Over revving

At what point does over revving a two stroke engine damage the engine?
i.e. how many revs above the WOT.(18hp Evinrude 1964 model)

 

There is no number, the manufacturer gives you the safe RPM range, above that and you're on your own.

 

I suppose the age and condition of the engine has a lot to do with how far to push it.
Not that that is my intention,but if the engine is capable of higher revs than advised,it's a point to watch I suppose.

 

That's called hot-rodding and even though it can make you go faster, the life of the engine will be severely reduced.

 

I believe the 1964 18hp Evinrude is rated 18 hp at 4,500 rpm. Operating range is 4,000 to 5,000 rpm.

Prop it so that it does not exceed 5,000 rpm.

 

Just what I was after VinTin,thanks

 

If the engine is rated at 18 hp at 4500 rpm, it is unlikely that this engine would have produced more horsepower at either 4000 rpm or 5000 rpm.

It appears that you do not have an abundance of horsepower for the boat that you are trying to get on step so it is probably not a good choice to prop for 5000 rpm where the actual horsepower available to the prop could be less than the 18.

Ie if the engine could have produced 20hp at 5000, then I am sure that Evinrude would have advertised it as such

So prop it so that the engine will max out at 4500 and you will get every bit of power out of the engine that it can produce

 

That's not the correct way to prop an outboard, you target the very top end of the range with a light load, this brings it down a bit when it's loaded with beer and other less essential supplies. Otherwise once you do load it down you're lugging the motor, they tend to self-destruct sooner when being lugged.

This also allows the boat to get on plane easier and faster.

 

It certainly is a tricky subject,on a road vehicle,adding a gear reduces the rpm and enables
a higher speed to be achieved.This of course,does not increase the bhp of the engine,but simply utilises it.Which,in theory,is why we have different prop sizes.
The common denominator is obviously to control the engines revs to prevent damage.
Until the weight of the boat increases more bhp is not needed.
Should top speed be the aim,then more bhp will be required,and be controlled by a suitable prop.
After spending years,trying to get more power out of road vehicle engines,my conclusion
is improving the power of a marine engine will only get results with the final drive,which is
the prop.
Very interesting subject.

 

The op's OTHER thread that he started stated that with his current prop his boat will not get on plane.
Over propped to be sure
An 18 hp on a 14 foot boat is not a lot of horsepower for a boat this size, assuming that it is a normal 14 FOOT boat, ie fibreglass, etc

The engine is rated at 18 hp at 4500 rpm, the max available hp so changing props does not change the output that the engine can at max produce at 4500

So lets say you pick a prop that can reach 5000 rpm in a normally loaded boat. Assume also that this engine will not produce 18 hp at 5000 as its advertised hp is 18 at 4500.

Take a guess at 16 hp at 5000 rpm and you match a prop to hit this rpm. Assume that a prop curve is linear from 4000 rpm to 5000 rpm, just to make it easier to understand.

Then the prop will absorb 16 hp at 5000 rpm and maybe 14 hp at 4500 rpm. mmm???

You are already underpowered because you cannot turn the 9 x 11 at enough rpm to get the boat on plane as the 9 x 11 takes to many hp ( or it is cavitating like heck) at sub plane speeds)

It makes not sense to pick a prop that will produce 5000 rpm which will use less than the available hp at the max horsepower at 4500 as advertised.

There is no argument that the 9 x 11 is overpropped, but the target should be to match the max hp with the max hp of the prop.

It is also unlikely that a difference of 500 rpm will "lug the motor" and "self destruct"

Certainly the prop must be propped to get the boat on plane, 9 x 10 or 9 x 9 or whatever. Assuming that either prop will get the boat on step, I stand by my original comment that if you pick a prop for 4500 rpm, normally loaded boat, then you will get the 18 hp out of the motor.

The parameters of this boat is that with the 9 x 11 or whatever the op has, it is on the other thread, is that he cannot get the boat on plane. Ie limited hp


Another aspect is the speed of the boat as the inlet velocity to the prop can change the thrust characteristics of the propeller significantly

I recently repropped a 12 foot RIB, 15 hp, efi, that had a 9 1/4 by 11 prop, all up weight with passengers was about 850 pounds and it would struggle to get on plane. A change to a 9 x 10 reduced top speed but got the boat on plane with little effort

 

Barry I believe what you are suggesting may well end in outboard motor destruction when run wide open with a light load.

And that is why it's not done the way you are suggesting.

You are also making light of 'lugging' this old two stroke outboard motor. That is a big mistake.

 

This is not the correct method for propping an outboard.

 

If you select the prop to 4500 rpm and 18 hp, that means that the engine rpm will not go past this rpm

WOT is 4500 rpm,
Do not forget that the op cannot get this boat on plane. He needs a prop that will enable the engine to produce 18 hp and I believe it was you who said that the range was 4000 to 5000 rpm

Lugging, just so we are on the same page, why don't you explain your definition of lugging an 2 stroke engine for a boat. Propping the boat as I have suggested will get the engine into your suggested rpm range.

 

I'm done.

 

I'm tipping the old motor would be much the same, tired.