientific survey of GB 36 Classics for sale on YW. US boats only.
Equal number of singles and twins for sale. Did not track condition or features.
GB 36 single: mid 80's, mostly lehman 120's, avg ask price - $85,000
GB 36 twins: some late 80's, lehman 120/135's, avg ask price - $120,000
GB32 single: variety; mostly lehman 135's, avg ask price - $74,000

Sparse listing of cruise speeds but 7/8 knots common with max of 9/10.
One report of 12 knot max with a Cat 325Hp.

Conclusion: Equal number of each sold for GB36 in the 80's.
There's probably a 10-20 % premium for twins
I can't find any empirical data indicating there is any significant performance advantage so I am guessing the decicions are based upon emotions.

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10-20% ?
Huge difference 85K to 120K, like 40%, never expected that much.

Nomad Willy said:

But re a cylinder .. after the bore is worn to the point that along w the rings where there is too much blow-by to have enough bmep to perform to new specs? If an engine is good for one re-bore then the re-bore is considered part of maintenance?

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Detroit Diesel 671's were designed to be re-sleeved. As long as you can get sleeves they will last 2 days past forever. Anyone who ran a few of them kept a set of sleeves in the shop.

But all sleeves didn't wear out at exactly the same time. There were manufacturing tolerances so one sleeve ID could have been oversize or the ID wasn't parallel so the failure mode was random. Same with exhaust valves. They were machined to tolerance, installed (probably without lapping) and run. All valves don't burn up at the same time.

But until the engine throws a rod through the crankcase, they are rebuildable (not counting volvo's) with replacement parts. I consider that normal maintenance but just as we amortize the cost of a new heating system we can also "amortize" the cost of a rebuild into the cost of the boat rather than considering it an expense item such as an impeller.

stroutmail said:

https://youtu.be/9Q7oxIbD3MY

Always wondered if anyone ever used an outboard for a "get home" engine? A remarkably small outboard will move a 36 foot trawler at 4-5 mph in smooth water.

And, for guys running twins...if you shut down one engine..How do you cool the stuffing box and transmission with the prop freewheeling?

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If the outboard is attached to your inflatable dinghy, I know it will work, especially if the outboard is powerful enough.
When I owned a 30' sailboat, powered by a 12hp diesel, I had a breakdown and had to use my "get home" outboard, a 15hp outboard attached to my inflatable. It got me home in relative comfort as I was able to stand at the tiller of the sailboat for the journey, getting back into the dinghy only when close to my destination.
When I got my trawler, I was unable to move it satisfactorily with the same outboard on the same inflatable. Luckily I haven't had to try my present combo of a 40 hp on a bigger inflatable, as my twins act as their own "get home"
I have run a whole summer on one engine, with a pipe wrench on the non-moving shaft.

Soo-Valley said:

10-20% ?
Huge difference 85K to 120K, like 40%, never expected that much.

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Kind of a WAG considering the lack of data. I presume brokers have data going back years and are thus able to suggest a selling price. I'm assuming most of the differential is condition or feature related (or seller dreams) but I think there is enough of a difference to suggest at least a 10% premium.

stroutmail said:

https://youtu.be/9Q7oxIbD3MY

Always wondered if anyone ever used an outboard for a "get home" engine? A remarkably small outboard will move a 36 foot trawler at 4-5 mph in smooth water.

And, for guys running twins...if you shut down one engine..How do you cool the stuffing box and transmission with the prop freewheeling?

Click to expand...

Some transmission set ups “tolerate” free wheeling, at least for a while. I think our 4788 has a ZF transmission and for which free wheeling apparently isn’t a problem (I can’t say for how long, the manual is on the boat and I’m home).

SoWhat said:

Kind of a WAG considering the lack of data. I presume brokers have data going back years and are thus able to suggest a selling price. I'm assuming most of the differential is condition or feature related (or seller dreams) but I think there is enough of a difference to suggest at least a 10% premium.

