As I said, below 10-15hp, weight is definitely less for 2-stroke. Above that, 4-strokes become lighter.

AND, I am only talking about comparing with OLD 2-strokes less than 10hp no longer manufactured.

Your Nissan weight is a numbers exception because the 9.8 was the same basic engine as both the 6hp and 8hp with different carbs - so you really have a 6hp engine for comparison. In 4-strokes, the 9.9 is the same as the 15hp. The 9.9hp 2-strokes available today from Tohatsu and Yamaha are also built on 15hp engines, so you won't even find a 2-stroke that weighs the same as your Nissan anymore. That 2-stroke Yamaha Enduro or Tohatsu 9.9hp is going to cost you 85-90 lbs (and you could go with a 15hp instead with no weight penalty).

Currently sold 6hp 4-stroke outboards weigh the same as your Nissan. So you see, the weight/HP is a game of numbers and manufacturing, not a game of what type of combustion cycle it has.

The old 2-stroke/4-stroke weight argument is specious because of this. When you nail down actual equivalent comparison of engine models sold today (2-strokes are sold almost everywhere outside the US), there is very little, if any at all, significant weight difference. All 2-stroke engines now from Yamaha and Tohatsu - right down to the 3hp models - weigh the same as their 4-stroke equivalents with the exception of the 6-8hp not being easily comparable.

Here are Yamaha's current 4-stroke specs: http://global.yamaha-motor.com/business/outboards/product/4st/small-spec/

Here are Yamaha's current 2-stroke specs: http://global.yamaha-motor.com/business/outboards/product/2st/15-2spec/

I concede there can be brand/model specific noise level differences. In general, however, we have found 4-strokes to be much quieter and smoother than equivalent 2-strokes.

Portability is not a primary concern for the majority of cruisers, where the dinghy is the main transportation used daily and for long distances. A very large percentage have 15-20hp engines. At most, the engine is only taken off for long passages, and hoists are fitted if needed. Most of us spend far more money on dinghy fuel than main engine fuel, so fuel economy=$$$ in our pocket.

BTW, I am not picking an argument here. Because we recently were in the market for a new engine, and have both 4-stroke and 2-strokes readily available to us here, I did a very exhaustive analysis that surprisingly disabused me of a lot of "common knowledge" I had around 2-stroke vs. 4-stroke engines. I am just presenting some of what I learned - all of which took me by surprise.

And yes, the "good old days" are always better, but they are gone now.

Mark