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  #31  
Old 10-02-2007
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Quote:
Originally Posted by newengsailor View Post
The Mercury engines under 30 hp are made by Tohatsu in Japan and I compared my 5hp Merc 4-stroke to a 5hp Tohatsu 4-stroke last summer and they are indeed identical. I believe the Nissan is also the same. My hull weighs approx 2400 lbs loaded and the 5hp is very adequate and hits hull speed. My son borrowed the motor while his Honda 8hp was being rebuilt on his 30' Catalina and it did not reach full rpms with the 5hp Tohatsu. It clearly seemed underpowered on that hull.

According to Mercury the difference in the 4/5/6hp engines is not just jetting, the carb itself is constructed differently on the inside. The props can also be slightly different. I have been toying with the idea of buying a 6hp carb just to try it out.

I think you have a case of too much boat for a 1-cylinder engine. When you're talking displacement hull such as a heavy sailboat the number of cylinders makes more of a difference than hp which also helps to explain why a 4hp 2-cylinder performs better than a higher hp 1-cylinder. My Mercury 5hp vibrates considerably more than the old 8hp 2-stroke Tohatsu I had. Mercury and the dealer both say it is operating normally. I've gotten used to it. I would say it is quieter than a 2-stroke though.

You might check the valve lash as the poster suggested above. This could definitely affect performance. If your boat is 3000+ lbs plus gear I would say that that is more likely your problem and you need to upgrade to a 2-cylinder engine either 2-stroke or 4-stroke. From your original post I thought your speed over ground difference would have been greater ... I don't think you're that far off and I'd bet that a 2-cylinder engine will be night and day difference.
I too think I have to much boat for the engine. It is in my opinion a very weak 6hp.. My boat is only 23.5 feet long. a very very substantial difference from a 30 ft boat. As I have said before I would not think much of it except that I have run a 4hp engine given its a 2 stroke and it ran stronger than the new engine. regardless of 2 stroke or 4 stroke I purchased the engine to move up from my current engine in power. The manufactures rate these things in hp. It is rated at 2 hp 50% more than my old engine and is weaker in pushing the boat than the 4hp engine. If nothing is wrong with the engine then it leaves me feeling as if the manufacture is playing some kind of numbers game with me. Misrepresenting their product. The biggest thing to me is that the engine on the water at full throttle misses max rpm by around 1400 rpm and nominal by around 900 rpm.. on a engine that is supposed to do 5500 rpm to put out 6 hp missing 900 rpm is about 20% of its rpm range. I would guess it is more than 20% of its power missing.

When some poor slob like me goes down to purchase a new engine for our boat we look at the one on it now and go.. hmm I want more.. If I have 4hp now I want more power.. 6 hp should be 50% more. Ok I will get that 6hp 4stroke.. they even have it specifically made for sailboats with 20 and 25 inch long shaft. cool.!!! so poor slob pays his money and orders it, gets it in.. diligently breaks it in and then takes it out and opens it up and .... ummm .ummm well its.. hmmm.. slower than the old engine?

Spend 2 months being polite and working your butt off for the manufacture to figure out why... to be told.. well its just not enough engine for your boat.. you obviously should have purchased a bigger engine.

hmmm... i thought I had? 50% bigger as a matter of fact.

I was semi clueless prior to this episode. I have educated myself a lot during it. It is a complex subject with what prop you have on it making a lot of difference, also the torque differences of 2 stroke vs 4 stroke making a difference. I think now that the manufactures or at least tohatsu in this case goes out of their way to play down the actual comparative performance of the engines.

When someone really cranks down and asks why... well its your fault.. your boat is to heavy, you must have stuff growing on the bottom (not), your engine mount needs to be moved up. They will not address comparative performance at all. And when asked why the engine will not produce nominal rpm they just keep saying the boat is to heavy. If the engine was producing 5500 rpm while pushing my boat with these props and it was going slow then the first thing I would do is jump in the water and look for the anchor I'm dragging.

