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Old 03-16-2019, 07:13 PM   #1
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Question Engine Swap help needed

New guy here from Ohio.
The Wife and I are Ready to buy a boat. Our intent is to motor around
here and there and all over.
Narrowed it down to a 35 0r 39 Mainship single engine. Have not found the boat yet.
Problem...300 and some horses apparently eat 3 to 13 gallons per hour.
I am thinking maybe an engine swap down to Maybe a 4 cylinder Perkins. Have my eye on a 77hp Perkins MFA4.300 302.2 cu in 4 cylinder tractor motor. Of course it would become all fresh.
I suppose a Lehman 120 would work but the weight you would save with the 77hp just seems like a neat thing to me. The shallower draft would be neat.
From the cruisers boat and using the small block chevy motor from an automobile I know the exchange is doable...maybe not with this Perkins motor but doable.
I am wondering IF there are changeover parts available AND if there are those folks that can help you through the process.
I think I have given you folks an idea of what I am thinking...
Hopefully I can get some good input and or places to search for what I want.
OR how ridiculously stupid the idea is.
Thanks and Regards in Advance, Gary
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Old 03-16-2019, 07:29 PM   #2
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There is a reason Mainship installed a 300hp+- engine, and not a 77hp Perkins. I recently saw a 395 with a 385hp Yanmar which at displacement/trawler speed was quite economical. Are you sure you know more about engine requirements than the builder/designer?
And balance the cost of the changeover against the possible fuel savings, a kind of cost benefit analysis.You won`t be just swapping engines, there are engine bearers,mounts, exhaust, gearbox mounts, fuel supply, raw water supply, prop shaft modifications, instrumentation and senders, lots of knock on changes to consider.
Maybe run it for a while after you buy one, and see how it looks then.
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Old 03-16-2019, 07:30 PM   #3
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Greetings,
Welcome aboard. Well, you can pretty well put almost anything in anything given enough time, patience and $$. My first and gut reaction is buy the Mainship and go easy on the throttle. I suspect it would take many, many years to save the amount of $$ it would take to swap to a smaller diesel engine.

Most definitely NOT ridiculously stupid, just a bit IMO.
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Old 03-16-2019, 08:00 PM   #4
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You are talking big bucks. First find a replacement engine then remove the engine, change the motor mounts and transmission as a minimum.

My suggestion followers others' suggestion..... Use what you have and run it at hull speed. Determine fuel consumption at the various RPM.

I doubt if you will get a return on your investment before you die.

I dont know the weight of the boat but I suspect 77hp would not satisfy you at all and then you would want to change the engine, motor mounts again.

A naturally aspirated Lugger, maybe, depending on the weight and transmission reduction and prop.
It is better to have reserve RPM that to run it WOT.
If you need a custom reduction via the transmission, that will be big bucks too.
Downsize your engine will make the boat more difficult to sell too.
Find your 'sweet spot' and run it there. Use the savings to buy fuel for your trips.
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Old 03-16-2019, 08:07 PM   #5
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Just drive it a hull speed or less

Quote:
Originally Posted by gfr51 View Post
New guy here from Ohio.
The Wife and I are Ready to buy a boat. Our intent is to motor around
here and there and all over.
Narrowed it down to a 35 0r 39 Mainship single engine. Have not found the boat yet.
Problem...300 and some horses apparently eat 3 to 13 gallons per hour.
I am thinking maybe an engine swap down to Maybe a 4 cylinder Perkins. Have my eye on a 77hp Perkins MFA4.300 302.2 cu in 4 cylinder tractor motor. Of course it would become all fresh.
I suppose a Lehman 120 would work but the weight you would save with the 77hp just seems like a neat thing to me. The shallower draft would be neat.
From the cruisers boat and using the small block chevy motor from an automobile I know the exchange is doable...maybe not with this Perkins motor but doable.
I am wondering IF there are changeover parts available AND if there are those folks that can help you through the process.
I think I have given you folks an idea of what I am thinking...
Hopefully I can get some good input and or places to search for what I want.
OR how ridiculously stupid the idea is.
Thanks and Regards in Advance, Gary
It takes the same amount of energy to push the boat at Hull speed, whether using 300 horse or 75 horse engines. I doubt you could motormuch more economically with smaller engine. 3 gallons an hour is not bad.

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Old 03-16-2019, 08:10 PM   #6
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Fuel costs are the cheapest part of boating. Besides, “you only pay for what you use.” If you want 300+ horsepower, it will take a certain amount of fuel to attain that, no matter the engine. Your hull speed will probably only need 75 hp or so, almost all engines will take the same amount of fuel to make 75hp., not much fuel at all. It will take you thousands of hours of boating to recover what a new engine will cost.
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Old 03-16-2019, 08:17 PM   #7
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I have actual experience of the cost of an engine swap.
It ranges. From insignificant to unaffordable.

