Single vs Twin: It's Baaaaack!

Art said:

Overheating crack the block, frozen coolant??

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Neah, this was well before any freezes.

He left on a weekend trip, noticed she seemed to be running warm (not hot), and pulled into a local marina (under power) and had a mechanic change the impeller. Went on his way and still seemed warm... didn't wanna ruin the weekend.

Once arrived noticed it was low on coolant again, topped it off and decided to take it slow and easy getting home. After a 40 mile run and within a few miles of home, temps began to climb even when lowering RPMS. For the sake of safety, pushed on and got back to his slip under power.

All that being said, he could have shut down and gotten a tow, but he did not. On a twin, he would have least had an option to secure the motor and get home under power. Only using the second motor to navigate the lanes, or come in as a single.

I'm a pilot. Airspeed, Altitude, Ideas.

Never run out of options.

SomeSailor said:

Neah, this was well before any freezes.

He left on a weekend trip, noticed she seemed to be running warm (not hot), and pulled into a local marina (under power) and had a mechanic change the impeller. Went on his way and still seemed warm... didn't wanna ruin the weekend.

Once arrived noticed it was low on coolant again, topped it off and decided to take it slow and easy getting home. After a 40 mile run and within a few miles of home, temps began to climb even when lowering RPMS. For the sake of safety, pushed on and got back to his slip under power.

All that being said, he could have shut down and gotten a tow, but he did not. On a twin, he would have least had an option to secure the motor and get home under power. Only using the second motor to navigate the lanes, or come in as a single.

I'm a pilot. Airspeed, Altitude, Ideas.

Never run out of options.

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Thanks for added info - If possible, I'd still like to learn... why does he / do you feel the block cracked?

Art said:

Thanks for added info - If possible, I'd still like to learn... why does he / do you feel the block cracked?

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I'd love to know too.
I think this is the 2nd story like this on TF recently.

One of our engine shutdowns occurred on the first longer cruise we took after acquiring the boat out of SFO Bay. On the last day of the trip we noticed the port temperature starting to climb so we shut it down and continued to our anchorage on the other engine. Called the diesel shop in Bellingham and told them what had occurred. They had us check the waterflow out the exhaust of the engine we'd shut down. At idle it appeared to be normal.

Our run home was some thirty or forty miles so the next day we tied off the shaft of the shut-down engine and came home on the other one. Took a little longer but no problems. Because we had decent waterflow from the shut-down engine at idle, the shop had suggested that when we got to the marina we could start up the engine and use it for maneuvering, which is what we did. (Yes, we remembered to untie the shaft-- we'd taped the throttle of the engine back with a bunch of blue tape to remind us.)

The shop then came down during the week and investigated and found the through-hull almost choked off by a mysterious crystalline growth. They had seen this only a few times before, always on boats that had come from out of state, and it was obvious it had started before we ever got the boat. They chipped it out and that was that. It has never returned.

But this is an example where having a spare engine under the floor turned what could have been a big, time-consuming and potentially expensive hassle into a non-event. In this case, we didn't even lose the maneuvering advantage of two engines.

Mark measured his and said it was 3 degrees.

psneeld wrote;
"I move the rudder and so does the autopilot so much more that the teeny tiny bit of rudder necessary to overcome the almost non-existent prop walk AT SPEED , that you can't even realistically measure it...at least not on the dozens (maybe more) of sngle engine boats I have driven."

I don't think it's much either but it all adds up.

Art said:

Thanks for added info - If possible, I'd still like to learn... why does he / do you feel the block cracked?

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He was getting combustion gas into the cooling jackets and they crack spread as the heat built. It was bad enough that didn't even reman the block. They ended up just scrapping it.

He's an older gentleman too, so the extent of his maintenance capability would be adding coolant, if he even did that.

manyboats said:

Mark measured his and said it was 3 degrees.

psneeld wrote;
"I move the rudder and so does the autopilot so much more that the teeny tiny bit of rudder necessary to overcome the almost non-existent prop walk AT SPEED , that you can't even realistically measure it...at least not on the dozens (maybe more) of sngle engine boats I have driven."

I don't think it's much either but it all adds up.

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not when environmentals make it lost in the noise.

and one boat does not make a design factoid.

The question has never been properly addressed as I see it. I've never heard of a vessel that had the proper power, engines and other variables to make a good comparison.

It's probably close enough that just the efficiency of the engine/engines would swing it one way or the other. As an example there was a 36' GB w twin 55hp Yanmars and if it was compared to a 36 GB w a 120 Lehman the twin would (no doubt in my mind) be more fuel efficient than the single. There is a four cyl Lehman but if there was a 3 cyl version twin 60 hp 3 cyl lehmans I'll bet they would be very close and quite likely the twin 60 hp Lehmans would be a bit more efficient.

But I have never heard of a boat/engine configuration that is that comparable.

LOL... Learning To:

Again, I Say:

Single - most economical... more ways than one!

Twin - greater maneuverability/safety ... more ways than one!

Single boats are great!!

Twin boats are great!!

Each Captain to their own...

That's the answer... What's the question??

Makes me wonder why most all commercial, freight-hauling ships have only one propeller for forward motion.

manyboats said:

Another reason a twin could be more efficient is that it's not having to counteract the propwalk w a deflected rudder creating more drag.

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But what about the resistance of struts and shafts, as well as a likely extra rudder? And for that matter, one propeller working to counter-act the other's prop walk.

