fuel burn gas vs diesel

Rated WOT is actually measured when loaded, not empty.

If you re-power with high-performance diesels, you could spend $100k or easily more to gain speed, efficiency and reliability. The problem is, you'll have a $60k boat. If you're heavily invested in her already, then maybe that's what you want to do. From a financial standpoint, it's hard to justify.

We have a river here that's been known to push sailboats backwards. Sailboats are different though - full displacement hulls and small, single engines and tiny props.

Semi-displacement (and most actually have planing hulls) trawlers with twin diesels can fight that current. They're also built to handle more severe weather than a SeaRay, so you're less likely to need to outrun something. Just food for thought...
 
Bias, rumor, mis-conception, half-truth and poor math.

It's not just CSR. It happens all over the internet, and on the dock. Makes me grumpy.

In the spirit of first-hand experience and data, I took this screen-shot this afternoon. (please forgive the analog fuel gauges, for whatever reason they won't register on this screen. Strangely enough, they work on the next screens, even though the settings appear to be the same)

y4mQTg7afZtHh0HKXaXBrzh96C3WeIFP7U8KJSSaoZOS-oyaZ63BJkbCc1956-aDWXdePwkexyGJAnnDOkmlAiqXoqC6Vkv3jaohcUMnwDRgybFQDEzvmrfTeeTCSE7XpYp0EUC_7lPNRs80CIWobClP-Hp0FI9G2t1UspdjZcjwsitcQE7GWjTqE_l7fOue_so0mbHV3SS9g3X1EhVZ8bPbg


Not too shabby for a big old (gasp) 40 foot gasser, eh? 177 gallons onboard is about half tanks. No water, not much waste. Light chop, clean bottom. She'll do even better if I pulled back even further and carried less fuel. This was about 3,100 RPM, which isn't an ideal speed. I can go considerably faster without much if any penalty in economy, but at almost 1,400 hours and 22 years old, I like to be kind to the ol' girls.

I don't know why some similar boats perform so poorly. I don't know why owners of those boats don't question it and do something about it. Actually I do know that: dock gossip and bad info from the internet sets their expectations and they just suffer.

Diesels can be awesome, but they're not an automatic choice for every boater at some magic threshold of LOA.

Yes this is typical of the fuel economy for the 400 with 7.4L's. Just try to get the guy with our boat but with diesels to believe it.
 
I am considering being a liveaboard on something like a 400 sedan bridge or possibly 390MY or something similar. Of course a trawler might be an option, but that's another topic.
For those of you that own either of those models, what do you get mpg? Looking for input from both gas and diesel owners to see the real world numbers and difference, thanks in advance.

Sorry I am late to the party, but I actually do own one of those.
During our Loop, I kept very accurate logs of just about everything, and the fuel burn ended up like this. In 7992 statute miles, we burned 5686 gallons of diesel for an average of 1.41 smpg. That includes running at 25mph (not much) a lot of 9-10 mph, sitting waiting for locks and bridges to open, everything....total miles divided by gallons pumped into the tanks. I did not attempt to break out the genset, it only ran about 200 hours during the whole trip and it would be a guess as to how much it used.
At 1100 rpm, my Cummins 6CTA's burn just under 5 gph for both of them combined. The boat moves at 9.8 to 10.2 mph depending on conditions.
At 2300 rpm, it burns 30 gph combined and moves at 25 to 26 mph.
At wot it moves at 31.5 to 32.5 mph. I have no idea what it burns at that speed because I don't run it there for any sustained period of time to actually calculate a burn rate.
I have reduced my prop pitch to achieve 2750 wot.
Cats will give better fuel economy at the expense of less speed. I don't run my boat hard so Cats would work better for me, I don't care about top speed, but that's my opinion and I like my Cummins none the less.
Maybe some of the Cat and gas 400 DB guys will let you know what they are experiencing.
Good luck
 
Repowering an older gas boat with diesels is a personal decision for sure. It can make sense if you gassers are worn out and you are willing to commit to the boat for many years. If you think you will recover the cost upon resale you are fooling yourself.
 
Bias, rumor, mis-conception, half-truth and poor math.

It's not just CSR. It happens all over the internet, and on the dock. Makes me grumpy.

In the spirit of first-hand experience and data, I took this screen-shot this afternoon. (please forgive the analog fuel gauges, for whatever reason they won't register on this screen. Strangely enough, they work on the next screens, even though the settings appear to be the same)

y4mQTg7afZtHh0HKXaXBrzh96C3WeIFP7U8KJSSaoZOS-oyaZ63BJkbCc1956-aDWXdePwkexyGJAnnDOkmlAiqXoqC6Vkv3jaohcUMnwDRgybFQDEzvmrfTeeTCSE7XpYp0EUC_7lPNRs80CIWobClP-Hp0FI9G2t1UspdjZcjwsitcQE7GWjTqE_l7fOue_so0mbHV3SS9g3X1EhVZ8bPbg


Not too shabby for a big old (gasp) 40 foot gasser, eh? 177 gallons onboard is about half tanks. No water, not much waste. Light chop, clean bottom. She'll do even better if I pulled back even further and carried less fuel. This was about 3,100 RPM, which isn't an ideal speed. I can go considerably faster without much if any penalty in economy, but at almost 1,400 hours and 22 years old, I like to be kind to the ol' girls.

