2 stroke vs 4 stroke outboard

[SeaStu1]

I'm thinking of changing my 4hp 2 stroke outboard for a 4hp 4 stroke outboard. I think l heard somewhere that the power on a 4 stroke will be less than a 2 stroke. Can anyone comment on this? Will l notice any difference in the power? Thanks.

 

[VicS]

I'm thinking of changing my 4hp 2 stroke outboard for a 4hp 4 stroke outboard. I think l heard somewhere that the power on a 4 stroke will be less than a 2 stroke. Can anyone comment on this? Will l notice any difference in the power? Thanks.

Power is power... and these days its measured at the prop shaft.

They behave differently which is why when it comes to big engines on sports boats 2 strokes are preferred. Acceleration is better esp for pulling a water skier etc. or getting up onto the plane

as far as you are concerned 4 hp is 4 hp. Its the other pros and cons that you need to consider

 

[SeaStu1]

Thanks Vic. In my case its for my Leisure 20. The power has never been an issue although some owners prefer 5 or 6hp instead but l have found the 4hp to be fine for what l do.

 

[Bosun Higgs]

Since a 2 stroke has twice as many firing strokes per 1000 revs than a 4 stroke but each power stroke is less efficient, the two have quite different power characteristics. But in something as mundane as an out board, the power developed is much more a matter of what the engine designer designed. So best thing to do is to look at the brochures and compare.

There is another major issue anyway - the design of the prop and the gearing. Many outboards are made for speedboats / ribs etc and not for plugging away at 4 knots on the arse of a sailing boat so the props and gearing are not satisfactory for that use and the bhp the engine develops never gets into the water so to speak. So check out the bollard pull of the o/b you are interested in.

The 4 stroke will likely be heavier and more thrifty with fuel.

 

[VicS]

Thanks Vic. In my case its for my Leisure 20. The power has never been an issue although some owners prefer 5 or 6hp instead but l have found the 4hp to be fine for what l do.

Yes I'd prefer a bigger engine than 4hp. I have 6hp Evinrude yachtwin on my Sea Wych..... I've seen people struggle with a 4 hp.

sail versions are worth considering when available

There are several differences between my Yachtwin and the standard 6hp.

Its got a finer pitch prop, which makes it better for a slow displacement hull ... like having a lower gear. The prop also has big mickey mouse ear blades so thrust is good at low speeds.

It has a modified version of a long shaft leg that has the exhaust outlet is higher. That allows the prop to be set deeper without creating back pressure on the exhaust. It also prevents the gases being sucked into the prop when going astern, so giving good thrust astern.

 

[Fire99]

I have a 10hp 4-stroke on my Snapdragon 23 and it's a touch overkill and bleedin' heavy.

IMO a 6hp would be a good compromise on a 20' and a fair bit lighter (but not 2-stroke light unfortunately)

 

[Seajet]

For boats like mine where the engine should be lifted out of the well and stowed on any decent sail, weight is important, and the difference between 2 stroke & 4 stroke 4hp's felt to me like the difference between 'easy' and 'can my back take it ?!'

Also do check any 4 stroke you consider, for its' stowage position; most 4 strokes are very fussy about how they are stowed, or there is the risk of lube' oil filling the combustion chamber - expensive if one tries to start, possibly bending con-rods etc, not to mention failing to run !

As VicS says, 'Saildrive' type engines are good, made for the job; they also usually have a more powerful electrical charger; Tohatsu do one - 6hp I think - which seems popular, don't know the weight but it will be heavy.

I use a 5hp 2 stroke Mariner and love it, used to have a 4hp 2 stroke Yamaha which pushed my 22' boat across the Channel a few times happily in calms; I did trim the prop' blades on both very slightly to get full revs'.

 

[CAPTAIN FANTASTIC]

You will notice a difference especially if the 4 hp four stroke is brand new. In general four strokes are higher torque, much more economical, quieter, heavier, more reliable and will start easier.

 

[MoodySabre]

My son has replaced his 5hp 2-stroke with a Yamaha5 4-stroke in the well on his Achilles 24 and it is much smother and quieter (a relative term). It can be laid down any way except head-down.

 

[Seajet]

Captain Fantastic,

I'd agree except for the reliability & starting; my Mariner - and the Yamaha before it - always start 2nd pull, and have never ever conked out on me, I've used each across the Channel Poole - Guernsey etc several times.

I do think maybe a 2-stroke requires a little more careful owner than a 4-stroke, possibly that's it.

 

[CAPTAIN FANTASTIC]

Captain Fantastic,

I'd agree except for the reliability & starting; my Mariner - and the Yamaha before it - always start 2nd pull, and have never ever conked out on me, I've used each across the Channel Poole - Guernsey etc several times.

I do think maybe a 2-stroke requires a little more careful owner than a 4-stroke, possibly that's it.

Seajet,

You are right; some people get on very well with them; perhaps I am not as careful.

 

[Seajet]

Captain Fantastic,

if that was you I saw up the mast yesterday, you seemed to be justifiably careful, but still rather you / him than me !

