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  #21  
Old 09-25-2007
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Quote:
Originally Posted by sailingdog View Post
I use a Tohatsu 3.5 HP four-stroke for my Achilles inflatable, and it works just fine.
I'm not saying this engine won't work fine for some applications. It cranks reliably, gets good fuel economy, the ultra long shaft is the bomb.. very very nice.. but the engine doesn't have the power of my 4 hp 2 stroke. For me this is end of story.. after 2 months working with the manufacture to figure out why. I made modifications to my engine mount per their request and tried the lowest pitch prop they sell for the engine and in the end they gave up and just said my boat is to big/heavy for the engine. I agree with them now after testing the second engine on my friends boat and getting the same performance numbers from it. My boat is to big and heavy for the engine. Its not to big and heavy for my 4 hp 2 stroke engine though.

As far as putting the engine on a dinghy.. it should work great for that. Just don't expect it to be a power house. According to all I have read and I have read, and thats a lot, it should run reliably for a long time. It should get great fuel economy. Don't expect it to be much quieter than a 2 stroke at full throttle though. Also it vibrates a lot which the dealer and manufacture both went out of their way to tell me is just normal for these one cylinder engines. If you can afford the weigh I would for sure go to a 2 cylinder engine. I wish now I had just for the quiet and smooth running. not to mention the extra power. After getting this engine and having issues with the performance both the dealer and manufacture started telling me that they thought I should have gotten the 8 or 9.8 engine for my boat.

The whole situation has left a bad taste in my mouth. The dealer I bought from on the internet was useless. When I first contacted them about it they didn't even bother to follow up with me after saying they would. Tohatsu USA, the manufacture did work with me for a long time trying to run down what the issue was. Even though I'm unhappy with the engine they did work with me and gave good customer service to a degree. their service center on the other hand was terrible, in my opinion incompetent. I spent a lot of money purchasing this engine and it hasn't met any of the criteria I had other than fuel economy. Quieter operation and more power than my old engine were flops. I am trying to return the engine but there is doubt over being able to do so. If I can't I'm stuck with 1400 dollars for an engine I could have spent 700 for in another new 4 hp 2 stroke.

Last edited by audeojude; 09-25-2007 at 09:39 PM.
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  #22  
Old 10-02-2007
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Audeojude,

Did you perform the same test on your old 4 horse that you did on the two 6 hp/4 strokes? It would be interesting to see the results. I too, have a Tohatsu 6hp, 2007 model, ultra long shaft on a North American Spirit 23, probably about 3400#s with all the s*** I've got on board. I get about 5.2 knots(GPS) in light air, and I can tell the unit is no where near top end..if I drop it down a bit, I can still get about 5, but again, light air, a headwind of any significance knocks it down a half knot or so, maybe a bit more. I have to agree with you on the economy.. on the first few "fillups".. I think the tip to the dockhand was more than the gas.. and I"m not the biggest tipper on the water. This is the only engine I have ever had on this vessel, so I have nothing to compare to; however, when I get it wet again next week, I intend to put an older 9.9 two stroke on for a comparison. Again, it would be interesting to see how those "bollard pull" results from your old 4hp/2stroke measure up to your newer 6. What would be even MORE interesting would be the same test between a NEW Tohatsu/Nissan, 4, 5, AND 6 hp.
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Old 10-02-2007
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Quote:
Originally Posted by messenger View Post
Audeojude,

Did you perform the same test on your old 4 horse that you did on the two 6 hp/4 strokes? It would be interesting to see the results. I too, have a Tohatsu 6hp, 2007 model, ultra long shaft on a North American Spirit 23, probably about 3400#s with all the s*** I've got on board. I get about 5.2 knots(GPS) in light air, and I can tell the unit is no where near top end..if I drop it down a bit, I can still get about 5, but again, light air, a headwind of any significance knocks it down a half knot or so, maybe a bit more. I have to agree with you on the economy.. on the first few "fillups".. I think the tip to the dockhand was more than the gas.. and I"m not the biggest tipper on the water. This is the only engine I have ever had on this vessel, so I have nothing to compare to; however, when I get it wet again next week, I intend to put an older 9.9 two stroke on for a comparison. Again, it would be interesting to see how those "bollard pull" results from your old 4hp/2stroke measure up to your newer 6. What would be even MORE interesting would be the same test between a NEW Tohatsu/Nissan, 4, 5, AND 6 hp.
I would be interested in how the 4 5 and 6 hp tohatsu/nissans compare against each other also.

I did do the tests on my old 4 hp johnson 2 stroke 20 inch shaft. It would generate 120 to 130 lbs of thrust consistently in a static / bollard test at full throttle. The tohatsu 6 hp produced an average 127 lbs of thrust with an 8 pitch prop in the same test. The tohatsu produced approx 160 lbs of thrust with the 6 pitch prop in the static test.

