2 stroke vs 4 stroke difference in gas!!!!!!

[MarioCoccon]

Hi, Iam deside of throw away my rotten, rust, damage,junk Atomic 4 and put a big outboard on my Newport 27 with hope to be close of 8knts. I have a friend with a Jeaneau 27 with a outboard 8hp at full speed he cruise at 6.5knts. I want 8 minimum. Ok the ?? is buy a 25 4 stroke or 2 stroke?? the difference in price is amazing a 4stroke Mercury cost $3,400 and a 2 stroke $2,200. The positive of the 4 is no smell, no smoke, less noise the negative a little bit more heavy and very expensive. How much it will be the difference with the two motors at full speed talking about gas???? If some that have any 4 stroke can say how much use by hour. With my 15hp 2 stroke I burn 6.6 galons in 5 hours almost full speed maybe 3/4 of throttle because I dont feel the difference between 3/4 and full speed. Thanks to the ones can help me.

[marshmat]

Firstly, the power range you're looking at.
That 8hp is pushing the Jeaneau at 6.5 knots, its 'hull speed'. This is as fast as that boat can go without attempting to climb to plane. Since neither the Newport nor the Jeaneau can reach a full planing state (at least not in any practical application), any attempt to go any faster than that 6.5 knots under power will be futile. So a 25hp outboard would be far more than your boat would need; a 9.9 would suffice to cruise comfortably at hull speed. (This is the same phenomenon you notice when you gun the 15hp to its maximum but see no increase in speed- the boat can't actually go any faster than it's going when that engine is at 2/3 power.) Using a 25 would not get you any more speed than a 9.9, but would increase your fuel consumption and make slow-speed cruising very difficult.
Where the extra power comes in handy is if you ever need to motor into a strong wind or storm. Here, having more thrust can be handy. But outboards are not much good for this, because they can lift out of the water as you go over the crest of each wave. An outboard on a sailboat only needs enough power to push the boat comfortably to hull speed in conditions where the motor stays in the water- any more is wasteful. If you need to beat into storms under engine, you want an inboard or S-drive whose propeller is firmly planted in the water at all times.
So if you go outboard, go with 9.9 or 15 hp. Four-stroke units are quieter and use considerably less fuel (up to about 30-40% less in some cases) than comparable two-strokes in this size range. (The difference is less notable with larger, direct-injected 2-strokes, but smaller ones are usually still carb types.) The 4-stroke will also usually have a flatter, lower-RPM torque curve, as opposed to two-strokes which tend to be more high-strung. As a sailboat kicker the 4-stroke is generally the better option.

[Buc]

As a very rough rule of thumb, in order to exceed what Marshmat calls hull speed -- in your case about 6.5 knots -- it is necessary to cube the horsepower to double the speed. As example, to double your speed from 6.5 knots to 13 knots, you would have to move from 8 horsepower to 512 horsepower.

As your wish is to make only 8 knots, it wouldn't take quite so much, but it would still require more than you would want to carry on a bracket on the transom. In addition, since most outboards develop their maximum horsepower at relatively high rpm, you'd need a propeller with a relatively flat pitch to allow the engine to reach maximum without overloading. 6.5 knots is a very reasonable speed for a Newport 27.

As an alternative, you might want to consider finding a rebuilt Atomic 4. They've been out of production for several years, but there are still thousands of them in use and, treated properly, they run forever. Since you're looking to replace gasoline with gasoline, that obviously isn't a problem for you. As I remember, the one Newport 27 I surveyed many years ago had the A-4 with a direct drive -- no reduction -- and seawater cooling. You should add freshwater cooling to increase longevity, but keep the direct drive to avoid having to make changes to your engine mounts. The cost would likely be less than a new 25-hp outboard.

