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Old 25-04-2016, 12:32   #1
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Switching out gas for diesel engines

Hey Guys,
I just bought a 1979 Sea Ray 360 with twin Mercruiser 454 gas engines. The engines are in good working order but use a lot of gas. They are late model engines. My plan is to swap them out with smaller twin diesel engines. I love the boat and has almost everything I need. Speed is not a factor here but faster is always better,but if I can maintain about 10-20 knots I'll be happy especially getting better mileage than the gas burners. Not sure which engines I'll go with but so far the Lehmans' seem the best. I'd appreciate any and all advice and thoughts. I was going for a trawler but just a little out of my price range. lol, Viking was my first choice but haven't won the lottery yet.
Thanks for any and all advice,pro or cons.
JohnyO
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Old 25-04-2016, 14:47   #2
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Switching out gas for diesel engines

Price it, and you will know why it's almost never done.
You will never save enough fuel to pay for the repower unless you use your boat a LOT, almost always it cheaper to sell what you have and buy in your case, that trawler.


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Old 25-04-2016, 15:06   #3
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Re: Switching out gas for diesel engines

I wanted to do the same thing with my 27 Albemarle sportsfish, pull two 350 gas engines and replace with diesels. The $40,000 minimum price tag stopped me cold.
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Old 25-04-2016, 15:30   #4
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Re: Switching out gas for diesel engines

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Originally Posted by a64pilot View Post
Price it, and you will know why it's almost never done.
You will never save enough fuel to pay for the repower unless you use your boat a LOT, almost always it cheaper to sell what you have and buy in your case, that trawler.


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This is good advice.
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Old 25-04-2016, 19:54   #5
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Re: Switching out gas for diesel engines

Thanks Guys, guess I didn't think this completely through.anyway I still got a good boat in good condition.My plan is to do the loop a time or two and if the gas mileage is good enough take her down the gulf coastline towards Panama.
I looked all over youtube and other sites for power boats cruising the Caribbean and the loop but not much out there. On the loop the are a few on youtube but they are mostly trawlers. I'm not bright enough to take the hint.Anyway I still am happy with my boat and I'll decide what to do when it comes time to swap out the worn out engines. Like I said they were put in a few years ago so there's still some life and adventures left in them. Who knows I might go back with the same engines when time rolls around.Stranger things have happened,right?
Anyway just wanted some feed back and I got it. Thanks again.
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Old 26-04-2016, 10:12   #6
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Re: Switching out gas for diesel engines

The rule of thumb is that a diesel engine has about a 20% fuel advantage over FI gas.
Out of that 20% you have to spend much more money on upkeep and when the diesel breaks it will be 3 X the cost of the gas engine to do major repairs, difficult to break even on a repower.
David Pascoe has some good white papers on this subject. Boatdiesel.com is another good source of support.
The plain truth is that power [all] boats are expensive to own and operate.
I keep thinking I could cut some logs in Minnesota and Tom Sawyer a raft to New Orleans for my big boat trip.
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Old 26-04-2016, 10:27   #7
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Re: Switching out gas for diesel engines

Twin 200+ HP diesels are going to be very expensive. You'll never break even. If you want you can run slow like a trawler and reduce fuel usage by a huge amount with your current engines. Even run one at a time.
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Old 26-04-2016, 10:56   #8
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Re: Switching out gas for diesel engines

I wasn't going to switch out the motors with comparable hp, I was going to down size the hp. I know I would have way less speed but thought the mileage would be a lot better going down from 330 hp to 170 or 150 hp each. Like I said I'm not so much interested in the speed as the fuel mileage. And when conditions are right I would run one engine and switch back and forth on a daily basis keeping both motors close to the same in use and hours. I will be moving from location to location after my loops are done and most of the maintenance I will do. I plan to put some solar panels out to ease the use of the gen set and most of my time will be in the water or fishing or just having a lil Cpt. Morgan and coke. I'm not one for getting drunk or toasted so I won't be staggering or going overboard in a stupor. I got my next 10 years planned out minus the changes that no doubt will occur.
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Old 26-04-2016, 11:04   #9
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Re: Switching out gas for diesel engines

There was a guy on this forum last year (?) who put a used Perkins 4-108 in a smaller powerboat. Not sure what his outcome was, but you might try to find that thread and ask him.
IIRC, the hard part is having enough HP to get the boat up on a plane. Not sure, but maybe you can note the RPM required as your boat is now to just get you up off that "shelf" prior to planing. Maybe you could determine the HP needed to do that. But I suspect it's a lot of HP.
BTW, I tested dozens of large HP outboards on new boats. (250 HP x 2) The most efficient use was at high speed. (other than just idling along) Why? Because at 3500-4000 RPM you were moving 35+ miles per hour. Yes you were slurping a lot of gas, but you were going a lot of distance! I suspect just barely on plane will not be fuel efficient.
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Old 26-04-2016, 12:38   #10
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Re: Switching out gas for diesel engines

