Fuel use - DD vs modern engine

[MikeP]

I know it is "Common Knowledge" that DD two strokes are very inefficient vs modern diesels. But in reading current boat reviews of boats in the same size range as our 53MY, frankly I see virtually no difference in the fuel consumption at equivalent speeds. Yes, the new boats/engines will go much faster than our 53's top speed of approx 18 Knots but throughout the speed range of the 53 - idle of around 5K to WOT 18K, the fuel use seems to be about the same.

Now admittedly, it's not like I read that many reviews but every time I do see one for a boat in the size range, I look at the fuel consumption and end up noting that there is essentially no difference.

The increased efficiency is real in the sense that they get a lot more power out of current engines of similar displacement to the DDs but I don't see any improvement that would be meaningful as far as operational fuel consumption at the speeds within the range of the original boat. Certainly, if one wanted to make a 53 MY (for example) reach 25 knots, there are new diesels that could do it but otherwise, I don't really see much difference when looking at fuel consumption figures for current boats.

Am I missing something or have I just been reading reviews of the wrong boats?

 

[captainwjm]

I've commented on the same thing. For how I would use a new boat, the fuel usage would be no different from my'89 8v92s, at least based on the published fuel use data.

 

[captddis]

Ditto here. I get .5 MPG@20kts. The new boats have a faster top end but are about the same at 20 kts cruise.

 

[Bill Allen]

Mines about the same. But I feel my DD are way less trouble then my other boats newer engines, all the electronics, intercoolers, turbo's and lastly fly by wire. Non of which my old 6-71 N's have and far less of a hassle to keep the running properly. Bill

 

[saltshaker]

You need to compare the same boat with different engines and then you'll see a huge difference. I was looking for a 54C w/ 1350HP 3412 Cats. This boat also came with 1100HP 12V92DDECs. The Cat boat would cruise easily at 2000RPM and 28kts burning about 80gph. Dropping down to 26kts brought the fuel consumption down to 67gph. WOT was 34kts and 125gph. The Detroit boat would run 25kts at 2000RPM and 90gph. WOT was 29kts and 130gph. That's a significant difference. I also looked at a 58ft Chris Craft SF. Boat was repowered with 1015HP Cat C18's and would cruise 26kts at 65gph and run almost 30kts WOT at just over 100gph. Boat originally had 1040HP 12V92TA's Owner claimed she ran about 28kts WOT at over 120gph and would cruise 23kts at 80gph.

 

[captddis]

I don't doubt your numbers, but on a new boat that is used in a boat test that is in my size range the cruise numbers are usually close except in the higher end.
They did a repower on a 48 Viking with Cummins a few years ago. I had just logged 2800 miles on a 48 Viking with 8v92 DDEC. The numbers at 24 kts were very close.

The exception I see is the IPS with Volvo's, they do seem more effecient..... until they need service.

 

[saltshaker]

I don't doubt your numbers, but on a new boat that is used in a boat test that is in my size range the cruise numbers are usually close except in the higher end.
They did a repower on a 48 Viking with Cummins a few years ago. I had just logged 2800 miles on a 48 Viking with 8v92 DDEC. The numbers at 24 kts were very close.

The exception I see is the IPS with Volvo's, they do seem more effecient..... until they need service.

I think a lot has to do with weight too. That 58 Chris lost about 6K-7K lbs from the repower. I guess it really depends on the boat and engine load. I think the Detroits aren't as bad in the lower RPM range but get much worse as the load increases. While this is also true of modern engines, I don't think the inefficiency increases as drastically. If you look at the NMPG published for many newer boats there's not that much of a difference, once they're on plane, as you increase engine speed.

 

[saltshaker]

I can't post a PDF but I have the sea trial data of the 53C Slane repowered with 715HP QSM11's. Two way average was 28.5kts 2550rpm WOT and 67gph. 2010rpm cruise was 23kts and 41gph. Engine load was only 84%-88% max. When the boat was fully loaded, she did perform about the same but the engine load increased to 95% max so there was a slight increase in fuel consumption. I don't know the numbers for a 12V71TI 53C but I'm sure they're no where near these.

 

[krush]

Well, this seems to be somewhat the saga I'm running into in the 40-50ft range. I'd like modern power, but options are limited. The few that do have modern engines (cummins, volvo, etc) don't really seem to have that great of a gain in fuel burn at cruise.

Now, doing slow 8-10kts the modern engine may burn less. Also the modern inline engine will weigh less and be easier to work on and POSSIBLY more reliable and quieter, but I'm not really sure if I should put as much weight on repower as I was before. And then if there is a noticeable price difference for new power vs original engines that still have plenty of life in them, it gets even more blurry.

EDIT: Also it could be poor repowers etc....

 

[saltshaker]

Well, this seems to be somewhat the saga I'm running into in the 40-50ft range. I'd like modern power, but options are limited. The few that do have modern engines (cummins, volvo, etc) don't really seem to have that great of a gain in fuel burn at cruise.

Now, doing slow 8-10kts the modern engine may burn less. Also the modern inline engine will weigh less and be easier to work on and POSSIBLY more reliable and quieter, but I'm not really sure if I should put as much weight on repower as I was before. And then if there is a noticeable price difference for new power vs original engines that still have plenty of life in them, it gets even more blurry.

The value in the repowered boat is more evident if you run at higher speeds. Then you'll see the benefit of increased speed, reduced noise, emissions and longer service life. We all hear how high output 16V92's are 2000hr engines. Another HOFer is looking at big MYs and has seen many with 5-6K hour strong 16V92's. Even the large engine dealer/repair facility in his area told him this is normal when the engines are cared for. Big difference here is how they're run. The MY crowd doesn't run as hard as the SF crowd. So my point is, if you're going to run them slow, most Detroits will do well compared to a similar HP modern diesel.

 

[Fanfare]

Another eye opener is the "mandatory" factory maintenance required to keep your new modern engine manufacturer's warranty. I have heard remarks of a $ 40-50,000 range in annual tuneups and parts replacements, filters, etc. These are on the new common rail engines. Of course it is all hearsay to me. Compared to Detroits, where I can pick my own service tech or just do much maintenance myself, it seems to dwarf the fuel savings.

Just sayin'.

 

[jim rosenthal]

The service costs on common-rail engines are higher, and fewer outfits can service them because I think you need the factory computers and tools. But up to 540 hp, you can get mechanically injected Cummins engines through their recon program. (I don't think any other engine builder has anything similar) And there's something to be said for service access etc.

With all that, if you own a MY and run it at moderate speeds it is very difficult to justify taking out healthy DDs and putting in anything else- at least it's hard to rationally justify it. Taking care of what you have and just putting fuel in the tanks is clearly the best way to go. Now if you own a SF and want to go faster, you'll have to spend the money on either the fuel or the engines, per Jack's posts.

It's a good argument for smaller boats. Although periodically I still lust after a bigger Hatteras convertible....