Switching out gas for diesel engines

[rwidman]

Buying a boat is a lot like choosing a wife. It's much better to get the one you want in the first place than to get one and then try change her to fit your needs.

[neptunesjester]

I did a gas to diesel conversion on an old Chris Craft. I pulled the 427s and installed to Lehman 120's The boat has been in my family for generations and there is sentimental attachment. I can verify from experience the cost of the diesel refit is high.

Things to consider
1) Engine bed reengineering
2) Fuel system reengineering for return lines
3) Cleaning/replacing your current fuel tanks - diesels do not react well to too much gas in the mixture.
4) Changing props for new hp/gear ratio

Those are just a few of the challenges.

On the flip side, the lehmans have been very reliable engines. I push the boat at hull speed (7.5kts) very economically (2.8 gph total fuel burn)

I have found the decreased need for maintenance more than offsets the slightly higher cost of parts - Lehmans are tractor motors with a very large parts supply that keeps costs down. If you go with non-turbo models I would say the maribe parts price is on par for marine gas engine parts. I rebuilt both the diesels prior to installation so I have some hard data in this regard.

The cost of running the boat has dropped significantly so once you handle the upfront costs, it is cheaper to operate.

If you like the boat and dont plan to sell it any time soon, go for it. I don't regret doing it.


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[LooseGoose]

i Have a 67 Chris 40 aft cabin equipped with twin 350 CID 260 HP engines which I run at 7-8 mph......I keep track of distances via gps, and I get 1.4 mpg at that speed....pretty good for a larger aft-cabin machine.

Get some data from actual use for awhile before performing your calculations....you might be very ok with wearing out current motors before changing to alternative power......BTW a "Crate" brand new GM Goodwrench engine is under $3 grand.....so very hard to justify diesels.

Check out re-built Yanmars on ebay

[captjcook]

Just for fun,

There is a 29' sportfish nearby (looks similar to Bertram) that could be had cheap, one blown, one tired 350's. Nice boat but it needs paint, which I know how...

I can buy used 1994 (non turbo, non computer controlled) 7.3 International diesels (F250/350 engines) on Craigslist etc for $600. Sleeved so a good start. I have a shop that will do total rebuild, including marinization for $2500 each. The right Borg/Warner velvet drives will handle the 200hp...I have not gotten as far as fit on the trannies, something would work. As a budget number call it $4000 each. $8000 for two diesels, 400+ hp. I have swapped multiple marine engines, rebuilt engine logs...so I have a good idea of what is involved. Another $2000 should cover diy.

Nice boat for me...yup, I know what I have! A Frankenboat to sell? More than likely!

Just don't see any return unless I keep it. Naw...not for me...




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[ranger58sb]

Originally Posted by Scout 30

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.

Almost anything running at idle or just above doesn't burn much fuel. The key is slow, nothing much to do with the hull form.

Anyway, it was just a guess. Another generalization: If a single-screw something or other, diesel, gas, whatever... burns 2 GPH at 1x(SQRT)LWL, a twin screw version of the same boat probably uses around 4GPH at the same speed. (That would be if all other things are equal, which is almost never the case.)

4GPH at 6 knots -- any boat, any hull form -- is cheap, comparatively speaking.

But then a planing hull might not be comfortable in all sea states at that speed. Of course, some displacement hulls aren't comfortable in some sea states, either.

Still, an already-owned 36' express (I think) cruiser can be relatively inexpensive to run, as is... slowly... compared to the cost of selling/buying/etc.

That's not a solution for everyone, of course.

Originally Posted by rwidman

Buying a boat is a lot like choosing a wife. It's much better to get the one you want in the first place than to get one and then try change her to fit your needs.

Yeah, but once you've already walked down that aisle...




-Chris

[Scout 30]

[QUOTE=ranger42c;2108202]Almost anything running at idle or just above doesn't burn much fuel. The key is slow, nothing much to do with the hull form.