Lucky for me, I swapped without needing to buy anything other than the newer engines, so mine was at insignificant cost.

I had a client who had to buy: new exhaust system, new engine mounts, new shaft logs, in new locations, new floor in saloon, new shafts, new propellers, new dash instruments, new wiring harnesses, probably more that I forget. When the bills passed $100k, and this is 15 years ago, he called a halt and ended up in litigation, at more expense.

You shouldn't want to go there.
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Old 03-16-2019, 10:44 PM   #8
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It sounds like economy is a lot higher on your list of preferences than a turn of speed, so why not consider a more economical boat to begin with. Unless the Mainship is you personal dream boat, there are plenty of used single engined vessels out there with similar space and features that include the economy you are looking for.
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Old 03-17-2019, 12:48 AM   #9
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I would not repower it with a small engine. Using the large engine at low power will give you almost as good economy as the small engine will. Besides with the big engine you can go faster if circumstances necessitate. You will never save enough on fuel to pay for the repower unless you happen to own the small engine already and do the install yourself. And you will kill the resale value of the boat. It will be different from every other 350 out there and will take a specific buyer to sell it to.
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Old 03-17-2019, 12:55 AM   #10
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If you reduce the engine weight by a significant amount, you may have to replace that weight with additional ballast to maintain stability, so you may want to run the idea by someone that can speak definitively before you assume you'll get a shallower draft.
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Old 03-17-2019, 12:56 AM   #11
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Quote:
Originally Posted by gfr51 View Post
Problem...300 and some horses apparently eat 3 to 13 gallons per hour.
Only if you use them. I have a 250 hp. Looking at my log I'm at under 1.5 gph over the last 600 hours. That's perhaps extreme, but there are lots of folks here running high hp motors at low throttle and getting acceptable economy. Not optimal but not worth the complexity and cost to downsize for most.

And I gotta say there are times when you might want the power on demand.
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Old 03-17-2019, 01:02 AM   #12
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Good advice from many experienced and knowledgeable folk.
The only thing I'd be wary of is if the existing engine is turbo charged and after cooled, if so you would need to give it a blast at full throttle every once in a while to clean out sooty deposits that build up around the turbo. Then of course you may get a taste for a bit extra power, we all do, its the little boy inside trying to get out of the sensible old guy who's on the wheel.
Seriously if you love the boat then go with it and enjoy your boating life to the full.

For those ever thinking of buying a Perkins 4236 (4 cyl 80hp M90).
A very worthy alternative is the British built JCB 4 cylinder 90hp it's a completely new tier 4 engine which they use in their diggers all over the world so spares are never a problem, fishermen love them for propulsion and being able to drive winches for their nets, they also make a 6cyl version.
Some of it attributes are that it's a very modern industrial engine design,reliable, parts are cheap and it has a gear driven high capacity hydraulic pump, which, if fitting/using hydraulic bow and stern thrusters you can run them all day with no cut-out problems.

I have a Vetus electrical bow thruster and whilst its very good the downside is the overheating cut out, which when you think about it you'd only use in excessively bad conditions, if it cuts out in the middle of tricky conditions its leaves you up the creek without a paddle. I've only ever used mine as an aide but I know some who are completely dependent on them and I could see the potential problems there.
The other problem is the dust from the carbon brushes which makes an awful mess as I discovered when I went to fit new brushes. I got around this by fitting an old sheet loosely (like a tent over the motor) to catch the dust and keep everything around it clean.
A hydraulic system has none of these faults.
Apologies for digressing but food for thought.
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Old 03-17-2019, 04:13 AM   #13
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Quote:
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=
A hydraulic system has none of these faults.
Apologies for digressing but food for thought.
I totally agree with differences between the electric and hydraulic systems. In my case, the American Tug came with the electric bow and stern thrusters. Would it be economical to change to a hydraulic system? I dont see how that could be.
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Old 03-17-2019, 06:40 AM   #14
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And now for the alternate opinion.
I repowered my boat from a 450 HP Cummins to a 135 HP John Deere. Was able to sell the old engine for almost what the new one cost. Used the same transmission and shaft. Replaced the exhaust system, new propeller, and rebuilt the engine bed with new mounts. All in with me doing most of the work, I probably have $10K in it. For my boat there was a substantial improvement in efficiency and it gave me a new engine to do extensive cruising with. Part of my decision was based on $4 per gallon fuel at the time, and a 5,000 to 10,000 engine hour use over my ownership of the boat.

In your situation, if you only plan to cruise locally (<200 engine hours per year),the fuel savings on an engine swap won't come anywhere near paying for itself.

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Old 03-17-2019, 06:45 AM   #15
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If better economy is the basic desire you can modify the installed gas motor for better low hp fuel burn.

