I hope it's ok to revive an old thread. I have had my hands in old two strokes recently. Re-building them. Testing them on my 3.3M SIB. Two are late 70's, one is a 1981 and another is a 1990. All two strokes 7.5, 8.0, and 9.8 hp model Mercs. My son owns a 9.8 HP 4 stroke. 2018 year. He also has a 3.3M SIB. He's close to my weight. Similar loads in both boats.
Each model of engine has positives and negatives. It really depends what your needs are. If you're getting up there in age and have to lug around an outboard, weight might be your #1 issue. If you have the money and little to no mechanical experience, your boat lives on a trailer a new 4 stroke likely suits you much better.
Prop pitch has a big effect on your boat's performance. Matching the right engine hp, engine weight, boat hull design and payload and meet your expectations is challenging. It doesn't come down to just 2 stroke or 4 stroke engine design.
2 of our 3.3M SIBs side by side in the lake recently, my son and I got to compare performance. My 7.5 hp engine had a 7" pitch 3 blade prop. His 9.8 4 stroke had a 9" pitch triple. My 7.5 2 stroke matched hix 4 stroke out of the hole. He pulled away after a 5 seconds. His 9.8 had probably 5 MPH top speed advantage.
Next was my 1979 9.8 2 stroke with a 8" pitch triple blade prop. 2 stroke had a very slight advantage out of the hole full acceleration. The 4 stroke was reigned I a bit but he still had about a 3 mph top speed advantage.
Next up was my 1990 8.0 hp 2 stroke with a 9" pitch triple blade prop. Same pitch and 3 blade prop as his 4 stroke 9.8. My 8.0 hp spanked his 9.8 out of the hole. Also beat him on top end speed. Probably 2-3 mph advantage for my 8.0 hp.
My 7.5 merc and 9.8 both weigh 64 bs. My 8.0 2 stroke weighs 71 lbs. The 4 stroke 9.8 weighs 85 lbs. I hope to put a 9" pitch prop on my 9.8 2 stroke. I think it will then also beat out the 4 stroke 9.8.
After a few hours on the lake yesterday we compared fuel burn. The difference between 2 and 4 stroke was considerable! The 4 stroke consumed less than half. We both traveled the same distance over about 3.5 hours of travel time.
When it came time to break down our boats and pack up, roll everything into the truck the 4 stroke felt like a heavy boat anchor. The 2 stroke was much easier to load and unload when we got home.
I picked my overall favorite. My 1990 Merc 8.0. It's quieter than my other 2 strokes and light weight. Though not as quiet as the 4 stroke. 8.0 top speed is good. A good overall performer. My back is weak. I had a herniated disc years ago. I'm in my mid 50's age. I don't want a boat on a trailer. I like the ease of rolling up and storing a SIB in my garage. The 8.0 works for me.
A 9.8 or a 15 hp 4 stroke would be nice but by myself, I just cannot handle the weight. 20 lbs doesn't sound like a lot if weight. But add 20 lbs to a 64 lb engine, it's a lot!
Benefits of each engine type in my experience:
4 stroke twin:
Fuel economy which also means longer range!
Quiet
Smooth running
No mixing oil & gas
Minimal pollution
Higher resale value
2 stroke twin:
Faster acceleration fires 2x every 1 complete crank revolution.
Light weight
Simple, easy to work on yourself
Cheap maintenance
Did I say light weight! Worth saying again
Low initial purchase price
Fun to work on!
Negatives of each:
4 stroke twin:
Higher initial purchase price
Fires 1x every crankshaft revolution
More frequent & higher cost maintenance, especially at a dealer
Higher level of technical skills required to work on it yourself
Oil changes and oil disposal
Weight
Slower acceleration
2 stroke twin negatives:
More pollution
Have to mix gas & oil
You probably shouldn't own one I you cannot fix it yourself. Often its unneconomical to pay a dealer
You should be pretty comfortable fixing if yourself
Increased fuel burn
Likely more spark plug changes over lifetime
Lower resale value
Just my 2 cents. For me at my age, weight is the single most important point. I will will throw good money at my 8.0 2 stroke to rebuild it myself, when needed. If I had to sell it after a rebuild I'd likely lose money. 2 stroke used prices aren't great compared to a 4 stroke. Yet I'm not rebuilding it to sell it. I'll rebuild it for my own personal use for years ahead.
For me the 2 stroke lives on! Cheers.
