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Old 01-29-2016, 08:59 PM   #21
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One more thing, sorry about being obnoxious, but go out on somebodies else's boat and see what you think (or charter one). Experiencing something can assist in decision making. If you do not like it and it is not what you want opinions are not worth squat.
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Old 01-30-2016, 12:17 AM   #22
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My present car is a slightly larger turbo diesel version of its gas NA predecessor, engine is 200cc bigger. The diesel version is much more tractable, more torque, unfazed by hills,uses much less fuel. Boats is boats, cars is cars, but it tells me something.
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Old 01-30-2016, 01:51 AM   #23
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Gasoline engine is relatively inexpensive to initially purchase (4 to 5X less than a diesel engine), and generally simpler to maintain during its useful life, and equally less expensive to replace after some 3000 to 5000 (generally no more) hours of carefully maintained and generally easy use. At equal HP output... Gas engine will consume a bit more fuel per hour than applicably sized diesel engine.

Diesel engine is relatively expensive to initially purchase (4 to 5X more than a gasoline engine), and generally more difficult to maintain during its useful life, and equally more expensive to replace after some 5000 to 15000 (or more - depending on engine type) hours of carefully maintained and generally easy use. At equal HP output...Diesel engine will consume a bit less fuel per hour than applicably sized gas engine.

Therefore... for your choice of engine type: You can take the statements above (YRMV - compared to my experience, with gasoline and diesel) as well as the hundreds of other variables in a particular boat's (and boat owners's) "needs" for one or the other type engine to make your decision.

Happy Boat-Power Daze! - Art

PS: As mentioned several times before in this thread - I too believe that at the point of some 20K +/- lbs dry and 40' +/- feet long gasoline engines should probably be supplanted with diesel engines. Much past that weight and size boat will almost surely function better with diesels. Our 34' Tollycraft tri cabin at 20K lbs loaded and with twin 255 hp Mercruisers functions perfectly well to fulfill our desires.
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Old 01-30-2016, 09:42 AM   #24
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I'd suggest the advantages of diesel are not limited to larger boats.

Our boat has been built with 315hp 454 gas, and with 260hp Volvo diesel. It gets about 40% more miles per gallon with diesel. A buddy with a 2859 Bayliner repowered from 454 gas to Yanmar 6LP diesel, and saw about the same advantage in fuel economy and thus range.

Even though we are only 26 feet, with limited fuel capacity (ours is 110 gal) the efficiency of diesel vs gas can be a huge advantage. Cruising the distances of the Inside Passage, diesel gives us far greater range on a tank, and thus much more flexibility in the routes we choose to wander. For us, the $7,500 higher initial cost for the diesel has been well worth it.
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Old 01-30-2016, 09:48 AM   #25
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Comparing an efficient diesel to an inefficient gas and running the boat differently isn't necessarily a fair comparison.


If you compare state of the art and power the boats so the performance is exactly the same and run exactly the same....then it is more fair.


For the OP, best to stick with generic concepts until he has a specific boat or powerplant in mind.
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Old 01-30-2016, 09:57 AM   #26
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torque turns the prop. At any speed the torque is continuously needed to spin the prop. Torque does the work HP is simply the measure of how much work is done over time.
Greater torque means moving more water is possible.
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Old 01-30-2016, 10:05 AM   #27
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Quote:
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Comparing an efficient diesel to an inefficient gas and running the boat differently isn't necessarily a fair comparison.


If you compare state of the art and power the boats so the performance is exactly the same and run exactly the same....then it is more fair.
My comparisons were based on running much the same way. At planing speeds of 18-20 knots.

Very much the same 40% advantage in fuel economy - in fact somewhat more - when running at 6-7 knots.
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Old 01-30-2016, 10:33 AM   #28
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torque turns the prop. At any speed the torque is continuously needed to spin the prop. Torque does the work HP is simply the measure of how much work is done over time.
Greater torque means moving more water is possible.

bayview that's a statement. I've heard it many times. I don't believe it. Torque is force like voltage. There can be lots of torque w/o any movement of the shaft but no movement will happen w/o work being done. If your boat is moving work is being done and that's hp .. not torque. Torque is a static value. Torque increases the shaft speed or accellerates a car or starts a boat moving but hp is what poweres the boat.