Anyway, it was just a guess. Another generalization: If a single-screw something or other, diesel, gas, whatever... burns 2 GPH at 1x(SQRT)LWL, a twin screw version of the same boat probably uses around 4GPH at the same speed. (That would be if all other things are equal, which is almost never the case.)

4GPH at 6 knots -- any boat, any hull form -- is cheap, comparatively speaking.





If you're idling at 6 knots docking must be pretty interesting. Hopefully idle speed is more like half that. However, I do agree, if you run a Sea Ray at 6 knots instead of 36 knots you'll burn less fuel. The best thing to do is hook up flow meters & figure out your mpg at different speeds.

I don't agree that hull form doesn't matter at slow speeds. If that was true sailboats would look like Sea Rays & would be able to motor at 35 knots. The truth is that with no rocker you're dragging water behind you which means you're using more energy to move your boat which means you're using more fuel. On top of that, with no keel a Sea Ray will not track straight at slow speeds. Doing the loop at idle speed in a Sea Ray would literally be water torture.

[ranger58sb]

Originally Posted by Scout 30

If you're idling at 6 knots docking must be pretty interesting. Hopefully idle speed is more like half that. However, I do agree, if you run a Sea Ray at 6 knots instead of 36 knots you'll burn less fuel. The best thing to do is hook up flow meters & figure out your mpg at different speeds.

I don't agree that hull form doesn't matter at slow speeds. If that was true sailboats would look like Sea Rays & would be able to motor at 35 knots. The truth is that with no rocker you're dragging water behind you which means you're using more energy to move your boat which means you're using more fuel. On top of that, with no keel a Sea Ray will not track straight at slow speeds. Doing the loop at idle speed in a Sea Ray would literally be water torture.

Didn't say hull form doesn't matter, just that it's a minor detail in the grand scheme of fuel consumption... if you already own a boat with whatever hull form you bought at the time. Using more energy? Maybe, but that's sometimes cheaper and often a lot easier than buying a different boat.

Sure a Sea ray will track straight at 6-7 knots. If sea states permit, and often on inland routes (like much of the Great Loop), they do. Easy. Even easier, set the autopilot.

When we troll, and when weather cooperates (mostly wind, since we have a permanent un-reef-able "sail" so to speak) we can track straight at 2.0-2.5 knots even on only one engine. Hours at a time. Easy.

-Chris

[a64pilot]

I've done the fuel flow meters on dual 454's on a planing Sportfishing boat.
I was certain most efficient speed would be just where she broke over on plane. Wasn't even close. I graphed it per single Kt of speed, by a big margin, most efficient was a dead idle, and I think we idled with both engines at about 5 kts, anyway right at the point where you began to leave a wake, fuel mileage dropped and continued to drop for every Kt of speed, the slope got real steep just before plane, and wasn't so steep on plane, but it did continue to climb.
Boat was very stable and had good steerage etc at 6 to 8 kts and that is what we ran at night on the way to the middle grounds, my Brother had seen a telephone pole floating in the Gulf one day, and that did it for him, no fast running at night, he was sure we would find that telephone pole.

Same exact thing on my last boat, a small center console outboard it read mpg directly when I interfaced the outboard with what was called a Merc monitor and it to the Garmin 740S.

But the boat ran slow just fine, no problems at all.


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[petter5]

Originally Posted by oleman

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.

Mostly, you will break al lot of gas engines before you will have to do a major repair of a proper diesel engine. Also fuel consumption is in most configuration far less withg diesel engines, and thereby increased range with the same amount of fuel. Back in the 90's I had a 22' Nimbus with Volvo-Penta AQ 311 (chevy V-8 350) gasoline engine that I at that time used extensive in my business. After buying 3 new V-8 engines in 5 years for this boat due to total engine break down (hard use) and I received a another new one on warranty when the 4'th engine broke down, I sold the replacement engine without packing it out of the box and at that time bought a new Mercruiser D254 with Bravo I drive. This engine still runs fine today after about 6000 running hours, only normal service and a replacement of the Bravo drive some years ago.

