fuel burn gas vs diesel

Your friend burned that much fuel because he cruised at a speed way higher than his optimal fuel efficient speed. His is about 21 knots and 3300 RPM. That will get him about 0.7 MPG. No way gas burns twice your diesel.

You are completely wrong. His boat will go more than 40 kits at WOT. We've made this same trip for three years. Some times he's gone alone and he has never used less than 320 gal round trip which is 1.6 NMPG
 
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).
 
You are completely wrong. His boat will go more than 40 kits at WOT. We've made this same trip for three years. Some times he's gone alone and he has never used less than 320 gal round trip which is 1.6 NMPG

I don’t know what to say to this other than “no”.

I’m also disappointed in Ron for providing data in a gas vs diesel thread. Isn’t that a ban-worthy offense?!?
 
Haha! In my defense, the word "gas" does appear in the thread title.

Banning is harsh. "30 days in the cooler. DIS....MISSED!", as Klink would say.
 
It’s true that a diesel boat will cost you more to buy than the same make/model with gas engines. But, you’ll also get more for it when it comes time to sell, and probably sell it faster too.
If you can lay out the price difference for the time you own the boat then go for the diesels.
Like you, I had never owned a diesel anything before and was very apprehensive about it. No regrets though now that I have them.
Also true that they cost big bucks if they break, but if you maintain them they don’t break too easy. But big block Mercruisers don’t come too cheap these days either.
Met a guy just yesterday with a 41’ Silverton. He just got it this season. It’s a real beauty, but gas powered. For what he uses it for, basically a floating condo, the gas boat makes sense for him. He works, commutes, and has no time to take boat trips. He put it in the slip in early May, took it out to a cove a short distance away for a few hours yesterday, and then right back to the slip. It was the first time he left the dock this year, and could possibly be the last. But he and his wife have enjoyed every weekend on it at the dock. Gas works well for him.
I take my boat out to either a cove or for overnights at a transient town marina at our local beach at least once or twice a week and take a couple of small trips a year. The days on the hook I like to run the diesel genny for a few hours. Hoping my wife stops working soon so we can do more 3-4 day trips, or even longer if she agrees to it. For the way I use my boat the diesels make a lot more sense.
 
For a reference, you can get a complete big block Mercruiser 8.1 crate motor for under $18k, under warranty. That includes a new FWC system.

Overhauling a comparable diesel will easily exceed that.

While true the fuel economy is better with diesels, depending on how you actually use the boat can make a gas engine an alternative.

I generally prefer diesels, but the majority of my boating is local so when my present boat came along, it was such a good deal it was hard to pass up. And the majority of diesels I had looked at mostly qualified as junk (that would be too kind)..

Ymmv
 
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You are completely wrong. His boat will go more than 40 kits at WOT. We've made this same trip for three years. Some times he's gone alone and he has never used less than 320 gal round trip which is 1.6 NMPG
I believe you made a typo....he'd be burning 1.6gal per mile getting .625miles per gallon.
 
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.
 
Now we know the fuel burn of the two is it worth putting in diesels when your gas engines need replacing?
 
You are completely wrong. His boat will go more than 40 kits at WOT.

i do not have personal experience on this but would be very surprized if a 38 foot yacht with twin 454 can go over 40 KNOTS ( so not statue miles )
 
Advertised speed in 1989 for our boat was 22 knots and that is what it did empty. Loaded with gas, water, 14 batteries, watermaker, inverter and 20 days of food it goes 18 knots. At 18 knots each engine burns 26GPH according to the fuel flow meters
 
Now we know the fuel burn of the two is it worth putting in diesels when your gas engines need replacing?
Advertised speed in 1989 for our boat was 22 knots and that is what it did empty. Loaded with gas, water, 14 batteries, watermaker, inverter and 20 days of food it goes 18 knots. At 18 knots each engine burns 26GPH according to the fuel flow meters
How/where we use our boats has been mentioned but it is really a bigger factor for some than others. If your range at any speed was 2+ times larger how would it affect your boating along Canada's west coast?

Do you have to make a side trip for fuel when you'd have liked not to? Is generator run time rationed? Are extra exploration miles limited by fuel concerns? Would you like to run faster during some portions of you trip but don't because you have to conserve fuel?

I have a hunch moving to a diesel boat would have you smilin' like a chessie cat.
 
I have a hunch moving to a diesel boat would have you smilin' like a chessie cat.

I agree - particularly a displacement diesel boat. Northern, it sounds like you're living the trawler lifestyle already. There are plenty of benefits to making it official.

I get nervous when "dock talk" suggests that lightweight, high-revving, pleasure-duty diesels are an investment to take lightly.

Old-school, low-power, naturally-aspirated diesels on the other hand? That's what people on the dock are actually talking about when they say "like to run", "barely broken in", "will outlast you..."

A friend of mine lives aboard his 44 footer powered by twin 150 Lehman's. That SOB smiles way too much.
 
We have no problem finding fuel. We normally travel at 12 to 1400 RPM. At that speed we burn 1US gallon per knot. The generator uses 3/4 GPH. We tend to run it 2 to 3 hours a day. Boat has fuel flow meters and total use meter. It is within 5% when we fill up. Based on that we can go 250 knots on a fill up. We run 2,000 to 2,500 RPM when we need to. That is about 6 to 8 GPH per engine and speed is 9 knots. We have been to Alaska with the boat and had no problem with fuel availability. Big fuel burn is at WOT and that was 4200 RPM originally and when fully loaded it is 3800RPM. Once or so a year we need to go WOT for an hour or so to get out of currents or bad weather that was not supposed to be there. A trawler would not go fast enough for us when required. Recently we were in 8 knot opposing current smooth water our ground speed was 3 knots. With a trawler we would have been going backwards.
Some years we have run 300 hours so about 2,000 gallons. This year we are at 70 hours and will put on another 70.
 
I certainly don't intend to talk you out of your boat. It sounds like you're happy with her and that's all that matters.

I am concerned about spending an hour at WOT while over-propped though. If you're scared enough of the conditions, that's when you least want to hear a loud BANG, but it's the time it will most likely happen!
 
Rated RPM is 4200 With boat empty we can get to 4200 but boat is never empty. There have been sections where we get current in 3 directions and that is when you need power. Have seen 42 foor sailboat flipped 180 degrees in current
Main question is to go with new gas engines or repower with diesel. Know some one with similar boat and it was 100K to go from gas to diesel and based on boat show talk it is 50K for gas including new generator. Our engines have 3100 hours. Compression is good and constant and oil use is 1/4 quart every 100 hours.
 

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