An old wives tale said that long stroke engines had more torque. Don't belive that either. Buick went from very long stroke to short in 1953. All the short stroke Buick engines made more torque than the old long strokers that preceeded them. The same is probably true for Chevy's. I've always said it SEEMED the short strokers didn't have much torque because the torque was overshadowed by all the increased power .. hp.

But like you I do believe torque has something more to do w power than a static force. I think I used to know but have long since forgotten. I'm hoping someone can relight my extinguished flame.
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Old 01-30-2016, 10:36 AM   #29
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Quote:
Originally Posted by psneeld View Post
Comparing an efficient diesel to an inefficient gas and running the boat differently isn't necessarily a fair comparison.


If you compare state of the art and power the boats so the performance is exactly the same and run exactly the same....then it is more fair.


For the OP, best to stick with generic concepts until he has a specific boat or powerplant in mind.
Agreed - especially for larger boats; e.g. those over 32' and/or over 20K lbs loaded. Of course hull shape plays heavily into efficiency levels too. Each boat/engine combo always has its own tricky nuances.
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Old 01-30-2016, 10:48 AM   #30
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Boattest.com recently did a review (well actually more of a features recital) of Mercury's new 3.0 liter 260 hp marine diesel engine which was derived from Volkswagen's TDI automotive diesel.


They "compared" it to a Mercury 6.2 liter 320 hp gasser which delivers roughly the same performance as the diesel. What was interesting was the pricing quoted:


The MSRP for the Mercury Diesel 3.0L V-6 260 is $33,533.
The MSRP for a Mercury 6.2L 320 ECT TowSports gas inboard is $20,550.


At least for these pairs of diesel/gassers the price is not 4-5 times higher for the diesel. OTOH, I don't think that the Mercury diesel will last the 5-15,000 hours quoted above. I think that if used the same way, both the Mercury diesel and the equivalent gasser will probably last roughly the same length of time. I did note that the EPIC 23V that boattest used to compare the two engines was propped very lightly for the diesel, which probably helps with longevity.


I also take issue with the "a bit more fuel for the gasser" statement above. It is a lot more fuel. The Mercury diesel produces 20 hp per gph of fuel at wot. The equivalent gasser produces about 12 hp per gph. At about 90 hp where boattest reported the "best cruise" for that engine, the diesel burns 4.3 gph, the gasser would burn 7.5 gph, 74% more.


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Old 01-30-2016, 10:52 AM   #31
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My comparisons were based on running much the same way. At planing speeds of 18-20 knots.

Very much the same 40% advantage in fuel economy - in fact somewhat more - when running at 6-7 knots.
Seems you surely made correct choice for boat to engine match-up! Congrats on your improved efficiencies.

Mid 1960's Dad and I replaced 155 HP Nordberg Knight big straight six with a Perkins straight six. Results were very pleasing on a 37' 20K lb + SD hulled sport fisher boat. Hard to have accurately compared everything as the Nordberg basically was worn out. Suffice it to say that the single Perkins had our boat cruise 12 to 13 knots with not too much fuel use (I believe we were getting nearly 3 nmpg - memory on stats vague after 50+ yrs.).

Soooo... Yes for sure... for certain budgets, in certain boats, used for certain reasons, under certain conditions, with certain power of engine chosen, to travel certain speeds, turning certain prop-size/blade-count diesel engine is the smart way to go! And for same reasons... sometimes gasoline engine is the smart way to go. I like both type engines - my choice per boat is greatly dependent on conditions present.
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Old 01-30-2016, 11:05 AM   #32
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I am just trying to understand what 'efficiently' mean.
Not trying to compare gas vs disel.
suppose i have two identical engines, both gas. Put them on two boats. Same gear etc, everything the same except one boat weights 10k and the other 5k lbs. Run both boats at 3000rpm for 500 hours. Would the two engines have the same wear and tear? Would they consume same amount of gas? If not, why?
chicagoq,
It's hard for diesel orented guys not to talk about diesels.

