why small outboards is so unpower?

[matmar6974]

I know that scooter 125cc-150cc have 9-12 hp . Why the outboards -like simple 4stroke machines-have only 5(tohatsu 123cc mfs5) or yamaha 6hp/197cc ? Either the china motors duck/scooter 110cc have 8hp. I have one of this.

 

[pine island fred]

Re: why small outboards is so unpower?

Never gave it much thought. More than likely it is for longevity and reliability. Outboard is expected to put out max continious power over a long period of time where as a scooter does not. How many hours are actually put on a motor scooter? The more you derate a motor, the longer it will last. Believe also that the small scooter motors are 2 stroke which produce power on each stroke and the 4 strokes you mentioned have one power stroke every second revolution. FRED

 

[Chris1956]

Re: why small outboards is so unpower?

some Outboards were designed to have just as much power as your scooters. For examply the Merc inline 6 1500 had 150HP in a 99 cu in (1622cc) displacement. That is 10.8 cc/HP, which is about what your scooters are.

Now the drawback of the smaller displacement outboards is less torque, and less torque is not a good thing in an OB. Those Merc inline six motors had a specific powercurve. If the prop had too much pitch or the boat was to heavy for the prop pitch and motor power, the Merc towers would not rev up at all. For example, I could pull two skiiers on my speedboat with a 21" pitch prop. I could not get above idle speed with a 23" pitch prop and a single skiier.

 

[Silvertip]

Re: why small outboards is so unpower?

Want another comparison? One of my RC airplane engines is just shy of 1 cubic inch (.91 to be exact). 2.2 HP but at 15,000 rpm.

 

[RogersJetboat454]

Re: why small outboards is so unpower?

HP is not everything. I would question what standards a Chinese scooters HP is rated by anyway.
Torque is what makes things move, horse power is a measurement of how quickly work can be performed.
I could buy a .21 R/C nitro engine rated for 3HP. That doesn't mean that you could hook it to a prop shaft and move a boat with it (other than a scale model), as it produces very little torque.

 

[matmar6974]

Re: why small outboards is so unpower?

Oκ. I thing that i learn some things because i'm not a mechanic.If i understood right the rpm and the torque are the most important.Either after of all those answers i found from the manual of scooter that the 8,5hp are on 8000rpm. I know that 500/1000 more rpm improvement a 5hp outboard to 6hp .So this explanation to me why the scooter have more power with same cc.Thanks.

 

[steelespike]

Re: why small outboards is so unpower?

Marine engines are constantly working like the are going up hill.Its likely they may routinely run for extended periods at peak rpm. A scooter is generally on the level and usually would operate at peak rpm anywhere from a few seconds to a few minutes.There are any number of outboards that developed more that 1 hp per cu in dating back to the 50s.Those below 30 lag behind a little possibly due to developement cost in relation to price.There are a few that make it to one for one or better in the 60s.Also motors under 30 are working harder pushing more pounds per hp all the time "uphill".

 

[TOHATSU GURU]

Re: why small outboards is so unpower?

Oκ. I thing that i learn some things because i'm not a mechanic.If i understood right the rpm and the torque are the most important.Either after of all those answers i found from the manual of scooter that the 8,5hp are on 8000rpm. I know that 500/1000 more rpm improvement a 5hp outboard to 6hp .So this explanation to me why the scooter have more power with same cc.Thanks.

Torque, is for all practical purposes irrelevant in an outboard discusion due to fixed gear ratios. Since you cannot vary the gear ratio, to take advantage of any special point in an engines torque curve, what makes an engine superior at X hurts it at Y. The only thing that is important is prop shaft hp at a given RPM.

 

[Silvertip]

Re: why small outboards is so unpower?

The displacement of an engine limits the maximum amount of torque and HP it can develop when naturally aspirated. A good example of this is the Chevy small block. Anyone that has ever looked under a hood knows that there are more HP ratings for a 350 (5.7) GM V8 than one can list here. It is all about the "state of tune". Torque on that engine is moved around the rpm band but is basically in the 290 lb/ft range to about 350 or so. HP on the other hand covers a much broader span. In 1979 that engine was rated SAE NET at about 165 HP. Performance versions of the crate motors and in Corvettes were rated in high 300's. Whether two stroke or four, when an engine goes into a boat it must be tuned to get the boat out of the hole, cruise at a comfortable rpm, and have a reasonable wide open throttle RPM -- and now the most important part -- all without a transmission. One gear. Even a scooter has a variable transmission so it's engine can be tuned much differently than one for an outboard. Try starting out from a traffic light with the scooter locked in high gear (if that were possible) and you will find out in a hurry what a slug it really is.

 

[Outsider]

Re: why small outboards is so unpower?

Outboard HP is measured at the prop, your scooter wouldn't be 9-12 HP if measured through a transmission ...