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Old 20-07-2010, 00:42   #1
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Outboards: Two Stroke / Four Stroke

I have placed this in the multihulls section, because more and more of the newer multihulls, power or sail or both are using outboards (properly positioned) instead of diesels for a number of reasons.

I designed my 40 ft motor/sailor trimaran for Outboards because they are lighter, cheaper, and can be lifted clear of the water when beaching.

I have now replaced my original purchase of 2x 30 hp four stokes with 90 hp two strokes.

After a lot of consideration I have chosen 2 X 90 hp Tohatsu Two strokes (old technology but latest design).
Not fuel injected(tldi) but good old, carb based two strokes,
Why.

Because they were half the price of the four strokes.
AUD $8,000 each versus $AUD $15,000 for Honda four stroke.
Reliable tests have shown me that if I run the two strokes at 4,000rpm they are only 10% less efficient than four strokes run at 4,000rpm.
Plus, where I am going (out of the way places) anyone can fix two strokes, where as four strokes etc are just too complicated.

I have saved $14,000 which will buy a lot of fuel, plus the two strokes are much lighter, which is important in a multihull.

The plan is to use one 90 hp for cruising (at 10 knots) and 2x 90 hp for that, get there quick, scenario (16 knots)

I will let you know how it all works out in the next couple of weeks.
I also have an inexpensive Oceanic lateen sail.
More on that later
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Old 20-07-2010, 00:56   #2
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I am a very, very big fan of two-strokes, and wouldn't have anything else on my dink or on my snowmobile, but I seriously doubt that the difference in fuel consumption will be only 10% compared to a four-stroke. A carbureted two-stroke marine engine is tuned rich anyway to avoid risk of overheating, and besides that wastes a good bit of fuel-air mixture out the exhaust port. As far as I know, the specific fuel consumption of carbureted two-stroke outboards is at least 50% more than comparable four-stroke, and can be as much as double.

We have a 25-horse Mariner two-stroke on our dink. I love it to death; it's a great motor, reliable, responsive, smooth. But it sucks down fuel at about the same rate as the main engine on the mother ship. Even babying the throttle, we can hardly get two hours out of a 10 liter fuel tank.
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Old 20-07-2010, 01:15   #3
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I have a new 15hp-2 stoke yamaha and can run most of the day on a about a gallon- love it
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Old 20-07-2010, 03:27   #4
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I have a new 15hp-2 stoke yamaha and can run most of the day on a about a gallon- love it
That's amazing. Maybe the new ones are not tuned as rich as the old ones. Our Mariner is about 10 years old. It is pushing an Avon 340 RIB with center console weighing about 100 -- 110 kilos empty. We like to get it up on a plane and then back off the throttle until it just stays there. Like that -- two hours on a tank. Maybe it would be more if we were not planing.

But don't get me wrong -- I love that motor, which weighs only about 50 kilos for 25 honest horsepressure. I would never want anything but a two-stroke on a dinghy.
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Old 20-07-2010, 05:15   #5
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That's amazing. Maybe the new ones are not tuned as rich as the old ones. Our Mariner is about 10 years old. It is pushing an Avon 340 RIB with center console weighing about 100 -- 110 kilos empty. We like to get it up on a plane and then back off the throttle until it just stays there. Like that -- two hours on a tank. Maybe it would be more if we were not planing.

But don't get me wrong -- I love that motor, which weighs only about 50 kilos for 25 honest horsepressure. I would never want anything but a two-stroke on a dinghy.
wow you have a gas hog there, maybe you can ajust it some- I have a 11ft HP floor it weighs somthing like 100lbs, I drive it like you do -just on plane- engine weighs around 80 pounds
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Old 20-07-2010, 06:10   #6
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Quote:
Originally Posted by beau View Post
After a lot of consideration I have chosen 2 X 90 hp Tohatsu Two strokes (old technology but latest design).
Not fuel injected(tldi) but good old, carb based two strokes,
Why.

