Difference between 4 stroke and 2 Stroke Outboards

[James W]

I have an ageing (1995) 5hp 2 stroke Yamaha that i've just serviced and runs ok at home, but I am a little worried about it for the season ahead as the no.1 engine for the boat. I know that 4 Stroke outboards are a little heavier and quieter, but what are the other differences?

Is it a case of light and shade or is there really not much difference?

Thanks!

 

[LadyInBed]

4 stroke need more TLC.
oil change, more care in handling/laying down.

 

[VicS]

I have an ageing (1995) 5hp 2 stroke Yamaha that i've just serviced and runs ok at home, but I am a little worried about it for the season ahead as the no.1 engine for the boat. I know that 4 Stroke outboards are a little heavier and quieter, but what are the other differences?

Is it a case of light and shade or is there really not much difference?

Thanks!

That they dont burn an oil/ fuel mix and therefore ( a bit) less polluting.

The whole design of a fourstroke makes it more efficient so you will get more hours to the gallon than you will from a twostroke. That would be my main reason for choosing a 4 stroke over a two stroke.

Downside apart from weight is perhaps the extra complexity of a four stroke, but it is a pretty well tried and tested principle, and the limitations on handling /laying down that has already been mentioned.

Only one area of performance that I am aware of where two strokes score and that is acceleration "out of the hole" which is why the modern fuel injected 2 strokes remain popular for towing water skiers.

There should, however, be plenty of life left in your Yahama if properly treated / serviced etc. I expect my 1984 6hp Evinrude to see me out!

 

[Spyro]

4 strokes less power per cc but more fuel efficient and quieter (unless it's a small air cooled Honda) no mixing of oil and petrol required.

 

[Beyondhelp]

Hmm,

The problem is it varies with generation of 2 stroke. Modern 2 strokes are MORE efficient, and more powerful for the same sized capacity than similar sized 4 strokes, at all RPM ranges. Evinrude E-tech and the new G2 series Evinrude are an example of this.

Take an older 2 stroke however which unless operating in their efficient range will be mostly similar or less efficient. At idle 2 strokes can be significantly less efficient.

2 stroke engines however do tend to be more reliable. There are so few moving parts, that provided you keep the oil mix right they will last until the point they fall to bits with corrosion, or fail due to lack of maintenance.

In all honesty if your engine is corrosion free, and in good health, I doubt you would gain much from changing to a 4 stroke.

 

[James W]

Thanks to all, for that, much appreciated.

It's the noise and smoke that are the main issues. My eldest son isn't happy until the outboard is switched off and we're sailing and so I do worry about any enforced need to motor. Is it true to say that a modern 4 stroke 4hp with a high thrust prop would be as powerful as a 5hp 2 stroke?

Thanks!

 

[macd]

The problem is it varies with generation of 2 stroke. Modern 2 strokes are MORE efficient, and more powerful for the same sized capacity than similar sized 4 strokes, at all RPM ranges. Evinrude E-tech and the new G2 series Evinrude are an example of this.

This is not a problem because it's irrelevant as far as the OP is concerned. The technology you describe is simply not available on small outboards, from Evinrude or anyone else. The smallest G2 Evinrude would sink his boat.

 

[Seajet]

JamesW,

if you don't want your 5hp 2-stroke I may well know someone who does !

4-strokes aren't ' just a bit heavier ' they are colossally ' who bolted this to the floor ?! ' type heavy...

On boats like mine where one needs to lift the engine from its well and stow it on longer trips, preventing drag and turbulence noise, 4-strokes are nothing short of a back injury inducing disaster; very poor for weight distribution trim too.

I have no qualms whatsoever with the reliability of well maintained 2-strokes, I have motored across from Lymington to St Peter Port in calms a couple of times.

Lots of smoke probably means too much oil or possibly a poor brand ?

 

[VicS]

Thanks to all, for that, much appreciated.

It's the noise and smoke that are the main issues. My eldest son isn't happy until the outboard is switched off and we're sailing and so I do worry about any enforced need to motor. Is it true to say that a modern 4 stroke 4hp with a high thrust prop would be as powerful as a 5hp 2 stroke?

Thanks!

Not really. 4 is 4 and 5 is 5 but the correct choice of prop is equally important to both to match the engine to the boat regardless of whether it is a 2 stroke or 4 stroke.

Outboard engine power is measured at the prop shaft in accordance with Icomia 28-83 http://continuouswave.com/whaler/reference/ICOMIA28-83.html

Small 4 strokes are not necessarily heavier than the same power 2 stroke. Some are, many are not vastly different, and a few 2 strokes are even heavier than some 4 strokes of the same power.
Seajet's comments are an over generalisation and an exaggeration.

 

[Skylark]

Thanks to all, for that, much appreciated.

It's the noise and smoke that are the main issues. My eldest son isn't happy until the outboard is switched off and we're sailing and so I do worry about any enforced need to motor. Is it true to say that a modern 4 stroke 4hp with a high thrust prop would be as powerful as a 5hp 2 stroke?

Thanks!

A smokey 2-stroke is usually too much oil in the fuel mix. For their simplicity and reliability, especially given the relative "abusive" environment in which they are used and operated, an older 2-stroke is something to keep hold of and treasure.

