Is there a perfect engine?

I could be rebuilding an older boat. I am trying to decide between repowering or renovating the existing engines. Obviously, there are numerous options. Are new motors really the best option or are there a great series of motors that are worth buying used and restoring/remanufacturing?

 

What do you have now?

 

It depends on what engines you have now. Most can be rebuilt in place. Changing engines is costly. It usually means cutting a hole in steel boats or cutting a hole thru the cabin in wood boats. Then you have all the alignment and hook up problems.
I've run almost every marine diesel made for boats and small ships. Even the old direct reversing engines.
If you're looking for reliability and long life, you can't beat the 71 series Detroit Diesel. I've been around these engines since the late 1950s. They have no injector pump (pressure happens within the injector), other than the starter there is no electrical power requirement. Once started, they run until out of fuel. Compared to most yacht engines, DDs are heavy duty. All have a roots type blower and higher hp models also have a turbo, some have dual turbos. Parts are available throughout the world.
The engines were used in industrial, trucking, generators, etc. Every city or port of reasonable size has someone that rebuilds heads, blowers, pumps, etc. You can find lot of parts on Ebay or the web.
They are not as economic as some, but last longer and parts are cheaper than most.
I have a pair of 671s in an 83' yacht and get about 8.3 gallons an hour at 10 knots in the ocean. I think that is economic.
Almost every month I hear a story about new engine failure due to circuit board or sensor problems. One commercial fisherman, 100 miles out, couldn't get his engine above idle.

 

"They are not as economic as some, but last longer and parts are cheaper than most."

The hassle with many DD installs is the engine is over sized for the load, costing efficiency.

A 2 stroke DD needs to be loaded to at least 60% of rated HP , regardless of the cruise RPM you select.

1200 is great the engine is really quiet,

1800 is the max at cruise for normal economy.

A good install will get 16 HP from each gallon burned in an hour, select properly and "forever" is pleasure boat engine life.

About 20 HP per cylinder (6-71) gives a long life so just match the number of cylinders to the expected cruise load.

 

I'd have to agree with FAST FRED. I ran my twin 6-71 NA's at 1325 rpm. Always except when docking. Noise was as would be expected from a Jimmy, but at higher rpm I'd get nervous from the racket. Of course my old boat had relatively bad (actually horrible) acoustic insulation installed. I found 5+ day cruises to get fatiguing from the noise. Learned a good lesson to apply to the next boat.

So anyway Northwindii, if you go for Detroits then ensure you spend good effort and money on proper acoustic treatment. Some Forum members have suggested removing the crappy marine air silencer box and replacing it with a truck style air filter, which can be further wrapped with a sound insulation blanket.

 

Humorously ...

An engine that was powered by electronic fund transfers to a fuel supplier ... no fumes or risk of fire, "unlimited" range (subject to having funds, of course), and your guests can help pay the tab if they want to go really fast.

 

I run at 1800rpm on my 671s. That gives 10 knots and is easy to figure dead reckoning. I don't notice the noise. I can have a normal conversation in the cabin over the engine room. The sound proofing was done in the 1970s. Lead sheeting makes the best base. Non combustible foam works good, too.
I ran mostly Detroits in Vietnam. 671s, 12v71s & 6v53s. We were young and our uncle bought the fuel. We ran as high as 2500. Sometimes more. Rings & sleeves lasted about 1000 hours. I used to take my sleep time on a hatch over 2 12v71s.
I also ran a tug with 4 671s on a single shaft.

 

Lepke, did you mean to say the rings lasted 10,000 hrs? Or was it only 1,000 hrs because you ran the engines so fast?

 

"lasted 10,000 hrs? Or was it only 1,000 hrs because you ran the engines so fast?"

Hours are NOT the measure of engine life , total fuel consumed IS.

Running at 10X the HP requires 10X the fuel so the engine only sees 1/10 the "hours" till worn.

No free lunch.

 

Was going to start a new thread

But I thought you may find this useful. other posts have stated some engines last longer ok. say you have the choice of two engines about the same physical size and in the power range you want which would you buy. This is what I found on the net.
1. An engine which is 50 cubic inches displacement should produce about 25bhp if it is intended to last. If it were to be tuned to say 60hp it wont last, could be a racing engine.
2. I found a commercially available engine of 125bhp of 122 cubic inches a ratio of 1.02:1. Most engines I found had a 0.3 to 0.5ish to 1 ratio.
3. The lower the ratio the longer the engine is likely to last hope this helps. mik

 

I did mean 1000. In Vietnam the 6v53s were in PBRs usually on 2 boat patrols. Depending on the patrol leader, we often ran at max throttle and short engine hours. The 671s and 12v71s were naturals and cruised at 18-2100. I've seen them run higher. They went much longer than 6v53s between overhauls. The boat captains were kids, mostly 18-23. Like kids everywhere when someone else is buying the fuel and paying for repairs.

 

There are many perfect engines, though they are seldom matched with their perfect boat!
First consideration should be whether the existing engines are a good match, then on to cost and availability issues.
We still have no idea what you are talking about, engine or boat.

 

My perfect engine would be a new 671... with a few simple systems to make them cleaner. My NA do about 250 hp. Would love to make a system to provide propane injection for extra power for short bursts.

The amazing thing is that DD still make are on a great engines list, their basic designed is over 50 years old. Many people have had them for decades, were most have gone through 3 or 4 sets of engines.

 

"My perfect engine would be a new 671."

With the aluminum block from a minesweep.to loose some weight.

The "perfect" propulsion package would come out of a landing craft.

(2)- 6-71 mounted to a gear box turning a single shaft.

Either engine can be disconnected from the drive with a simple lever.

By replacing one 6-71 with a 2-71 or 3-71 for ocean transit very economical operation could be had.

Inshore with a thick wallet and fuel available the 6-71 would push and the smaller would have a DC generator head bolted in place for house power.

KISS

 

I've thought for quite a while how well the DD would take to some modernisation:
-high pressure common rail
-modern injectors
-electronic timing control