4stroke v 2stroke

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thaxter

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Need to replace the outboard on my 20ft Privateer. the old one is a mariner 5hp which has done the job quite well for a number of years albeit a little lumpy and noisy. All the advertising blurb tells me I should now be thinking of a smoother, quieter 4stoke, however the more experienced tell me they are not so quiet and not so smooth (being single cylinder and firing differently) As my engine is fitted in an inboard well I want it as quiet as I can get. Would I be better off moving up to a 6hp so that I can get a twin cylinder and buying a 2stroke while they are still available?
Also would I benefit much by going for a 'high thrust' prop.

Would appreciate your words of wisdom!
Stuart

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We had a 7.5 Evinrude on our last (24 ft.) boat. It was very quiet and never let us down once in over ten years. replaced it with a mariner 10 - also very good. I found that there seem to be a range of power outputs for engines which are very similar in weight. Just bigger cylinders I suppose. I got the one with most power for a given weight. 4 strokes seem a lot bigger and heavier for a given power but I suppose they are more efficient. I fitted lower pitch props to both and they seemed to work well.

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I think as you say 2 cylinder outboard is a must for smooth quiet operation. I have a 20 year old Evinrude Johnson (2St) and there has never been any doubt about starting or problem with the power head. Of course you get problems with water pumps and water passage corrosion and blockage but that would be the same with a 4 stroke. If you don't use the motor much then 2 stroke is fine for a lot of use 4 stroke would give better fuel consuption. Put the sails up will

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Hi Stuart
I have a 24 foot yacht with a 9.9
However my 3 1/2 Tohatsu (purchased for the dinghy) pushes it along nicely
BUT I generally use it only as far as the end of the creek (10 mins)
Yes it is a little noisy and there is some vibration.
Undoubtably a two stroke twin would be smoother as it has almost perfect balance (except a rocking couple) On most, the 6 and 8 are the same block as are the 10 and 15.
You have to ask yourself "Do I use it enough to justify the extra cost and weight of the larger engine?"

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by mirabriani on 05/11/2004 09:02 (server time).</FONT></P>
 
a 2-stroke twin has the pistons moving alternately whereas a 4-stroke has them moving together so a 4 stroke is inclined to more vibration. (don't know about outboards but jap bikes have had balance shafts to cure the vibration for many years now)

any outboard on a sailing boat will benefit from a finer-pitched prop as the standard ones are coarse pitch to be able to work at planing speeds. you'll get less cavitation and more thrust.

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Same reason an old british twin would shake your goolies off! (Two pistons rising together.)

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I have got a twin cylinder Evinrude 6 which is now very old. It is really is a very quiet engine - I would be surprised if a 4 stroke was any quieter. As for Tohatsu 3.5hp, had one and sold it - didn't like it at all - ok for the price, but noisy, and tinny - I guess you pay for what you get.

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FWIT, I have been down the road of changing from 2t to 4t and now I am back to 2t.

"Logic" being OB is only used for the tender so weight is an issue, storing the 4t was a pain in the a$$ as iit (they - I had 2 of them) had to be stored upright of laying on the tiller side, never allow prop to be higher than power head lest oil gets
into the cylinder.

Little merc 3.3 more than enough for the dingy and can be lifted with one hand - great when lifting the little bugger off the dingy and puting on its mount on the pushpit.

Fuel consumption was much less with the 4t 4hp yamaha. I would estinate arounf 50% less or even more

Noise level was not that much different but the sound was. The 4t was slightly quieter (the 4t did not have the distinctive burble of the 2t).

As both are/were single cylinder engine they both vibrated.

I am now back to 2t and much happier.

If I was going for a bigger engine I would probably go for a twin cylinder 2t if usage was limited but if I could cope with the added weight and storage problems and envisaged extended running hours I would look at a 4t probably either merc or Suzi.

I believe Suzuki have overcome the oil problem at least on their smaller 4ts.

