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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Outboard
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  #121  
Old 04-02-2008
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Rockter will become famous soon enough
Cardiac...
You are being much too hard on this owner.

The manufacturer of an outboard will not state power at the prop... what you call shaft HP I reckon.

It would be better to state power at the prop shaft. That would do for me. Bollard pull is of some use, but you will not measure power from it.

This outboard sales stuff is like trying to sell a motorcycle engine without knowing the wheel size, the final reduction at the chain, the weight of the bike and the weight of the rider(s). The only way to sell that engine is to state its power at the crank (or gearbox would do) at stated crank rpm... that's really all you can do.

The questions that Audio asks are all very reasonable. I asked a few of them when I first bought an outboard, back in 1990.

Bollard pull has become some sort of standard, but it is so dependent on the prop choice at that moment.
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  #122  
Old 04-03-2008
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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.
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  #123  
Old 04-06-2008
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Horsepower is horsepower it is a standard for doing work.

This motor started as 4 HP and went to a 5 HP and then to a 6 HP with out a change in volume or compression.

Let us see if we can find out where the horse power went!
29.92 inches of Hg at 59 F is what is used as a standard day to measure from for aircraft performance purposes and dynometers correct reading for these standards and more.
Less pressure equals less HP.
More temperature equals less HP.
More alcohol mixed in the gas equals less HP. (If not jetted for it)
Premium fuel when regular is called for equals less HP. ( You are not doing you engine a favor)
Adding an accessory after the HP has been established (charging system) equals less HP to the prop.
Let's deduct 5% for manufacturing variables such as a minor difference in compression ratio, lump in the intake track, cam timing variance, piston height, deck height, valve clearance, gasket alignment just a bunch of differences between the original prototype and the production engine.
Now the big hit, 10%! The engine was designed and tested to meet EPA air quality laws on pump gas without alcohol. It was jetted very lean then. Now with the addition of up to 15% alcohol before they have to call it GASAHOL(?) it is running so lean that it cannot equal the HP of the test engine by a long shot. Add to that the fact that most marinas only carry 91 octane and you could loose even more.
How about 3% for the alternator, it takes HP to make electricity.
2% for the weather.
So there you have it, 20% loss or 1.2 HP. Now your 6 HP motor is only putting out 4.8 HP.
What about the power of my 4 HP 2 stroke I hear. Well, two stroke engines are delivered jetted rich because they run leaner during the break-in period because the extra oil leans out the fuel mix. Two strokes are still jetted rich to protect them from over heating and sizing or galling a piston. The new gas is leaning them further and bring them into the sweet spot for max power but any more and they will start to self destruct. I will guess your 4 HP is putting out closer to 4.4 HP (10% increase and no loss from an alternator)
Who to blame?
Blame the manufacture 5%, you for 3%(charging system), the government 10% (epa and fuel) and God 2%.

On second thought don't blame God (cuz he will give you some great sailing days to make up for it).

Let's try to drop your losses to 7% or less.

The easy stuff first.
I am sure you have fresh gas of the 87 octane the book calls for.

Remove anything that obstructs your view of the intake side of the carburetor. Now turn the handle all the way open and check the throttle plate is 90^ to the air flow and has not over shot the mark and started to close. If it has look for a stop screw or linkage and adjust.

Next take a plastic straw and slide down the carburetor throat, holding tight to the side and see if hangs up on any lips that may protrude at the junction of the engine and manifold or carburetor and manifold. I hope not because the carburetor will need to come off.

Is there an air filter or flame arrester for the carburetor? Make sure it offers no restriction. Put every thing back together.

Did you find anything? Get out the instruments and run a base line pull test. Log and add the barometer and temperature readings.

Disconnect the battery, any change?

So now you have broken the dock lines and shot out into the bay looking much like a torpedo running straight and hot! You can stop testing now. Nope?

Take off the prop and let's check the rev limiter, the brochure says it has one. I guessing the motor should top out at over 6,000 rpm. It should rev this high without the prop if not then the problem may be electrical.

I say that before we get into jetting we throw off the lines and go for a sail. This is a good time to see if any small changes have gotten us over the hump. Nope? then on to jetting.
Jetting, or re jetting as it can be called is not hard if you use the rule of two.