A different cam, different intake manifold and good 2 bbl carb and modern ignition would be cheap and easy.

Remember too any repair or rebuild work on the gas engine will be far cheaper than similar work on a diesel.

Spark plugs at $3.00 vs rebuilt injectors at about $75each , a new factory short or long block may cost less than a fuel rebuilt injector pump.

And of course the mechanical noise on board will be far lower than most diesels , so the time underway cruising will be more enjoyable.
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Old 03-17-2019, 07:55 AM   #16
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RE: Engine swap...Let's switch gears

I think I confused the last guy, I apologize.
Apparently the consensus is to not mess with mother nature! Although motor swaps are commonplace in the automotive world and well within my personal realm of abilities.
Two thoughts here...
1)The older diesels are much more simple in operation IE..
Our 1972 220D Mercedes owners manual, for instance, stated that if you cannot find diesel fuel, you can fill up the tank with regular gas and pour a quart of motor oil in the tank with it. Whoulda thunk?

With the newer diesels, even though they are much more efficient, you are at the mercy of all of the Electronic sensors and gizmo's and it is my understanding that if the electrionics fail you may well be dead in the water? Where once the older diesels started they putt along until they ran out of fuel.

2) What about a variable or 2 speed transmission?? I read where caterpillar marine couples some diesels with a variable speed transmission. Yanmar (sp)also?
Any personal experiences with any or all of the above?
Regards in advance, Gary
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Old 03-17-2019, 08:01 AM   #17
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Quote:
Originally Posted by gfr51 View Post
Problem...300 and some horses apparently eat 3 to 13 gallons per hour.
I am thinking maybe an engine swap down to Maybe a 4 cylinder Perkins. Have my eye on a 77hp Perkins MFA4.300 302.2 cu in 4 cylinder tractor motor. Of course it would become all fresh.

As others have said, run slow -- below maximum theoretical hull speed, probably around 6-6.5 kts for a Mainship 350/390? -- and your fuel consumption will be way down at the lower end of that range. And you'll have optional power available if needed.

As others have said, repowering down will cost a lot of $$$ that you won't recoup (and it would probably impact resale value, too). Takes a lot of savings in fuel to cover the cost of a repower like that; maybe more than your lifetime.

As others have said, repowering may well cost $$$$$$$ instead of $$$ if you also have to replace gears, shafts, etc.

IMO, buy that boat, leave it as is and run it according to your fuel preferences... or find a different boat.


Quote:
Originally Posted by gfr51 View Post
1)The older diesels are much more simple in operation IE..
Our 1972 220D Mercedes owners manual, for instance, stated that if you cannot find diesel fuel, you can fill up the tank with regular gas and pour a quart of motor oil in the tank with it. Whoulda thunk?

With the newer diesels, even though they are much more efficient, you are at the mercy of all of the Electronic sensors and gizmo's and it is my understanding that if the electrionics fail you may well be dead in the water? Where once the older diesels started they putt along until they ran out of fuel.
You won't notice much of that. New diesels pretty much work, , too, as is. Certainly something can go wrong, electronics can fail... but they usually don't... and towing insurance can add piece of mind.

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Old 03-17-2019, 08:41 AM   #18
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Some have noted the high cost of repowering and have given some data. Let me provide manufacturer data for fuel consumption for two different engines: the high output 6LY 400 hp Yanmar and the low output 80 hp John Deere NA 4045 engine.


Both produce about 17 hp per gph of diesel at 60 hp output.


How can a 400 hp engine be as efficient as an 80 hp one? Two reasons. The first is that they both make that 60 hp at about the same rpm which means that parasitic rpm related losses will be similar. But the JD has 23% less displacement which should mean a parasitic loss advantage. Well the second reason is that the slight turbo boost of the Yanmar at low rpm compensates for that displacement disadvantage.


The engines discussed above are both modern Tier 3 compliant engines. With older NA engines such as the Continental and Mercedes diesels discussed above, the modern Yanmar will actually have a fuel consumption advantage at similar power outputs.


So don't think you are saving fuel by repowering. You are just spending a lot of money and destroying your boat's resale value.


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Old 03-17-2019, 09:06 AM   #19
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I think you should go for it and keep us informed. You appear to want to spend a bunch of money on whatever boat you get, so why not?
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Old 03-17-2019, 10:20 AM   #20
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In my opinion resale would be extremely difficult with a 77 hp motor.
The very first Mainship 350 singles (as they were originally called) had a low HP Yanmar, I am going to say it was around 175 to 200 hp. Perhaps it was the first one built, don't recall exactly. I talked to the guy waaay back that had one and he hated it because it was way to slow. Couldn't get out of it's own way. Barley made hull speed.
I'm sure Mainship knew it was a mistake, as they quickly started building them with more power.
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