Remember a 4t has many more moving parts than a 2t - more to wear or go wrong or need adjusting (valves).

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Would you recommend using a Tohatsu 3.5hp on a 2 1/4 ton long keeled 26ft yacht?

I am told a 2hp will get you home if the wind dies, but wondered if you could confirm this? Obviously only needed in light wind flat water.

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the majority on this site would advise pretty big outboards even for fairly small cruisers, e.g. 9.9 for your size of boat. personally i don't agree as i had 5hp on my 22ft sonatas then changed down to a 4 with no noticeable loss of go. a 3.3 will most certainly get you home in calm conditions provided not too much chop or tide. your main problem will be that long shaft versions are uncommon in this size and the standard shaft may well pop out of the water and lose drive.

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Don't know where Snowleopard got his m/cycle info, but almost every Japanese 4-stroke parallel twin since the 50s has had it's pistons 180 degrees out of phase. If they rise and fall together (like almost all British parallel twins), they vibrate badly - like a single.

I don't know a thing about Japanese twin cylinder 4-stroke outboards, but would be surprised if they weren't phased at 180 degrees, too. Worth checking.

Surely main merit of 4-strokes is their far better fuel economy. Drawbacks are extra weight and cost. But I gather that Thaxter had better decide fairly soon, since 2-stroke marine engines will effectively disappear from the market.

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I found a 9.9 2 str. would even give me 5 1/2 knots on a 26ft. sloop. Reliable, smooth, quiet even after 10 years. Don't go to 4 str. until you have to.
Ken

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Hi MUS
Snowleopard has answered most of your question and I agree.
I would add that your advice is correct. A friend has similar yacht to mine and his 2.2 drives it along in flat water. However most peeps will warn you to beware, you do not want to be caught out in a blow with a piddly little engine. So ideally you need a bigger backup.
Regards Briani

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I used a Yamaha 2hp to push my 4 ton cat (no wind or tide), but is really too small. Dotnt go for the yam 9.9 hi-thrust, this years is built on the 15hp engine, so weighs lots more than the older engine. If it was me, I would be looking at the hi-thrust Yam 8hp, and know that it would have the power to push me against wind and tide.

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There are many regulations coming in/came in regarding 2strokes. The manufacturers had responded by making their 2stroke more environment friendly/more fuel efficent. There will still be 2stroke in years to come, just that they will be a totally different to our 2stroke.

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That's true -- the regs coming into force in 2006 will bring widespread use of direct (fuel) injection technology and so on. But as I understand it, for economic reasons this is likely to adorn high-end (ie high power output) engines, rather than the ones likely to be used for tenders and smaller yachts. There the choice will be predominantly 4-stroke. Certainly the days of the cheap, light and simple 2-stroke are seriously numbered.

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And what damage do the number of small 2 stokes do to the environment when SUV's and 4x4's choke the roads guzzling Saudi oil fields by the minute? I wish the bureaucrats who make these laws to ban 2 strokes would look at context and impact. Why not tax aviation fuel for instance!

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>I wish the bureaucrats who make these laws to ban 2 strokes would look at context and impact.

But that's exactly it I think - a US study of pollution, aimed at hitting automakers, found that 2 stroke outboards were way over represented in total pollutant outputs - they made a substantial reduction in airborne pollution by outlawing 2 strokes, to say nothing of the improvements in water quality ......

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you're right about the bike engines - almost all were 180 deg.

Don't know of any 360 deg 4T outboards (pistons rising together), certainly my Yamaha F8 wasn't...

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With respect, this is garbage... Very few parallel twins run 180 cranks, because that gives worse balance than 360 crank. A few (TRX yam and Triumph Bonnie America) run 270 deg cranks. 360 gives you one power stroke per rev, anything else gives you an odd arrangement. They also tend to have a single coil and ign set up and use the "wasted" spark (2nd spark on exhaust stroke) "system"

2 strokes of course are all 180 cranks.....

<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"
 
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