Top speed is the same as the previous AQ 311, a little bit more time to reach plane but the engine has always been dependable as a Swiss chronometer and the fuel consumption in liter is far better than the 20% improvement oleman refers to.

Also, if you have decent boat and want to go for a long trip, it's about weight. Normally most people thinks about Diesels as heavyer than gasoline engines and that is true for the engine itself. But if you fill up the fuel you will need for let's say a 200 nm trip, due to much larger consumption, the Gasoline boat will have a much larger weight at the start of the trip than the diesel one that will require significantly less fuel for travelling the same distance - or can do a much longer trip on the same amount of fuel.
For me, the only advantage of the V-8's is cheap price in initial purchase but much more expensive in use if you include repairs, hazzle and fuel consumption, and I wouldn’t even dream of going back to the old style V-8 petrol engine. Yes, for a small 18' Sea ray or a big powerboat just for fun it is the right thing since you always will be close to your local gas station and repair shop,,, :-)

[lonesoldier0408]

So, smaller diesel generator sets connected to electric drive units.
Trim adjustment via electric drive rpm. Just a thought.
Tied to a jet drive, you could plumb bow thruster abilities.

Did anyone else see a unicorn?


Still surrounded by anchors.

[valhalla360]

If you aren't truly committed to this boat, consider something like the old Mainship trawlers.

Most were single diesel. I haven't priced them lately but 8-10yrs ago a 79-80 would run $25-30k which is less than your repower is going to cost.

[GrowleyMonster]

Originally Posted by lonesoldier0408

So, smaller diesel generator sets connected to electric drive units.
Trim adjustment via electric drive rpm. Just a thought.
Tied to a jet drive, you could plumb bow thruster abilities.

Did anyone else see a unicorn?


Still surrounded by anchors.

This would NOT be more efficient. In fact, unless it was very carefully engineered, it could be significantly less efficient. It does offer a lot in terms of operational flexibility, though. For instance, no minimum idle speed. Ability to go short distances on battery only. Supplementation with solar power. Easy changeout of the genset when necessary, and freedom to mount it wherever it is convenient. Instant power and instant reversing with no warmup. Lower initial cost than a new diesel prime mover, or a pair of them, for sure. Incidental presence of a very large battery bank that would facilitate genset downtime at anchor for maintenance. Only one genset needed, and two relatively inexpensive motors and controllers, instead of two diesels, though two gensets would be very nice to have.

Bow thruster? Sure. Why not? But if you set up your props to turn outboard instead of inboard, a bow thruster is absolutely not needed. I ran crew boats for a while and they do not have bow thrusters and we could crab sideways onto of off from a dock with surprising expediency by putting the rudders hard over and reversing the screw on the side toward which the boat needs to walk, and coming ahead on the other. Most multiscrew motor yachts are set up with both screws turning inboard, instead. This allows excellent and very intuitive pivoting but makes crabbing sideways (or walking as it is called in the oil patch) difficult or impossible, and so in that case a bow thruster is a really nice thing to have. with this setup, you come ahead on the screw on the side toward which the stern of the boat must go, and astern on the other, swinging the stern in the proper direction while also using the bow thruster to push the bow in the same direction. So yeah, bow thruster. Unless you set your port wheel to be a right hand screw and your starboard to be a lefthander. In this case if you know how to drive a boat, a bow thruster will actually be seldom used. With electric drive, there is no reversing gear and the motor is quite happy to turn in either direction unless there are problems with the cooling fan only working in one direction. So right hand / left hand is not a difficult thing to change to however you like it. Just reverse the polarity of the fwd/rev control and swap the props. Easy sneezy. Retrofitting a bow thruster involves too much hull cutting for my comfort. Better to just buld some sort of up/down sliding mount for an electric outboard at the bow, if you just got to have bow thrusting available. Or be a real man and do without.