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I agree your hunch is closer, just the numbers say 40%. Otherwise many who want two will buy singles at that spread.

Gb36 single vs twin is a great baseline example. Not sure though if percentage or real dollars is the right comparison. But I wouldn't be surprised if $30k - $40k, is the right uptick with all other things being equal. Of course, not all twins are the same - defevers are well protected running gear and accessible engines on outboard side. Not so many other twins.

Lindbergh crossed the Atlantic with one engine.

stroutmail said:

https://youtu.be/9Q7oxIbD3MY

And, for guys running twins...if you shut down one engine..How do you cool the stuffing box and transmission with the prop freewheeling?

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As far as cooling the shaft seals, in my case PSS, both engines send water to both seals, so running on one engine cools the seals on both shafts.

Regarding freewheeling the dead-side prop, most Twin Disc gears are OK up to 8 hours if you monitor the temperature with a laser thermometer or similar. The TD owners' manual has the models and the freewheeling procedure. Other gears like the Allisons M-20's I had behind Detroit 871's can't be freewheeled at all, which requires that you lockdown the shaft. Not sure about ZF.

markpierce said:

Lindbergh crossed the Atlantic with one engine.

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Yup---just after the guys in the three engine S35 crashed.

SoWhat said:

Detroit Diesel 671's were designed to be re-sleeved. As long as you can get sleeves they will last 2 days past forever. Anyone who ran a few of them kept a set of sleeves in the shop.

But all sleeves didn't wear out at exactly the same time. There were manufacturing tolerances so one sleeve ID could have been oversize or the ID wasn't parallel so the failure mode was random. Same with exhaust valves. They were machined to tolerance, installed (probably without lapping) and run. All valves don't burn up at the same time.

But until the engine throws a rod through the crankcase, they are rebuildable (not counting volvo's) with replacement parts. I consider that normal maintenance but just as we amortize the cost of a new heating system we can also "amortize" the cost of a rebuild into the cost of the boat rather than considering it an expense item such as an impeller.

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I was a Co-Op Engineering Student at General Motors Institute, sponsored by Detroit Diesel in 1968. So 6 weeks in Flint at school and 6 weeks in the factory on Outer Drive in Detroit. One of my favorite assignments was the "Engine Repair" department where engines that failed dyno testing would be disassembled and inspected and repaired. I got proficient enough to take a 6-71N completely apart and re-assemble in less than a full day. Wonderful engines. Developed a reputation for leaking oil, but otherwise built to last forever if you keep replacing the worn out parts. Sounded awesome. Turbo charging and electronic fuel injection controls kind of obsoleted them, but I seriously looked for a boat that was powered by one.

Since I intend to maintain the engine in my boat myself--having room to work was a major factor in choosing a single engine.

Very few comments about range. I always thought the attraction of a "trawler" was how far it could go on a tank of fuel? Single engines win that contest don't they?

stroutmail said:

Very few comments about range. I always thought the attraction of a "trawler" was how far it could go on a tank of fuel? Single engines win that contest don't they?

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Got four fuel tanks. Presumably I'm able to boat to Seattle from San Francisco, although I've no such urge.

stroutmail said:

Very few comments about range. I always thought the attraction of a "trawler" was how far it could go on a tank of fuel? Single engines win that contest don't they?

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Range is a good/bad thing. If you don't use it, fuel goes stale.

Personally, my benchmark is 1500 nms range which will get me from Panama Canal to Florida. Or Socal to Acapulco to Panama.

To answer your question though, singles are more efficient than twins, but not by much. More about how the boat is setup and tankage..

stroutmail said:

Very few comments about range. I always thought the attraction of a "trawler" was how far it could go on a tank of fuel? Single engines win that contest don't they?

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Not necessarily. Range varies more with the design intent of the boat than anything.

