Repropping to lower top speed??

Hi,

I have what is probably an unusual question regarding repropping.

For background I have a 70' Neptunus motor yacht powered by 2 Cat 3412E's @ 1350 hp each. This is not an express boat but a skylounge motor yacht with a semi planning hull. Current power/prop configuration has it run at 7 kts at idle and up to 29 kts WOT. I don't have the fuel consumption numbers handy but from memory she burns about 20 gph at 9 kts and about 200 gph at WOT.

I mostly run her at slow speed for fuel efficiency and comfort, and never run her at high speed because it's ridiculous to suck that much fuel. I'm never in that much of a hurry. BTW, I shouldn't say "never" as I know it's bad for the engines to run slow all the time, so I do run her up a couple times a day to clean out any carbon build up.

It's my understanding that diesels run most efficiently at a specific loaded rpm. Being turbocharged engines, I would think that's with the turbos spooled up. My turbos come in 1400-1500 RPMs, and I normally run slower than that running 12 - 14 kts or so. Curious if anyone on this forum has thoughts on trying to reprop to get the slow speed RPM's up to have slow speed running in a better operating speed range.

I'm not trying to turn the vessel into a trawler but just make her a little more efficient at the expense of a high-speed level I don't need. Appreciate any thoughts/opinions.

Thanks

Matt

Thanks for all the replies. Just to clarify my thought was not to try to get the efficient engine speed to 14 kts, because as someone stated that's not an efficient hull speed, but to get 10-11 kts more efficient for the engines by upping the RPM's. I totally agree "if it aint broke don't fix it" but is running at close to idle for long periods (days/weeks) even with occasional run ups good for the engines.

I dug up my initial test numbers and at 10.3 kts I'm at 24% load and 18 gph. At 29.5 kts I'm at 84% load and 145 gph (my initial 200 gph was a bad memory lol). I have numbers in-between and am happy to share if interested.

I do have spare props so could play with different configs if I get that far. Anyway, all this is more a thought experiment so again I appreciate the thoughts added to the pot

P.S. I just ran a couple fuel consumption calcs. These numbers wont surprise anyone on here as your all experienced but just for fun, reference using a hypothetical 500-mile run.

@10.3 kts, 18 gph, running time 48 hours. Total fuel used 864 gal
@21 kts, 71 gph, running time 24 hours, Total fuel used 1704 gal
@24 kts, 90 gph, running time 21 hours. Total fuel used 1890 gal

P.S.S. Okay I had to plug in idle speed just for fun
@8 kts, 7 gph, running time 71 hours, Total fuel used 500 gal

Just jumped back on after being offline a few days to see a few more comments. Again, thank you for all the comments, very informative to get other people's opinions and experiences.

Capt J I've owned and operated yachts for more than 30 years up and down the west coast, Mexico and the east coast. So yes, I know what it costs to buy, maintain, fuel and yes even reprop a yacht. Why you need to toss in condescending comments about if you can't afford fuel don't buy a yacht or buy an RV is beyond me. This was a simple question looking at ideas, options and opinions, not snide comments. So, no I don't plan on buying an RV because in your opinion looking at ways to save on fuel is somehow beneath that of a yacht owner.

I do appreciate your comments, but I did say "from memory 200 GPH" and that was based on me thinking 95 gph per engine WOT, okay, I was wrong, its 72 gph/engine. I then quickly corrected it when I checked my charting so you got me, my memory wasn't so good. Somehow that justifies negative judgements on my experience to the level of those previous comments? And even if I was a newbie and asking a completely stupid question, I'd still expect cordial explanations, like you and most people offered up. Separately why does 12-14 kts indicate some lack of experience? Although not as efficient as 8kts, it's below the curve for significant fuel flow (planning speed) so I gain 50% more speed over most efficient at a nominal additional cost in fuel consumption. Another reason for that speed is since acquiring this boat a lot of my running has been in populated areas with some open stretches followed by slow speed or no wake zones then back to open. So, to run up to 20 kts then drop to 8 then back up to 20 doesn't feel efficient. 14 to 8 then back to 14 feels right and is more comfortable for the passengers. So, I find that speed a balance. If there is something I'm missing, please let me know.

Back to my original question and the topic of this thread. My initial thought on this idea came after realizing that trawler yachts run 9 kts all day very efficiently and they do it at around 1800 rpm which has the turbo spooled up. No this isn't a trawler and before someone says "well you should have bought a trawler" the thought was simply, is it possible, and if so, does it make sense to get the engines into their more efficient operating rpm at a slower vessel running speed. If the ultimate answer is no, fine, that's why I'm here collecting opinions and tapping other people's experience. BTW I have spare props so the thought was if it makes any sense to play around with pitching I would do it with those and see how she runs. This also would allow me to put her back in normal running condition should I ever sell her.

I'm not saying I'm an expert captain and know everything there is to know about yachts but somehow for 30 years I've been able to maintain (mostly myself), upgrade and run my yachts all over the place...and amazingly I was able to afford it lol

Thanks again all!!

Looks like it's time to wrap up this thread. But again, I'll thank everyone for their opinions and information. Pascal thanks for the detailed response. Other than the 14 kt item the run pattern you describe is almost exactly what I do now in open water conditions. I run at 10 kts with a run up to 20 or 22 mid-day and again at the end of the day. Therefore, using your example if I was running nonstop to the Exumas I would be running at 10, with 4 run ups.

Not sure where some of the discussions went off the rails or where have I argued with any opinions. All I've done is present an idea and facts as I know them. In other words, I HAVE NO OPINION TO ARGUE, just an idea. The 12-14 kt thing seemed to generate some back and forth but is really irrelevant to my initial question. For reference at 14 kts I'm burning 36 gph and yes I know it's less efficient then at 10 kts but more efficient than on plane. But again, who cares, that wasn't the question I started this thread with.

So, some variety of opinions, mostly to just run the way I'm doing to possibly doing some tweaking. Being an engineer, I love to experiment, so we'll see where this goes. Maybe I'll try something, maybe not, but either way I appreciate all the responses.

Cheers,

Matt

I think you've done the math correctly and I respect Pascals numbers (I'm assuming from the Johnson he previously ran) but my boat burns slightly different. My math has @14 kts burning 36 gph making it .38 NMPG. For 10.3 kts @ 18 gph I get .57 NMPG. So definitely worse but that's a choice. Worst speed is between 14 and 16. Next time I run her I'll get the numbers for 15, 17 & 19 just to round out the table.

Time for my second cup