If you operate the engine mostly at full throttle, then 5 hp is more than 4 hp in every sense. If you have issues with acceleration, then a 4 stroke power delivery over its rev range may well "feel" beefier. 2-strokes notoriously don't start delivering much power until higher revs. That said, the difference in small horse power engines is tiny.

 

[Iliade]

Four stroke engines are immeasurably nicer to live with: quieter, nicer tone, half the fuel consumption (in the sort of motors used to push small sailboats), no plug fouling, happy to run at low revs for hours on end, less smelly, less dirty, easy starting, realistic electrical power output, though there may now be 2t with that.

But...

Heavy, with a big lump of heavy added.
Very slightly more complicated servicing.
Expensive
Very fine carburettor low range jets which can block easily with dirty fuel or corrosion products causing the starting problems some suffer.
Did I mention that they are heavy?


That said, I have had a Tohatsu 9.8 which weighed a ton, but happily pushed my >3.5 ton Centaur along at 5 kt against a headwind. It was 100% reliable for several years until it got dunked. Thereafter I struggled to get the fuel system completely clean and corrosion-free.

I had a 4 hp Yamaha for my tender, but it got pinched in France. Bought used. Not too heavy and 100% reliable. I bought a new one there the next day and it is still running impeccably three years on. It'll do 12 kt on flat water, one up.

For a displacement boat a four stroke with a fine pitch prop is far better than a two stroke with a coarse prop.

 

[johnphilip]

I too was thinking of upgrading my old 2 stroke Mariner but I was not sure if I wanted to leap to a 4 stroke, perhaps a move in stages would be the way?
Unfortunately I am struggling to find a marine 3 stroke unit readily available.

 

[James W]

I too was thinking of upgrading my old 2 stroke Mariner but I was not sure if I wanted to leap to a 4 stroke, perhaps a move in stages would be the way?
Unfortunately I am struggling to find a marine 3 stroke unit readily available.

^^

 

[sailor211]

Is oil in the sump still a problem. I thought that 4stroke had to be kept upright or the oil goes where it is not required. So you cannot just throw it on the deck as you deflate your dingy on a bouncy morning. Or just drop in the bot of the car.

Is there a solution to this problem. I am a bit lot to learn new habits!

 

[Pete54]

I have a variety of smaller 2 strokes and one 4 stroke a Honda 9.9. The Honda is massively heavier. It can only be laid down on one side and before you do that it MUST have all the fuel drained from the carb to avoid the idle jets blocking when the fuel evaporates (ask me how I know!). If you place it in the wrong orientation due to heeling it does not lose the oil, but will refuse to start until the oil drains and the plugs are cleaned. Compared with the 8hp Johnston it replaced it's only advantage is the cleaner running at idle. Would I buy another 2 stroke to replace it? As soon as a sensibly priced one comes up.

 

[GrahamM376]

Is oil in the sump still a problem. I thought that 4stroke had to be kept upright or the oil goes where it is not required. So you cannot just throw it on the deck as you deflate your dingy on a bouncy morning. Or just drop in the bot of the car.

Is there a solution to this problem. I am a bit lot to learn new habits!

They can be stored with one side down but not the other, usually labeled. We went from a Malta 3 hp 2 stroke to a Suzuki 6 hp 4 stroke, big mistake. Much heavier engine, slow on acceleration compared to 2 stroke and noisy at high revs + the paint fell off it just out of warranty. Sold that and bought a 9.8 2 stroke Tohatsu which is the same weight, quieter and of course much faster. Only advantage to small 4 strokes is less fuel but more complicated so likely to cost more in repairs in the long run.

 

[catalac08]

For me noise is the only downside of the 2/4 stroke choice. I had a 30 hp 2 stroke Tohatsu main engine and it never missed a beat in 7 years heavy use. I changed it for a 4 stroke 20hp Tohatsu that is much quieter and uses less fuel. Noise may not be such an issue on a smaller 2 stroke. I think with 2 strokes you have to be more careful to run the petrol dry to avoid any gel accumulating in the float chamber.

 

[ProDave]

Thanks to all, for that, much appreciated.

It's the noise and smoke that are the main issues. My eldest son isn't happy until the outboard is switched off and we're sailing and so I do worry about any enforced need to motor. Is it true to say that a modern 4 stroke 4hp with a high thrust prop would be as powerful as a 5hp 2 stroke?

Thanks!

That depends a lot on where it is mounted.

My 4HP 2 stroke hangs off the transom. And yes it's noisy and I prefer to be sailing.

BUT I was on a friends boat last year with a 4 stroke in a well in the cockpit. Boy was that noisy by comparison with mine, I would hate to have that configuration.

 

[James W]

That depends a lot on where it is mounted.
BUT I was on a friends boat last year with a 4 stroke in a well in the cockpit. Boy was that noisy by comparison with mine, I would hate to have that configuration.

Bugger, that's my configuration.....

 

[Colvic Watson]

2 stroke every time. So much lighter and easier to service. An absolute pain when a 4 stroke dumps its oil, and that includes when they're put the right way up. They are harder to start than 2 strokes, sometimes being nasty and biting you back, snatching the cord. My 10hp 80's vintage 2 stroke runs quietly and smoothly and has a lovely power curve. But, for cruising engine the fuel consumption is an issue.