 

[CAPTAIN FANTASTIC]

Captain Fantastic,

if that was you I saw up the mast yesterday, you seemed to be justifiably careful, but still rather you / him than me !

No, It was not me. Yesterday I was in Southampton Water, fixing the wiring of the mast head light on the switch panel. My Macwester is slow, so I need to have a light to be seen when everyone else is zooming everywhere.

 

[sailorman]

I'm thinking of changing my 4hp 2 stroke outboard for a 4hp 4 stroke outboard. I think l heard somewhere that the power on a 4 stroke will be less than a 2 stroke. Can anyone comment on this? Will l notice any difference in the power? Thanks.

one uses less fuel, costs more, need more maintaining, heavier, harder to start.
Then there is the 2T
guess wot i have

 

[rwoofer]

You will notice a difference especially if the 4 hp four stroke is brand new. In general four strokes are higher torque, much more economical, quieter, heavier, more reliable and will start easier.

I always thought that 2 strokes had more torque not less for the same power. Anyone know the right answer?

 

[Bru]

We just swapped out the 8hp 2 stroke Suzi on Brigantia for a shiny new 6hp 4 stroke jobby and I don't somehow think we're going to be regretting the decision!

We haven't had a chance to give it a good burn as yet but it's incredibly quiet in comparison to the old engine and my impression from a couple of short hops to and from our mooring is that it shoves the boat along a good deal quicker at rather lower revs - in fact, we might be after changing the standard prop for the fine pitch optional prop to reign things in a bit!

Expectation is that it'll use rather less than half the fuel of the old outboard based on a conservative estimation of the realities behind the manufactuers claimed figures (which would suggest it will use about a quarter of the fuel!)

The single cylinder 4 stroke engine is almost exactly the same physical dimensions as the old twin cylinder 2 stroke and a few kilos lighter. It doesn't run quite as smoothly, for obvious reasons, but as mentioned above is much quieter. If lifted out of the well it would have to be stored on the correct side or upright. (Not something we plan to do routinely during the season as it's not easy to lift the engine over the mainsheet horse and into or out of the well and we haven't got anywhere suitable, other than the well, to store the engine when not in use. One of the longer term jobs on the task list is to look into the possibility of being able to lift the engine vertically out of the water)

Finally, the 6hp Suzuki is exactly the same dims as the 4hp and 5hp engines and this is the same scenario from the other manufacturers. Might as well, I figured, spend the extra hundred quid or so on the more powerful model

 

[Giblets]

Stu, I have a Tohatsu 4T 6HP Saildrive on my Leisure 17 which barely runs above tickover (unless being swept into Langstone Harbour on a fierce flood and heading towards a moored fishing boat but we won't go into that! . Nearly had me boat up on the plane though! )

The engine is nice & quiet and has a good charging coil for the battery but does weigh in at 25 kg though.

 

[VicS]

Stu, I have a Tohatsu 4T 6HP Saildrive on my Leisure 17 which barely runs above tickover (unless being swept into Langstone Harbour on a fierce flood and heading towards a moored fishing boat but we won't go into that! . Nearly had me boat up on the plane though! )

The engine is nice & quiet and has a good charging coil for the battery but does weigh in at 25 kg though.

thats lighter than my 6 hp 2T Yachtwin which is 27.4 kg

 

[Lakesailor]

I've had 4 stokes, Honda, Yamaha, Mariner and now Tohatsu. All on 15 to 19ft boats on the lake.
I've also run a Tohatsu 3.5 2T on all those boats. And a 4hp 2T Evinrude on one.

The 4 strokes are, in my experience, quieter, more economical, smoother (except the Honda) and more powerful (with standard prop) than the 2Ts.

The argument that you need a bigger HP model is flawed in that once you are at displacement speed extra power is noiser, thirstier, but no quicker.

On my 15ft bog-about boat it does 5 knots at about twice tickover speed and full speed (6 knots) at possibly half throttle. I can't imagine any situation in which full throttle would provide anything extra. Strong headwind?

 

[Seajet]

I've had 4 stokes, Honda, Yamaha, Mariner and now Tohatsu. All on 15 to 19ft boats on the lake.
I've also run a Tohatsu 3.5 2T on all those boats. And a 4hp 2T Evinrude on one.

The 4 strokes are, in my experience, quieter, more economical, smoother (except the Honda) and more powerful (with standard prop) than the 2Ts.

The argument that you need a bigger HP model is flawed in that once you are at displacement speed extra power is noiser, thirstier, but no quicker.

On my 15ft bog-about boat it does 5 knots at about twice tickover speed and full speed (6 knots) at possibly half throttle. I can't imagine any situation in which full throttle would provide anything extra. Strong headwind?

All that's spiffing, as long as one doesn't have to lift the thing to stow it or take it home, and the boats' trim / performance doesn't mind more weight at the stern if left there, which can lead to boat pitching, transom dragging or displayed engine being pinched...

Boats having to leave the engine in a well due to weight - in the past largely due to daft oversize engines, now maybe standard 4 strokes - make me cringe, not only thinking of the drag but also the corrosion.

Roll on electric jobs !