[Ike]

If you google Atomic 4 you will find a bunch of web sites dedicated to atomic 4. Most are about maintaining and repairing but a few are sites for drop-in replacement engines. Universal, that built the atomic 4, makes a replacement engine that is a drop-in but is a diesel. There is even a forum that deals with nothing but Atomic 4. I used to be a member but after I answered the same questions about a thousand times I disenrolled. People just don't seem to take the time to go back and read the old threads. Huh? That sounds familiar!

[Buc]

The "drop in" replacement isn't a drop in for the direct drive Atomic 4. The shaft coupler is a few inches below the A-4's; this requires raising the engine mounts or moving the prop shaft. The mounts are spaced identically, but that's all.

Also note, Universal is now marketed by by Westerbeke. I haven't looked at them for a few years, but their engines were all based in the Kubota tractor engines and available in 1, 2 and 3 cylinder versions. I put one of the 3-cylinders in a boat in the late '80s, though not to replace an Atomic 4. If the current engine is much the same, it's a winner. Still, a rebuilt A-4 would be cheaper (Google Moyer Marine) and a real direct replacement.

[mattotoole]

Originally Posted by Ike

There is even a forum that deals with nothing but Atomic 4. I used to be a member but after I answered the same questions about a thousand times I disenrolled. People just don't seem to take the time to go back and read the old threads. Huh? That sounds familiar!

Yes it does! But there's a good reason for this, besides laziness -- poor webcraft. Most web forums do not have useful search. Either it doesn't give useful results because of bad backend software design, or the web page is poorly designed so the search feature is not convenient to use. Even this forum has the search buried in a drop-down menu.

[Seafarer24]

I'd recomend the Yamaha 9.9 4-stroke long-shaft, if you really want an outboard. Also not mentioned is that a 4-stroke will far outlast a 2 stroke.

[wave1235]

I run a 9.9 four-stroke extra long shaft high thrust on my Skipper 20. Will do hull speed at half throttle in flat water. You will need all the power you can get running in to a head sea though. Get the high thrust model as the larger propeller is the most important feature. I believe I could use even less pitch than the stock prop for just a little more efficiency but that would require a bit more diameter or possibly some cup to the blade. Big waves take a lot of power to motor through. Don't short yourself on power. I get between ten and twenty mpg depending on sea state. I haven't seen a two stroke that can come close to that. I am a bit disappointed in the amount of vibration from my Yamaha though. I wonder if the Honda is smoother? I would like to hear from owners of Honda outboards of similar size.

[MarioCoccon]

Hi and thanks for reply, thats the reason I want no less than 18 my friends boat run at 6.5 but when the wave came slow to 2.5 to 3 no power, I think a 18hp have more torque than a 2 stroke and it will push the motor better, the motor I looking for is the Tohatsu/Nissan 4 stroke 18hp if anyone here with one of them, i want to konw the reliability and noise. Thanks

Originally Posted by wave1235

I run a 9.9 four-stroke extra long shaft high thrust on my Skipper 20. Will do hull speed at half throttle in flat water. You will need all the power you can get running in to a head sea though. Get the high thrust model as the larger propeller is the most important feature. I believe I could use even less pitch than the stock prop for just a little more efficiency but that would require a bit more diameter or possibly some cup to the blade. Big waves take a lot of power to motor through. Don't short yourself on power. I get between ten and twenty mpg depending on sea state. I haven't seen a two stroke that can come close to that. I am a bit disappointed in the amount of vibration from my Yamaha though. I wonder if the Honda is smoother? I would like to hear from owners of Honda outboards of similar size.

[MarioCoccon]

if anyhere with a boat between 6000 to 7000 pounds get in close to 8 knts motoring???

[wave1235]

At a s/l ratio of 1.34, eight knots is a boat almost 36 feet long. The only way to achieve such speed with a shorter boat without a lot of horse-power is to have a very narrow boat for its length or to have sufficient surface area flat enough to plane or semi-plane. One may add flat wings to the back of a hull as some bartender boats do to help plane or simi-plane with sufficient hp. If velocity above hull speed is desired then a planing hull is generally the answer with the increased power required to provide lift to the hull.