In my mind, recreational boats with multiple engines are designed for speeds faster than hull-speed, and many purchasers consider that first over redundancy. On the other hand, there are many reasons for purchasing a particular boat other than the number of engines. ... Can't imagine fitting multiple engines in the Coot's engine room. Even a genset would make things extra crowded. Also, multiple-engined boats seem to have higher profiles as living spaces tend to be above the engine room. High profiles cause their own problems. ... Presume this technician is installing the Coot's air compressor, right elbow on the water heater.

SomeSailor said:

I'm a pilot. Airspeed, Altitude, Ideas.

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Why should/do most private planes be equipped with only one engine?

And for that matter, one propeller working to counter-act the other's prop walk.

PROP WALK is your FRIEND!!!, if you have a single and are docking or short turning or leaving a slip!

With most of our cruising now brown water ,the singles ability to not be trashed by debris , and pick up pot lines works best for me.

Realistically light weight high speed diesel singles top out at about 400HP -650 HP,
so if a 1/2 nmp/g planning boat was my desire ,
I guess there is a point where a second motor would be needed.

Some things never change

This thread proves that some things never change.....debating single vs twin.
The other classics are UP vs DOWN galley, and Which anchor is best?
I think which anchor is best holds the world record for longevity.
R,

Dont forget Chain vs line for the anchor rode !

Mark,
The reason most ships are SS is economics. Simpler and cheaper.
On our trawlers the twins can have significantly larger props greatly increasing the prop disk area and increasing efficiency unless extremely low powered. This increased efficiency should over come the drag of shafts and struts.
Mark says "In my mind, recreational boats with multiple engines are designed for speeds faster than hull-speed" Some truth in that but I think usually the twins are popular for redundancy. Most people know that a twin is considerably safer in the event of an engine failure. Most private planes are singles for economy and singles are MUCH safer in the average light plane because when an engine quits in a small twin it becomes uncontrollable. When this happens on take off usually everybody dies. I think this just happened last week.

SomeSailor,
Yes. Your options are really excellent w one engine running and extremely poor w no engine running. How so many TF members fail to get their head around that is a mystery to me.

Art the horse is obviously not dead.

Those big single screw ships have engines two or three stories high that can be segmented or multiple engines that drive the same shaft. Most of the well built big cruise ships have as many as 4 pod drives, I suppose those Carnival customers wish they were on one...

Despite my own choice in boats, I like single screw boats a lot; like a one engine airplane, the maintenance has to be intense, but it should be on a twin too, so there's a savings. My boat would do just fine with one engine given our cruising style. Unfortunately there weren't any singles that provided the other design aspects we absolutely wanted, if there were, I would have bought one.

It's funny...once you accept the fact that you may break down as a single engine at some point, and plan for it, it's no where's near as bad as people think. The good new is it happens so rarely to good cruisers....and they are often on their way again lickety-split...

manyboats said:

Art the horse is obviously not dead.

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Maybe not, Eric, but it needs to be shot; and put our of its misery. #860

Moonstruck said:

Maybe not, Eric, but it needs to be shot; and put our of its misery. #860

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Quick - Shoot the horse! - - >
Before all guns/ammo get confiscated!!

markpierce said:

Why should/do most private planes be equipped with only one engine?

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Because one is cheaper, lighter, less complex and simpler to operate and so is easier for beginners to master.

Mark-to add to Eric's comments re ship having a single screw-many now are diesel electric, not direct drive. Main reason is that torque curve on an electric engine is almost perfectly flat, better low speed operation. Multiple diesels driving generators to power the electric drive motor.

Marin said:

Because one is cheaper, lighter, less complex and simpler to operate and so is easier for beginners to master.

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That sounds like a pretty good argument for buying a trawler!

I Luv Twins

No matter what type fuel!!

SomeSailor said:

Neah, this was well before any freezes.

He left on a weekend trip, noticed she seemed to be running warm (not hot), and pulled into a local marina (under power) and had a mechanic change the impeller. Went on his way and still seemed warm... didn't wanna ruin the weekend.

Once arrived noticed it was low on coolant again, topped it off and decided to take it slow and easy getting home. After a 40 mile run and within a few miles of home, temps began to climb even when lowering RPMS. For the sake of safety, pushed on and got back to his slip under power.

All that being said, he could have shut down and gotten a tow, but he did not. On a twin, he would have least had an option to secure the motor and get home under power. Only using the second motor to navigate the lanes, or come in as a single.

I'm a pilot. Airspeed, Altitude, Ideas.

Never run out of options.

Click to expand...

I have friends who are pilots. They tell me the three most useless things to a pilot are:
1) All the air above you
2) All the runway behind you
3) All the fuel back at the hanger

Never heard of all the other engines.

Is there a nautical equivalent?

Walt,
Why is SanDiego purple?

alormaria,
All your engines that have quit.

SeaHorse II said:

That sounds like a pretty good argument for buying a trawler!

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Or at least one with only one engine.

manyboats said:

Walt,
Why is SanDiego purple?

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We think it has something to do with the San Onofre Nuclear plant which lies just 45 miles to the north of San Diego. There have also been reports of young girls growing breasts on their backs. Although they are not very popular, the boys say they're awfully fun to dance with.

A little off the subject but is there a diesel repair and maint. class that can be taken. I am on the 5 year plan and want to be able to work on the engine if need be for general maint. upgrading or adding systems, trouble shooting issues and minor repairs if needed. I live in the Stuart, St. Lucie Fl. area Thanks