Where is Gasolene power today?

[FAST FRED]

As A rule of thumb I figure we get 20 hp from every gallon of fuel run thru a Modern diesel engine.

I used to figure 10hp out of a gallon of gasoline , but with todays fancy engines what would be a good number for say a 100 or 250 hp engine?

As most outboards are small car engines stuck on a drive , do they run the same power / gallon as gas inboards?

Am looking for the lowest cost way to produce power , but when the price of a new marine diesel is $25,000 for a VW , and $3500 for a auto conversion , I'm concerned with the diesel payback .

200 hours a year & maybe 10 years of use.

FAST FRED

[SteamFreak]

pick a used one from a junk yard where you find old work trucks... strip it and convert there... cost you ALOT less.

[ted655]

Seems to me a fellow can be penny wise & pound foolish just thinking about mpg & motor cost. It's not that simple. required size in proportion to performance, Purpose of boat, area of use, distance/rande covered, isolation/dependability of operation, expected weather/water conditions, safety/insurance rate, repair/maintaince costs, speed/torque range. hours of constant operation, life span, cost of controls & auxillary equipment. I'm sure I've left plenty out. A used 1970-1980 40hp Johnson/Evenrude is the cheapest most common type motor. So it isn't as fuel thrifty as a new 4 stroke or FI 2 stroke! If you have to pay an extra 8K for them, that money will buy one heck of a lot of gas for the older model. So... another factor in cost is the "pay back" time of the newer technoligys. The break even point is usually MANY years for the recreational user.

[Willallison]

Fred - I've always gone by the 20hp/gal/hr rule too. Though these days the modern diesel can do a fair bit better than that. For petrol, I generally would look at about 15hp/gal/hr. But again, modern petrol engines can often do a bit better than that.
And of course, then come all the variables - like load - that can swing the numbers heavily one way or the other.
Further, there are other considerations too. Some people have an aversion to the flammablilty issues of petrol. If range is a primary consideration, then diesel starts to make sense as your tankage will (generally) be less. Availability of the two fuels in the places you are likely to travel. Etc, etc
At the end of the day though, the reality is that if you have to pay as much as double for a diesel engine, then unless you are a commercial operator, or a VERY prolific recreational cruiser, you are very unlikely to recoup the cost differential

[SteamFreak]

Ted has a point there... its really something that should come down to a question of honestly assessing how long you'll have the vessel and only if its a real long time (in terms of operating hours) should you look into a new, highly efficient engine specificaly suited to the vessel being used in whatever manner you desire... otherwise.. the el cheapo engine that eats a gallon more per 2 hours of operation may be far more cost effective if you only ever use the thing for 4000 hours.
(4000hrs)*(1gal/2hrs)*($2.50/gal)=$5000 so if you save $8k by purchasing the el cheapo, even with additional gas cost your end savings is $3k)

[Willallison]

hmmm - but there's also the small issues of piece of mind and reliability to consider too...

[bilgeboy]

Well, I've owned a boat with twin diesels for 5 years (1987 vintage). Never had a problem starting the engines, never quit unexpectedly, never idled down to zero. I had a problem turning them off at the slip...but that was an electrical problem traced back to user error. I love the things. They have blown smoke, and I have spent some money on them. They don't scare me and we have a nice relationship. I change the replaceables on schedule, and the twins love me back. I approach the key with the intention to start, and they roar to life. I don't think the starter motor even gets involved.

I have owned an Evinrude 225 (1989) for a month, now. Compression and spark check out. I rebuilt all 6 carbs, new plugs, new impeller kit, fresh gas...wont start. I'm going to kill the starter trying to get her going. Then she starts with a cloud of smoke to fill Wrigley Field, idles and revs nicely, and just as I leave the trailer, she quits...won't restart. I am told I just need to get a "feel" for the 2-stroke, and all will be well. I "feel" like shooting it.

I want to love her, but she won't let me. Not the first time!

I am a diesel fan all the way. I think conversion is a great way to go...if you have a clue. I asked the guys at Cummins Northeast just a few days ago what the difference was between the highway engines and marine engines, and the answer was "nothing". He was at the parts counter, but I figure he'd been around. The marine diesels are overpriced, but the market supports it. You can't blame the manufacturer for getting what he is able to. They should be more affordable.

Gas outboards are nice an light, and I couldn't imagine a dink with a diesel on her. I am holding out some hope for the 'rude.

Mike

[FAST FRED]

Im a diesel fan too , but our current 3000 lb 6-71 would be really out of place in a boat of 8-10,000lbs.

With light weight diesels the RPM seems to be the same as a gasoline at cruise ( guess I will have to get used to 3500rpm rather than 1500 at cruise).

If a modern gas job can get 15 or so HP from a gal of fuel , I am guessing the diesel efficency is simply the denser fuel and higer compression ratios.

If correct a turboed gas engine would come closest to diesel efficency.

I am familiar with marine conversions of truck diesels , which for my purposes would be easy. I prefer dry stack exhaust and keel cooling .

The gasoline option looks best for a fast boat , especially at $20,000 cheaper!!

Any sugestions as to WHICH gas package should be given consideration?

And to expand the thread , propulsion can be a std prop and shaft , which I "Rule of Thumb" at 25 lbs thrust per hp , after the 3rd or 4th prop attempt.

The out drives with dual counter rotating props claim a 30% improvement in fuel use at cruise. Is this just advertising dept BS?

Or do they actually get more thrust from a gallon of fuel?

FAST FRED

[wave1235]

Love the four stroke outboards for quiet, mileage, maneuverability, and relative freedom from the threat of explosion. One tenth of a gallon per horsepower per hour is still pretty true for mine. Diesels will use about a third of this rate. The greater energy in diesel and the higher thermal efficiency of the compression combustion allow this. The ideal for me would be a four stroke outboard with connections for keel cooling. This would provide the benefits of the outboard with greater longivity and the ability to provide heat and defrost while underway.

[Richard Hillsid]

Originally Posted by Willallison

hmmm - but there's also the small issues of piece of mind and reliability to consider too...

Yes i agree.

[hartley]

I think 20hp per u.s gallon is a bit optimistic even on a late model diesel ,two things need to be considered namely engine rpm and engine loading,the following figures are from a respected local publication.
Volvo Penta D2-55 a 55 bhp engine at 3000 rpm....at 2600 rpm with the prop absorbing 34 hp the consumption is 8 litres per hour ...using 4.5 litres per imperial gallon that works out about slightly less than 19 hp per imperial gallon ,hoowever when converted to U.S gallons ,using 3.8 litres to the gallon it works out to 16 hp per U.S gallon.At the other end of the scale a Cat C9 turbo and common rail injection ...very up to date ....at wot producing 500 hp its consumption is 103 litres per hour ,which is 23 imperial gallons per hour which works out to 21.7 hp per imperial gallon ,converted to U.S gallons it works out to 18.5 hp per gallon ,pretty close to Fred's estimation ...however it is necessary to add 10% to these figures on a planing hull ,at different rpm to account for increased engine loading ....I'm tired now ....cheers

[FAST FRED]

The diesel engine Mfg seem willing to give fuel consumption info , if only a full throttle and "ideal" prop curve.

They guard the "fuel map" or BMEP from public view , although its the only tool usefull to setting up a cruising , non full throttle vessel.

The commercials are happy to plan on 80% load and 90% rated rpm , and go out working for thousands of hours.
But thats not what a cruising boat usually does.
I agree sizing the engine like this is the "most efficent" but most rec. boaters want a bit of extra power for really adverse conditions, so they don't slow as much.

Has anyone found a good power/gph graph for common gas engines?

FAST FRED