Fuel consumption

[jcatkeson]

My 1983 Compac 23 was bought complete with a four stroke Honda 8 hp (which is way too much power). I have an opportunity to change to an electric start 2 stroke Mercury of 7.5 hp (probably still too much power). I would prefer the Honda or better yet a smaller motor but the electric start is a plus for Nanci. Electric starts don't seem to come in smaller sizes. As there are times when getting home is looming and the wind is not cooperating, my question concerns fuel consumption. Any data on this?

[Salty19]

I don't have any specific data to share, but generally speaking a 2 stroke motor will consume to the order or 50-100% more fuel than a 4 stroke.  It depends on the prop, rpms and what motor you're using.

I actually see this as a good thing for sailboats since gas tends to not get used much.  With ethanol in fuel nowadays, you MUST use fresh fuel or you'll have carb trouble in short order.  Having a higher consumption motor will encourage using fresh fuel.  Unless you are going to motor a lot (ie a long channel or just to make time), or go on long cruises, the higher consumption isn't going to be that big of a deal.  If you're motoring 5 hours a week or more or plan to go a long ways between marinas, it probably makes sense to use a 4 stroke as you'll use the gas up before it goes bad. Otherwise I like the 2 stroke.

The mercury is really a Tohatsu (japanese=quality) branded as a Mercury. Nothing wrong with Honda...I won't use them myself as I've had a lot of trouble with their lawnmowes and generators.  However a nice Honda VFR interceptor graces the garage and it's abuot as perfect as a bike can get.

Also 2 strokes weigh a whole lot less and require less maintenance at the expense of having to mix oil in the gas.

This is just my opinion, others will probably disagree with me.

[BobK]

I have read the fuel consumption varies as little as 10% to as high as 25%.  I use the 8hp Honda on my 23.  I prefer it because of noise and no smell.  I don't believe the fuel savings would buy a case of beer a season since it is the auxillary power.
Bob K

[Craig Weis]

If I could, I'd drive a Mazda Rotary. So smooth you don't even feel it running, but boy do they blubber out oil and fuel, Even have a rotary R/C rotary engine...way cool I can hold it in my hand and run it. And there is a diesel conversion for normal 2 stroke R/C engines too.

"10-25%?  It has to be more than that?"
I'm not sure what that means?

"the fuel passages in the carburetors are larger in two stroke engines"
Why? Every time the piston comes up it's fueled and driven back down but the air/fuel mix ratio remains the same 14.7:1. Every pulse into the combustion chamber of X amount of cc's is the same. Regardless if it's a two stroke or a four stroke. Why would the fuel passages be larger, but rather 'worked' more times per minute. "Instead of every fourth time the piston comes up, every time".

"a 2 stroke motor will consume to the order or 50-100% more fuel than a 4 stroke"
True but it's making power every stroke. So why wouldn't it burn more fuel per equal running time? I fly R/C aeroplanes. Both Rossi 2 stroke and OS Max four stroke 0.40 cubic inch engines. Checked on the ground with my electronic tach...
I can turn a smaller prop and less pitch at 21,000 rpm on the ground. More rpm in the air once the prop unloads in the air on the two stroke.
I can turn a larger prop and more pitch at 10,500 rpm on the ground. More rpm in the air once the prop unloads in the air on the four stroke.

The same plane and four stroke 0.40 cu inch engine is so much more fuel efficient [22 minutes vs 9 minutes] but it is so much slower. I mean if I was just flying point to point my distance covered in nine minutes would be so much more then the distance covered in 22 minutes. It's a wash as far as energy used, X amount of distance vs X amount of fuel used is about equal. But as always resistance through air is squared as speed doubles. I wounder what is is for water?

I'm thinking when I'm really booking 14 miles across the Bay of Green Bay to the otherside my 5hp 2 cycle Mercury full open at about about 5200 rpm uses about 1/2 gallon [50:1 fuel/oil] an hour at a 5.4 knots, S.O.G. via the chartplotter. I burn about 7 to 9 gallons a season. My tank is 3 gallons.

