single vs dual engines
- Thread starter jann
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markpierce
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FF
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MY"DREAM BOAT would have a twin engine transmission package from a USN LST.and a CPP.
The two engines would be from the same mfg ,just different sizes .
Say a 3-71 and a 6-71 or a more modern choice.No turbo,no electric injection.
The boat could take the ground ,(unlike most twins) and would operate with an efficient engine at cruise or flank.
OTS ,used rebuilt ,repairable worldwide,cheap to keep.
FF
The two engines would be from the same mfg ,just different sizes .
Say a 3-71 and a 6-71 or a more modern choice.No turbo,no electric injection.
The boat could take the ground ,(unlike most twins) and would operate with an efficient engine at cruise or flank.
OTS ,used rebuilt ,repairable worldwide,cheap to keep.
FF
SomeSailor
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There's some argument of longevity with twins as well. Neither motor is working as hard as a single would through most of it's life.
markpierce
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Darn. Why do so many tell me it is bad to be easy on a diesel engine rather than to work the engine harder (like 75% of max RPM while idle is about 30%)?
Last edited:
psneeld
Guru
Maybe in commercial applications...or hard run sportfish...but most trawler guys underuse their engines or use such little power from them they are more likely to die of old age than wear out from op hrs or stress.
SomeSailor
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I guess it's the subtle difference of getting hull speed out of a 37,000# displacement hull with a single engine or with twins. To get mine to 9 kts on a single motor is pushing it, but at 1650 RPMs both motors purr.
I suppose the bigger benefit would be the redundancy of twins (fuel system, belts, cooling, electrical casualty).
I suppose the bigger benefit would be the redundancy of twins (fuel system, belts, cooling, electrical casualty).
Nomad Willy
Guru
SomeSailor,
If you have a full displacement boat you should'nt be running at hull speed. You'll burn half as much fuel and make much less noise and wake about one knot slower. Hull speed is only attainable at full throttle. You probably have a semi-disp hull.
If you have a full displacement boat you should'nt be running at hull speed. You'll burn half as much fuel and make much less noise and wake about one knot slower. Hull speed is only attainable at full throttle. You probably have a semi-disp hull.
SomeSailor
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True hull speed is the most efficient point you could operate any hull. Many people describe the maximum speed of their boat as hull speed.
I am speaking of hull speed (8-9 knots I'm guessing), but I can push to about 11 kts at higher RPMs.
I can achieve that at around 1650 RPMs. (WOT is 2400-2450)
I am speaking of hull speed (8-9 knots I'm guessing), but I can push to about 11 kts at higher RPMs.
I can achieve that at around 1650 RPMs. (WOT is 2400-2450)
FF
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"Why do so many tell me it is bad to be easy on a diesel engine rather than to work the engine harder (like 75% of max RPM while idle is about 30%)?"
This is because combustion pressure BEHIND the rings is required for the rings to seal properly.
This requires a fairly heavy load , it does not happen well at low loads.
Square cut rings (instead of trapizoid in high performance) , and the frequent higher compression pressures of a non turbo engine at low speed are a help.
More frequent oil changes will get rid of the acids in the blowby.
A SLOBBERING engine is the result of the abuse of underloading, BUT this is not common in yacht "trawlers" as the engine source is usually tractor or taxi engines , which are far more accepting of this as a design requirement.
Run a 700hp REAL TRAWLER at just above idle for hours on end and the service life drops BIG TIME!
FF
This is because combustion pressure BEHIND the rings is required for the rings to seal properly.
This requires a fairly heavy load , it does not happen well at low loads.
Square cut rings (instead of trapizoid in high performance) , and the frequent higher compression pressures of a non turbo engine at low speed are a help.
More frequent oil changes will get rid of the acids in the blowby.
A SLOBBERING engine is the result of the abuse of underloading, BUT this is not common in yacht "trawlers" as the engine source is usually tractor or taxi engines , which are far more accepting of this as a design requirement.
Run a 700hp REAL TRAWLER at just above idle for hours on end and the service life drops BIG TIME!
FF
Nomad Willy
Guru
SomeSailor wrote:
"True hull speed is the most efficient point you could operate any hull. Many people describe the maximum speed of their boat as hull speed."
The approx best speed for a full disp hull is about .75 to 1 knot below hull speed. on a full disp boat max speed w the correct amount of power is hull speed or a small amount over. Like 1/2 of a knot.
What kind of boat do you have and what is it's water line length?
