Nissan / Tohatsu 6h 4 stroke

[audeojude]

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.

[PBzeer]

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

[audeojude]

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.

[sailingdog]

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.

[Timrodabaugh]

If you have the outboard incorrectly proped for the application and cant get to the peak power RPM how is that the manufactures fault again?

[audeojude]

Originally Posted by Timrodabaugh

If you have the outboard incorrectly proped for the application and cant get to the peak power RPM how is that the manufactures fault again?

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.

[audeojude]

Originally Posted by sailingdog

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.

[Timrodabaugh]

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

[audeojude]

Originally Posted by Timrodabaugh

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.

[sailingdog]

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.