I wonder if anyone uses a diesel O/B ? Air or water cooled.

skip.

[Salty19]

10-25%?  It has to be more than that?

Two strokes intake and combust fuel at twice the interval of 4 strokes.  Also there is blowby; where as the fuel is being drawn into the combustion chamber through the intake, some unburnt fuel is expelled out the exhaust at the same time. Just the way they are designed. Less efficient.

Typically..and one would have to check this, the fuel passages in the carburators are larger in two stroke engines.  This is to allow for sufficient oil/fuel mix to lubricate the engine as the primary goal with the secondary goal being to keep up with the increased demand for fuel (twice as much combustion).

With that said, my numbers were estimates and could be way off.

The reason question is how far are you planning to go between marinas?

Skip's figures match up closely with a 4hp 2 stroke Johnson I used to have.  When I switched to a 3.5 4 stroke, economy went way up...about 1 quart for every hour of use at 1/2 throttle.  That's about a month and half of use for me, thus my fixation on fresh fuel.  Burned about 2.5 gallons all season-the lawnmower would get the old gas.

[kickingbug1]

  unless your local regulations have prohibited two strokes, thats the way to go for three reasons. simplicity-two strokes have no valves and the associated hardware required. reliability-as long as the fuel is pretty fresh they will run forever, and they weigh a hell of a lot less which is important on any sailboat. plus,they actually produce more power as every stroke is a power stoke and the associated mechanical rolling resitance i.e. cams etc is absent.

[TedW]

I agree with kickingbug on the reliability.  My Nissan 8 has been incredibly reliable for the past 7 seasons.
It has never failed to start and pushes the boat well in most conditions. (Lake Huron).  The prop sometimes comes out of the water in big waves.  For winterizing I drain the oil and store it in a warm basement.  Very simple.

[Bob23]

   I also have a 1988 Nissan 8 hp 2 stroke on my 1985 23/2. It starts every time on the first pull, even when I brought it out of winter storage. I'll never get rid of that motor. I might use 10 gallons a season. That's not too bad.
   Skip: I'd heard that Yanmar was going to produce a diesel outboard but I doubt it would be small enough for the hp that we commonly use.
   Salty: Trouble with Hondas? Strange. I have a generator and a  powerwasher. Both perform flawlessly. Even my 1984 Accord Hatchback runs great. For fun, I borrow my brother in laws 2001 S-2000. That helps cure my need for speed. Love the 9000 rpm redline. Too bad Honda stoped importing them.
   Returning to the issue, like most of you, I like the lighter weight of the 2 stroke feels better than a heavy 4 stroke back there on the transom. My mantra is "If it works, don't fix it". I'll be a Nissan 2 stroker for a long time, I hope.
Bob23
   

[Bob Condon]

I have had a Tohatsu for 3 years and it has been not been very good. I had the discussion on the CP19 site,
but now that the carberator is tunes and clean, and I only use 93 octane fueld with Marine Stabil,
I hope things will be better . I am also excited to hear that the engine is very reliable because
when it is not reliable, it is consistently not reliable!

I also run my fuel out whenever I am done with the boat whether a day or a week to the next use..

===
I have a yanmar 12 HP in my Cape Dory and I can not beleive that they can build
a "lightweight" diesel. Yanmar makes a great diesel !

[Salty19]

Yep, Bob--I've had no luck with Honda small engines.  I know this is unusual!   Will never buy one again! My generator uses oil and refuses to start sometimes (I've rebuilt the carb twice).  The lawnmower is also difficult to start and has no power.  Actually I gave it away to a neighbor in favor of a Toro which has been flawless.

With that said they make great cars and bikes! Have had many Honda bikes and they all have been real good.  Considering a Civic Si but probably will stick with the fire-breathing MR2 turbo for awhile.  If you come out, I'll have to take you for a ride in it (much quicker than an S2000).

