Prop pitch

Youve seen a merc grab a piston--- not necessarily from over proping.
Running an engine 200 400 rpm low is not serious at all.

 

2 stroke outboards can't be compared to diesels and 4 stroke petrol engines, they definately do not like overloading. our case was probably a combination of overload and poor quality fuel , a lot of the petrol we buy here is rubbish. all i know is , we had no problems until i put the larger stainless prop on.

 

So stainless props are a problem ?? but stainless props are surposed to be much better and stronger then aluminium !!. so why did it cause the problem ???

 

sorry to cause confusion, it was a larger pitch prop, i shouldn't have said what it was made of.

 

Sorry i couldnt resist the temptation to pull your leg and leave out some of the quote about it being a bigger pitch . !!

 

Oh --so its the fuel now then. well can you explain the mechanical reason for the 2 stroke merc piston to 'pick up' with iether over loading or bad fuel?

 

jfair,
If you want to take advantage of all the power your engine can produce, then I'd advise going with conventional wisdom and prop the boat so it will top out at whatever the manufacturer's recommended RPM is. This should be with the boat loaded to what you would consider an average load and a healthy engine in a good state of tune. This also means that you have to have an accurate tachometer.

If this power level is less important than fuel economy is, then I agree with Frosty's posts, however Eyeschulman does make a good point.

The statement is often made is that if you over prop an engine so that it cannot attain the manufacturer's full rated horsepower RPM it will be overloaded, lugged, or whatever term you like, at all speeds.

I have a major problem with that statement. First of all because it is way too general. What kind of engine in what kind of boat are we talking about here? Heavy displacement hull, semi planing cruiser, planing high speed boats? Outboard engines? Inboard? Gas? Low speed naturally aspirated diesel? High speed turbo intercooled diesel? How about the amount of power in the boat, and how it relates to the owner's particular boating style? Way too many variables to make that kind of general statement.

I do know one thing for sure though. Gasoline inboards are generally quite inefficient at light loads, efficiency increases with load to where it peaks at about peak torque and maximum manifold pressure. This is not best economy, but best efficiency. You can "over prop" to where your cruising speed is the best compromise between engine revs and manifold pressure, taking into consideration what you expect for engine life without overloading the engine at all. But you have to know what you are doing, have access to your engine's power and torque curves, have good instrumentation, and realize that you will not be able to get full rated power out of this engine any more. If you try to, you will be in an over load situation and you will damage your engine.

Dave Gerr's "Propeller Handbook" touches on this subject and is well illustrated with the relationship between an engine's power and torque curves and a propeller power curve.