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Old 18-10-2013, 00:46   #1
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Newbie needs help

I'm planning to purchase a 36-40' boat to live on in one of the marina's in Southern Calif.. I'm new to boating and need help with some basic information. I plan to go on couple excursions per month. One of the local islands is about 165 mile round trip and the other is approx. 50 miles. I know that gas engines are far less expensive and will run far less hours between rebuilds than diesels but what is the number of hours you can run the a gas engine with "average " maintenance? Also, what is the "average" for diesels?

Is there a rule of thumb for the average fuel consumption per mile for gas vs. diesels?

Any help would be appreciated.
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Old 18-10-2013, 04:15   #2
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Re: Newbie needs help

forget about gas inboards,get a diesel.
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Old 18-10-2013, 04:52   #3
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Re: Newbie needs help

I ran a sports cruiser with a Volvo 275hp V8 inboard for a season, engine based on one of your GM small block engines I believe. The fuel costs were horendous and the fuel meter would record 90 lph (24 US gals) at speed.

We sold it thankfully just before the credit crunch and bought a yacht. There might be a case if you were just living on board and not planning on going anywhere other than a local beach / anchorage once in a while, but a 165m trip will cost a packet and you may not have the tank range to get their and back with petrols, never mind the problems of petrol vapour and electrics in a saltwater environment.

Atoll's advice as usual is spot on but just to add I might have to buy some diesel for the yacht next year and I only topped up the tank in Aug 2012.

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Old 18-10-2013, 05:20   #4
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Re: Newbie needs help

Greetings and welcome aboard the CF, mikekomm.

Diesel fuel has a higher energy density than gasoline.
One gallon of diesel contains approximately 147,000 BTUs of energy, while a gallon of gasoline only has 125,000 BTUs.
This means it takes more gasoline to equal the power output of diesel, making diesel engines more efficient per gallon of fuel burned.
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Old 18-10-2013, 05:21   #5
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Re: Newbie needs help

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Originally Posted by mikekomm View Post
I know that gas engines are far less expensive and will run far less hours between rebuilds than diesels but what is the number of hours you can run the a gas engine with "average " maintenance? Also, what is the "average" for diesels?

Is there a rule of thumb for the average fuel consumption per mile for gas vs. diesels?

No great rules of thumb, Mike, but roughly you could postulate maybe 1000-1500 hours for a very well maintained gas engine, and maybe 5-10,000+ plus for a diesel. When the gas engine reaches that, it's often a throw-away, while the diesel simply gets a major overhaul. Very, very rough estimate, of course... and at the low end, a poorly maintained gas engine can go south at 5-700 hours.

Expect something like .5 NMPG for gas, .75 NMPG for a planing diesel, and up to 2-3 NMPG for diesel in a displacement hull.

YMMV, all over the map, but that could at least let you decide what ballpark you want to shop in.

Easier to get empirical info once you home in on a specific engine candidate.

If marinas out there are similar to some other places around the country, you might look into liveaboard slip availability first...

-Chris
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Old 18-10-2013, 05:54   #6
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Re: Newbie needs help

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No great rules of thumb, Mike, but roughly you could postulate maybe 1000-1500 hours for a very well maintained gas engine, and maybe 5-10,000+ plus for a diesel. When the gas engine reaches that, it's often a throw-away, while the diesel simply gets a major overhaul. Very, very rough estimate, of course... and at the low end, a poorly maintained gas engine can go south at 5-700 hours.

Expect something like .5 NMPG for gas, .75 NMPG for a planing diesel, and up to 2-3 NMPG for diesel in a displacement hull.

YMMV, all over the map, but that could at least let you decide what ballpark you want to shop in.

Easier to get empirical info once you home in on a specific engine candidate.

If marinas out there are similar to some other places around the country, you might look into liveaboard slip availability first...

-Chris
Whew! Damn you get horrible mileage on your diesel! I run a 40 foot 10 ton Jeanneau sailboat. with a 55HP 4 cylinder Yanmar. I get right around 2 liters per hour running at 6 knots (assuming not too heavy seas). I have a 130 liter tank and only fill it once a year. And then it takes about 30-40 liters (unless I've done some really heavy motoring). I realize you guys are talking motorboats - but sheeeeeit, that's bad
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Old 18-10-2013, 09:19   #7
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Re: Newbie needs help

Over the years been able to get between 3 to 5,000 hours for gas and a diesel 6 to 10,000 hours or about double. If planning on being a live aboard you DO NOT want gas as diesel is a lot more forgiving. Another concern is the new Ethanol gas does not have a long storage life, which is starting to have an effect on tanks and engines. However, the new low sulfur diesel also has a short live. However, poor maintence will kill all engine early!


How much fuel you burn is directly effect by your speed. Most sail boats and long range boat/trawler/ships go max hull speed. (Square root of the length on the water X 1.3) the longer the boat the higher the hull speed. It takes a lot of power to get over its bow wake, and keep it up on plane. The Eagle is a full displacement, 51 ft on the water and 45 tons and at hull speed, 7 to 10 knots, gets 2 to 3 miles per gallon or 4 to 6 gallon per hour. If you had a power boat that could get over its bow wake ot wold be a lot more. So keep the speed down!
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Old 18-10-2013, 09:45   #8
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Re: Newbie needs help

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Whew! Damn you get horrible mileage on your diesel! I run a 40 foot 10 ton Jeanneau sailboat. with a 55HP 4 cylinder Yanmar. I get right around 2 liters per hour running at 6 knots (assuming not too heavy seas). I have a 130 liter tank and only fill it once a year. And then it takes about 30-40 liters (unless I've done some really heavy motoring). I realize you guys are talking motorboats - but sheeeeeit, that's bad

Just one of the compromises about boats. I can actually get places today don't have to trip all over shrouds and stays and sheets and so forth, and have two decks with picture windows all round

-Chris
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