I'm sure it is obvious i am unhappy with tohatsu. I believe that these engines are weak, not that they don't run or aren't reliable, just that they are over reported for what power they produce. I believe that tohatsu knows this and seriously downplays it.

I like the way the trolling motor company's rate their products in lbs thrust. I would love to see outboard manufactures do the same. They could rate the thrust for each engine with each of the prop choices available. I think this would introduce some meaningful numbers for end users. I don't think that the hp ratings now on the engines are meaningful numbers giving my experience with this engine.
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  #32  
Old 10-02-2007
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Nissan 6HP 4 stroke.

I caught the dealer before he shipped the 6 HP OB and changed my order to the 8 HP 2 cylinder model with electric start and 20" shaft. The 9.8 HP seems to have the same specs as the 8HP but probably detuned. I don't believe I really need the extra long shaft or more HP at extra cost.

If it was just one of you complaining about the 6 HP OB I would have purchased it. But when a couple of you complained, you convinced me.

Thanks all for your input.

Marv

PS: Has anyone tried the tunnel attachment? The two stainless steel sheet metal pieces that screw on to the cavitation plate. It should help under powered engines.
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  #33  
Old 10-02-2007
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I have a 4hp Tohatsu and it runs for crap it is loud and will not idil. Give me a 2 stroke any day.
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  #34  
Old 10-03-2007
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Tohatsu 6 hp

I too regret buying a Tohatsu 6hp this summer. Mine vibrated so badly at first that I couldn't hold on to the handle. The vibration was several inches and was both at idle and under load in gear. My dealer replaced the prop with another from Tohatsu and that helped some but it still vibrates more than any other 2 stroke I've ever owned in the past.

My worst complaint is the inability to tighten the rotation wing nut to prevent rotation. I took it back to the dealer and they replaced the relavent parts but now its even looser. At low speeds there is absolutely no resistance and the motor immediately flops 90 degrees to the side and I promtly turn and hit the dock. I have it on my Beneteau 210 which weighs 2400 lbs.

I tried to get the customer service at Tohatsu to take this one back but they have refused even though there is a guarantee.

The 6hp does move my boat at 6 + knots where my old Merc 5 two stroke moved at about 5.2 knots. It is not as quiet as it should be compared to a friends older Merc 5 hp four stroke. This older engine is absolutely solid, no vibration and is quiet. I think they used to make them better.

My new outboard is the only new one at our lake this summer and is all the talk at the club. We skippers like to get on one boat and sail together and they all laugh at my shaking motor. Tohatsu won't sell any more around here! They should be more helpful!
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  #35  
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Marv,

