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Nissan / Tohatsu 6h 4 stroke

112K views 261 replies 50 participants last post by  Ulladh 
#1 ·
I had the opportunity to work with a friend on his boat yesterday and I took the inline scale and a tachometer with me to test the performance of the engine in a static thrust test. We also did this with both the stock 8 pitch prop and a 6 pitch prop.

The tachometer is a Sendec unit good to 7000 rpm in 10 rpm increments.
The scale is a 3000 lb loadcell scale accurate to the nearest 1 lb increment.

I have been wanting to do this for a while as I have the same engine only marketed under the tohatsu brand and it has run weak since I got it. I had assumed that there were problems with my engine but based on the test results I think that there is a much larger issue.

The Nissan engine produced identical test results in RPM and thrust for the static tests with both the 6 pitch and 8 pitch props as my Tohatsu. Both engines with the 8 pitch prop could only turn 3700 to 3800 rpm and produce an average 124 lbs of thrust. With the 6 pitch prop they both turn about 4300 rpm and produce an average of 165 lbs of thrust. With the 6 pitch prop this does produce more thrust at low rpm and in static tests. However on the water this does not equate to better high end performance. The drop in pitch actually needs the engine to turn more rpms to maintain the same speed at the top end. Unfortuanatly even with a 6 pitch prop the engine does not produce enough power to even match the manufactures specified nominal 5500 rpm.

For this engine to output 6 hp the engine has to be at 5500 to 6000 rpm. On the water not tied to a dock the engine will produce a little more rpm.. It will with the 6 pitch prop get to around 4700 rpm. This is still 800 rpm below the 5500 rpm nominal and 1300 rpm below rated max rpm. The difference in 800 rpm on the thrust produced is large, 1300 is simply huge.

My conclusions based on these tests is that Tohatsu/Mercury/Nissan all the names this engine is branded under are over reporting the power produced. Their 6 hp engine produces approx the same output as a 4hp 2 stroke engine.

When I replaced my old 4hp engine with this one I was looking for more power, better fuel economy and queiter operation.

1. Power

The power is the equivalant of a 4hp 2 stroke engine based on static thrust tests (based on testing two engines-one Nissan and one Tohatsu) at the dock. On the water on the same boat that the 4hp engine was pushing the week before , the boat actually lost about 10% of its through the water performance. Going from a top average over the water speed of 5.76 knots to 5.35 knots. I had actually lightend the boat during this time period also in cleaning out a bunch of stuff that had accumulated on the boat. My expectations were sadly let down with this engine. It not only doesn’t give me the extra safety margin I had looked for in a engine 50% more powerfull than my old one it actually is just slightly less powerfull.

2. Fuel Economy

I have to give the new engine top rating for comparative fuel economy. A three gallon tank will give almost 30 nautical miles range with the new engine. This is more than twice the range I had with the old engine. It meets the stated .5 gallon per hour fuel consumtion give by the manufacture.

3. Noise and Vibration

I was expecting a much quieter operation with the new engine and it failed to give that. It is almost as loud as the 2 stroke at WOT (wide open throttle) which is the normal operating speed needed to fight the currents here. Also the engine vibrates even more than the two stroke. The Manufacture told me that this is normal as it is a one cylinder engine. The vibration is bad enough that it vibrates the whole boat at speed and if holding it the engine tiller handle will numb your hand after a while becuase of the vibration.

Another effect of the vibration and a badly designed friction adjustment on the unit is that it will not hold itself straight unless you hold onto the tiller handle. ( the friend with the Nissan version reports the exact same issues) At low rpms the torque of the engine running will instantly turn the engine to the side if not held and at WOT it will gradually twist sideways. For those in dingys and such that use the engine to turn/steer with this shouldn’t be that much of an issue as you are constantly holding the engine tiller handle anyways, but on sailboats (which it is marketed to in the long and ultralong shaft designs) it is a major problem. We have a boat tiller or wheel that we steer with, not the outboard. If not closely monitored it will introduce course changes when your not expecting it while under power.

I would imagine that this data holds true over the entire line of the 6hp engines having these model numbers. Mercury also puts these engines out but I don’t have the model numbers offhand. As to the 4 and 5 hp engines that are based on the same engine and chasis, I have no test data to say how they perform. I can only think that they will be quite weak, as well as having the same issues with vibration and noise.

MFS6BS, MFS6BL, MFS6BUL for the tohatsu’s
NSF6B1, NSF6B2, NSF6B4 for nissan

I would be interested in others experiences with these engines.
 
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#2 ·
I have the 2hp Nissan branded Tohatsu, and it has worked well for me on a Walker Bay 8 dinghy. Tohatsu is the manufacturer of the Nissan/Mercury and a few other 4 strokes under 10hp.