Back to outboards, I just picked up a used, but never started Yamaha 8hp 2 stroke/2 cylinder.  Should run a heck of a lot smoother than the 6hp 4 stroke.  I'm not real impressed with the 4 stroke Nissans (Tohatsu). Hard to start and seems like the power of a 3-4hp motor...very sluggish and vibrates like crazy past 1/2 throttle.  I'll probably hang onto it for a backup.

[Bob Condon]

I also have a 1994 S2000 in mint condition with 9000 miles. (yes... 9K miless)
Never been in the rain or snow... always garaged

damn little rocket! I am thinking of selling it to reduce the toys...
hard to keep everything running well

[brackish]

I almost think I should knock on wood with this one.  Honda tiller, Honda pressure washer, four strokes, never do anything to them leave the fuel in the tank year round, no stabil, don't use them but a couple of times a year, always start on the second pull and run perfectly.

My Suzuki 9.9 four stroke has also run perfectly.  I do try to put only alcohol free gasoline in it, but not always possible.  If the motor had not come with the boat, I would have a 6 or 8 hp to try to cut down the weight.  I have never needed the power.  Last week at BEER, when the wind did not cooperate or when approaching anchorages I ran it at about 1/4 throttle and made about 5 knots. 

I do notice that there is a slight difference between the two tanks I have for it.  I have a Suzuki OEM with a 5/16" fuel line, it is a 3.2 gal with a manual vent.  Motor runs smooth as glass.  I also have a Bombardier 6 gallon tank with a 3/8" fuel line "self venting" whatever that means.  Motor tends to surge a little bit varying RPM.

Fuel consumption is not important in my world.  Last year I burned about 4 gallons total.

[Shawn]

"Back to outboards, I just picked up a used, but never started Yamaha 8hp 2 stroke/2 cylinder.  Should run a heck of a lot smoother than the 6hp 4 stroke.  I'm not real impressed with the 4 stroke Nissans (Tohatsu). Hard to start and seems like the power of a 3-4hp motor...very sluggish and vibrates like crazy past 1/2 throttle.  I'll probably hang onto it for a backup."

Have you had the Tohatsu looked at? Doesn't sound right, esp. the vibration after 1/2 throttle.

I replaced a Johnson 8hp 2 cylinder 2 stroke with the Tohatsu SailPro 6hp 4 stroke and love the Tohatsu. Starts second pull, plenty of power, quieter, slightly lighter and will go a lot slower at idle (idle speed is about half the 2 strokes) in gear then the Johnson which is handy when docking. The only thing I really liked about the Johnson more was the front mounted shift control.

Shawn

[Bob23]

Bob:
   Before you sell your S2000, consider donating it to the Bob23 Land Travel Relief Fund. This is a worthy fund, believe me.
Salty:
   A former employee bought a new Civic Si but drove it like an old lady! Some crimes deserve mandatory life sentences. It was a nice car and I couldn't help but reflect on how far the Honda car company has come. I was in the foreign car repair business when Honda released thier first cars. Total junk. Air cooled, if my memory serves me well, 10 inch wheels, and so small it made a VW beetle look like a Buick Roadmaster wagon. We had one come into our shop and it was given to me to determine why it wasn't running. How about fried pistons? yep, they were toast. I looked at that car and said to myself "This car company is going nowhere." Yeah, I have the same foresight when picking stocks.
   But, lest I be accused of being a hijacker, back to outboards. I had a Tohatsu 3.5 2 stroke on my Seapearl 21, loved it and even after it was submerged in salt water (don't ask), and serviced by a good mechanic, ran like a champ and I assume continues to do so. Other than that and my current Nissan 8, I have dabled in a few old Evinrudes and Johnsons. As you all are aware of, outboard motors were created to try mens patience.
   Bob23

[Craig Weis]

I bought a Honda lawn mower once. No power. I sold it after one season for $15 and bought a Simplicity 6.75 hp Briggs and Straton OHV four stroke. A world of difference! Starts first pull. I liked it so much I bought a Simplicity rider with hydrostatic transmission. Another sweat ride.

skip.