Good thing you did with a 5,000 lb hull. You will be very happy with the 8hp 4-stroke 2-cylinder... its a great deal smoother when compared to a single cylinder like the 6hp. The extra cylinder will also give you the needed push for your application. The only downside is it does weigh more but I think you will find it tremendously more powerful over a single banger. If I were buying a new outboard I'd go with a 5hp again because I like the weight and the vibration doesn't bother me as it does some of the others here but if my hull was heavier (like my son's Cat 30) I'd opt for the 8hp or 9.8hp. The extra cylinder makes all the difference in the world. You can even see how a 4hp 2-cylinder performs better than the 6hp 1-cylinder that Audeojude is using. It just can't handle the heavier loads as well. The Mercury "9.9hp" (2-cylinder 4-stroke) is the same as the Tohatsu/Nissan "9.8hp" (not sure why there is a .10 difference in the rating) at any rate it is a sweet engine and runs so smooth and quiet. Several guys have these in our marina under all three "labels" and they love them.
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  #36  
Old 10-03-2007
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in reading of your efforts to obtain the expected performance of your nissan/tohatsu 4s 6hp , I have this to say.
1 never trust salespersons unless they were successful engineers first! just collect the info they have and decide on a commitment later.
2 your 6hp engine is rated for 6hp at the output shaft with a metering device not a propeller not even a squirrelcage.
3 your engine may still develop 6hp on your boat provided that you load the engine properly.
4 your are not going to change the shape of your boat I presume so you will have to change the pitch of the prop. and unless you are a toolmaker such as myself and have a lot of time and patience (not I) with this issue you may have to wait like myself for someone to offer a lower pitch prop that will unload our engines just enough so that they will reach the engines rated horsepower/rpm range without overspeeding. with a relative water speed of 5.5 to 6.5, knots because of our hulls (mine a catalina 22),I would be surprised to see the likes of a massproducer such as nissan to take on the challenge.
5 as an aircraft inspector as well, I must confess that I questioned the salespersons until I became certain that there was no data to be obtained in the USA through them that would allow myself to believe the 6hp model would achieve the rated horsepower/rpm with a relative water speed of 6 to 7 knots with the finest pitch prop available then. a few months later I purchased the 6 hp and the then finest pitch prop of 7in.
I used a slack tide ,gps,photo tachometer and reflective timing tape on the flywheel to gauge performace on my catalina 22 with the 7in. pitch prop installed.after starting to break in the engine per the owners manual,I realized the engine would not reach the rated horsepower/rpm with my hull/prop configuration.so much for complete engine break in. no surprise.
best performance was obtained briefly just to get readings in each direction. hull speed=6.15 knots at 4300 rpm.hull speed=5.73 at 4000 rpm.I run 4000 max. until I get a much finer pitch prop-when it is available.

6 I believe you were sold a fine piece of machinery as was I (mine a nissan 4s 6hp) and hopefully we will both come across the correct prop for our hull application.
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  #37  
Old 10-03-2007
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I have a 9' RIB, and had a 1984 Johnson 8 hp on it until last year. With two people the boat would practically leap out of the water onto plane. I ended up buying the 6 hp Tohatsu, because I coouldn't tolerate the extra weight of an 8 hp. The boat is such a dog now it barely gets on plane with a single 150 lb teenager. I have tried all different trim angles, and a tiller extension to move weight farther forward to bring the bow down. Nothing has worked. I havent tried a different prop as it doesn't get used for much other than general dinghy duty. This application is nothing like pushing a heavy boat. I haven't minded the vibration or the noise but...
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  #38  
Old 10-03-2007
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I wish someone had started a thead like this before I bought my engine. I searched the internet for days for reviews of these engines and couldn't find much. Most of them were of tohatsu engines in general and people had a lot of good things to say about longevity etc... the only decent review of this specific engine was the practical sailor one and now I know it was seriously incomplete.

Buyer beware. If I were someone looking at a sub 8 hp engine I would look at engines other than tohatsu now. If possible i would also never buy a one cylinder engine.
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  #39  
Old 10-03-2007
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This isn't just limited to Tohatsu engines btw. I have a Honda 20 four-stroke on my boat... and until I got it propped right, it wasn't really capable of giving me the performance it should have. IIRC, the stock prop is 9.625" x 10". It wouldn't get anywhere near the WOT range of 5000–6000 that Honda recommends. The prop I finally ended up using on it is a 9.5" x 7", and at WOT it settles in around 5500 rpm.

You've also got to remember that most of the the smaller OB motors, regardless of whether they're two-stroke or four-stroke, aren't designed to push a large 22'+, 3000 lb. sailboat. The ideal outboard motor for that purpose would have a much lower gear, and use a larger diameter prop. Most of the outboards are geared at about 2:1, and you'd really need one geared at 3:1 or so with a 12-13" prop IMHO, but the market for such a beast is so small that no manufacturer is going to make an outboard like that.
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  #40  
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Audeojude why don't you put your 6hp for sale? The 6hp will work for lots of smaller boats such as mine and your engine is practically brand new so you should get top dollar. Then buy an 8hp or larger. I think the 8hp is roughly about $300 more and you would be much happier with it.
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