Currently at 38 20 10 N 76 27 42 W , Solomons, MD
 
#3 ·
This engine has been dependable and it cranks easy but the one I have and the one I tested two days ago is seriously underpowered. With the stock 8 pitch prop at WOT (wide open throttle) it only gets to about 4300 rpm while on the water. This is 1700 rpm short of max. and 1200 to 1300 short of what the manufacture called nominal 6hp at 5500 rpm.

I went back and forth for months with them trying to find if something was wrong with the engine and it was a mess. The national tohatsu people were friendly and worked with me on it but the local service center left me feeling I knew more about the engine than they did.

Eventually the national tohatsuUSA people gave up and quit.. I now understand why. There is nothing wrong with the engine I got other than it is not manufactured to meet the claimed performance. If they had labeled this engine 4hp I would have beleived it.. but 6.. not even close. I spent several hundred dollars getting the equipment to test thrust and rpm with this thing just to make sure I wasn't crazy.

In the end tohatsu basically said that my boat was two big for the engine and that you can't compare a 4 stroke engine to a 2 stroke engine. They didn't appreciate me saying I wasn't comparing a 2 stroke to 4 stroke but a 4hp to a 6hp engine.

In the end I think the engines run well and get good fuel economy. However I also think they are not made to even meet tohatsu's own performance claims. Also if looking for a quieter engine than a two stroke this isn't it.
 
#4 · (Edited)
Audeojude-

Part of the problem is that you can't really test the output of the prop sitting at the dock. The top-end RPM will be lower than if the boat was in motion IIRC. The torque and performance curves of two-stroke and four-stroke engines is very different and it is hard to compare them directly IMHO. Also, if you're not getting to the manufacturer's recommended RPM range for WOT, how is it that you expect to get the full HP out of the engine. The missing two HP that you're complaining about is probably provided by the 1700 RPM that you're short of the WOT recommended range by.

BTW, I have both a 20 HP Honda four-stroke and a Tohatsu 3.5 HP four-stroke outboard engines.
 
#7 ·
Audeojude-

Part of the problem is that you can't really test the output of the prop sitting at the dock. The top-end RPM will be lower than if the boat was in motion IIRC. The torque and performance curves of two-stroke and four-stroke engines is very different and it is hard to compare them directly IMHO. Also, if you're not getting to the manufacturer's recommended RPM range for WOT, how is it that you expect to get the full HP out of the engine. The missing two HP that you're complaining about is probably provided by the 1700 RPM that you're short of the WOT recommended range by.
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.
 
#6 ·
It is propped to the lowest pitch prop made for the engine and it can't reach nominal rpms. Um... ok... lets see... yeah I do blame the manufacture.

Both the old engine and the tohatsu are rated at hp out at the prop. This is what Tohatsu USA told me even. So 4hp is more powerfull than 6hp now. I guess math is not a strong point for me.

All I know is that the engine propped with the lowest pitch prop made for it wont reach it's nominal rpm. My old 4hp engine will push the boat faster than the new 6hp engine.
 
#8 ·
If your old 4hp outboard is correctly proped it does not surprise me that it out performs the incorrectly proped 6hp outboard.

There is a high thrust prop available from mercury for this outboard par t# 48-812951A1
8 3/8 diameter and 6” pitch with a different blade design than the standard props
 
#9 ·
If your old 4hp outboard is correctly proped it does not surprise me that it out performs the incorrectly proped 6hp outboard.

There is a high thrust prop available from mercury for this outboard par t# 48-812951A1
8 3/8 diameter and 6" pitch with a different blade design than the standard props
I will have to check that out. I do have the 6 pitch prop from tohatsu on the engine. Im not sure if that is the same. Ok the prop I have is 7.9 inches in diameter and 6 pitch. I'm thinking that adding to the diameter at the same pitch will actually decrease the rpm as it will take more force to turn it. However at this point I would be willing to try just about anything as being stuck with a less powerfull engine than I had bites chunks.
 
#10 ·
If you can find a 5" or 4" pitch prop, that would be a good thing, since each inch of pitch adds about 300 RPM to the WOT speed.
 
#11 ·
Although larger in diameter the blade design is more suitable for a displacement hull application than the props you are using which are more semi planning to planning applications.

The Mercury 6hp is the same outboard as the Tohatsu 6hp and Nissan 6hp so consulting with a local mercury dealer could be helpful, they have been known to let you return a prop that is not suitable after you try it as long as there is no damage to it.

Although your outboard was manufactured with a leg long enough to use on a sailboat, the gear ratio in the lower end is not ideal for swinging a large prop for slow speed operation which is why the Yamaha high thrust outboards have such a low gear ratio.

It would appear that with your 4hp you got lucky with the torque curve, gear ratio, and prop combination for your hull speed, and the 6hp is just behind the power curve for this application.



