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Old 04-06-2015, 09:29 AM   #1
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Power down-grade

On a 6cyl diesel, how difficult would it be to down grade the power, Similar to what Cadillac did in the 80's and currently doing again with their V8-6-4?

I don't need a 120hp, so can I shut down 2 cylinders, return the fuel and open or remove the intake valves to reduce the compression on those 2 cylinders? If so, which cylinders?
I'm looking for fuel economy and only need 80Hp. I've thought about exchanging the engine with someone or company, however the 6cyl is to heavy for me to do by myself, so that's out of the question.

I don't have the funds to re-power.

Any suggestion?
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Old 04-06-2015, 09:36 AM   #2
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just pull back on the throttle.. Keep the bow all the way down and stern wake to a minimum.
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Old 04-06-2015, 09:38 AM   #3
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That would be a LOT of work for almost no gain. A 6 cyl diesel producing 80 hp uses a tiny fraction more fuel than a 4 cyl producing 80 hp. I wont speculate on how a modified 6 cyl would run. If it doesnt work, now you need another engine. Why not reduce fuel use by going slower? That is the much proven way.
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Old 04-06-2015, 09:51 AM   #4
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Thanks for the advise. I plan on cruising around 6 knots; 32 'trawler w/full displacement hull. Is that going to give me the 1-1.5 gph I'm looking for?
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Old 04-06-2015, 09:51 AM   #5
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I agree with all of the above. Unlike gassers, diesels are reasonably efficient at light load. You can generally produce 15 hp per gph of diesel at 1/4 load vs 18 hp per gph at 3/4 load.

What is the waterline length, weight and desired cruising speed of your boat. That will determine what pct of the 120 hp you will use and how much fuel you will burn.

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Old 04-06-2015, 10:02 AM   #6
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You answered most of my questions as I was typing my previous post.

Assuming 15,000 loaded weight and a lwl of 30' I plugged these values into the boatdiesel power required calculator and got 18 hp required to go 6 kts and 30 hp to hit a 7.3 kt displacement speed.

So you are way down on the power curve- about 15% of 120 hp. That might go as low as 12 hp per gph depending on the engine which is 1.5 gph to go 6 kts so it barely meets your criteria.

120 hp in a 32' displacement hull is way too much, I agree. Repowering with a modern 50 hp engine like the Yanmar 4JH might improve that by 1/4-1/3 gph. You will have to cruise for more than 10,000 hours to pay out the cost though.

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Old 04-06-2015, 11:06 AM   #7
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I.ve thought of putting to sleep some cylinders to downsize a FL120. Never thought anybidy else would think of that. I'm the one that usually comes up w those far out ideas. Sometimes I get so far out of the box I can't find the box.

The issue as I see it is that you only need 35 to 45hp depending on weight and hull design. If your boat's hull looks much like a sailboat you could do w less than 35hp. Check out some sailboats the same size and displacement. If you're boat looks more like a fishing boat or harbor tug but is definitely FD go w 45hp.

With the crankshaft journals arranged for a 4 stroke 6 cyl configuration I'm thinking taking two cylinders out will not leave you w a balanced engine. You'd be better off pulling the throttle back and perhaps going to smaller injectors and running a hotter thermostat.

My idea was to take out 3 cylinders. Leave in the valves but take out the pistons. The remaining 3 cylinders will no longer need to move those two pistons back and forth. The firing order will be evenly spaced and the remaining three cylinders will be under a normal load. Basically you'd have a 3 cyl engine of about 50 - 55hp. It would be a lot of work but almost all re-powers are also a lot of work. But it would cost you almost no money. You should burn 1.25 to 1.5 GPH. Your 3 cylinder engine will have a very heavy flywheel and crankshaft. Could be quite smooth.

You will be taking a chance as strange things may happen as you'd be upsetting many apple carts. A hot cylinder next to a much cooler one may over time blow a head gasket. Certainly not likely but many things will be mismatched like volume of coolant and oil .. size of water pump ect.

But it would be an interesting experiment and a great conversation source. A smaller engine is the obvious choice but costs money and re-locating exhaust pipes and engine mounts is a lot of work too. How much time have you got on this overpowered boat and could you live w it as is. Spend your time doing other things.

Lastly are you the chap in the Gulf Islands w a steel double end lifeboat that posted about 4 years ago?