THE NUMBER ONE RULE IS TO ALWAYS SET THE FLOAT LEVEL PER FACTORY RECOMMENDATIONS BEFORE ANYTHING ELSE!!!!!!!!!!!!

The Rule Of Two
Always go up two sizes.
Improvement? Go up two more sizes.
Keep going up by two until you start loosing power then go back one.
Go back one more if necessary.

Now there is a slight chance that the carb is running to rich and you will have to lean it out. The rule of two still works.

I will bet you won't find a large array of jets for this carb so you will have to drill them. The factory will probably have a leaner high altitude jet but none larger than stock. Take your stock jet or a replacement to a good hobby store and check for availability of letter sized drill bits(I think they are the smallest). The trick will be finding a size that will fit in your jet and then use the rule of two and buy three larger ones. If you don't have a ---PIN VICE---get one there too. ( holds tinny bits so you can drill with your hands or put into a regular drill.---Use your hands)

You can solder the hole closed and then re drill it smaller if you are good and don't get any solder on the threads.

Good luck
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  #124  
Old 04-06-2008
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Rockter will become famous soon enough
Timebandit...

I needed you to set the mixture on by olde 500 cc 2-cycle Suziki. Move to Scotland. It took me days to get it to idle and pull from low down.

This is good stuff, but advanced. It really is too much for the average (or non-average) owner. The two-cycle motor is notoriously vulnerable to weak mixture. Do you really think they sold it running that weak? I understood it fries the piston if so. I am forever checking plug colour. So far I have been lucky. Is it not easier to lift the needle a notch and settle for richer low-to-mid range?
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  #125  
Old 04-12-2008
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Rockter

The carburetor in question is more like a lawn mower then motor cycle's.

Audeojude's
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  #126  
Old 04-21-2008
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Another opinion

Using a Mercury 4hp (same as Nissan/Tohatsu) on a Grumman 17' square stern canoe proved to get excellent results. I know it's not a sailboat but here goes anyway. After fully broken in, the oil was changed (10w-30) and the valves were adjusted to specs. They were quite loose after the break in process. With a tachometer used for checking small engine RPM we set out to fine tune the engine. With 2 persons aboard it would top out at 19 knots at 4700 rpm. Not too bad we thought. But, my friend thought we could improve on that. He ordered a carb and carb base plate for the 6hp version from the merc dealer and we swaped the units. No other changes were made. The standard prop is a 7.8x7. This proved to be an improvement in power. We were able to get 5600 rpm with a GPS speed of 22 knots.
Another good motor is the Yamaha 4hp. I just got one for my canoe (same model). It produces the same power as the 4hp Mercury but is an 8 lb lighter motor. Not a big deal to you sailboat folks but it is when you have to carry these things around like we do. Another thing I like about the Yamaha is, it doesn't pull to the right like the Mercury does. That is a common tendency for outboards but the Yamaha seems to have solved it.
The true beauty of these new small motors are their economy and enviromental cleanliness. We can run around for 2 hours on less than a 1/2 gal. of fuel. And if we are trolling, it just sips it. And...2 strokes are banned on most lakes anymore and for very good reason. They are stinky, polutting, and just plain wrong considering availability modern technology.
As for vibration..we use tiller extensions with these boats and the dampens it quite well.
I hope this helped..
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  #127  
Old 04-28-2008
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Quote:
Originally Posted by Ajaxman View Post
Using a Mercury 4hp (same as Nissan/Tohatsu) on a Grumman 17' square stern canoe proved to get excellent results. I know it's not a sailboat but here goes anyway. After fully broken in, the oil was changed (10w-30) and the valves were adjusted to specs. They were quite loose after the break in process. With a tachometer used for checking small engine RPM we set out to fine tune the engine. With 2 persons aboard it would top out at 19 knots at 4700 rpm. Not too bad we thought. But, my friend thought we could improve on that. He ordered a carb and carb base plate for the 6hp version from the merc dealer and we swaped the units. No other changes were made. The standard prop is a 7.8x7. This proved to be an improvement in power. We were able to get 5600 rpm with a GPS speed of 22 knots.
Do you have any more info on this conversion? I tried PMing you but it wouldn't let me because I don't have 10 posts. I read online that I jsut needed to replace the jets and I tried this but it didn't work. Then I heard you can just replace the carb but I haven't heard anything about a carb plate. Please send me any info you have. Did your friend REALLY do this and it worked? If you could send me any part #s or prices it would be greatly appreciated. -Chris M..... Please email me vtx531(@)gmail.com
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  #128  
Old 07-06-2008
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My dealer finally solved the problem of the motor not holding straight when running. He filed a flat spot on the shaft and changed the bolt/wingnut to one that tightens down on the shaft to positively hold the motor. Very tight now. The downside is that to rotate the motor now you have to reach around and loosen the wingnut. Works fine on a sailboat since we steer with the rudder.
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  #129  
Old 07-15-2008
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I had the opportunity to take my engine out on a 8 ft rib dinghy a couple weeks ago. up on plane with just me in the dingy the engine just barely hit 5000 rpm at full throttle, still a 1000 rpm below the max rpm of 6000 rpm. I still say these engines are marketed as more powerful than they actually are. This is fine if it is what you expect. it runs good, gets good fuel economy etc. but it sucks paying 6hp prices for a 4 hp engine.