-Tim
 
#12 ·
Well, there may be a problem with the engine. We have a Tohatsu 6hp 4 stroke for our 9 ft AB RIB and I always thought it was slow.

A few weeks ago we were using it on long stretches on flat water, versus the usual short hops to the boat in the mooring field, we were going wide open then all of a sudden the RPMs increased and the boat took-off. It has not done that before and not done it since. Why would it do that? Is there an adjustment? Is something clogged?
 
#14 ·
I use a Tohatsu 3.5 HP four-stroke for my Achilles inflatable, and it works just fine.
 
#21 · (Edited)
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.
 
#15 ·
It sounds like you have a heavy boat - what is the Type/Brand? Length? and weight of the hull? If your previous engine was a 2-cylinder engine and you're trying to compare it to a single cylinder that may be where you problem lies. 2-cylinder and 1-cylinder engines will act very differently when trying to push heavy loads.
 
#19 ·
I have a Beneteau F235... Factory says it is 2300 lbs but most owners that have weighed them would say 3300 lbs or better.

yes 1 cylinder is different than 2 cylinder and two stroke vs 4 stroke is different. I would expect a 6 hp 2 stroke to run stronger than a 6hp 4 stroke by some small margin. but not a 6 hp 4 stroke to run weaker than a 4 hp 2 stroke.
 
#16 ·
It's the prop

The real answer to this problem is a 4.5-5" pitcjed 3 bladed prop or a 3.5-4" pitched 4 bladed prop. You would then reach WOT, attaining full HP which would increase HP out put and speed (to a point on displacement boats). Thrust would go way up, and fuel economy only slighty down. Another thing to remember, 4 cycle engines don't like WOT for long periods of time, so you would never cruise at 5-5500 RPMS or you would burn up the engine. But then 2 cycle engines don't like being run at below 1/4 throttle or they foul their plugs after a while. My engine does not twist, I have the bolt that holds the collar where the engine turns tight and I do not lube this point ever. If yours is twisting, clean out the grease here with some brake cleaner and it won't turn on you anymore, nor will it steer by the tiller well unless your a gorilla.
 
#20 ·
The real answer to this problem is a 4.5-5" pitcjed 3 bladed prop or a 3.5-4" pitched 4 bladed prop. You would then reach WOT, attaining full HP which would increase HP out put and speed (to a point on displacement boats). Thrust would go way up, and fuel economy only slighty down. Another thing to remember, 4 cycle engines don't like WOT for long periods of time, so you would never cruise at 5-5500 RPMS or you would burn up the engine. But then 2 cycle engines don't like being run at below 1/4 throttle or they foul their plugs after a while. My engine does not twist, I have the bolt that holds the collar where the engine turns tight and I do not lube this point ever. If yours is twisting, clean out the grease here with some brake cleaner and it won't turn on you anymore, nor will it steer by the tiller well unless your a gorilla.
I will try the trick with brake cleaner.. the manual states though that the friction adjuster there was not designed to hold the engine straight when in use. :(

As to WOT... thats rough.. cause if I can't get about 40% more power out of the engine it will be run at WOT for 98% of the time as that is what it now takes to get the 5 NM against 2+ knots of current each way most of the time to get to where you can sail. Against the current over the ground speed will be 3.3 knots now. hour and a half trip or better one way. With the old 4hp 2 stroke I would be doing right at 4 knots over ground and I had to run it WOT to get that. Getting the new engine I hoped to be able to run it at about 3/4 power/throttle and get the same speed... sucks to be me... WOT and it is slower by a lot. Even when dropped down to the 6 pitch prop it is slower by a lot.
 
#17 ·
SD - What size and model of Achilles do you have? I am looking at buying one for next year, and may buy now with the sales, and good $ exchange.
I am looking at the 9'6" or 10'4" in either the wood floor, or the inflatable floor. I will be towing most of the time, but will store in / on the boat for some trips. Weight difference is about 30 lbs more for the aluminum roll-up.
re: moters, looking at a honda 2hp (light and internal tank, just to get out to the boat / mooring) or 5hp (heavy but fun, able to cruise around a little faster)
Thoughts?
 
#18 ·
Northeaster-

I've got the little R2U Achilles. It's a two-person boat, which I mainly use for local weekending. For long term cruising, I'm probably going to be using my 12' portabote. :D Much more durable.
 
#22 ·
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.
 
#23 ·
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.
 
#24 ·
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.
 
#25 · (Edited)
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.
 
#27 ·
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
 
#30 ·
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.
 
#29 ·
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.
 
#31 ·
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.
 
#32 ·
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.
 
#34 ·
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!
 
#35 ·
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.
 
#36 ·
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.
 
#37 ·
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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