What is it's rated power and displacement?
I suspect you have a semi-displacement hull or you would'nt be able to achieve 11 knots. Can you post a picture? Preferably out of the water and of the stern (best) or side.
Proper power for a FD boat is 3 to 5hp per ton and top speed is about hull speed.........1.34 X (the square root) of the water line length.
There is no such thing as "true" hull speed. Hull speed is hull speed. And while I'm at it there is no such thing a revolutions per minutes. It's all about one minute ....only one.
"True hull speed is the most efficient point you could operate any hull. Many people describe the maximum speed of their boat as hull speed."
The approx best speed for a full disp hull is about .75 to 1 knot below hull speed. on a full disp boat max speed w the correct amount of power is hull speed or a small amount over. Like 1/2 of a knot.
What kind of boat do you have and what is it's water line length?
What is it's rated power and displacement?
I suspect you have a semi-displacement hull or you would'nt be able to achieve 11 knots. Can you post a picture? Preferably out of the water and of the stern (best) or side.
Proper power for a FD boat is 3 to 5hp per ton and top speed is about hull speed.........1.34 X (the square root) of the water line length.
There is no such thing as "true" hull speed. Hull speed is hull speed. And while I'm at it there is no such thing a revolutions per minutes. It's all about one minute ....only one.
Anode
Guru
Sure there is. Unless you stop after one minute.
If you're running 1800 revolutions per minute, then you're running 3600 revolutions per 2 minutes, then 5400 revolutions per 3 minutes and ...
Nomad Willy
Guru
You better hide under that chair Chip or I'm going to take a shot at you.
Haha......I did'nt even think about someone coming back w that.
Haha......I did'nt even think about someone coming back w that.
after much wrenching thought...I decided to go with twin yanmars for my mainship.
I just can't see my self going to the Dry Tourtugas or the bahamas with a single engine. Especially since I already had a failure ( 2 times the engine would not start ) with my current 30 ft. mainship
I just can't see my self going to the Dry Tourtugas or the bahamas with a single engine. Especially since I already had a failure ( 2 times the engine would not start ) with my current 30 ft. mainship
skipperdude
Guru
I think if you only haveone engine you are more on top of things.
I'm a single and I watch it like a hawk.
Just say'in.
Who was it said "The engine was running fine when it quit."
SD
I'm a single and I watch it like a hawk.
Just say'in.
Who was it said "The engine was running fine when it quit."
SD
psneeld
Guru
Wouldn't start or quit underway?
Sure diesels can be finicky to start sometimes...but the beauty of them is start'em, give them air and fuel...and they usually run till you shut them down...thus why many of us have only one.
I had the following problems while away
1) I had a bad pilot starter relay ( burned out )on the yanmar. I now know how to jump start if it should happen again.
2) bad Control Relay ( mainship cheap mexican relay ) that sends power to the computer not alowing the engin to start. I love the mainship ship but they bought a lot of cheap support stuff.
anyway...I've had it with single engins!!
1) I had a bad pilot starter relay ( burned out )on the yanmar. I now know how to jump start if it should happen again.
2) bad Control Relay ( mainship cheap mexican relay ) that sends power to the computer not alowing the engin to start. I love the mainship ship but they bought a lot of cheap support stuff.
anyway...I've had it with single engins!!
Nomad Willy
Guru
I have a single and wish it were a twin.
skipperdude wrote
"I think if you only haveone engine you are more on top of things.
I'm a single and I watch it like a hawk."
THAT IS TRUE .... but still I think twins are far more resistant to bringing a boat to be dead in the water. And one can back up straight too.
skipperdude wrote
"I think if you only haveone engine you are more on top of things.
I'm a single and I watch it like a hawk."
THAT IS TRUE .... but still I think twins are far more resistant to bringing a boat to be dead in the water. And one can back up straight too.
JD
Guru
Were not both of the problem relays supplied by Yanmar? The one relay has been a problem with the 4JHT series Yanmar for over 15 years now. Ask Hunter Marine how many sailboats had the same problem.
Now if we want to discuss why Yanmar over something else that is a different issue. But there is nothing cheap about Yanmar's they cost a bundle. In the late model (2004-2009) 34t's the Yanmar was Standard and Cummins was a 20k option. The reason was that when Mainship was powering the 34t's Yanmar gave them a break. Not so much on price but they supplied the engines on consignment and Mainship didn't pay for an engine until the dealer paid for the boat from Mainship. Strictly a smart business decision.