I have also tested a second one of these engines with identical results.. my opionion is that Tohatsu/Nissan/Mercury full well know that these engines don't run to spec and just ignore it. I got hammered in my credot card dispute with them. The local service center wouldn't test the rpms. just cranked it up and said it was running good/smooth even though I asked them to check the rpms under a load. When I got a third party marine place to verify that the engine was not running to spec they (Tohatsu/dealer?) had their legal department land on them like a ton of bricks and withdraw their supporting documentation for my case because they were not certified to do those tests. I found it interesting that they felt the need for that level of response on a simple credit card dispute over one engine. My personal opionion is that they don't want a precedence set for this even though they are aware that it is an issue.

I would imagine it is very rare for someone to go to the lengths I did to verify the correct operation of one of these engines and just normally isn't an issue for them. As some have said elsewhere I am admittedly a bit anal about details such as this. So every one beware. These engines as a model across all three brands are about 20% less powerful than claimed by Tohatsu/Mercury/Nissan. It's a lot of money to spend extra on a 6hp engine to only be getting about 4hp.

One of these days I am going to buy the high thrust prop from tohatsu guru and see what it does. Right now the boat is still sitting in the yard. Economy sucks and I can't afford to keep her in the water right now.
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  #130  
Old 07-10-2009
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It's been a year since the last post here but I've recently purchased a 6hp Tohatsu (came with the boat I recently bought) so I read the whole thread and found it interesting. Audeojude, do you have an update for us?

When I was negotiating to buy my boat, I had figured on selling the 6hp Tohatsu and buying a dedicated H/T engine. But I'm not so sure anymore, although my boat weighs about 3000+lbs. I'm thinking of trying the 8 3/8" HT thrust prop before I spend the $$ on a new engine. Even if I have to have it pitched lower than the available 6" pitch (lowest available pitch on the market if I understand correctly) it seems like a reasonable roll of the dice to me.

I agree with Audeojude that it seems odd that anyone would market, as 6hp, an engine that has no available prop that'll allow it to reach the RPM needed for the rated HP. On the other hand, I disagree with Audeojude that the engine CAN'T be 6HP since he's apparently never been able to test it at the rated RPM level. -shrug- or maybe I missed something (it's a long thread lol!) Seems to me he just needs a prop pitched to something that's not a standard off the shelf pitch.

Anyhow, if anyone, especially Audeojude can give me an update on the discussion here, and any tests and/or findings regarding this subject, I would appreciate it. I like my little Tohatsu engine although, even before I bought it, I've considered it unsuitable for pushing my sailboat. But if the H/T prop makes enough difference, I can just extend the shaft (currently it's 20") and add a charging system and save quite a bit of money (and weight) over buying a new H/T 8hp Yamaha.

Rick
BTW, this is my first post here. I hope I did this right.
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