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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance > Engines > Outboard
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  #101  
Old 11-05-2007
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Rockter will become famous soon enough
The choice of prop is everything for an outboard.

The engine manufacturer will state the power of the motor, and it's just that... the rate at which the motor will do work. It's probably measured at the crankshaft, and using a dynamometer. It will be stated at a certain RPM of the motor too.

On the water, the power PRODUCED will be the product of the forward propulsive force and the velocity of the boat.

The problem with the static test, is that the prop will simply have the wrong "gearing" .... after all, you are stirring water, and the boat is not moving.... and that is very different for when you are moving.

For example, your outboard when tied to a crise liner of 60,000 tons will simply be a food whisk... stirring water ("underspeeding").

Tied to a long thin rowing boat, it will be moving so quickly that it will be overspeeding... it would be capable of going faster, but cannot take enough "bite" in the water.

Everything is prop choice.
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  #102  
Old 11-05-2007
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To a degree I agree different props do have an effect. However this engine as a class (Tohatsu, Nissan and Mercury versions) doesn't seem to have the power to turn at nominal rpms with any of the stock props on it. At the dock with a static test or on the water moving. You do get better rpm on the water under power but not up to nominal.

I would like to run some tests with it on a dingy. So far almost everyone is running it on small displacement boats. We have had a couple post in on small boats or dingys and not be terribly happy but they haven't been able to give any detailed results such as rpm and speed vs boat weight and type.

I also am still planning on testing with the prop tohatsu guru has spoken of as soon as I find out if im returning my engine or not. Life has been busy lately and this has all been on a back burner for the nonce. Once I get some time I will jump back into generating some more hard data with dingys and the max thrust prop on sailboats.
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  #103  
Old 11-05-2007
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"I have a Nissan 6hp 4 stroke. It has been dissappointing as well. I've also had it back to the dealer several times and have talked to a factory rep a bunch too. Mine doesnt idle well. Idle stop is set and either it runs too fast or dies, pick one. I dont even know if you can adjust the low speed jet? It also hesitates greatly when you hit the gas? Anyone else? I have it on a grumman sport boat and I get "slipping" at high speed, played around with all different pitch props Nissan supplies, to no avail. Really scratching my head over this one. Might have to do with the boat but the hesitation still sucks."

You probably have an obstruction in the carburetor. The carburetor has fixed jets that are not adjustable. The solution is to have your dealer replace the carburetor.
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  #104  
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The problem is similar to having a car with one gear, and you are having to change wheel sizes to suit the application.
A heavy, resistive car needs small wheels.... a light, non-resistive car needs big wheels to get the best out of it.
In the bollard test of the outboard, that is similar to having the car tied to a bollard and spinning the wheels, both large and small. Big coarse-pitch prop equals big wheels. Small, fine pitch prop equals small wheels.

If the wheels are too small, they spin easily, and the motor cannot see enoughy resistance to give its max power .
If the wheels are too large, the motor labours, cannot reach optimum revs and so cannot give full power... there is too much resistance.

Max power is stated at a certain rev with throttle wide open.

The throttle must be wide open, or the motor is not giving its best.

The load must be adjusted to give optimum rev with throttle wide open. This is easy with a dynamometer measuring shaft power in the test 'shop, but not easy with a bollard pull test, as it is not practical to measure the power when the motor is stirring water.

It could be done in the water if you could measure the forward propulsive force of the motor where it bolts to the transom... with a load cell, or something. Let the speed settle, then multiply that force by the speed of the boat, and you've got the power output at that moment.

The equation is...

H = u*F

H : power (typically Watt)
u : boat velocity (typically m/s)
F : measured force (typically Newton)

End of sermon.
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  #105  
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Sorry folks, I should have said that is the USEFUL power delivered by the prop.
There is a lot of useless water-stirring still going on, that every prop sufferes from.
In fairness, it is not a direct comparison to shaft horse-power from the motor, as prop efficiency needs to be factored in, and is ever-changing.
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  #106  
Old 11-27-2007
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Pigster Prop

Anyone Have Results Running That Pig Prop With This Outboard On A Boat That Has A Hull Speed Of 6 To 7 Knots ?
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  #107  
Old 01-25-2008
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Smile Outboard performance

One of the things I found out when working in manufacturing is that the performance of identical products coming off a production line can vary enormously from one item to the next. If a customer wanted a performance gaurantee we had to test the item before we shipped it to him. The "certified" test results would be sent to the customer along with the tested product. With a normal customer we would ordinarily sell him something that would be "oversized" to be sure that it would at least meet the advertised performance. To make sure you avoid this problem you have to buy a product that has enough capacity to take into account manufacturing variables. Thanks. Steve S.
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  #108  
Old 01-25-2008
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Quote:
Originally Posted by sstuller View Post
One of the things I found out when working in manufacturing is that the performance of identical products coming off a production line can vary enormously from one item to the next. If a customer wanted a performance gaurantee we had to test the item before we shipped it to him. The "certified" test results would be sent to the customer along with the tested product. With a normal customer we would ordinarily sell him something that would be "oversized" to be sure that it would at least meet the advertised performance. To make sure you avoid this problem you have to buy a product that has enough capacity to take into account manufacturing variables. Thanks. Steve S.
I have tested more than one of these engines now and they give identical performance. If it was just one engine that was running really weak then I would be very disappointed in the dealers and tohatsu's response to an engine that produces only 2/3 of its rated power. If this is just how the engines' run; then I am very disappointed in their honesty as a manufacture. Regardless, you could correctly say I am disappointed in the product and in the support of the product i got from them.

