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??

[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.

[Beaver]

You can put a $10,000 motor on the back of a $5000 boat and you still pretty much have a $5000 boat. On a resell and operational level its hard to justify (financially) replacing a good running two stroke with four stroke. I happen to love old two strokes.

[Bushwacker]

Your original question didn't fit your answer, so I changed the question to what I think you really intended to ask! There are really 2 separate issues here. 1. The one in the new title above. 2. New 4 stroke vs. new DI 2-stroke?

I think you've answered question 1, the new vs. old issue, pretty accurately! You'll NEVER be able to recover the cost of a new motor simply by a ~10% reduction in fuel cost with the slightly better efficiency of the new motor on a typical recreational boat! You're already getting pretty decent mpg, which just stretches out the payback period even further! It's the same basic issue as trying to decide between gas or diesel, with 2-3X more cost for a ~20-50% efficiency improvement, on a new bigger boat! I went through the same thought process when I was contemplating a repower on my boat, trying to decide if I should replace a reliable, good running 31 year old 2-stroke with a new motor that would cost over twice as much as I paid for the original rig! (albeit in MUCH LARGER 1975 size dollars!) The bottom line is, unless you use the boat nearly every day for something like dive charters or commercial fishing, the fuel savings is insignificant!

There's a more important and difficult cost/benefit analysis that nobody on the forum can do for you: i.e., is the benefit of (fill in the blank here . . . less noise & smoke, smoother idle, more power, improved reliability and parts availability, etc.) worth all that cash and extra weight?! (Even the new DI 2-strokes are a lot heavier than the old ones!) In my own situation, I had run the old motor for so long that I was simply tired of all the noise and smoke and premixed fuel hassle that I was ready for some newer technology! Also the ethanol issue can be a problem for the older fuel systems not designed for it. However I also did some boat shopping to see what sort of boat I could end up with for equivalent $. (the cost of a new motor & bracket added to what I thought I could get for my current boat.) I found that I could get something BIGGER and maybe a little newer, but I concluded that NOTHING (with the possible exception of a repowered Moesly 21!) could match the unique combination of seaworthiness, trailerability, and cruising amenities of a 20' Seafari with a new motor and a bracket! The old motor also had an aftermarket power trim system that had died, for which parts were no longer available. And despite good compression, with over 1000 hrs on it, cold starts required use of starting fluid, and I suspect it needed new reed valves, which was a lot of $ to put into such an old motor. Basic reliability was not an issue, because I knew that old motor so well I could tell whenever it needed attention and it was so easy to work on that it never left me stranded; I could always get it going whenever I did have a problem. Although the new motors are very reliable and run great, they're not nearly so simple and easy to work on!

As for question No. 2, the issue of NEW 4 strokes vs. DI 2-strokes, that's a question that has been and will continue to be debated on this and other forums ad infinitum! The performance and reliability differences are quite small and, with the E-Tec's, there is no significant noise difference either. I personally think the excess weight of the 4-strokes is a potential problem for the 18-20' hulls, especially with a bracket, since, unlike modern boats, they weren't designed for nearly that much weight. That's not so much of an issue on the 23's, which were often fitted with twin O/B's that were almost as heavy as the modern big single O/B's. Good luck with a tough decision (and convincing the wife!) I'm afraid I may not have been much help! Denny