Great Debate: 4 stroke vs. 2 stroke - MPG

[77SceptreOB]

I know this is a much talked about subject, but I wanted some opinions and FACTS of the subject.

I have a bunch of friends who are DYING TO SPEND MY MONEY, or lack of it as I explain to them. Many believe that the gain in effeciency of a 4 stroke will more than pay for itself in better fuel mileage vs. the monthly cost of buying a NEW engine. I for one don't believe this wishfull thinking logic, BUT, am open minded enough to hear some facts from the crowd. So here goes:

I have a OLD 2 stroke Yamaha (1988 Yamaha Excel - 225hp) It burns about 11 gallons/hr cruising at 4200 RPMs and pushes my 1977 23' Sceptre right at 31 mph. That nets out at
an effecient 2.81 miles per gallon. The attached picture, taken today (with a light load) shows a little better at 2.91 miles per gallon.

My well heeled and intended friends SAY a four stroke would burn 40% less fuel at cruising speed. If true, which I DONT believe, that would mean a F225 would burn around 6.5 gallons per hour. Again, unbelievable and unachievable fuel burn rate...IMHO. I believe MAYBE, and I mean Maybe, a F225 might burn as little as 9 GPH. That would be a 18% gain in effeciency.

I use my boat for about 5 months of the year, accumulating approximately 80 hours/year (Shown on my hours gauge) Of the 80 hours, I would estimate 55 hours are at cruising speed. So it works out this way:

55 hours x 11 gallons/hour = 605 gallons of fuel annually

605 gallons x $4.50/gallon = $2,722 of fuel consumed annually (At Cruise)

Got to add the oil too. At 50:1 mix (Which is probably high consumption) So that is 12 gallons of YamahaLube @ $25/gallons = $300.

So add the fuel and oil and I spend about $3,000/year on fuel (At cruise)

So lets assume that a 4 stroke can and will make a 20% gain in effeciency. That is 8.5 - 9 GPH at cruise.

55 hours (cruising) x 8.8 GPH = 484 gallons used annually x $4.50/gallon = $2178/year

So there is a savings of about $845 annually using a 4 stroke vs. 2 stroke

A Yamaha F225 is approx $19,000 rigged and ready to go...

So $19,000/845 = 22.5 years to break even given all other things equal.....

Ok, before all the 4 strokers start throwing tomates, rotten bananas, and empty 2 stroke oil containers at me; I agree a NEW 4 stroke offers a lot of intangibles such as:

Better reliability & Safety if offshore
Better for the environment
Quieter
Etc, Etc.

BUT..... it takes A LOT of use to justify a NEW 4 stroke.....

Your thoughts?????

[countshock]

I think that you may be ignoring the much greater efficiency of 4 strokes at idle and trolling speeds. Not saying that your calculations are wrong, just that old 2 strokes waste a ton of fuel at lower speeds.

[Fr. Frank]

2.9 mpg for a 23' is VERY good. I don't think you'll get more than 8-10% improvement with a fourstroke, at best.

[Islandtrader]

Wait til Bushwacker chimes in...

[Blue197320]

i know what you mean about people wanting to spend my money on my boat. 1991 23 contender with a 01 ox66 yamah, people tell me how much quieter and more fuel efficent a 4 stroke would be. i got the boat for 9k with no corrosion on the motor. he paid about 20k for his boat with a 4 stroke. it would take alot of trips to make up that difference. if you use the boat 10 times a year its not worth it to me. as far as reliability goes, i start my outboards every 2 weeks for a few minutes and maintain them myself. i would not hesitate to take them out at any time.

i know this is a little off topic but it looks like you have the fuel flow meter on your hds. i just installed that on my boat but have not had a chance to take it out and run it. how do you like it so far and how accurate is it??

[DonV]

Hummmm.....2.91 on a carbed engine. I'm shocked and impressed. My 225 Merc EFI burns right at 12 gal/hour @3800rpm with a medium load. I figured the best I was getting was 2 mpg. However my 90 hp Yamaha carbed engine loves gas, she runs perfect all the time, just loves gas.

[BigLew]

I'd say to my Friends,

Thank you for thinking about my well being and I will seriosly consider it when I conclude it is time for a new engine. For now and for ME, I have all the money invested in my boat that I feel I want invested in it.

[77SceptreOB]

I know this is a little off topic but it looks like you have the fuel flow meter on your hds. i just installed that on my boat but have not had a chance to take it out and run it. how do you like it so far and how accurate is it??

I like it, seems to be accurate. It is very responsive. My MPG goes from best = 3.05mpg at 29mph/3800rpm@9.5gph (My Boat/engine does not like to run this speed, wants to speed up or fall off and slow down) to worst = 2.16mpg at 39mph/5200rpm@18gph (WOT)

Hummmm.....2.91 on a carbed engine. I'm shocked and impressed.

Thanks. My Yami is VERY light weight, only 411 pounds according to the owners manual

I think that you may be ignoring the much greater efficiency of 4 strokes at idle and trolling speeds. Not saying that your calculations are wrong, just that old 2 strokes waste a ton of fuel at lower speeds.

Maybe. But to the best of my knowledge my motor idles and trolls at 1 GPH and at 4.5 mph so that is 4.5mpg. How does that compare with 4 strokes?

I don't think you'll get more than 8-10% improvement with a fourstroke, at best.

My thoughts, exactly...

[bitsamonkey]

I have a hard time believing your mpg's. How many times have you calibrated that sender by filling the tank to the brim? It took me at least three or four times to get it within a gallon when filling up when I had one of those on my last boat. If it is spot on, then congratulations, don't change a thing.

[77SceptreOB]

Originally Posted by bitsamonkey

I have a hard time believing your mpg's. How many times have you calibrated that sender by filling the tank to the brim? It took me at least three or four times to get it within a gallon when filling up when I had one of those on my last boat. If it is spot on, then congratulations, don't change a thing.

We used to have a Flo-Scan analog gauge, and the burn rate was the same in gallons per hour. The Speed is controled by GPS, so we know the speed is accurate. The rest is just math....so I feel the #'s are accurate. I am very happy with the numbers. My Yami has a new powerhead on it with about 500-600 hours of engine time.

As for the HDS fuel usage sensor, I don't think it has any "adjustment" feature to it. Nor do I think the HDS-7 head unit has any adjustment either. But, based on the burn rates being the same for the HDS and the flow-scan, I feel comfortable enough that it is pretty accurate.