The 4hp 2 stroke would push my boat untrimmed at 5.65 to 5.75 knots over water consistently in good conditions. Trimmed It would push the boat at 6 to 6.2 knots over water. I have done 30 mile trips up the waterway under power and seen these speeds consistently over the entire trip with the old engine.

The 6hp 4 stroke would push my boat 5.35 to 5.4 knots and get to around 5.6 when the boat was trimmed with the 8 pitch prop. With the 6 pitch prop it would push the boat 5.45 knots and about 5.6 when trimmed. lowering the pitch did bring rpm up some on the water, but still left it much below the manufactures nominal 5500 rpm (6000 max rpm). The drop in pitch increased torque but because of the lower pitch did not increase top speed significantly. If the engine had been powerful enough to turn at the full 5500 rpm with these props then I would expect to have seen results consistent with what I expected of a 6hp engine. I ran the theoretical numbers through several online prop calculators and with my boat and these props running at their rated rpm I should see around 6.3 knots under average conditions and closer to 7 with the boat trimmed.

The online calculators did predict fairly accurately the performance that I actually see with the 4hp 2 stroke engine.

My observations lead me to think that the 6hp 4 stroke is putting out about the same energy as the lower powered 4hp 2 stroke and that in the high end of the rpm range that it is putting out very significantly less power. The torque curve for a 2 stroke has more power higher in the rpm range. I would expect in a normal straight up test between at 6hp 2 stroke and a 6hp 4 stroke that the 2 stroke would mildly outperform the 4 stroke in the top end. I would not expect a 4hp 2 stroke to outperform a 6hp 4 stroke though.
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Old 10-02-2007
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Horse power from a motorcycle drag racers point of view.
I measure HP in .001 seconds and .001 MPH changes.
The things that change these figures are temprature and altitude and humidity. Since it appears that you are sailing on the ocean and not a lake at 6,000 ft. here is my guess.
Carburator jetting, I think you are running lean.
Why?
The pump gas is not just gas any more. It has some amount of alcohol in it. It takes twice as much alcohol as gas to make the same amount power.
The more alcohol the leaner the motor will run so a bigger jet will be required. E-85 will require almost twice the amount fuel (by weight) than straight gas.
You have the equipment and the mindset to try rejetting. Just get the two next size larger jets for the carb and see what happens.
Fuel injected motors with oxegen sensors can modify their fuel curves via the ecu but carburator engines cannot.
I just think you motor is nosing over because it is to lean to make the hp.
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Old 10-02-2007
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It's very possibly/probably is running lean and if my dispute with returning the engine doesn't work out I probably will go that route. I just think it is a shame that out of the box it doesn't perform to the manufactures own specifications.

As said above I would love to test the 4hp 5hp engines against the 6hp. My understanding is that the engines themselves are identical. I have been told that it is only the jets that are different in the carbs. I have also been told that they are different carburetors on the different models. But the manufacture themselves will tell you that other than something in the carburation being different that the engines are identical for all three models.

I had actually had the thought at one time that my engine mistakenly got jets for a 4hp engine in it. However since testing the second 6hp engine and getting the same test results I no longer think that is likely.

Last edited by audeojude; 10-02-2007 at 03:05 PM.
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Quote:
Originally Posted by audeojude View Post
Oh I so totally agree with you.. That 1700 rpm is exactly where my 2 hp im looking for is. Based on getting the same results on two engines now. I'm of the opionion that the engine is not producing enough power to turn the lowest pitch prop made for it at nominal rpms. This to me means the engine is under powered to do what it is called for from the factory to do.

regardless of all the numbers (rpm/thrust etc) at the end of the day the biggest difference to me is that a 13 year old 4hp 2 stroke engine will push my boat faster than a brand new 6hp 4stroke.
My expertise is in much larger engines, but I have to ask about the noise...is it mechanical, or intake/exhaust roar? On engines with very small cylinders, the valve lash can make an enormous difference in performance. Check the lash, check the spec and if you find that the lash is on the loose side or looser than spec, try tightening it to the tight side of spec. If the main jet in the carb is replaceable, go up one size and retest. if it's not replaceable, consider the cost of a new carb and decide whether or not to drill out to the next size (you sound capable of using good science). Unless the engine is truly not engineered to produce 6, then only a few things can restrict the power that badly. Do the spark plugs have any color at all? If not, certainly you can benefit powerwise by adding some fuel. I know how manufacturers are fast and loose with power numbers, but 50% off sounds like some sort of restriction.
Here's a good one-I had a motorcycle once I thought too slow, and checked everything. A friend of mine, called in frustration found that the throttle wasn't being pulled open all the way. Check to see that you're not working with 2/3 open carburetor.
\eric
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Old 10-02-2007
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You guys are making me very depressed. I just ordered the 2008 6HP Nissan OB engine for my Morgan 25 which weighs 5,000 pounds. The Nissan dealer says the only options for the prop is 7, 8 or 9 inch and the engine comes with the 8 inch prop.
Some questions for Audeojude: What year engine did you test? A 6" pitch prop is not offered on the new engines. I'm currently using a 3.5 HP Nissan 2 cycle outboard and it pushes the boat at 5 knots with no winds blowing. Where can I buy a smaller pitch prop? Are you sure your instrunents are calibrated properly? I might still be able to exchange the OB for the 8HP model but don't want the extra weight and I also wanted the extra long shaft.
Please respond ASAP