I don't know why some similar boats perform so poorly. I don't know why owners of those boats don't question it and do something about it. Actually I do know that: dock gossip and bad info from the internet sets their expectations and they just suffer.

Diesels can be awesome, but they're not an automatic choice for every boater at some magic threshold of LOA.

Thanks for the post, impressive for cruise. What mpg do you get at trawling speed?
 
Thanks to all for the continuing education. This thread has gotten me much less "afraid" to go diesel, but now I have a change in direction that will probably take that option away for the immediate future. The job opportunity was changed to part time which is a no-go for me, so I won't be moving to the coast for now. However this whole process got my wife and I thinking and we are seriously considering becoming liveaboards at an area lake.

I can find no diesel offered anywhere at any lake in our area so for the time being that probably limits us to looking at gassers for the time. I'm still focused on the relative size I mentioned in the first post but have opened up to considering other brands. Tomorrow I'll be looking at a Cruisers Yachts 3650 in Virginia.
 
Well, I’m still interested in keeping this thread going. I, as I’m sure are others, am looking at the “next boat”. Right now I run a 320DA with twin 350MAG MPI’s (5.7L). Perhaps a bit under powered for a fully loaded cruise, but she runs great and I keep her bottom clean.

In looking to go “up” (in every sense of the word) to a sedan bridge I’m considering everything from 36 Sedan bridges to 420’s. So I’m in that grey area we are discussing. Still mainly focusing on the Sea Rays, as I’ve looked at others and most other brands have something that doesn’t thrill me. My big concern is that I could not financially weather a blown Diesel engine. I treat my engines well, take good care of them, don’t run them hard, but still things can happen. And while I could afford a diesel boat, and would love the longer range and economy of diesels, the thought of a major engine problem scares the crap out of me. Would boat insurance cover a blown head gasket, sea water intrusion, or swallowed valve (Assuming I adhered to a regular maintenance schedule, of course) o a 15 year old boat?

My boating is about 50-70 hours, on the Chesapeake Bay. But I’d love to expand that, too.
 
You are worrying about things that don't usually happen. Since 1988, I've had Sea Ray boats kept at a large Sea Ray dealer on the Gulf coast. In that time, I've seen several diesel engines rebuilt or replaced. Only one of those engine failures was due to a mechanical defect, all others were a result of inadequate or incomplete maintenance, usually leading to overheating, or from overloading the engines.

Additionally, while the cost of a new crate diesel engine will break the bank, a typical top end rebuild is more in the $6K-$7500 area (except for Detroit diesel 2 strokes).

Go shopping but pay a lot of attention to the service records and maintenance history and on the sea trial, be sure the engines will reach the rated WOT rpms.
 
You are worrying about things that don't usually happen.

Welcome to a peek inside my head! Lol. When I don’t know a lot about something, I think about all possibilities.

And good perspective, thanks. I’m treading carefully because the “ideal” boat for me (as of today - it may change tomorrow) is a 2004’ish 420DB. Many came with the 480ce engines and were over propped by Sea Ray, resulting in ticking time bombs. So understanding maintenance history, configuration, etc is critical.
 
I too experienced your thoughts before pulling the string. It is kind of like falling in love....if you don’t take the first step, you never will. Have the engine manufacturer do the survey and trust them. The sea trial on ours lasted 2 hours or more. After it was done, I thought if it did not blow up then, it never will as I will never run it like they requested it be run.

We could not be happier!

Bennett
 
Been buying Diesel every day for the last 5 days. Will be buying for the next 6 days every day, We are not quite halfway on our trip. Tommorrow will be close the half way when we get to Charleston SC.

Would not be doing this on a gas powered boat.

Oh, one other tidbit. Pulled into a marina on the St Johns river, close to the inlet so we have minimal time getting out to turn north in the morning.

Just happened to hit the tide at the worst time. Moving in towards the dock, asked the dock master what the current was that I was seeing,. 5.5 knots he said. Yes, the river was rolling.

Docking in 5.5 knots was an interesting experience. Had to back into an inside slip on the backside of the fuel dock,.

The torque of the diesels, the ability to swing big props. Made all the difference in the world. Fuel mileage is not the only consideration,

Mark
 
Pete, I'm confused. 200 mile trip, 320g consumed. 200/320= 0.625 mpg...right (?)

That's more in line with the numbers I used to get with my 1989 340DB with 7.4s, slogging along at a tad under 19mph. Heavy boat with lots of wind resistance (I had floscans on that boat).

Yes! I meant to type 0.6, not 1.6. Lol. Thanks!
 
10 4 - 40 MPH is laughable let alone knots.

hows 38.5Mph against a 3.5mph current.. GPS speed (multiple GPS's) my 390DA with HO's 1/2 tank of gas, 1/4 tank of water and basically empty of everything else (first week I got the boat) (clean bottom, freshly tuned props..) Housatonic River In CT near Sikorsky Bridge.. heading up river against an outgoing tide..
 

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