Oh and as to what cylinders to de-activate? On a normal inline 6cyl engine I'd retain #2, #4 and #6 to keep the crankshaft forces close to the flywheel. But you're working w an engine that has known overheating problems w #6 cylinder. So you may want to retain cylinders #1, 3 and 6. I have no idea what cylinders to keep for converting to a 4 cyl. I suspect the crankshaft journal crank pin locations will not allow it to be done and retain even a remotely balanced engine.
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Old 04-06-2015, 11:19 AM   #8
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Our FL 135 SP will burn 1.7 gal @ 1600 RPM and produce 6.5 knots. The 4 cylinder FL has a larger flywheel then the 6 cylinder in an attempt to make it run smooth. I agree, slow down to save on fuel.
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Old 04-06-2015, 11:39 AM   #9
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You are right that Cadillac's previous attempt at dropping cylinders for fuel economy was not successful. Balance factors aside, improper oiling, upsetting the air & exhaust flow, and carbon were some of the problems encountered. I recall many years ago, a shop I worked in, rebuilt a 6 cylinder diesel compressor on which 3 of the cylinders formed the compressor & 3 the engine. It was a straight 6 and it ran very smoothly. Unlike the Cadillac, the manifolds were divided and the intake and exhaust were not disrupted like Cadillac and the cylinders were always compressing so there were no oiling problems.

For automotive use, this seems like a bad approach. I would think a small, strong engine, just large enough to make highway speed on the level, then reinforced by super and/or turbocharging for those short term jackrabbit starts and passing situations is a more cost effective approach.

Beebe's book showed a proposed 2 engine, 1 shaft, boat in which a single engine would be used, running at max SF consumption while long range cruising. The second engine would be engaged (via cog belts) for local travel above hull speed. So 4 of the 8 cylinders available were completely removed, friction and all.
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Old 04-06-2015, 12:45 PM   #10
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I think you'd throw it out of balance enough to make it and the driveline fail in relatively short order.
It takes x amount of fuel to make 80 hp whether you have 4 , 6 or 10 cylinders. I don't think you'd be able to quantify the savings.
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Old 04-06-2015, 12:51 PM   #11
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We just purchased an Acura with a V6 engine that automatically drops to 3 cylinders for efficiency gains. Absolutely seamless...and I've purposely tried to detect the transition. Forget the days of the Cadillac fiasco.
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Old 04-06-2015, 01:11 PM   #12
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Yep....more Auto maker are reducing cylinders a highway speeds, including Cadillac; though I hope it's better then the one in the 80's..... what a nightmare that was....lol
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Old 04-06-2015, 01:19 PM   #13
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I know when I was driving big-rig trucks and idling at truck stops, my truck would burn nearly 1 gph at idle. I didn't realize the Ford Lehman would burn only 1.5 gph at 1800 rpm's.
In that case, I will keep the 120HP engine. It wouldn't be cost effective to replace it.....And see why people should join & post to TF

Thanks all for the input!
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Old 04-06-2015, 03:36 PM   #14
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bankerboy1,
The engine speed is only part of the fuel burning element. The other is how much load is applied. 1800rpm in neutral an FL won't burn much fuel. Only enough to move all the parts and replace the heat lost. Full load at 1800rpm and you'll burn about 5.5gph. One guy in a boat burning 2gph at 1800rpm is running at a light load whereas one burning 4gph is probably over propped and under heavy load. So when someone says he's burning X GPH at 1800rpm it dosn't mean much unless you know his load. If he says his WOT rpm is 2500 on an FL (rated rpm) then you have a good idea about his load ... especially if you are running the same engine w the same WOT rpm. But "out of the blue" if someone says they burn 2.2 gph at 1800rpm it means almost nothing.
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Old 04-10-2015, 07:39 AM   #15
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Sounds like you are the perfect candidate for a cruse prop.

While it may not save much fuel, 10% or 15% of 1 or 2 GPH is hard to quantify , but the engine life and especially the noise on board while cruising will be way down.

If you can find a HP/RPM graph for the engine look for what RPM 18HP and 30 HP are about 75-80% of the load at that RPM. Look for the RPM where 40 HP can be taken from the Mfg data.

That is your target RPM for LRC , long range cruise..18Hp (25 from the Mfg) will be at a lower RPM , but it would give no reserve for climbing waves or 40K of breeze.

Besides a new prop you will need an EGT gauge (about $100) and a bit of nail polish on the tach to give a NEW red line .

Most engines are car or truck marinizations so are quite smooth at 1500 or so, start looking about there.
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Old 04-10-2015, 11:40 AM   #16
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Interesting thread! What ever one does to reduce power and save fuel (except slowing down ) costs something. Those costs have to be recouped by the savings, might take quite a while to do that.




This Season however we all got a break in per hour operating costs, due to lower fuel prices.. Lets take advantage of it, and get out and enjoy the waterways.


Cheers JEP




















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Cheers JEP
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