Marv
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Old 10-02-2007
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Quote:
Originally Posted by elacruze View Post
My expertise is in much larger engines, but I have to ask about the noise...is it mechanical, or intake/exhaust roar? On engines with very small cylinders, the valve lash can make an enormous difference in performance. Check the lash, check the spec and if you find that the lash is on the loose side or looser than spec, try tightening it to the tight side of spec. If the main jet in the carb is replaceable, go up one size and retest. if it's not replaceable, consider the cost of a new carb and decide whether or not to drill out to the next size (you sound capable of using good science). Unless the engine is truly not engineered to produce 6, then only a few things can restrict the power that badly. Do the spark plugs have any color at all? If not, certainly you can benefit powerwise by adding some fuel. I know how manufacturers are fast and loose with power numbers, but 50% off sounds like some sort of restriction.
Here's a good one-I had a motorcycle once I thought too slow, and checked everything. A friend of mine, called in frustration found that the throttle wasn't being pulled open all the way. Check to see that you're not working with 2/3 open carburetor.
\eric
Noise:
i don't think it is exhaust as that comes out underwater through the prop. To be honest im not sure.
I am not going to adjust internal components on the engine till i know whether I can return it. However I have thought about going with a larger jet. The throttle I believe is opening all the way. It is actually all the way open at about 3/4 throttle on the throttle control. Turning the throttle control more than that does not increase rpm or change sound of engine at all. The manufacture states that this is normal behavior and that at about 3/4 throttle on the throttle control is actually full throttle.

I would love to test more of these engines to see how they rate comparatively. When it was just mine I thought something was mechanically wrong with it. After testing the second one and getting the same test results I am no longer convinced that it is broken or malfunctioning. Just weak, as in it just runs weak from the factory.
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boat is probably too heavy for a single banger, also agree with lash comment above

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.
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Old 10-02-2007
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Quote:
Originally Posted by marvshirley View Post
You guys are making me very depressed. I just ordered the 2008 6HP Nissan OB engine for my Morgan 25 which weighs 5,000 pounds. The Nissan dealer says the only options for the prop is 7, 8 or 9 inch and the engine comes with the 8 inch prop.
Some questions for Audeojude: What year engine did you test? A 6" pitch prop is not offered on the new engines. I'm currently using a 3.5 HP Nissan 2 cycle outboard and it pushes the boat at 5 knots with no winds blowing. Where can I buy a smaller pitch prop? Are you sure your instrunents are calibrated properly? I might still be able to exchange the OB for the 8HP model but don't want the extra weight and I also wanted the extra long shaft.
Please respond ASAP

Marv
Marv,
My tohatsu 6hp engine is a 2007 engine. The nissan 6hp i tested is a 2005 model. Unless they changed the shaft that the prop goes on you should be able to buy any prop sold for the nissan/tohatsu/mercury engines in the 4/5/6 hp range.

Someone on this discussion mentioned a mercury power prop that fits these engines. I have not tried it but it would be what I went to next if I do anything. The tohatsu 6 pitch prop is 52 dollars if i remember right ordered from tohatsu.

For what its worth my boat is much lighter than yours and very sensitive to weight on the transom I bet. If I were you I would go to the 9.8 which has the ultralong shaft and is two cylinder engine. I am told it runs smooth as silk and very quietly.. I do not have personal experience of this though. I dont think you are going to see a huge difference on a 5000 lb boat of the added 30 lbs of weight between the 6hp and 9.8hp engines. I don't believe the 8hp comes in the ultralong 25" shaft model

I would think that you would move boat speed up to 6.5 knots or thereabouts if you are getting 5 with a 3.5 motor. However take this with a grain of salt as I do not have hands on experience with the 8 or 9.8 engines.

My knot meter and gps both are in agreement as to what speed the boat is going. the currents where I am are very consistent and my gps speed always agrees with the knot meter + or - the current. On the ocean with no current they will both agree to .05 of a knot.

given the 1 to 1.5 hour trip each way to go sailing I have a lot of accumulated data with the old engine and the new engine on the water with it running. I pulled the new engine when I decided to try and return it just before my 2 months grace period for disputing a credit card charge was up. In the two months working on this engine with the manufacture and in the normal course of operations where I am I put in excess of 40 hours on it. I was disappointed in it.

Sadly i had a opportunity for a 9 or 8? hp honda 25 inch shaft outboard 6 months prior to purchasing this engine for the same price. It was used only 20 hours on it. I bitterly regret not having the money to purchase it then.
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