The difference is that the boat weighing twice as much will burm twice as much fuel. For a FD hull it's almost totally as simple as that. For SP hulls it gets a little more complicated and for planing hulls even more complicated.

As for the wear on the engines the one pushing the boat weighing twice as much will have more wear. Maybe only a tiny bit more depending on operating temps and many other variables. But if the harder working engine was below it's continous power rating and the lower loaded engine was not suffering from the effects of light loading (and gas engines seldom have a problem w that) the engine wear should be nearly the same ........ I M O.
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Old 01-30-2016, 11:11 AM   #33
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bayview that's a statement. I've heard it many times. I don't believe it. Torque is force like voltage. There can be lots of torque w/o any movement of the shaft but no movement will happen w/o work being done. If your boat is moving work is being done and that's hp .. not torque. Torque is a static value. Torque increases the shaft speed or accellerates a car or starts a boat moving but hp is what poweres the boat.

An old wives tale said that long stroke engines had more torque. Don't belive that either. Buick went from very long stroke to short in 1953. All the short stroke Buick engines made more torque than the old long strokers that preceeded them. The same is probably true for Chevy's. I've always said it SEEMED the short strokers didn't have much torque because the torque was overshadowed by all the increased power .. hp.

But like you I do believe torque has something more to do w power than a static force. I think I used to know but have long since forgotten. I'm hoping someone can relight my extinguished flame.

IMO -Torque is the movement initiation force available and applied. Horsepower is the working coefficient that keeps the movement ongoing once a prescribed level of movement has been attained... via applied torque.

Side note: Torque adds to and builds momentum to reach a set energy level desired. HP then works to uphold the inertia for continuing said energy level. In other words, regarding boats; torque is for acceleration to desired speed and an amount of engine's HP then comes into play for maintaining the speed reached. That said - Torque is created by HP, HP is not created by torque.

I'm sure someone has time to delve deeply into definitions of Torque and HP as well as their interactions of togetherness regarding making props spin through water. Unfortunately, right now. I do not.
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Old 01-30-2016, 11:21 AM   #34
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Originally Posted by djmarchand View Post
Boattest.com recently did a review (well actually more of a features recital) of Mercury's new 3.0 liter 260 hp marine diesel engine which was derived from Volkswagen's TDI automotive diesel.


They "compared" it to a Mercury 6.2 liter 320 hp gasser which delivers roughly the same performance as the diesel. What was interesting was the pricing quoted:


The MSRP for the Mercury Diesel 3.0L V-6 260 is $33,533.
The MSRP for a Mercury 6.2L 320 ECT TowSports gas inboard is $20,550.


At least for these pairs of diesel/gassers the price is not 4-5 times higher for the diesel. OTOH, I don't think that the Mercury diesel will last the 5-15,000 hours quoted above. I think that if used the same way, both the Mercury diesel and the equivalent gasser will probably last roughly the same length of time. I did note that the EPIC 23V that boattest used to compare the two engines was propped very lightly for the diesel, which probably helps with longevity.


I also take issue with the "a bit more fuel for the gasser" statement above. It is a lot more fuel. The Mercury diesel produces 20 hp per gph of fuel at wot. The equivalent gasser produces about 12 hp per gph. At about 90 hp where boattest reported the "best cruise" for that engine, the diesel burns 4.3 gph, the gasser would burn 7.5 gph, 74% more.


David
Interesting report. Wonder if Boattest worked as an unbiased outside lab, in other words - any bias applied regarding gas or diesel? And, ouch! The fuel use stats sure do not resemble previous years' reports I've read... but... there are some really great new-age design improvements on diesels being accomplished. On gas engines advancements are also in process.
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Old 01-30-2016, 11:30 AM   #35
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You can have torque without rotation...

Your 2:1 transmission will double the engine's torque minus a bit for gear loss.

If you look up your engine spec sheet, the max performance will be about where peak torque output is. You can add more air and fuel and get more HP and a bit more speed, but lose in efficiency. This is assuming that it is properly propped, BTW.