More on that later
Guessing the most important issue, is based on your boats design is to get the right pitch on the props. Are you using the standard props?
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Old 20-07-2010, 15:22   #7
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I have seen some independant tests that clearly show a newer model two stroke only uses 10% more fuel than a four stroke if, if it is running in the 3,500 -4,500 rev range. Where the four stroke wins is at trolling speed.
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Old 21-07-2010, 02:30   #8
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I have seen some independant tests that clearly show a newer model two stroke only uses 10% more fuel than a four stroke if, if it is running in the 3,500 -4,500 rev range. Where the four stroke wins is at trolling speed.
That's really interesting -- can you provide a link?
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Old 21-07-2010, 22:53   #9
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10% difference in fuel use at 4,000 RPM? Not my experience.
I repowered my 5.3 m plate alloy Sportfish fishing boat in 2006 using a 90 hp 4-stroke Suzuki. The original 90 hp 2-stroke Yamaha was very similiar to Tohatsu non-TLDI 90 hp that Beau is considering. At 4000 RPM it used around 17l per hr. Being an older motor this is similiar to the tested fuel useage of a new Tohie (Tohatsu M90A – Boat Reviews, Tests & News - BoatPoint Australia)
The new Suzuki DF90 pushes the same boat at the same speed at 4,200 RPM and uses 12.5 l /hr. That's around 26% less fuel.
As Beau says the difference is far greater at troll speeds.
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Old 22-07-2010, 00:22   #10
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I got my figures from Australian "Fish and Boat" by Peter Webster.
How old was your 90 hp yahama.
Currently I have 2 X 30 hp Honda four strokes and they use 10 liters and hour each at 4,000rpm
I was figuring on 20 liters and hour from one 90 hp Tohatsu at 4,000rpm.

The fact is, I saved $14,000 by buying 2 X 2 stroke rather than 2 X 4 stroke.
The honda dealer answered by questions of "why so expensive" by saying "it costs money to SAVE the environment".
I was not impressed.
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Old 22-07-2010, 00:39   #11
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In California and maybe the rest of the U.S., 2 strokes are being or will be outlawed because of pollution standards.
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Old 22-07-2010, 00:40   #12
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In California and maybe the rest of the U.S., 2 strokes are being or will be outlawed because of pollution standards.
This applies only to new engines produced.
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Old 22-07-2010, 00:56   #13
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The evinrude etec and Tohatsu TLDI have better pollution standards than four strokes.
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Old 22-07-2010, 01:26   #14
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The evinrude etec and Tohatsu TLDI have better pollution standards than four strokes.
Please do nto attempt to confuse the politicians with facts.



Dockhead said;
Quote:
...the specific fuel consumption of carbureted two-stroke outboards is at least 50% more than comparable four-stroke, and can be as much as double....
These figures are greatly inflated, the 4 stroke is not going to save that much fuel... unless you are (as others have said) comparing consumption at very low RPM.

My experience is consistent with others here, there is an improvement in economy, but not as significant as the weight increase...

My current 4 stroke replaces a 2 stroke... and will be replaced with another 2 stroke.
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Old 22-07-2010, 01:26   #15
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10 l/hr fuel burn (TEN!) from a Honda 30hp? Wow!

My first thought is "What's wrong with the installation?" Prop ratio or condition? Mount height? 4 barrel Holly carbie fitted? (Joke)

I trawled through some old F & B mags (an Aussie fishing and boating monthly) and didn't find a Honda 30 test, but a few tests of interest. I love the accurate fuel useage data that Webster does on his boat tests.

Yamaha 40hp (4 st) on Allycraft 435: 6.1 l/hr @ 4000 RPM
Yamaha 60hp (4 st) on Horizon 490 : 9.7 l/hr @ 4000 RPM
Honda 20 hp (4 st) on Horizon 3.4m: 3.3 l/hr @ 4000 RPM
( max fuel burn: 7.2 l/hr @ 6100 RPM)
Honda 50 hp (4 st) on Signature 460C: 8.7 l/hr @ 4000 RPM

I'd guess that 10 l/hr is maximum WOT fuel burn for a Honda 30. Not a good idea to run any engine for too long like that.
If you want to go the outboard way, look closely at gear ratios besides prop pitch. Honda sell a 20 hp in Australia that has an ultra long leg (26 inch I think) and low ratio gearing. It is a popular fit to catamarans, especially the Orams in the 12 -14 m range in this part of the world. Even though I don't have personal experience with this engine, I've heard good reports.
Methinks the Tohie 90 on the back of a big cat will make a lot of noise, froth and smoke, chew through the juice and not push all that well.
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