Nissan / Tohatsu 6h 4 stroke

[audeojude]

Originally Posted by FijiSomeDay

The "rotation tension adjust screw" does not hold firmly, appears to be a design defect for this application. I'd like it to be fixed in place. I will consider milling a flat spot on the shaft as described above.

That is not a design flaw..
I thought the same thing and tohatsu pointed me back at the manual and it did clearly state that it is not intended to fix the motor in place but to only be used as a friction device to adjust how easily the motor turns. They recommended that I improvise a mount from the boat to one of the holes on the engine designed for attaching a steering system. Or from the rudder to the engine so that the rudder could control the engine direction.

[carball]

Outboards are mandated by Federal Regulation to state hp at the prop as crankshaft rated hp is useless as a measurement in outboards due to fixed gearing.

[speedsquare]

I`ve read lots of this thread. I will just add comments on my own experience. J/27 with '06 Nissan 6hp 4s L/S - std prop. This engine pushed the boat at 6kts at a little more than 1/2 throttle. The tension screw held the engine in place. I don't think that the engine vibrates any more than any other single cylinder engine. I will upgrade to the Hi Thrust prop to improve efficiency and give better response. I don't have any comparrison, but from my expereicne it is a sound engine. My only caveat would be that you carefully inspect your engine bracket from time to time. I found a major crack in a critical part that needed to be welded.

[INMA]

I have a 5hp Mariner outboard with a high thrust propeller. It was great in forward and useless in reverse due to cavitation in the exhaust gases.

I have modified my propeller and another yachts propeller and have fixed the problem with reverse. This suits all Tohatsu/Mercury8Mariner based 4, 5 and 6hp outboards.

Here is an explanation of the modification.

http://www.trailersailerplace.com.au...p=74217#p74217

There is no way I am posting all that information on this site. If someone knows someone at Tohatsu or Mercury, let them know there is a simple fix for their lack of reverse on the small high thrust propeller.

[TohatsuGuru]

Your modification reduces the propellers efficiency in forward to help reverse. As engines are more frequently run in forward the modification isn't practical for most users. That's not to say that you or someone else would not be willing to gain reverse thrust at the expense of forward...Just not most people.

[tommays]

prop venting is nothing new

[INMA]

Tohatsu Guru, there was no noticeable change in thrust in forward, if anything the engine noise might have been a bit quieter but I did hull speed with a 24 foot yacht towing a 10 foot Walker Bay dingy.

Another 26 foot yacht with the same propeller reported hull speed as well so I have no evidence of reduced thrust in forward.

In addition even if there was some loss of forward thrust the availability of a reasonable reverse thrust is worth it. The standard Tohatsu high thrust propeller is considered unsafe for use on heavy boats by some owners, hence the search for improvement.

I am also aware that hydrodynamic devices like propellers develop their greatest efficiencies on the surfaces with the greatest water velocities, any loss of thrust due to gasses escaping in forward through the venturi would be at the root of the blades which do very little work.

You were very quick with negative comment I wonder whether you have modified a propeller and tested it or is it your expert opinion.

Fact is I copied a similar modification from a larger Tohatsu outboard that was useless in reverse before the modification, there are at least 4 propellers with similar venturis where the owners were unhappy with the standard reverse thrust and after the modification we are all impressed with the improvements. That is fact.

If you have a Tohatsu propeller that works in reverse for the 4, 5, 6, 8 or 9.8hp outboards please let us know. Anyone that can add positive information on this issue is welcome to comment but don't trash my post without facts.

Tommas, I am new to this forum and do not have the rights to use links. I already admitted to copying other's work there are few things truly new in this world.

[INMA]

Tommas, I Googled "prop venting" and learnt Mercury do that stuff on their high speed products. Its a smart idea.

The venting I used only works in reverse to assist sucking the exhaust gasses away from the flow over the propeller.

Looking at the Mercury information their vents do not change the performance at the higher engine speeds which tends to confirm the venturis on the smaller propeller won't change much at full engine throttle.

Thanks for that information, its a good day when you learn something.

[pvanv1]

Originally Posted by INMA

If you have a Tohatsu propeller that works in reverse for the 4, 5, 6, 8 or 9.8hp outboards please let us know. Anyone that can add positive information on this issue is welcome to comment but don't trash my post without facts.

Indeed the HT prop for the 4/5/6 doesn't improve reverse much, if anything, over the standard high-speed props. However, for the MFS8/9.8, there are two props that do. I use them on sail applications all the time. They are 3B2B645110 PROP 8 & 9.8 HP (8.7 DIA X 5 PITCH)-4 BLADE (HIGH THRUST), BLACK $69.67 and 3B2B645120 PROP 8 & 9.8 HP (8.7 DIA X 7 PITCH)-4 BLADE (HIGH THRUST) BLACK $69.67. They have oversized hubs (to relieve the exhaust in reverse), large, flat blades (with larger area at the periphery), and give stellar performance in reverse. They are made by Solas for Tohatsu.

You can also get a similar prop for the MFS9.9/15/18/20 from Solas.

[TohatsuGuru]

Originally Posted by INMA

Tohatsu Guru, there was no noticeable change in thrust in forward, if anything the engine noise might have been a bit quieter but I did hull speed with a 24 foot yacht towing a 10 foot Walker Bay dingy.

Another 26 foot yacht with the same propeller reported hull speed as well so I have no evidence of reduced thrust in forward.

In addition even if there was some loss of forward thrust the availability of a reasonable reverse thrust is worth it. The standard Tohatsu high thrust propeller is considered unsafe for use on heavy boats by some owners, hence the search for improvement.

I am also aware that hydrodynamic devices like propellers develop their greatest efficiencies on the surfaces with the greatest water velocities, any loss of thrust due to gasses escaping in forward through the venturi would be at the root of the blades which do very little work.

You were very quick with negative comment I wonder whether you have modified a propeller and tested it or is it your expert opinion.

Fact is I copied a similar modification from a larger Tohatsu outboard that was useless in reverse before the modification, there are at least 4 propellers with similar venturis where the owners were unhappy with the standard reverse thrust and after the modification we are all impressed with the improvements. That is fact.

If you have a Tohatsu propeller that works in reverse for the 4, 5, 6, 8 or 9.8hp outboards please let us know. Anyone that can add positive information on this issue is welcome to comment but don't trash my post without facts.

Tommas, I am new to this forum and do not have the rights to use links. I already admitted to copying other's work there are few things truly new in this world.

Negative?

I didn't think it was negative. Just a casual warning that to get one thing you do give up another. As I said, it's not for everyone. And yes, I have modified propellers to evacuate exhaust gas. It's a very old technique with limited results. I grant that even a modest increase for some people is a major achievement. Just keep in mind it's not new and has side effects. I hope it continues to work for you and others...I just don't think it is worth the forward thrust loss that you didn't notice.