Short stroke vs long stroke.
It depends on the RPM operating range of your engine. If the bore is wider than the stroke, that's a short stroke, high RPM engine.

longer stroke than bore, and that's a lower RPM engine.

The energy to reverse the piston in the cylinder is lost, so doing it less often is usually more efficient. If you lighten the piston crown / apron, and have everything as light as possible you can compete on a short stroke engine. But with everything cut to the bone, you lose the sledgehammer durability. That's why the older engines maxed out at 1800 RPM. Later it went to 2300 and then to 2800 as Turbos came about. Even though the HP output went up, they had to work hard to move the torque curve past about 1600 RPM.

Then the short stroke diesels came around with 3400-4800 RPM bands, usually with turbos and sometimes after-coolers. All the Yanmar engines are short stroke high output engines.
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Old 01-30-2016, 11:33 AM   #36
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My comparisons were based on running much the same way. At planing speeds of 18-20 knots.

Very much the same 40% advantage in fuel economy - in fact somewhat more - when running at 6-7 knots.
But I still suggest that the engine efficiency may not be fair.

My example. My 1986 37 Silverton convertible came with 3208s custom from the factory. I never heard the stats on any other (but there might have been) but gassers with carbed 454s.

My boat got 1NMPG at 20 knots while the gassers got 1/2NMPG. The diesel had a lower top end of about 3 knots...but who cared.

Yet when these same boats were repowered with the newer fuel injected Mag 5.7s, their mileage got within a few tenths of a NMPG that the old diesel was. Had I repowered with newer diesels...the spread may have increased again.

One of the few boat models that I have direct knowledge of with similar sized powerplants of gas/diesel.

Of course the diesel Silverton was something like 3000 pounds heavier and sat 4 inches or more deeper in the water...but boat for boat...that's my experience with boats/motors as close as you can get.
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Old 01-30-2016, 12:01 PM   #37
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Good question - were the engines and boats comparable vintage/technology/design?

My Volvo is a 24-valve electronically controlled 260hp 1997 KAD44P. Boat is a 25'7" moderately deep-V (19 degrees at the transom), 11,000 lb (loaded).

My Bayliner 2859 buddy's 454 gasser was a fuel-injected 2001. He repowered it with a relatively simple (no electronics) 300-315hp 2002 or 2003 Yanmar 6LP. His boat 28' 9", a few degrees less deep-V, 13,000 lb loaded.

Engines very comparable vintage.
Both boats moderate-speed planing hulls.
We cruised the Inside Passage together, at the same speeds.

As for the comparison of gas vs diesel Bounty 257's - those numbers came from the naval architect who owned Bounty and built my boat.
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Old 01-30-2016, 12:21 PM   #38
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Fair enough ...but goes to show you that not all boats and engines pan out to simple concepts.


If 40% gains could always be achieved by diesel ..I would think it would be a much harder sell by the boating industry....even 3rd party "experts"....
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Old 01-30-2016, 12:25 PM   #39
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Manyboats:
torque is not just static it is continuously available at different levels over all RPMs. Torque is what does the work HP measures that work. Without torque at every RPM the engine would stop.


Torque and HP are interrelated but HP does not exist without torque while torque may exist without HP.
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Old 01-30-2016, 12:43 PM   #40
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Let me thow some data into the fire:

A 50 year old NA DD 6-71 makes about 18 hp per gph of fuel. A similar era carbureted gasser makes about 10 hp per gph.

A modern common rail, Tier 3 diesel makes about 20 hp per gph. A modern high compression, EFI gasser makes about 13 hp per gph.

So both have made significant improvements over the years in fuel economy, but the relative advantage of the diesel has stayed about the same.

Automotive engines should continue to improve with things like variable valve timing, cylinder cut out at low loads, highly turbocharged low displacement engines, etc. But these mostly affect automotive use.

Diesels may also improve, but I think that the improvements will be minor and in the case of automotive diesels they may get worse due to NOx emissions controls, a la VW.

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