Stroutmail wrote;
“Very few comments about range. I always thought the attraction of a "trawler" was how far it could go on a tank of fuel? Single engines win that contest don't they?“

Trawlers are coastal cruisers and long range is not a big priority or even a priority at all for most. Wint our little Willard we can go to Ketchikan from Seattle. But it would require running 24-7 and no stopping or anchoring. And if we solwed to 5 1/2 knots we could go forth anchoring as usual. Plenty of range as there’s lots of places to stop and get fuel.
And in most coastal areas in the US there are many more places to get fuel than in the PNW.

Re fuel burn singles v/s twins a twin would be quite To more likely to have better range than a single if she was powered the way twind should be powered .... that is w the same total hp. But for some dumb reason twins and singles in typical trawlers of the 70’s and 80’s the twins had the same engine as the singles. So how’s a boat w twice as much engine going to be equal or better fuel burn wise as a single? If the GB 36 had 190 cu. in. 60hp engines in the twin it would almost certainly beat the single in burn rate at a given speed.

Ken E. said:

As far as cooling the shaft seals, in my case PSS, both engines send water to both seals, so running on one engine cools the seals on both shafts.

Regarding freewheeling the dead-side prop, most Twin Disc gears are OK up to 8 hours if you monitor the temperature with a laser thermometer or similar. The TD owners' manual has the models and the freewheeling procedure. Other gears like the Allisons M-20's I had behind Detroit 871's can't be freewheeled at all, which requires that you lockdown the shaft. Not sure about ZF.

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Our ZF45A are good to freewheel. Also, the PSS don't need water upto a certain speed (11kn I think it was?) and since on one engine we can only push to about 8kn, we haven't got the cross-over PSS water connections.

Nomad Willy said:

Trawlers are coastal cruisers and long range is not a big priority or even a priority at all for most. Wint our little Willard we can go to Ketchikan from Seattle. But it would require running 24-7 and no stopping or anchoring. And if we solwed to 5 1/2 knots we could go forth anchoring as usual. Plenty of range as there’s lots of places to stop and get fuel.
And in most coastal areas in the US there are many more places to get fuel than in the PNW.

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Good point. If I run 6.5 kts or so, even with my fuel guzzling gassers, the only place I know of in North America that might be questionable in terms of range with a good reserve is the Alaska run. And even that is probably do-able.

mvweebles said:

Range is a good/bad thing. If you don't use it, fuel goes stale.

Personally, my benchmark is 1500 nms range which will get me from Panama Canal to Florida. Or Socal to Acapulco to Panama.

To answer your question though, singles are more efficient than twins, but not by much. More about how the boat is setup and tankage..

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Beebe provided a definition of passage-maker of 2400nm range at 7.4kn, self sufficient for two weeks. I've amended it slightly with powercats in mind to be 2000nm at 6-8kn. Everyone has their own ...

Singles use less fuel than twins - up to a certain speed. That's not necessarily "efficiency" unless you focus only on fuel used and forget weather travelled through, time of departure and arrival, extra/less engine maintenance due to lower hours, and many other factors.

If you want to limit the discussion to only the speeds in which the boat can go on one engine, and then compare that to the fuel used with one engine vs two engines, you'd be usually right to say one engine is more efficient... unless the weather and sea conditions slowed you down to a speed that wasn't "efficient" in other ways where cranking up the second engine to go the same speed would help - like getting out of the way of a low pressure.

stroutmail said:

https://youtu.be/9Q7oxIbD3MY

Always wondered if anyone ever used an outboard for a "get home" engine? A remarkably small outboard will move a 36 foot trawler at 4-5 mph in smooth water.

And, for guys running twins...if you shut down one engine..How do you cool the stuffing box and transmission with the prop freewheeling?

Click to expand...

One issue might be that outboards are generally gasoline. And a majority of trawlers are diesel . . . so your supply of gasoline might not be enough to get you home.



On Freewheeling one prop. Check with the manufacturer of your transmissions. Some, like Velvet drives tolerate freewheeling just fine with no time limit according to the manufacturer. Others, not so much!