My flat out recommendation to anyone going from two stroke to four stroke is to multiply your current hp by 1.5 to get equivalent performance. You will need that extra 50% power just to stay even with your old engines output.
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  #109  
Old 01-26-2008
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I think that the support that you got was what was warranted under the situation. IE. There is nothing wrong with the engine, so there was nothing that they could do for you. In the end this is about your dissatisfaction with the performance of the engine based on your expectations. You made the decision to purchase this engine without any advice or consultation with either the dealer or the manufacturer. If you had asked you would have been told that four-stroke engines develop their power differently than 2-strokes and that hp ratings are at WOT and that if you cannot develop the RPM you will not get the hp. The fact that no one else in the U.S. is complaining about the power output of this engine should be a big sign to you that your complaint is unwarranted.

Does that mean that you are 100% wrong. No, it does not. But, your complaint should be to the Federal Gooberment that determines HOW the engines are rated. Prop-shaft rated HP is how all outboards are rated. It's done at WOT. And that's it. It's not a plot, mistake or stupidity by all the manufacturers. It's Federal law and even the Feds are caught in the technology problem between four-strokes and two-strokes. When they came up with the way outboards would be measured no one considered that 20 years later 4-stroke engines would actually be out on the market in any kind of quantity. The solution would be to have three hp measurements in the specifications for every outboard. Say one at 1/4 throttle, one at 1/2 throttle and the last one at WOT. Then you would be able to cross compare brands and models with some idea of actual performance differences. Call your congressman and get that lazy, worthless, piece of crap to do something about changing the regulation.
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Last edited by TohatsuGuru; 01-26-2008 at 07:46 PM. Reason: Furthered the rant
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  #110  
Old 01-26-2008
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I will agree with you to a point on support from tohatsu usa. They did try to work with me over the phone actually they went out of their way to help me. However once I took the engine to a service center they dropped the issue with me when the service center said it works fine. I'm sorry but that service center isn't competent. It's a shade tree dealership that somehow got to be a service center. I wish I could show you through my eyes the workshop and the conversations I had with the owner. You would understand why I say this if I could show you. Dealing with the people around this transaction has been a mixed bag..on the con side a dealership that would not return my calls or emails until forced to and an incompetent service center. on the plus side tohatsu usa phone support did "try" to be helpful. Also your dealership who wasn't even involved in this transaction has impressed me with your willingness to back the product and try to help. I will give credit where credit is due but I will also call a spade a spade.

In the end the engine in operation doesn't have the umph to turn the lowest pitch prop made for it at 5500 to 6000 engine rpm which is where it develops it rated hp. Point blank.

Pre purchase. I did talk with the salesman about the application I was putting it to and what engine I was then using that It was going to replace with it.

One of the biggest irritations with the whole support thing was the total non response by the dealer up till the point I filed a credit card dispute.

I have at this point lost the credit card dispute and I do still have the engine. Since I can't afford to purchase another one I will just have to live with this one. It doesn't give me the extra power I had hoped to get by buying an engine 50% more powerful than my last one. It actually performs just under the old one.

I don't doubt that that shaft turning at 5500 to 6000 engine rpms will produce 6hp. As I guess the government certifies? However my engine and the other engine I have tested could not turn the stock 8 or 6 pitch props with enough power to get even close to that rpm. Maybe those two engines weren't personally certified by the government.

I have fought this thing out as far as I can. In dealing with the whole credit card dispute, I have found being the little guy sucks but thats about par for the course for this whole thing. I'm not willing nor have the resources to pursue it further so I'm going in to in this situation now focus on the future and give what has gone before a rest.

As we have spoken of before I will be ordering one of the Special High Thrust props you sell sometime this spring when money is a bit better and I am able to get my boat back in the water. I will run all my tests again with that prop and report results and on the water performance to contrast it with the stock 8 pitch and the stock 6 pitch that have been tested already. As I am very interested in seeing what kind of difference we will see. We have a local prop shop here that I'm also thinking about talking to about a custom prop. maybe a slightly bigger 5.5 or 5 pitch prop... I think I will try and take the prop off the old engine in also and get them to tell me what the pitch and size are compared to the 8 and 6 from tohatsu. Actually I can probable take a ruler and figure that out myself. maybe something to play with soon.

I am also going to try and borrow a dingy and run the on the water tests with the engine on it to see exactly how it performs with a super light boat with each of the props. I also plan on doing a pull test of my boat with another boat and the inline scale to give me solid numbers on a hull speed vs (friction resistance) curve for my hull.

i do bitch about things I think should be bitched about but I don't do it based on "I feel" I do it based on here are some concrete numbers. Every step of the way I have used modern technology to test and measure what I have reported in a scientific manner. Hopefully by this summer I will have the next set of numbers ready to be published. Maybe with the high thrust prop we will get some pleasant numbers. I hope so.

Last edited by audeojude; 01-26-2008 at 10:33 PM.
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