Quote:
Originally Posted by Johnyo View Post
Hey Guys,
I just bought a 1979 Sea Ray 360 with twin Mercruiser 454 gas engines. The engines are in good working order but use a lot of gas. They are late model engines. My plan is to swap them out with smaller twin diesel engines. I love the boat and has almost everything I need. Speed is not a factor here but faster is always better,but if I can maintain about 10-20 knots I'll be happy especially getting better mileage than the gas burners.
Quote:
Originally Posted by Johnyo View Post
Thanks Guys, guess I didn't think this completely through.anyway I still got a good boat in good condition.My plan is to do the loop a time or two and if the gas mileage is good enough take her down the gulf coastline towards Panama.
I looked all over youtube and other sites for power boats cruising the Caribbean and the loop but not much out there. On the loop the are a few on youtube but they are mostly trawlers.
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I wasn't going to switch out the motors with comparable hp, I was going to down size the hp. I know I would have way less speed but thought the mileage would be a lot better going down from 330 hp to 170 or 150 hp each. Like I said I'm not so much interested in the speed as the fuel mileage. And when conditions are right I would run one engine and switch back and forth on a daily basis keeping both motors close to the same in use and hours.

A dock neighbor replaced worn out gassers with new, slightly smaller diesels. Included genset replacement. Cost $100K or so. Boat was initially worth maybe $50K. A few years later, he sold the boat for approx. $50K.

Going fast on one engine isn't all that great, mostly due to the steering issues with a small rudder and small prop. We troll on one engine often, but even then steering against a high wind doesn't always work.

You CAN save fuel money: drive it like a trawler (slow) when conditions and sea states permit. That hull form won't work perfectly that way all the time, but when it does, enjoy the scenery.

-Chris
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Old 26-04-2016, 13:22   #11
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Re: Switching out gas for diesel engines

I have to agree with everyone else on this. UNLESS you are okay with operating in pure displacement mode only, in which case much smaller and economical diesels are available. A pair of under $10k Betas might push you along fairly well but not a chance you will ever get up out of the hole, of course.
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Old 26-04-2016, 13:40   #12
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Re: Switching out gas for diesel engines

If speed isn't a priority then sell what you have and buy either a true trawler or power cat. You are paying a massive penalty in hull shape, comfort, and space for those large engines and if you are doing the loop a couple of times slowely there is no justification for it.
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Old 26-04-2016, 19:03   #13
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Re: Switching out gas for diesel engines

The cost of smaller diesels is not the only thing that doesn't make sense here. V shaped hulls are designed to go fast, not slow. There's no keel so at slow speeds they walk all over the place. At 10 knots your making a huge wake which means wasting power & the bow is climbing. You have to give it a lot of power to get it up on plane & I doubt you can get much below 25 before it starts dropping off plane. So repowering with smaller motors could make it hard to get up & stay on plane. However, repowering with diesels of similar hp would give you better fuel consumption although, as, previously noted, it would cost a fortune. If you want to run in the teens you need a hull designed to do that. That means a Downeaster / Lobster Boat type hull. Fine entry, full keel, soft chines & a flat stern. Not as fast as your boat but more economical to run & more seaworthy. Or you could go with what we've got, a displacement hull. We burn about half a gallon an hour at 7 knots but can't get past 8.
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Old 27-04-2016, 05:31   #14
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Re: Switching out gas for diesel engines

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There's no keel so at slow speeds they walk all over the place.

At 10 knots your making a huge wake which means wasting power & the bow is climbing.


Or you could go with what we've got, a displacement hull. We burn about half a gallon an hour at 7 knots but can't get past 8.

It's not all that dire. In protected waters and comfortable sea states (much of The Loop, as I understand it), his hull will do fine at slow speeds.

His theoretical maximum "hull speed" is around 8 knots. Running the boat at 6-7 knots is cheap, no wake, no wasted power... and I'd guess he'd be burning only about 2 gallons/hour total (both engines), maybe less.

-Chris
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Old 27-04-2016, 07:11   #15
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Re: Switching out gas for diesel engines

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It's not all that dire. In protected waters and comfortable sea states (much of The Loop, as I understand it), his hull will do fine at slow speeds.

His theoretical maximum "hull speed" is around 8 knots. Running the boat at 6-7 knots is cheap, no wake, no wasted power... and I'd guess he'd be burning only about 2 gallons/hour total (both engines), maybe less.

-Chris
That's amazing! Who'd a thought a Searay with 2 gas engines would burn less fuel than a typical trawler with a single diesel at displacement hull speeds? I'm gonna have to rethink this displacement hull thing.
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